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Five Decades Ago From the УTransuranicsФ to Nuclear Fission.

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Five Decades Ago: From the “Transuranics” to Nuclear Fission **
By Gunter Herrmann*
The discovery of nuclear fission is one of the most outstanding episodes in the history of
chemistry: It starts in the spring of 1934 when Enrico Fermi and his group irradiate uranium
with neutrons and seem to succeed in going beyond uranium, the then heaviest known element,
reaching the first transuranic element; it continues with Otto Hahn, Lise Meitner and Fritz
Strassmann who believe to have found additional transuranic elements; with IrPne Curie and
Paul Savitch who observe an activity which somehow does not fit into that scheme; again with
Otto Hahn and Fritz Strassmann who first identify this activity as radium but then on the 17th
of December 1938 after rigorous chemical tests realize that the activity is instead barium, thus
discovering the fission of the uranium atom into two lighter nuclei; and with Lise Meitner and
Otto Robert Frisch who explain nuclear fission on the basis of an already known nuclear
model; Otto Robert Frisch finally performs a physical experiment on the 13th of January 1939
which corroborates the fission of uranium. This discovery of nuclear fission is not only an
event of historic dimensions, it is also an excellent example of how science evolves, not by
successive logical steps but rather through strange detours.
This element should be radioactive and, if it is again a
p--emitter, might even decay into another transuranic element, 94 [Reaction (3).
1. The “Transuranic Elements”
1.1. Rome 1934: Element 93, Eka-Rhenium?
When Enrico Fermi and his group irradiate uranium with
neutrons in the spring of 1934 they do not figure at all on
fissioning uranium into two medium sized nuclei. Their intention is quite different: They want to go beyond the heaviest naturally occurring element in the periodic table, uranium (atomic number Z = 92) and to reach element 93, the first
transuranic element. This became feasible after Fermi and
co-workers succeeded in obtaining radioactive isotopes of a
number of elements by irradiation with neutrons, which had
been discovered only shortly before. In contrast to a-particles, which were previously used to induce nuclear transformations, the uncharged neutrons attach very easily to the
positively charged atomic nuclei, even in heavy elements.“, 1’
In this case one obtains isotopes of the starting elementr2’31
which subsequently undergo p--decay. In p--decay the isotope emits an electron from the nucleus and is transformed
into the element with the next higher atomic number. The
stable silver isotope 109 ( Z = 47), for example, captures a
neutron to form silver-110 which decays by p--emission with
a 24s half-life into the stable cadmium-1 10 [Reaction (l)].
If the same kind of reaction could be induced also with
uranium, one would expect formation of element 93 [Reaction (2)].
2 ; : ~
Prof. Dr. G. Herrmann
Institut fur Kernchemie der Universitat
Fritz-Strassmann-Weg 2, D-6500 Mainz (FRG), and
Gesellschaft fur Schwerionenforschung
Planckstrasse 1, D-6100 Darmstadt (FRG)
Based on a paper “Discovery and Confirmation of Fission” presented at
the conference “Fifty Years Research in Nuclear Fission” in Berlin on
April 4, 1989.
Angew. Chem. Int. Ed. Engl. 29 (1990) 481-508
This concept seems indeed to be correct: In several publiFermi et al. report on at least five p--emitters with half-lives of 10 s, 40 s, 13 min, 40 min and about 1 d
which are observed in their Rome laboratory after the irradiation of uranium with neutrons.
It is not sufficient, however, to simply rely on the detection
of p--emitters. One has to prove the formation of the new
elements in a more direct way which can only be achieved by
chemical methods. For this purpose one relies on similarities
between the elements of the periodic table: One attempts to
prove that the activities,[*’ which are ascribed to the
transuranic elements, behave chemically in a way which is
expected according to their location in the periodic table.
This concept is shown in Figure 1.
The task of element identification is quite difficult, because the p--emitters are produced in tiny amounts which
are neither visible nor weighable. The only means of identification is their radioactivity which allows one to detect even
single atoms. These problems, however, did not appear for
the first time, because some of the naturally occurring radioactive elements were also found in such small quantities.
Therefore, as early as at the end of the last century, radiochemists had to find out when trace amounts start to behave
chemically in the same way as macroscopic quantities.[’’
Radium, for example, is co-deposited with a number of insoluble barium compounds, because its corresponding salts
are also insoluble. Later, when radioelements were accessible
[*] In the historical context we use the then common word “activity” or
“body” to name a radioactive species; the more modern expressions are
“nuclide” or “isotope”. We also use the old nomenclature of isotopes even
though it is somewhat misleading. Mesothorium 1 (MsTh l), for example,
is not a thorium but a radium isotope. We will, however, usually add
today’s nomenclature with element symbol, mass number, and often also
the half-life; for example: Mesothorium 1, 5.8-y radium-228.
Verlagsgeseilschaft mbH, 0-6940 Weinheim,1990
48 1
in visible amounts, these findings obtained with unweighable
quantities were confirmed. On occasions, however, irregular
chemical behavior had also been noticed.
I,$ sI:
' ; 1 I6fd lsJb
l6/10 Is: : , 1
I,'" 1
Fig. 1. The periodic table of the elements in 1934. Missing elements at that time
are shown as shaded boxes. The present symbols for the elements are used,
except for masurium (43).
Thus, for the activities ascribed to transuranic elements it
has to be shown that they behave in the same way as their
homologous elements during characteristic chemical operations. For these tests Fermi et al., and all other groups who
perform experiments with transuranic elements, rely on two
assumptions which seem to be justified by both theory and
experiment but which later turn out to be wrong: First, as a
result of nuclear reactions-whether they occur spontaneously during radioactive decay or whether they are induced by particle irradiation-the atomic number changes
only slightly. Thus, only elements in the vicinity of the initial
element can be formed. Second, in the periodic table thorium, protactinium, and uranium are placed in the 4th, 5th
and 6th group of transition elements; therefore, the sought
after element 93 should belong to the 7th group of transition
elements, it would be eka-rhenium, and the elements 94
through 96 would be eka-platinum metals. Therefore, one
has to prove chemically that elements to the right of uranium
are formed and that elements in its close vicinity to the left
can be excluded.
All experimental facts known until thenL3]suggest that the
products of nuclear reactions are confined to the vicinity of
the original nucleus; a differing phenomenon had never been
observed. This view is also reflected in a then introduced
illustrative description of nuclear reactions."'] But also theory corroborates this view. According to Gamow's theory of
a-decay,[121one of the early triumphs of quantum mechanics, even the a-particle with its two nuclear charges can hardly tunnel through the electrostatic Coulomb barrier of the
nucleus; for larger fragments this barrier increases rapidly
and tunneling becomes extremely unlikely." 31 This reasoning is indeed correct; only decades later is the extremely rare
emission of carbon-I 3 nuclei from radium-223 discovered.[I4*The crucial point is, however, that the breakup of a
heavy nucleus into two fragments of similar size should not
be treated as a tunneling process. Quantum mechanics was
more of an obstacle than a help for the discovery of fission.
In the periodic table thorium is so closely related to hafnium, protactinium to tantalum, and uranium to tungsten that
an ordering different from the one shown in Figure 1 is not
seriously considered, even though a second series of f-elements similar to the rare earths have repeatedly been postulated since Niels Bohr's Nobel talk in 1922;[15]however, this
series is suspected as being rather to the right than to the left
of uranium.116-191In contrast, A. von GrosseI2'I explicitly
argues again in the early thirties that the elements 93 and 94
might have properties completely different from eka-rhenium and eka-osmium.
The presumption that element 93 is eka-rhenium is not
easy to integrate into chemical separation procedures, because one has to work with the least familiar group of the
periodic table. The two homologous elements 43 and 75,
masurium (later technetium) and rhenium, were discovered
just a few years earlier by Walter Noddack, Ida Tacke (who
later married Noddack) and Otto Berg.[21, Even though
the properties of rhenium have not yet been well understood
it is already available in small quantities, in contrast to masurium of which hardly anything is known. For the eka-platinum metals a complicated chemistry has to be expected as
well, as is obvious from their homologues.
Therefore, in order to chemically prove the identity of
element 93, Fermi and his coworkers focus their attention on
manganese. As a characteristic reaction they choose the precipitation of manganese dioxide in strong nitric acid by
adding chlorate; this is not a fortunate choice, because the
precipitate has a large surface and all kinds of elements get
attached to it. Initially, however, their concept seems to be
correct: the 13-min activity, which for such experiments is
the best suited one of the new activities, is found in the
Giinter Herrmann was born in 1925 in Greiz (Thiiringen). From 1946 until 1956 he studied
chemistry at the University of Mainz where he received his doctorate for work on nuclear fission
under the supervision of Fritz Strassmann. In 1962 he habilitated and became privatdocent at
Mainz. In 1967 he was offered but did not take up an appointment asfull professor at the Kansas
State University in Manhattan. Since 1968 Herrmann has been full professor and Director of the
Institut fur Kernchemie an der Universitat Mainz, and, since 1970 he has in addition been
directing a nuclear chemistry research department of the Gesellschaft fur Schwerionenforschung
Darmstadt. His main fields of interest are: rapid methods of chemical separation, nuclearfission
and reactions between heavy atom nuclei, exotic atomic nuclei. the search for superheavy elements, radionuclides in the environment. The honors conferred on him include, inter alia: since
1984 corresponding member of the Academie der Wissenschaften und der Literatur zu Mainz;
1987 Miller guest professor at the University of California in Berkeley; 1988 Nuclear Chemistry
Award of the American Chemical Society.
Angew. Chem. Int. Ed. Engl. 29 (1990) 481-508
apparently she is speaking of a search in nature. In a Supplementary noteL3’]she discusses two discoveries of element 93.
The first case,[411a natural element 93 in the Joachimsthal
pitch-blende, is closed quickly because she does not detect
element 93 in the original samples either chemically or by
X-ray spectroscopy. In the second case, namely Fermi’s[’]
element 93, she objects that only the heaviest known elements were excluded; Fermi ought to have compared his new
element with all the known elements. According to her own
studies, a large number of ordinary elements are being coprecipitated with manganese dioxide; in her opinion the coprecipitation with rhenium sulfide is not a strong proof either.
Ida Noddack note~:[~’~[*I
“Es ware denkbar, da13 bei
1.2. Objections and Their Consequences
der Beschienung schwerer Kerne mit Neutronen diese Kerne
Objections are being raised. Von Grosse and A g r ~ s s [ ~ ~ - ~ ’ ]in mehrere gr@ere[**l Bruchstiicke zerfallen, die zwar Isotope bekannter Elemente, aber nicht Nachbarn der beascribe Fermi’s activity to element 91, protactinium, which
they succeed in coprecipitating completely with manganese
strahlten Elemente sind”.
dioxide and to a large extent with rhenium sulfide. Also
Can this statement be taken as the prediction of nuclear
Fermi et al. meet some problems in connection with protacfission, as it is done sometimes?[42,431 Not really, because
Ida Noddack herself does not consider her proposition of a
tinium and its coprecipitation with manganese dioxide.”. 4 *
new and surprising nuclear reaction to be that meaningful to
Being aware of this controversy, Otto Hahn and Lise Meitner
test it experimentally. She seems more concerned, according
feel challenged, since they are the ones who found the only
protactinium isotope which in nature is available in weighto contemporaries, to render the detection of element 93 as
able quantities, the 3.3 x lo4-y protactini~m-231,[~*~
inconclusive; her objection is not relevant for the future dewho had also named the element. Moreover, Hahnc2’] had
v e l o p m e n t ~ . [She
~ ~ ]may plan herself to search for eka-rhenidiscovered with protactinium-234 the phenomenon of nucleum in nature, since the 7th group of transition elements is
ar isomerism, namely the existence of two nuclei with the
already well known to her. Her objection is hardly noticed in
same numbers of protons and neutrons but with different
Rome[451and Berlin;[461Strassmann even calls it a chance
half-lives of 1.17 min for the metastable (m) and 6.75 h for
coincidence.[48] This ignorance is certainly influenced by
the ground state (g). Lise Meitner is immediately attractNoddack’s dubious and irreproducible discovery of the natuedf30.3 1 1 by Fermi’s neutron-induced nuclear reactions; she
ral element 43.145.50* ’ I 1 When Ida Noddack later complains
persuades[32.341 Otto Hahn to resume their collaboration
about this attitude in a note,[521Hahn and Strassmann reafter twelve years of interruption and to focus attention on
frain from submitting an already formulated harsh reply[501
the transuranic elements.
and leave it to the editors to react.[53]The relationship beIn a first note1351they present evidence in favor of Fermi‘s
tween the Noddacks and Hahn and Meitner remains reservinterpretation: the 13-min and the 90-min activities can be
ed, also caused by political controversies,[501even thoughseparated from protactinium in two ways. First, both activior maybe because-they work in the same city.
ties can be coprecipitated with platinum sulfide, with protactinium remaining completely dissolved. The second proce1.3. Berlin 1935-1938: Elements 94 through 96,
dure is just the opposite, where the new activities remain
the Eka-Platinum Elements?
dissolved if protactinium is co-precipitated with zirconium
From now on further studies of the transuranic elements
phosphate. To obtain these results the transuranic activities
mainly carried out by Otto Hahn and Lise Meitner; they
are mixed with natural 6.7-h protactinium-234g and in the
soon ask the much younger Fritz Strassmann to join them in
course of the subsequent separation the distribution of the
their efforts, since he has shown a talent for difficult chemical
three activities among different fractions is determined. The
separations. No photograph exists from that time showing
two transuranic elements are more similar to platinum than
all three together, so one has to combine two separate phototo rhenium and they might be chemically different,1351the
graphs-Figure 2-or use a later common one-Figure 3.
13-min activity could possibly be element 93, the 90-min
The experiments are performed in three different rooms on
activity element 94. Fermi et al.136*371
obtain similar results
the ground-floor of the Kaiser-Wilhelm-Institut fur
and presume the decay of the 13-min into the 90-min activity.
Chemie[***l in Berlin-Dahlem, the plan of which is depicted
This was the last word from Rome with respect to the “urain Figure 4.
nium case”; they leave it to those who are more experienced
in radio~hemistry[~*]
and turn to an exciting new problem,
[*] “It is conceivable that during the irradiation of heavy nuclei with neuthe intense reaction of slow neutrons with atomic nuclei, a
trons these nuclei may disintegrate into a number of larger [**I fragments
phenomenon which is also discovered in Rome.
which are isotopes of known elements but not necessarily neighbors of the
elements irradiated originally.
Ida Noddack‘s
is of a more conceptual nature.
[**I Emphases as in the original.
In an article[401dealing with yet unknown elements in the
[***I The Kaiser-Wilhelm-Institut fur Chemie is heavily damaged by bombs in
periodic table she conjectures that the transuranic elements,
February 1944. The institute and its members temporarily move to Tailfingen in Wurttemberg. After the war the institute finds a new home in
with increasing atomic number, should tend to be more and
Mainz and IS called the Max-Planck-Institut fur Chemie. The Berlin
more short-lived and rare, with the even numbered elements
building, after reconstruction, now serves as the Otto-Kahn-Bdu of the
94 and 96 possibly still within the realm of being found;
Freie Universitat.
precipitate, in contrast to uranium and the lighter elements
down to lead, as is verified with radioisotope^.^'^^^
13-min activity also coprecipitates with rhenium sulfide in
strong hydrochloric acid;[’* 6 - *I this is a reaction by far more
specific than the precipitation of manganese dioxide. The
90-min activity chemically behaves very similarly to the 13min activity. The conclusion that elements heavier than uranium have been found, is presented by Fermi”] only reluctantly, however, as is obvious from the title of the publication; he is not content with the more definite public statements of his mentor car bin^."^]
Angew. Chern. Int. Ed. Engl. 29 (1990) 48f -508
Fig. 2. Orlo Hahn (1879-1968) and Lise Meirner (1878-1968) at an institute
outing, ca. 1934/35, and Fritz Sfrassmann (1902-1980). Berlin, ca. 1936 (after
The equipment is very simple. Near the front of the table
depicted in Figure 5 two brick-like lead blocks can be seen in
which the counters are housed in order to reduce the radiation background. In the right block, which is open, one sees
a cylindrical Geiger-Muller counter. The samples to be measured are glued on “boat-shaped” lead holders and placed
underneath the counters; samples, lead holders, and absorber foils can be found between the two blocks. The counters are made of a 0.01 cm thick aluminum tube of 6 cm
length and 2 cm diameter, the two ends of which are closed
with rubber plugs and gas-tightened with pizein wax. The
central counting wire, made of a piano string, is fixed in the
same way, as is also a glass tube which is used for pumping
and filling the counter with an argon-ethanol-mixture 10: 1
at a pressure of about 10 kPa. The wire is at a positive potential of about 1500 volts, to collect the electrons which are
created by P--rays ionizing the counting gas; to increase the
signal height the electrons are accelerated in the strong electric field of the counter forming an electron avalanche.
In the center of the table one sees two amplifiers with two
amplifier tubes each; to the left of both amplifiers is a mechanical counting unit which registers the number of electrical pulses. Each counting unit is equipped with a stop-watch,
which is not shown on the tab1e.I4*)The counters are coupled
to the amplifiers via high-impedance resistors, which can be
seen to the right of the tubes; the resistors consistIS6]of a
glass tube filled with toluene and imbedded in paraffin. The
power supplies producing the necessary voltages are located
at the back of the table; the voltages for the counters are
supplied by a chain of anode batteries underneath the table.
Neutrons are produced according to Reaction (4) by irradiating beryllium with a-particles from natural radioisotopes.
The neutron sources which consist of six brass tubes of about
6 cm length can be seen in the upper right part of Figure 5.
Each one is made of a fused glass tube containing beryllium
powder and either about 100 mg of radium sulfate or shortlived radon gas, which is freshly obtained once a week by
separation from radium performed in a separate hut in the
institute’s yard.[561If slow neutrons are to be used for irradiation, the sources are installed in a cyclindrical paraffin
block (upper right) on top of which the sample is to be
placed. The flux of thermal neutrons is about lo4 neutrons
per cmz . s.
Fig. 3 . Frizz Srrussmunn, Lise Meirner, and Otro H u h (from left to right) 1956
in Mainz at the inauguration of the Max-Planck-Institut fiir Chemie (from
Finally, a filter flask is to be seen in the lower right of
Figure 5 ; this is used for rapid filtration of radioactive precipitates in the so crucial radiochemical experiments. Below
the filter flask one sees the historical note-book “Chem. 11”.
As experiments are performed at quite different levels of
activity, strict precautions have to be followed in order not
to accidentally spread radioactivity, as this would make sensitive measurements impossible in the long run. ToiIet paper
is affixed to the doors of coffee rooms, rest rooms, and corridors and is supposed to be used when touching door handles. In the lecture hall and the library one finds light and
dark colored chairs for those working with “strong” and
“weak” radioactivities. One does not shake hands either; if
practiced at all, greetings are reduced to the touching of each
others little finger tips. Laboratory tables are covered with
absorbent paper; for this purpose waste paper from the
printing office of the Reichsbahn[*’ is well suited and is
available cheaply. On the other hand, the institute has quite
an advanced local telephone system to facilitate communication without direct contact.
The natural radioactivity of uranium creates a severe
problem. To obtain statistically significant count rates of the
artificial activities-about a hundred pulses per minuteusing the weak neutron sources ten- to hundred-gram samples of uranium have to be irradiated. These sources contain
[*] National Railway.
Angew. Chem. Int. Ed. Engl. 29 (1990) 48l-SO8
Fig. 4 Kaiser-Wilhelm-Institut fur Chemie at Berlin-Dahlem, plan of the ground-floor. Irradiations with neutron sources were carried out in room 29,
Hahn’s “private laboratory”-room 20-was used for the chemistry, and in room 23 the radioactivity was measured; Lise Meitner’s office was located
between rooms 7 and 11, Otto Hahn’s ofice was on the first floor (from [55]).
Fig. 5. Some of the Berlin team’s instruments, now in the Deutsches Museum in Miinchen: Geiger-Muller counters (front part), pulse amplifiers and
mechanical counters (middle part), high voltage supply (rear part), neutron source and paraffin block (upper right); additional details in the text. The
instruments shown here were located in the rooms 20, 23, and 29 of Figure 4.
Angeu. Chem. Int. Ed. Engl. 29 (1990) 481-508
four to five orders of magnitude more activity in the decay
chain (5).
The energetic p--rays of 1.17-min protactinium-234m pass
through the aluminum tube of the counters. It is certainly
possible to chemically separate protactinium-234 and its precursor thorium-234 from uranium; however, both activities
are growing continuously from the decaying uranium, which
therefore must be purified immediately before the irradiat i ~ n .For
~ ~this
~ ]purpose uranium is precipitated with a mixture of ammonia and ammonium carbonate in the presence
of some iron and is dissolved again as a complex uranyl
carbonate; thorium- and protactinium-234 remain attached
to the precipitated iron hydroxide. From the filtrate, ammonium diuranate is precipitated; this is usually used for the
The more the Berlin group progresses with the radiochemical analysis of the uranium-born activities and with refining
the separation techniques, the more puzzling are the results.
During the next three years new transuranic activities
are discovered;[58*
591 and the respective formation react i o n ~ , '6~0 3~ '1, decay properties,[621and chemical behavior
In May 1937 the results are presented in
detail in two publication^.[^^.^^] A few days before Lise
Meitner's escape from Germany the last joint note[641which
includes a new activity, the 60-d eka-iridium, is submitted.
The scheme of transuranic elements now looks as depicted
in Figure 6. Three p--emitters are assigned to uranium. Two
,,Eka-Re ,,Eka-0s ,,Eka-Ir ,,Eka-Pt ,,Eka-Au
2,2min -+59 m k - t 66 h - t 2,5 h +
40sec +16min +5,7h
23 min -+
Fig. 6. Decay chains of uranium and of the transuranic elements produced by
irradiation of uranium with neutrons; situation as ofJune 1938 according to the
last joint publication by 0.Hahn, L. Meirner, and E Sfrussrnunn [MI.
are the origin of two long p--decay chains with seven
transuranic activities; one chain ends with element 95, ekairidium, the second extends even to element 96, eka-platinum. No decay product is found for the strongest activity,
the third uranium isotope with 23-min half-life, even though
it is clearly identified chemically as uranium and shown to be
a p--emitter by using a cloud chamber;[57] thus, it must
decay into element 93. As even in pure samples of this activity no decay product is found, Hahn et al. presume it to be a
long-lived e k a - r h e n i ~ m . ' Finally,[611
all uranium activities
are ascribed to the same isotope, uranium-239. However,
such a triple nuclear isomerism which even continues in the
decay products is difficult to understand; this is certainly
realized by Meitner, Hahn, and Strassmann.16'I Another peculiarity is the stability against cr-decay in the chains approaching a region where a-decay should increasingly dominate. However, no a-emitters are observed among the
transuranic elements.[651This stability is theoretically explained[661by the assumption that transuranic nuclei might
be elongated rather than spherical. The nuclei in that region
are indeed elongated, as is found much later. The same results and an additional product with a 17-h half-life~65J
obtained at the Berkeley cyclotron, which has a neutron flux
larger by many orders of magnitude. Several p--emitters are
also produced [671 when uranium is bombarded with energetic neutrons.
The scheme of parallel decay chains is based on a number
of different arguments, which, taken together, make up a
reasonable picture. First of all one has the cloud chamber
e x p e n m e n t ~ [which
~ ~ * prove
~ ~ ~ that p--emitters are formed.
Genetic relationships within the chains are obtained by
chemically isolating one of the activities and by tracing its
decay into another activity; for example, 16-min eka-rhenium decays into 5.7-h eka-osmium[611and 66-h eka-iridium
into 2.5-h e k a - p l a t i n ~ m . [Even
~ ~ ] in cases where such relationships are difficult to establish, those activities already
present after a short irradiation time should be near the
beginning of a decay chain; they cannot be formed from the
decay of later members. Using this argument the 10-s, 40-s,
2.2-min, and 16-min activities are ascribed to either uranium
or eka-rhenium.[611An additional criterion is the influence of
the neutron energy on the production of the activities. The
two long decay chains can be produced with either thermal,
slow, or fast neutrons without any noticeable difference, the
23-min activity, however, only by resonance capture of neutrons with 25 electron volts energy.[611The production
cross-sections obtained for the three chains seem to be only
compatible with uranium-238 as the primary nucleus rather
than with the much less abundant uranium isotopes 235 and
234.[61]Thus, the conclusion is that all three chains emerge
from uranium-239. During experiments which focus more on
aspects of nuclear physics chemical methods are also used for
disentangling the reaction mixture.
The main emphasis in identifying the unknown activities
is, however, placed on their chemical properties, still with the
assumption that one is dealing with uranium, eka-rhenium,
and eka-platinum elements. A large variety of methods is
used in these chemical studies,'63' either based on group separations which focus on all four transuranic or all three
eka-platinum elements, or on methods which are very
specific for individual elements. In the case of the first and
the last members of the chains, the chemical studies are quite
straightforward. The 16-min, 23-min, and 59-min activities
dominate after a short irradiation and are still long-lived
enough to allow for more complicated chemical operations.
If, on the other hand, one carries out the irradiation and then
waits for a long time, only the 66-h activity and its 2.5-h
decay product remain. The treatment of the intermediate
products is more problematic due to the complex mixture of
decaying and growing activities. It also seems easier to distinguish between uranium, eka-rhenium, and the group of all
eka-platinum elements rather than between the individual
eka-platinum elements.
The almost quantitative coprecipitation of all transuranic
elements with platinum sulfide in medium-strong hydrochloric acid is thought to be a very characteristic reaction; in this
way, transuranic elements can be separated from large
amounts of uranium and strong activities of natural decay
products.[63]In the framework of the periodic table of that
time, this sulfide formation is considered to be the most
Angew. Chem. Inr. Ed. Engl. 29 (1990) 481-508
striking difference between the transuranic elements on the
one hand and uranium and the cisuranic elements on the
other. The 23-min activity can be
from the filtrate of such a sulfide precipitate together with the insoluble
sodium uranyl acetate, which is characteristic of uranium. If
the precipitation from the bulk of the irradiated uranium is
performed quicker, a shorter-lived activity is noticeable;
thus, also the 40-s activity seems to be a uranium isotopef631
according to this nice early example of fast radiochemical
separations of complex mixtures.
As expected, eka-rhenium is found electrochemically to be
the least noble transuranic element; in contrast to the ekaplatinum elements it is not deposited on noble metal foils
placed in solutions nor can it be reduced to the metal state
with bismuth as carrier. With some reagents the transuranic
elements can even be sequenced. In a precipitation of uranium by sodium hydroxide the following order of coprecipitation is obtained: eka-0s > eka-Ir > eka-Re > eka-Pt. If
precipitation is performed with rhenium sulfide in strong
hydrochloric acid the order is eka-0s > eka-Re 2 ekaIr > eka-Pt. In contrast to uranium and the cisuranic elements the transuranic elements have in common that they
are volatile at medium and higher temperatures. If a sulfide
precipitate is heated under aerobic conditions, the volatility
increases in going from eka-rhenium to eka-platinum; if it is
treated with aqua regia to form chlorides before heating, the
sequence is reversed. Finally, two very specific reactions
seem to corroborate this scheme: eka-rhenium coprecipitates
with nitron perrhenate and eka-platinum with ammonium
platinum hexachloride; thus, element 93 obviously forms a
species analogous to perrhenate while element 96 forms one
analogous to platinum hexachloride.
Hahn, Meitner and S t r a ~ s m a n n [finish
~ ~ ~ their article
about the chemistry of the transuranic elements by discussing similarities to and differences from rhenium and the
platinum metals. The similarities prevail and the trends seem
to continue plausibly. There should no longer be any doubts
about their place in the periodic table and, above all, their
difference from all other known elements is firmly establ i ~ h e d . [This
~ ~ ' conclusion, however, is premature; what one
could not imagine was that a complex mixture of lighter
elements could simulate these results.
The Berlin team refrains from suggesting names for the
transuranic elements. The group in Rome instead feels confident enough to start using the names ausonium for 93 and
hesperium for 94.[37681
Enrico Fermi, too, uses them when he
receives the Nobel Prize on December 10,1938, partly for the
synthesis of new elements. In the published version of his
lecture[691he adds a footnote, noble indeed, that after the
discovery of nuclear fission-a few days after the lecture-all
transuranic elements should be reexamined because they
might in fact be fission products. Fermi does not return to
Italy. Shortly before, a detailed review article appears dealing with the transuranic elements.[701Even the discovery of
element 93 in nature by X-ray-spectroscopy is again reported.[71. 721
1.4. Interlude: Radium from Thorium?
Not only uranium, but also thorium is especially interesting regarding activation by neutrons. In this case the focus is
Angew. Chem. In{. Ed. Engl. 29 (1990) 481-508
not on transuranic elements which should not be accessible
from thorium; still there is some connection with our topic,
as we shall see later. Radiochemists were fa~cinated,'~~.
instead, by the possibility of reaching the long sought 4n + 1
decay series via reactions like (6) and (7).
For the three radioactive decay series starting with uranium238, uranium-235, and thorium-232 the mass numbers of all
members of the respective chains can be divided by either
4 n + 2,4n + 3, or 4 n. Therefore, a 4 n + 1 series is postulated
which, however, is not found in nature.
Upon irradiation of thorium one has to cope with all nine
members of the 4 n decay series.[751Even when thorium samples are used which have been repeatedly purified from decay products over many years[761so that these have almost
disappeared, one still has to perform cumbersome purifications and additional test experiments.f73*
75. 781
Fermi and his co-workers'" 81 are again the first ones to
start with experiments; they find two p--emitters with halflives of 1 min and 15 min, respectively. The latter is soon
corrected to a value of 24 min.f36*371
This activity has the
characteristic chemical properties of thorium. It coprecipitates with thorium fluoride and thorium peroxide and can be
separated from all other elements between lead and uraniHahn and M e i t ~ e r 'confirm
~ ~ ] the two activities and
also identify the longer-lived one as being thorium-like. As
the short-lived activity is produced with fast neutrons they
assign it to radium-229, which is formed by an (n,u)-reaction
according to Equation (7); they argue that an additionally
observed I 1-min activity might be its actinium decay product.
But soon the situation becomes more complex, as in the
case of uranium. In Paris, IrPne Curie, Hans von Halban and
Pierre Preiswerk first find
then a total of
B--emitters with half-lives off8'] 1 min, 2.5 min, 12 min,
25 min, and 3.5 h, respectively. They assign them all to thorium and its neighbors, for example the 3.5-h activity to actinium, because it coprecipitates with lanthanum fluoride.[*']
Finally, a 42-h actinium is added.[781It remains controversial
whether the 4 n 1 series is believed to have been found first
in Berlin or in Paris; this may be the reason why the Berlin
group remains quite reserved1"' towards the following work
of the group in Paris.
The mixture of activites, however, becomes more and
more puzzling. Lise Meitner, Fritz Strassmann and Otto
Hahnr751show that 23-min thorium is formed via a resonance capture (n,y)-process (6) and correctly conclude that it
must be thorium-233. They identify its decay productagain correctly-as 27-d protactinium-233. With fast neutrons they even find three B--emitting radium isotopes
(Fig. 7), one of them with a 0.8 min half-life; all three are
chemically identified as radium and can be connected with
three actinium isotopes. They are presumed to be formed by
a Th(n,u) Ra reaction (7), although (n,u)-reactions have up
to then been observed only for light elements;c31the instability of thorium against u-particle emission, apparent in its
*32Th( t z , ~ )
20 bis 30 Std.
Fig. 7. Three isomeric decay chains for radium and actinium formed by irradiation of thorium with fast neutrons; after L. Meitner, F Strussmunn, and
0.Huhn 1751, May 1938.
natural decay, is taken as an argument for this explanati or^.[^^] It seems to be further corroborated by a
dealing with energetic a-particles which appear during the
irradiation of thorium with neutrons; in Meitner’s group,
however, a search for these prompt a-particles is not successfu1.[821The activities of the Berlin group are confirmed by
using much stronger neutron sources, and a new decay product, 24-h thorium-23 1, is added.fs3] Since natural thorium
mainly consists of thorium-232, all three radium activities
have to be assigned to radium-229 and its decay products to
actinium-229 (Fig. 7). This triple nuclear isomerism is, however, difficult to understand>751as is already the case with
uranium (Fig. 6).
Today, the 4 n + 1 seriescs4]is known to start with neptunium-237, which, due to its half-life of 2.14 x lo6 y, does not
exist in nature any more; neither does the rest of the decay
series. Among the nuclides which are correctly assigned in
these early experiments, 27.0-d protactinium-233 is indeed a
member of the chain. but not 22.3-min thorium-233.
1.5. Paris 1937-1938: The 3.5-hours Activity
Let us now go back to the artificial activities produced
from uranium. The cumbersome purification of uranium
prior to irradiation can be omitted if the energetic p--rays of
the natural decay product, protactinium-234m, are discriminated by covering the sample with a copper foil through
which these p--rays cannot pass. Then, naturally only those
activities can be detected whose p--rays are even more energetic. Irene Curie and Paul Savitch,IS5]in the summer of
1937, decide to proceed thus in dealing with the artificial
uranium-born activities. In addition to the known 40-s,
2-min and 16-min components they findcs6’a new activity
with 3.5 h half-life as depicted in Figure 8. The 3.54 activity
appears without absorber foil, too, if the activity of irradiated and non-irradiated uranium is followed with a compensation method. It is not noticed, however, that a very similar
activity appears also upon irradiation of thorium with neutrons.f801The 3.5-h activity does not coprecipitate with sulfides in acid solutionfs5]and can be separated this way from
the transuranic elements reported until then. It seems to
follow the natural decay product of uranium, thorium-234.
This assignment to thorium from the reaction (8) cannot
be confirmed in Berlin, however, where again-as already
before[611-thorium is not found. A letter is written to the
researchers in Paris suggesting the withdrawal of these results.‘87’
Fig. 8. The 3.5-h activity found in uranium irradiated by neutrons and measured directly as published by l. Curie and P.Smirch 1861 in Paris, July 1938.
Upper curve: activity measured with the sample covered by a copper foil of
0.48 g . cm-* thickness, depicted linearly as a function of time; lower curve:
build-up of natural decay products in uranium, also depicted linearly; middle
curve: difference between the two measurements in semilogarithmic representation with 16-min “eka-rhenium” at the beginning followed by a pure decay with
3.5 h half-life.
A folIowing note by I. Curie and P. Savitch[”I is indeed
modified with regard to this after they succeed in chemically
separating the 3.5-h activity from thorium and protactinium.
What they further find is even more remarkable: the chemical properties of the activity are similar to those of the lanthanoids; it is likely that it is either actinium, the heavier
homologue of this group of elements, or a new transuranic
element with properties completely different from eka-rhenium or the eka-platinum elements. In the next notets9]additional lanthanoid properties are ascribed to the 3.S-h activity, for example its coprecipitation with fluorides and
oxalates. As they are able to separate the 3.5-h activity chemically from actinium I. Curie and Suvitch favor the hypothesis of a new transuranic element, even though they find it
difficult to understand. For this separation the activity is
isolated with lanthanum as a carrier. Natural radio-actinium
is added followed by a fractional precipitation of lanthanum
oxalate. The 3.5-h activity is found in the head fraction and
actinium-as expected-in the tail fraction.
All these results along with some others are presented in
detail in the following publicationfs6]which is supplemented
by a note[g01about the maximum P--energy of the 3.5-h
activity, 3.2 MeV. I. Curie and Savitch present additional
chemical evidence against actinium. The similarity to the
lanthanoids is summarized:f861“Dam l’ensemble, les proprit t C s de R,.,, sont celles du lanthane, dont il semble jusqu’ici
qu’on ne puisse le separer que par fractionnement”.[*l One is
I*] “On the whole the properties of R, 5h are those of lanthanum, from which,
it seems so far, it can be separated only by fractionation.”
Angew. Chem. Int. Ed. Engl. 29 (1990) 481-508
tempted to think that if I. Curie and Savitch had simply
omitted the second half of the sentence the puzzle would
have been solved ; the observation of fractionation, however,
is correct, as we will see below. According to a later remark[911I. Curie and Savitch even consider uranium breaking into lighter elements; yet, they reject this explanation
because the 3.5-h activity as well as the transuranic elements,
represented by the 16-min eka-rhenium, show the same behavior with respect to different neutron energiescs6]and thus
seem to be related. In conclusion, albeit with some hesitation, they favor the hypothesis of a transuranic element with
completely different properties and probably being at the
beginning of this series of elements:[86]“On pourrait imaginer, par exemple, que ce corps a le nombre atomique 93 et que
les corps etudies par Hahn, Meitner et Strassmann ont des
nombres atomiques de 94 a 97”[*1 This, of course, is a challenge for the group in Berlin. At last, enough details are
known to critically test the foundation of these results. In
Berlin one is sceptical, as Otto Hahn tells FrPdPric JoliotIrPne Curie’s husband-when they meet in May 1938 during
a congress in Rome.[”* 921 In Berlin, somewhat jokingly, one
speaks of “ C u r i ~ s u m ” . ~ ’ ~ ~
2. Alkaline Earth Metals from Uranium
2.1. Berlin, Autumn 1938: the “Radium” Activities
As soon as the detailed publication by 1. Curie and
Savitch[s61appears in the third week of October 1938, Hahn
and Strassmann are able to reproduce the 3.5-h activity.1931
The best method, as recommended in Paris, to chemically
isolate the activity is its coprecipitation with sparingly soluble potassium lanthanum sulfate. Strassmann immediately
notices[48]that if radium should be present it would coprecipitate as sulfate and later during fractionation appear in
the head fraction, as prescribed for the 3.5-h activity. Radium could be formed if thorium from a U ( n p ) Th reaction
disintegrates by a-decay [Reaction (9)].
+ 0‘ n .---- ;cy + *;;Th
5’i;Ra & ‘;$AC
Finally. actinium could result from p--emitting radium. An
a-emitting thorium may well not have been noticed before.
This hypothesis would serve to trace back the puzzling chemical properties of the 3.5-h activity to a mixture of already
known elements. Of these elements, radium can be identified
more easily than a lanthanum-like element. Strassmann suggests,’76-94) not to coprecipitate radium as usual with barium
sulfate, which has a large surface, but with barium chloride
instead; this salt crystallizes from concentrated hydrochloric
acid as beautiful, colorless, and particularly pure needles and
can easily be reprecipitated after having been dissolved in
water. With this elegant method radium can quickly and
unambiguously be identified; it is accompanied only by
Within a matter of days Hahn and Strassmann obtain their
first important results, as Hahn notes on October 25, 1938 in
[*] “One could imagine. for example, that this body has the atomic number 93
and that the bodies investigated by Hahn, Meitner, and Strassmann have the
atomic numbers 94 to 97.”
AnKen,. Chem. Inr Ed. Engl. 29 (1990) 481-508
Fig. 9. Beginning of the note-book “Chem. 11” by 0. Hahn and E Strassmonn
[l03]: Chemical separation for “Weitere Priifung des “Cu-Sa” auf Ra-Ac”
(“Further tests of “Cu-Sa” for Ra-Ac”) on November 1, 1938, written by E
Strussmunn; separation of a radium fraction from irradiated uranium by precipitation of barium chloride from strong hydrochloric acid. “Cu-Sa” represents the 3.5-h activity of I . Curie and P. Suvitch [86].
a letter to L. ~ W e i t n e r . [The
~ ~ l“countdown” for the discovery of nuclear fission has started; it is documented in the
a plain booklet bound
laboratory notebook “Chem. 11”,[961
in wax-paper of about 18 x 24 cm format and 94 pages.[*]We
are well informed about the subsequent events by a series of
letters exchanged between Otto Hahn and Lise Meitner,
which were edited under somewhat different aspects.197*
Whatever seems important to one of them is written down
and the answer often arrives within two days. It is especially
informative to read the scientific part of the letters and the
laboratory notes in chronological order.c981Hahn urgently
asks for explanations of these unexpected results, Lise Meitner for more information. All this occurs in quite a troubled
period of time, as one notices clearly from Hahn’s autobiographic notes of 1945[761and, in a more condensed and
dramatic way, from his pocket-book of 1938.[991Even when
Lise Meitner is still with him, Hahn has already isolated
The author is indebted to Ernst Berninger for a copy of the booklet ‘Them.
11”. A previous note-book “Chem. I” seems to have been lost. The subsequent books starting with “Chem. 111” and the corresponding larger decaycurve drawings are stored in the Landeshauptarchiv Koblenz, legacy Fritz
himself c ~ m p l e t e l y ; ~ ’no
’ ~one
~ else in the institute is told
what is about to happen. It is the time of the Reichskristallnacht. Some of the institute’s senior scientists are fanatic
supporters of the “National Socialists” (Nazis), others, like
Fritz Strassmann, succeed in remaining aloof though it
meant facing disadvantages. A potential successor for Hahn,
who is considered politically unreliable, is already in the
institute. In this atmosphere the problem of safeguarding
Lise Meitner’s property has to be solved. Hahn’s wife Edith
starts to suffer from a mental disease and Hahn also feels
completely exhausted at times. The exciting results with uranium distract him and save this extremely difficult situat i ~ n . ‘The
~ ~ secluded
Fritz Strassmann, whom he also trusts
politically, cares for the continuous progress of the experiments.
“Chem. 11” starts on November 1, 1938, with a notef1031
shown in Figure 9. To verify whether “Cu-Sa”, the 3.5-h
activity of I . Curie and Savitch, might be radium or actinium,
the respective homologous elements barium and lanthanum
are separated from the bulk of irradiated uranium and accompanying “transuranic elements” are then removed with
platinum sulfide. To this purpose 8 g of uranium are irradiated with slow neutrons for one day, dissolved in hydrochloric
acid, and barium chloride is precipitated from the resulting
solution. After reprecipitating barium chloride and dissolving it again, platinum is separated as sulfide and lanthanum
as hydroxide. One half of the barium-radium fraction is
immediately processed to yield a sulfate, the remaining half
is put aside to allow for actinium to be formed and is later
separated into a barium-radium fraction and a lanthanumactinium fraction.
The barium-radium fractions are especially active. The
decay curves indicate the presence of even three radium isotopes, having half-lives of about 25 min, 110 min, and several days; they seem to be related to three actinium decayproducts, as shown in Figure 10. Hahn and Strassmann soon
mehrere Tage 89AC3
Fig. 10. Three radium isomers and their respective actinium decay products in
the barium fraction of neutron-irradiated uranium; note by 0. Hahn and I?
Strassmann [104], November 1938.
present details of these act!vit:es. The editor. P a d
Rosbaud, realizes the significance 0 -scme of the results reported at the end and has the publication appear already in
the next issue on January 6, 1939. However, four out of the
five printed pages deal with interesting but not really spectacular results. At first, one only notices that the title speaks
of “alkaline earth metals” instead of “radium” as in the case
of the preceding note. In the text it again says “radium”,
mostly in quotation marks, as well as in the decay chains
(Fig. 1 I) which are now supplemented. Proof is shown of all
,.Ra 11”
14 _C z
,,Ra 111“
,,Ra IV“
86 j, 6 M m .
< 30 Min.
A C I1
AC I11
< 40 Std.
Fig. 11. The decay chains of Figure 10 in the detailed publication by 0. Hahri
and F Strassmann [lo51 from December 22,1938, in a more complete form and
with the radium symbols in quotation marks.
activities by decay curves which are obtained after different
irradiation and separation times; Figure 12 presents an example. The chemical separation is discussed in detail and it
is stressed that the activity precipitated with the barium salts
can only stem from radium, “wenn man das Barium selbst
als allzu unwahrscheinlich vokrst auDer Betracht
as it still says in the beginning of the text.
2.2. Berlin, December 17, 1938:
Radiochemical Detection of Barium
The big surprise comes only at the end of the
one hardly suspects it in a publication entitled “Uber den
Nachweis und das Verhalten der bei der Bestrahlung des
Urans mittels Neutronen entstehenden
Erdalkalimetalle”.[**l Hahn and Strassmann report on some recent
investigations which they only publish with hesitation. They
have decided to prove more rigorously the occurrence of
radium. Since Marie Curie’s time it has been known how to
achieve this:” 0 6 ] Upon fractional crystallization of certain
barium salts, for example the chloride, radium becomes enriched in the head fractions and, after many repetitions, can
even be obtained in pure form. Hahn became acquainted
with these methods during his time with Ramsay in London
and discovered his first radioactive species this
way,[1o91the 1 -9-y thorium-229, then called radiothor. Until
the thirties, the study of such coprecipitation reactions was
one of his main research interests.[91
Presumably these control experiments are motivated by
some doubts which theorists raised against the inherited
isomerism.[‘31Already then nuclear isomerism is correctly
describe these findings in a note.[1o41All three radium activities are ascribed to radium-231 ; thus, as already in the case
of the activities isolated from thorium and ascribed to radium-229, an inherited triple isomerism is assumed. It is obviously not recognized that the half-lives are quite similar in
both cases, probably because different radium isotopes are
postulated. Strangely enough, the (n,a)-reaction (9) of uranium already occurs with slow
this effect is observed for the first time.
In the following publication which is submitted to “Naturwissenschaften” on December 22, 1938, Hahn and Strass-
,,RaI“ ?
[*] “ i f one does not consider the quite unlikely candidate barium for the time
“On the identification and behavior of the alkaline earth metals formed
upon irradiation of uranium with neutrons”
Angew. Chem. Int. Ed. Engl. 29 (1990) 481-508
Fig. 12. One of the decay curves from the same publication [IOS] as Figure 11: The three radium isotopes after four days of
irradiation. The measured activity is depicted logarithmically versus time (lower curve), its initial decrease with a ten-times extended
time axis (upper curve). The initial decrease is mainly due to 86-min “Ra III”, the subsequent rise is caused by the 40-h “Ac IV”,
the decay product of the long-lived 12-d “Ra-IV”, which dominates the final decrease.
ment of radium in a smaller portion of barium the counting
understood as an effect of nuclear spin[”ol-large spin difefficiency is to be improved. This is crucial for the long-lived
ferences hinder the transition from excited nuclear states to
the ground states; but it is thought to be very unlikely that
12-d component in Figure 12 which is only evident by growth
such an effect is preserved in a decay chain. Therefore, startof its 40-h daughter substance.
ing with the idea of deformed transuranic nuclei,[66]another
The first fractionation is performed on November 25,
kind of isomerism is suggested,“ namely the possibility of
1938. Figure 13 depicts the notes of this amazingly simple
experiment.[”’] 8.5 g of uranium are irradiated over night,
nuclei having the same numbers of protons and neutrons but
with different deformations and, in turn, differences in halfdissolved in strong hydrochloric acid, and barium ions (1 g)
lives. Much later, this shape isomerism is indeed found for
are added to the solution. Barium is precipitated as chloride,
the real transuranic elements I’ via their spontaneous
the precipitate, in turn, is dissolved in water, and hydrochlonuclear fission. The formation reaction is a second point
ric acid is added again; after cooling, barium chloride is
which raises
it is understood by many to be
precipitated in several portions. The barium precipitates are
a very peculiar, successive detachment of two a-partimeasured one after the other and, as can be read at the end
c l e ~ ~’ I’7~] .[cf. (101, although Huhn and S t r a ~ ~ r n a n n [ ’ ~ ~of~ the protocol, are converted into barium carbonate after
about two hours. In addition, further elements are isolated,
which we will not discuss, however.
The data of the first measurement of each of the three
explicitly point to one or more a-emitting thorium isotopes
barium chloride precipitates are collected in Figure 14.The
as intermediate products. But Huhn also sometimes specucorrected count rates are placed in the next to the last col‘16] For H ~ h n [ ’ ~
, the
lates about an (n,2a)-rea~tion.[~~*
umn. They develop continuously within error limits when
most substantial doubts are probably the ones that he hears
switching from one fraction to the next. The half-life turns
in Copenhagen on November 13-14, 1938, when he visits
out to be about 40 minutes; thus, mainly the short-lived
Niels Bohr to give a talk; on this occasion he also meets[99]
species “radium” I1 and I11 are present. No sudden decrease
Lise Meitner and her nephew Otto Robert Frisch, who works
can be perceived between the first and second and between
as an experimental physicist in Bohr’s institute. In the Berlin
the second and third fractions. In contrast to what was exinstitute, too, doubts are being raised,“ “1 some based on
pected originally, “radium” is not enriched in the first fractheoretical arguments, others because prompt a-particles
tion. No comment on that is found in the notes; in the headfrom an (n, a)-reaction are not
upon irradiation
line, however, “Ra” is put in inverted commas-Figure
of uranium with neutrons.
13-whereas in the following notes and in the letters of
Later, however, Huhn mentions also a more trivial reason
Hahn-Meitner it is not.
for the fractionations
94, ’I Due to the thickness of the
The next fractionation is attempted on November 28, this
barium samples the P-rays of especially the most long-lived
time with 86-min “Ra 111” and by using barium bronew activities are strongly absorbed. Therefore, by enrichmide,1’201in which radium is enriched to a larger extent than
’ ’
Angeu Chem hi.Ed. Engl 29 (1990) 481-508
Fig. 13. First chemical fractionation of the radium activity from irradiated
uranium with barium as carrier element, November 25, 1938 [119]: Beginning
of the chemistry protocol concerning “Weitere Versuche uber die Mischkristallbildung des “Ra” mit Ba Salzen u. Suche nach Folgeprodukten aus Ac-LaNiederschlag” (“Further experiments about the formation of mixed crystals of
“Ra” with Ba salts and search for decay products from Ac-La precipitate”),
written by F Sfrassmann. The irradiated uranium is dissolved in hydrochloric
acid. Barium chloride is precipitated from the solution, dissolved again in water, and, after cooling, precipitated in fractions. After a quick measurement the
precipitatesareconverted into bariumcarbonate. Note thequotation marks for
Ra in the title.
in the chloride. Due to the longer half-life the fractions can
now be measured alternately and thus, the count rates can be
compared better. Again, the activity is distributed equally
among four fractions. The third attempt, from December 6
to 8, again makes use of the bromide but is performed with
the long-lived “Ra IV”; the results are the same as beIn a first hypothesis the unexpected behavior of radium is
assumed to be caused by its extremely low concentrations,
much lower than at the time when the fractionation of radium was studied with radiation detectors which were far less
sensitive. In the “active” laboratory on the 1st floor
starts to fractionate radium in amounts which were common
earlier, When switching to the small amounts typical of artificial isotopes no difference is observed-the behavior is as
N~~ there is only one
consequent step left: The
chemical behavior of the artificial and the natural radium
isotopes has to be compared directly-the famous indicator
experiment by Otto Hahn and Fritz Strassmann on December 17, 1938, a Saturday!’221 86-min “radium” obtained
from uranium is compared with natural radium-228,
mesothorium 1. This long-lived radium isotope, half-life
Fig. 14. Some of the data of the fractionation experiment from November 25,
1938 11191: First indication that the “radium” isotopes from uranium do not
behave like radium because they are not enriched in the head fraction. The first
measurements of each of the three successive barium chloride fractions are
depicted; the samples were placed on different holders (Sch) and measured with
the same counter (2.27). The columns represent from left to right: 1 date, 2
time, 3 elapsed time, 4 counts on the meter, 5 difference between the counts and
duration of the measurement in minutes, 6 count rate in counts per minute, 7
count rate corrected for counting losses, 8 count rate after subtraction of the
background of 15.0 counts/min, 9-n
the right-normalized elapsed time.
During these 43
the activity-a mixture of z‘radium33
II and III--decreases
continuously over all three fractions without the sudden decrease between successive fractions that is expected for radium. Remarks concerning the chemistry
were written by Hahn, other notes by Irmgard Bohne. Clara Lieber, and f:
5.8 y, decays into the short-lived 6.1-h actinium-228,
mesothorium 2. The discovery of radium-actinium-228 was
Hahn’s first important achievement in Berlin.[123-1251The
Angew. Chem. Inf. Ed. Engl. 29 (1990) 481-508
C U J L ; J ~ -
3;‘ U
Fig. 15. Discovery of nuclear fission at Berlin on December 17, 1938: The
indicator experiment “Ra 111-Msth 1”; comparison of the behavior ofartificial
and natural radium in the fractional crystallization of barium bromide. Eight
lines are sufficient for Otto Hahn to describe the chemical procedure [126]:
“15,5 [g] Uran wird am 16. XII. abends bis 17. XII. morgens % 8 Uhr bestrahlt.
Dann 2 112 Stunden das “Ra 11” (14 Min.) zerfallen lassen. Dann das 86 MinRa I l l frisch, also frei von Ac I und Ac 11, hergestellt. Dieses mit einer hinreichenden Menge vorher von Th B C und Rdth befreiten Menge von Msth 1
[ =5.8-a Radium-2281 versetzt. alles gemeinsam mit BaBr, [-] % 2 g Ba-fraktioniert. Konzentration der Saure bei den Fraktionen blieb ungefahr gleich”.
(15.5 [g] of uranium was irradiated from the evening of Dec. 16 until the morning of Dec. 17 s= 8 a.m. Then “Ra 11” (14 min) was allowed to decay for 2 1/2 h.
Then the 86-min Ra I11 freshly prepared, viz. free of Ac I and Ac 11. To this a
suff~cientlylarge amount of Msth 1 [ = 5.8-y radium-228) was added from which
Th B + C and Rdth had been removed beforehand; the complete mixture was
fractionated with BaBr, [ -1 % 2 g Ba. The concentration of acid for the fractions remained approximately constant”.)
experiment is simple in principle and the record of the chemical part is correspondingly short;“261it is reproduced in
Figure 15. As before, barium bromide is precipitated in fractions.
By the evening of the same day the artificial radium in the
three fractions of barium bromide has already decayed; for
several days the growth of actinium-228 is being measured
until it is in equilibrium with its “mother substance” radium228. In Figure 16 the data“”] for the first fraction of 500 mg
barium bromide is depicted; facsimiles of the two remaining
data sheets can be found elsewhere.[’281 The conclusion[’291-Figure 17-is drawn by Hahn at the end of the
data sheet for the third barium bromide fraction and even
more concisely in his note-book:[991“Indikatorversuch Msth
1 + unser Ra 111. Ra 111 reichert sich nicht an. Msth 1
stark !!’’.[*I
Angew. Chem. h t .
$i-bc;Jbi -t.$~tyr
[‘I ”Indicator experiment Msth 1
+ our Ra 111. Ra 111 is not enriched. Msth 1
Ed. Engl. 29 (1990) 481-508
Fig. 16. Counting data for the first of three barium bromide fractions from the
indicator experiment radium 111-mesothorium 1 using 500 mg BaBr,, measured o n December 17, 1938 from 11: 15 h until 20:Ol h; measurements of the
growth of mesothorium 2 from mesothorium 1 until December 21 are included
[127]. The columns represent from left to right: 1 date, 2 time, 3 elapsed time
in minutes, 4 meter counts, 5 difference between the meter counts and counting
time in minutes, 6 count rate in counts per minute, I count rate after subtraction
of the background, 8 elapsed time in minutes and in hours. “K8, K 16” are
remarks added later concerning the corresponding sheets with decay curves.
The notes were taken by I. Bohne, 0. Hahn, C. Lieber, and F. Strassmann.
p4 A 4 k Cb
4hi. 4
t q:
; la@)
,Id +A4
Fig. 17. Otto Hahn’s conclusion from the indicator experiment radium I I I mesothorium 1 of December 17, 1938, at the end of the data sheet of the third
barium bromide fraction [129].
,,Die Auswertung (siehe Kurven-Blatt und beiliegende Zettel)
:I11 fur Msth 1.6
ergibt Fraktion I :I1
fur Ra I11 1:1,2
fur Msth
=80:70(-80):65-70 bei schlecht stimmenden Kurven!“
fur Ra 111
(“The evaluation (see decay-curve sheet and attached paper)
I :I1
:I11 for Msth 1.6
yields fraction
for Ra I11 1:1,2
for Msth
for Ra I11
%80:70(-80):65-70 for curves not well matching!”)
On the evening of the following Monday, December 19, he
informs Lise Meitner:[’301 “Zwischendurch arbeite ich,
soweit ich dazu komme, und arbeitet Straljmann unermiidlich an den Urankorpern, unterstiitzt von Lieber und
Bohne. Es ist jetzt gleich 11 Uhr abends; um 1/4 12 will
StraBmann wiederkommen, so daD ich nach Hause kann
allmahlich. Es ist namlich etwas bei den “Radium-Isotopen”, was so merkwiirdig ist, daB wir es vorerst nur Dir
sagen. Die Halbwertszeiten der drei Isotope sind recht genau
sichergestellt; sie lassen sich von allen Elementen auDer Barium trennen; alle Reaktionen stimmen. Nur eine nichtwenn nicht hochst seltsame Zufalle vorliegen : Die Fraktionisierung funktioniert nicht. Unsere Ra-Isotope verhalten
sich wie Bu. Wir kriegen keine eindeutige Anreicherung mit
BaBr, oder Chromat etc. Nun habe ich vorige Woche im 1.
Stock ThX [=224Ra]fraktioniert; das ging genau, wie es
sollte. Dann haben StraBmann + ich am Samstag eines unserer “Ra”4ostope mit Msth 1 als Indikator fraktioniert.
Das Mesothor wurde programmaDig angereichert, unser Ra
nicht. Es konnte noch ein hochst merkwiirdiger Zufall vorliegen. Aber immer mehr kommen wir zu dem schrecklichen
SchluB: Unsere Ra-Isotope verhalten sich nicht wie Ra, sondern wie Ba. Wie gesagt, andere Ele[mente], Trans-Urane,
U , Th, Ac, Pa, Pb, Bi, Po kommen nicht in Frage. Ich habe
mit StraBmann verabredet, daB wir vorerst nur Dir dies
sagen wollen. Vielleicht kannst Du irgendeine phantastische
Erklarung vorschlagen. Wir wissen dabei selbst, daB es eigentlich nicht in Ba zerplatzen kann”.[*]This part of the
[*] “Whenever I have the time I do some work and Strassmann works tirelessly on the uranium bodies, assisted by Lieber and Bohne[**’. It is
almost 11 p.m.; at 11:15 h Strassmann will return, so I can go home at
last. You know, there is something so strange about the “radium isotopes” that for the time being we only want to tell it to you. The half-lives
of the three isotopes are quite well determined; they can be separated
from al/[***’elements except barium; all reactions are fine. Except oneunless there are some really odd coincidences. The fractionation does not
work. Our Ra-isotopes behave as Ba. We do not obtain an unambiguous
enrichment using BaBr, or chromate etc. Last week I fractionated ThX
[ = 224Ra]on the 1st floor; this worked exactly as it should. Then, on
Saturday, Strassmann I fractionated one of our “Ra”-isotopes with
Msth 1 as indicator. Mesothor was enriched as expected, not so our Ra.
All this may still be due to a very peculiar coincidence. But more and more
we come to the frightening conclusion: Our Ra-isotopes do not behave
like Ra but like Ba. As I said, other ele[ments], trans-uranics, U, Th, Ac,
Pa, Pb, Bi, Po are ruled out. I agreed with Strassmann that we only tell
you about all this for now. Maybe you can suggest some phantastic
explanation. We know ourselves that it cannot really break into Ba.”
CIara Lieber was an American guest scientist, Irrngard Bohne was a tech-
nician at the Kaiser-Wilhelm-Institut fur Chemie.
Emphases as in the original.
Angew. Chem. Int. Ed. Engl. 29 (1990) 481-508
Uber den Nachweis und das Verhalten der bei der Bestrahlung des Urans
mittels Neutronen entstehenden Erdalkalimetalle .
22. Dezember 1938
Fig. 19. Last three columns of the first historical publication by Otto Hahn and Fritz Strussmann [lo51 where nuclear fission is still announced hesitatingly; it was
submitted to “Naturwissenschaften” on December 22,1938, and appeared on January 6, 1939. The figures not belonging to this part of the text are taken out and the
layout is slightly modified. Emphases by the author. The “vorerst” (earlier in the text, see end of Sect. 2.1) and the paragraph “Was die . _ .ergibt 239!” were added
by Hahn on December 27, 1938, upon correction of the proofs [139]; in the paragraph “Als Chemiker ... vorgetauscht haben” the expression “Gesetze” (“laws”) was
replaced by “Erfahrungen” (“experiences”), since (natural) laws were not encountered.
letter is depicted in Figure 18. Hahn mentions that they want
to check whether the “actinium” isotopes growing from “radium” behave like lanthanum.
When the letter is written this
is already
being carried out. The 40-h “actinium IV” is obtained from
the long-lived “radium IV”, which is still present in the samples of December 17, and is mixed with natural actinium-228
and lanthanum carrier. Upon fractionation of lanthanum
oxalate in nitric acid solution natural actinium, as is expected, becomes strongly enriched in the tail fraction whereas
“actinium IV” from uranium is distributed uniformly: it is
All these results are presented in the last three columns of
Hahn and Strassmann’s first historical
are depicted in Figure 19. The authors still hesitate in the
often quoted passage “Als Chemiker ... als Kernchemiker”[*I to replace radium, actinium, and thorium by the
elements barium, lanthanum, and cerium. The results about
these fission products, however, are surprisingly good; comparing the decay chains in Figure 1 1 with today’s situation,
“As chemists . . . as nuclear chemists”.
Angen,. Chrm. I n [ . Ed. Engl. 29 (1990) 481-508
Figure 20, one notices that everything was determined correctly; only chain I1 later had to be divided into two similar
chains. Soon it is realized in Berlin that the barium activities
emerge from short-lived cesium and xenon precursors.
It is probably quite unique that a result of such significance was presented so indirectly and with that much understatement. This may explain why some of the later publications by others sound as if Hahn and Strassmann had not
really noticed what they had found. Also recently it is arg ~ e d [ ’ that
~ ~ ’it was only after Lise Meitner’s positive comment that Hahn was encouraged to formulate more explicitly
the breakup into two nuclei. However, already in his letter of
December 19 (Figure 18) he speaks of “Zerplatzen”.
The peculiar presentation of the discovery of nuclear fission can, however, be accounted for by the hurry in which
this manuscript[1051
was written. As Hahn explains later,’1351
he starts with an almost finished text about the radium isotopes, a more detailed publication of what was reported prev i o ~ s l y [ in
’ ~a~ note.
The results of the indicator experiments are added at the end. The mention of “barium” in the
title is considered instead of the initially chosen “radium”,
but then the whole manuscript would have to have been
rewritten; the choice of “alkaline earth metals” is a way
There is no time for major modifications because on
December 21 all the personnel go on a 2-week Christmas
vacation; in view of the competition with the Paris group it
seems to be risky to wait this long. The evidence for barium
instead of radium is written down on December 21 -22 and
the manuscript is picked up[991by Rosbaud on the evening of
the 22nd for publication in “Naturwissenschaften”. The hurry does not allow for a careful checking or for detailed arguments. Thus, strange inconsistencies remained unnoticed :
The printed text says that the indicator experiments were
carried out with “langlebigen RaIV”[*] and with “AcII
(H. Z. rund 2,5 Stunden)”,[**] as one can read in Figure 19;
what was used in fact was 86-min “Radium 111” (as is obvious from the protocol, Figures 15 through 17) and[’32]40-h
“Actinium IV”. This inconsistency is never commented on
Fig. 20. The historical barium-lanthanum decay chains and the respective
precursor nuclides today 11331. “Ra 111” is Ba-139 and “Ra-Ac IV” are Ba-La140, “Ra-Ac-11” later turned out to be complex, namely Ba-La-141 mixed with
Ba-La-142; the presence of a very short-lived “Ra I” was also realized correctly.
The @--decayproceeds diagonally from the lower right to the upper left with
constant mass number: stable nuclides are shown in black.
On December 21 Hahn informs Lise Meitner about his
intention to publish quickly and writes that he is sending a
copy of the manuscript to her.[’ 361 He submits the paper
without waiting for Lise Meitner’s comment on his letter of
December 19 with the unexpected results. He receives her
reply“ 371 on December 23;[’381she writes that the assumption of such a drastic breakup appears difficult for her to
understand but that in nuclear physics there have been so
long-lived Ra IV
AcII (half-life about 2.5 hours)
many surprises that one cannot really call anything impossible. During the many years of collaboration Lise Meitner
became acquainted with the accuracy of radiochemical
methods in such a classic system like radium-barium; therefore, she is more inclined to give up nuclear-physics dogma
rather than to question the results.[341She also asks for the
transuranic elements which may have caused Hahn on the
same day to formulate a short paragraph “Was die ... ergibt
239!”[*] when modifying[991 the manuscript somewhat;
upon correction on December 27 this paragraph is added to
the text.[’391 There he indicates, Figure 19, that the
transuranic elements might be lighter rather than heavier
homologues of rhenium and the platinum metals, because
the sum of the mass numbers of barium and masurium is 239,
the initial mass. Thus, a breakup into two large nuclei rather
than a breakup into many nuclei is clearly referred to here.
Hahn realizes,[’39] however, that the argument is problematic, since not the sum of masses but the sum of the
nuclear charges instead is of importance: krypton, element
36, is the complementary fragment of barium, element 56,
rather than “masurium”, element 43. The suspicion, however, that the transuranic elements might in fact be lighter
homologues, for example element 43 instead of eka-rhenium,
is correct.
The decay curves of the indicator experiment of December
17 are presented in the second historical publication by Otto
Hahn and Fritz Stras~rnann,[’~~]
which is submitted on January 28, 1939. This time the title clearly indicates the main
theme: “Nachweis der Entstehung aktiver Bariumisotope
aus Uran und Thorium durch Neutronenbestrahlung.”[**]
Chemistry and data are described and discussed in detail for
the correct isotopes. In part A of Figure 21 the radioactivity
of the three barium bromide fractions is shown to decrease
first, followed by an increase towards plateaus of different
height; curve I depicts the data from the protocol, Figure 16.
Part B indicates how this increase of actinium-228 in the
radium indicator radium-228 is corrected for. After subtracting this contribution the resulting decrease is shown in part
C: The data points for the three fractions fall on one single
decay curve for 86-min barium 111. The reason why Hahn is
not completely satisfied with the curves, as he mentioned in
the note book-Figure 17-, is obvious: the last data points
for fractions I and I11 in part C are clearly above the decay
The contents, however, of this publication are even more
profound: “The experiments described in Hahn and Strassmann’s “second” paper rank among the most careful and
unambiguous ever carried out in radiochemistry.”[’411Hahn
and Strassmann now demonstrate the whole range of their
radiochemical virtuosity; between January 4, when work is
resumed at the Kaiser-Wilhelm-Institut,~’4z1
and the formulation of this second manuscript a wealth of results is obtained. The fractionations are repeated with other activities
and other barium salts: 86-min barium I11 is mixed with
natural thorium X, 3.7-d radium-224, and fractionated as
chromate; this works extremely well, since natural radium is
enriched in the first fraction by a factor 19.5 with respect to
“As to . . . yields 239!”
“Proof of the formation of active barium isotopes from uranium and
thorium by neutron irradiation.”
Angew. Chem. Int. Ed. Engl. 29 (1990) 481-508
Fig. 21 Decay curves of the radioactivity in the three barium bromide fractions I,II,III from the indicator experiment barium 111-mesothorium
1 of December 17,1938; they were published in the second historical paper by Otro Hahn and Fritz Strussmunn [140] on January 28,1939. The count
rate is displayed logarithmically as a function of time. The raw data are depicted in part A, compare the protocol of fraction 1 in Figure 16. The
initial decrease is followed by an increase until radioactive equilibrium is established between the indicator mesothorium 1, 5.8-y radium-228, and
its decay product mesothorium 2, 6.1-h actinium-228, in this section the decay curves differ appreciably, because the radium indicator is enriched
in the order fraction I > I1 > 111. Part B shows, how this growth is subtracted. Then, part C results with three practically identical decay curves
for the initial portion, the barium III,86-min barium-139. In contrast to natural radium, its activity is distributed equally among the three fractions.
The conclusion shown in Figure 17 is based on these curves.
the second whereas the barium activities remain unchanged.
The same procedure is performed for long-lived barium IV
and radium-224. Finally, for barium TV a cyclic experiment
is performed, namely a successive preparation of barium
compounds: chloride, succinate, nitrate, carbonate, again
chloride, iron(rn) mannite, and again chloride; the activity of
equal amounts of barium is again constant for all samples.
Joy in experimenting and not-as is alleged[’341-lack of
certainty is the likely reason for these variations. This is
reflected in Lise Meitner’s comment :[1431 “Euere Versuche
stellen eine wunderbar geschlossene Beweiskette dar und es
ist fabelhaft, was Ihr in diesen kurzen Wochen alles gemacht
The 15-min and 4-h “radium” from thorium, Figure 7, are
also identified as barium by fractionation as barium chromate using radium-224 as indicator; thus thorium fissions
too.[1401At last the “radium” decay-chains emerging from
thorium are interpreted correctly. The right mass number
139 is assigned to barium 111 from uranium, for barium IV
the mass number 140 is assumed. It is still left open whether
the barium isotopes from uranium and thorium are identical,
even though the half-lives are quite similar.
The main focus is, however, on the search for complementary fragments of uranium in the vicinity of krypton[’401
which, via p--decay, might turn into rubidium, strontium,
and yttrium; this important aspect is also underlined by Lise
Meitner and Otto Robert F r i s ~ h , “ of
~ ~which
Hahn is informed by a preprint when the corresponding radiochemical
experiments have just started.[’451 By precipitation of the
nicely crystallizing strontium nitrate using fuming nitric
acid, strontium is isolated, from which yttrium is separated;
both are active. A radioactive noble gas is found which decays into an alkaline metal that is active as well; it is temporarily left open whether the two are krypton and rubidium or
xenon and cesium. When uranium breaks into two fragments
of equal size palladium, element 46, should be formed; however, neither palladium nor the neighboring element silver
“Your experiments are a beautifully consistent series of proofs and it is
marvellous how much you could achieve during these few weeks”.
Angen. Chem. Inr. Ed. Engl. 29 (1990)481-508
are found among the fission products. For the first time the
strange asymmetry of uranium fission, namely the breakup
into a lighter and a heavier fragment, is thus apparent-a
fact which will raise questions for both experimentalists and
theorists for years to come. Only later[’46Jare products of
symmetric fission into fragments of equal size detected after
irradiation with energetic neutrons.
Naturally, the question as to what to think of the earlier
transuranic elements is also discussed. In the course of the
intensifying correspondence between Hahn and Meitner[97,983
this problem is raised again and again with changing points of view-and not without some ill feeling between
the two. For Lise Meitner this point is especially important,[’341because if all the transuranic elements were in fact
fission products her work during the last years would be lost
without her having the opportunity to participate in a discovery of even greater significance. After additional chemical experiments and some reflection, the suspicion1’ that
the transuranic elements are lighter homologues of rhenium
and the platinum elements is dismissed for the time being; it
is expressed instead that the transuranic elements continue to
exist.[’401We will come back to this later.
3. Confirmation of Nuclear Fission
3.1. Physical Experiments and Theoretical Explanations
Through letters and a copy of the manuscript Lise Meitner
and Otto Robert Frisch know what has happened; the scientists at the institute in Berlin, however, are informed on1 ~ “ ’’’]
~ . on returning from their Christmas vacation at the
beginning of January 1939 when the “first” publication appears. Lise Meitner meets Otto Robert Frisch at Christmas
1938 in Kungalv on the western shore of S ~ e d e n . [ ~ ~ . ’ ~ ’ ]
When walking together out in the snow they come upon a
very simple explanation[’44] of this unexpected nuclear reaction. The separation of large fragments from heavy nuclei
should not be viewed as a quantum-mechanical tunnel process;” ’] another concept[’481which describes nuclei as liquid
drops has to be applied. Nuclei, too, have a surface tension
which protects them from disintegrating due to the repulsive
electrical forces between the protons. For the uranium nucleus, however, this protection has almost vanished. Thus, after
capture of a neutron, such a nucleus first starts to vibrate
strongly, then elongates, and finally divides into two nuclei.
These two fragments repel each other electrostatically with a
kinetic energy of about 200 MeV, as can be estimated from
the nuclear radii and charges. This energy is indeed available,
since two nuclei of medium size are bound stronger than
uranium just by this amount. Thus, energetically this process
is possible; due to its similarity to the fission of cells they call
it “fission”,[’441after a suggestion by the biochemist W A .
Meitner and F r i ~ c h express
their ideas in a
publication whose essential paragraph is depicted in Figure 22.
Disintegration of Uranium by Neutrons:
Type of Nudear Reaction
a New
It Seems therefore possible that the uranium
nucleus has only ernall stability of form, aud may,
after neutron capture, divide itself into two nuclei
of roughly equal size (the precise ratio of sizes depending on fmer structural features and perhaps partly on
chance). These two nuclei will repel each other and
should gain a tot81 kinetic energy of c. 200 MeV., 68
calculated from nuclear radius and charge. This
amount of energy may actually be expected to be
aVd8ble from the difference in packing fraction
between uranium and the elements in the middle of
the periodic system. The whole ‘fission’ process can
thus be described in an essentially clessical WSY,
without having t.o consider quantum-mechanicel
‘tunnel effects’, which would actually be extremely
small, on account of the large rna88e% involved.
h 3 E &ITNEB.
Physical Institute,
Academy of Sciences,
0. R. firsca.
Inytitute of Theoretical Physics,
Jan. 16.
Fig. 22. Interpretation of nuclear fission in the framework of the liquid drop
model of atomic nuclei by Lise Meitner and Otto Robert Frisch according to
their ideas developed in Kungalv. Part of the publication submitted on January
16, 1939 [144].
he had not been taking the liquid-drop model that seriously[’501 because it was initially developed to describe static
nuclear properties like the nuclear masses without paying
much attention to its dynamic aspects. On the telephone,
Meitner and Frisch then prepare[’471a note which is drafted
by Frisch on January 6 and modified after a discussion with
Bohr; on January 7 , with Bohr already at the railway station
ready to depart for the U.S.A. by train and ship, Frisch gives
him the first two pages of the note.[i511Bohr promises not to
release anything before the note is published.“
Meanwhile Hahn and Strassmann’s “first” publication has
appeared. Frisch discusses the new phenomenon with G.
Pluczek who is very sceptical about it.[’”] This motivates1’471Frisch, who at first had not intended to perform an
experiment, to think of a possibility of physically detecting
the fast fission fragments. In a gas they should create an
unusually large number of ions, by far more than a-particles
for example; this should yield extraordinarily large signals.
The corresponding experiment is very simple[i471taking
only two days, January 13 and 14, 1939. In Figure 23 the
important part of the protocol[’521is shown, in Figure 24 its
author. The note about the interpretation of fission[i441is
not submitted until Frisch, on January 16, is able to report on
his experiment also:[’ 541 “By means of a uranium-lined ionization chamber, connected to a linear amplifier, I have succeeded in demonstrating the occurrence of such bursts of
ionization”. Using an oscilloscope he observes large pulses
above the background of cl-particles from uranium which he
concludes are caused by nuclei of at least a mass number of
70. Thorium produces the same effect.
Frisch does not show a picture of the fission-fragment
pulses; the picture in Figure 25 is taken during the first
is81 of nuclear fission on the American
continent in New York on January 25,1939. The most direct
way of identifying the fission fragments escaping in opposite
directions is to use a cloud chamber. One of the first two of
such e x p o ~ u r e s ~ ’1601
’ ~ ~is shown in Figure 26.
The notes by Meitner and Frisch “Disintegration of Uranium by Neutrons: a New Type of Nuclear
and by Frisch “Physical Evidence for the Division of Heavy
Nuclei under Neutron B0mbardment”[”~1 do not appear
until February 11 and 18, 1939, respectively; they had not
insisted on rapid publishing.[341Meanwhile, however, the
phenomenon of nuclear fission has become known through
Hahn and Strassmann’s first p ~ b l i c a t i o n . ~ The
’ ~ ~ 1first ones
to react in Berlin are the
they develop
ideas similar to the ones of Meitner and Frisch discussing[i61]the energetics of the process and reasons for the
formation of just barium.
Within a short time the huge pulses of fission fragments
are observed at several places, in New York,[’”] BerkeBaltimore,[’631Washington,[’641and Vienna;[’651in
Berlin the experimental physicists react somewhat later.“
Two quite different groups of fragment energies are found,
which correspond to the light and heavy fragment; their
mean kinetic energies are already determined surprisingly
well by W Jentschke and t;: Pr~nkl[’~’]
to be 61 and 98 MeV,
resulting In a kinetic energy release of altogether about
160 MeV.
I;: J ~ l i o t [ ’in~ Paris
~ ] chooses another way for a physical
detection of the fission process; he had learned early of the
to be the complemenIn addition, krypton is
tary fragment and the activities previously ascribed to the
transuranic elements are rather considered to be related with
lighter homologues of rhenium and the platinum metals; this
would make the isomeric chains superfluous. The similarity
is realized between the “radium-actinium’’ chains from thorium, Figure 7, and the activities identified as barium-lanthanum, with the conclusion that thorium is fissile, too. At
the end they discuss the 24-min uranium; its existence, they
argue, demonstrates that not all vibrations of the uranium
nucleus would lead to f i s ~ i o n . l ’ ~ ~ l
Back in Copenhagen, Frisch informs Bohr in detail on
January 3, 1939 and on the next day Hahn in a brief letter.[’491Bohr’s reaction is
“Oh what fools we
have been! We ought to have seen that before”. Presumably
Angew. Chem. i n f . Ed. Engl. 29 (1990) 481-508
38% 3
3897 0
?7 4
\P.0 9
Fig. 23. The first physical detection of nuclear fission by Otro Robert Frisch in Copenhagen on January 13, 1939; upper half of the protocol
sheet “Uran 2” [152]. A uranium sample is installed inside a hydrogen-filled ionization chamber of 1 cm depth and 3 cm diameter and is
irradiated with neutrons from a radium-beryllium source, 100 mg Ra. The electrical pulses are transmitted by a thyratron, whose threshold
voltage rejects pulses of the natural a-particles and only accepts larger pulses. The upper line of the text says: “Gelbes U-hydroxid auf Cublech
mit H,O aufgeschmiert und in Kammer gelegt”. (“Yellow U-hydroxide smeared on Cu-plate with H,O and put into the chamber”.) The
subsequent lines at the top and in the middle contain information about the eletronics used. Readings are alternately taken without (1,4,6,8)
and with the neutron source “Ra” (2,3,5,7). The numbers of the third column represent the time, those of the fourth column the meter rating,
and those of the fifth column the number of registered pulses. The first fission event between 16:34 and 16:50 h is welcomed with “Hurrah”
Pulses only appear with the neutron source installed.
Fig. 25. Oscilloscope picture of the strongly ionizing fission fragments obtained with an ionization chamber during the irradiation of uranium with
neutrons; a background of natural x-rays from uranium-238 can be seen [156].
The picture was taken upon the first observation of nuclear fission on the
American continent by H . L. Anderson, E. T Booth, J. R . Dunning. E. Fermi.
G. N . Clasoe, and E G. Slack in New York on January 25, 1939 [155].
Fig. 24. Otto Robert Frisch (1904- 1975) around the time of the “Frisch-experiment” [153].
Berlin results through a letter from 0. R. Frisch addressed to
his friend H . von Hulbun who works with Joliot; it is not
possible to establish[’68.1691 exactly when this happened.
For his experiment on January 26, 1939“701Joiiof makes use
o f the pronounced recoil of fission fragments caused by their
large kinetic energy. Uranium or thorium is spread onto the
Angen. Chem In!. Ed. Engl. 29 (1990) 481-508
surface of a hollow cyclinder which contains a neutron
source. Within 3 mm distance from the surface a bakelite
cylinder is mounted concentrically; on the latter, radioactive
products are found after irradiation which could have
reached there only by recoil. Joliot’s work “Preuve experimentale de la rupture explosive des noyaux d’uranium et de
thorium sous I’action des neutrons”[’6711*1
is reported to the
[*] “Experimental proof of the explosive rupture of uranium and thorium nu-
clei caused by neutrons.”
Fig. 26. One of the first photographs of the detection of nuclear fission using
a cloud chamber with three pairs of fission fragments flying in opposite directions. The stereo picture was taken at Berkeley [159];
left and right.
consists of two halves,
French Academy of Sciences on January 30, 1939, even before Meitner and Frisch’s publications[’44. 541 appear. The
experiment by E. McMi/lan~’711
at Berkeley is quite similar;
he already determines the penetration depth of fission fragments in aluminum foils and cigarette paper which corresponds to 2.2 cm if converted to air at normal pressure. This
is less than for a-particles even though the kinetic energy is
far larger; fission fragments carry a much higher charge and
therefore interact with matter more strongly.
In the U.S.A. the situation is dramatic.[150*1721 0n the
journey Bohr is accompanied by L. Rosenfeld, a theoretical
physicist with whom he has intense discussions. He does not
tell him, however, of the obligation to be silent. Thus, on
January 16 the physicists at Princeton, Rosenfelds first stop,
learn of the big news.“ 731When on January 20 Bohr still does
not know whether Frisch’s interpretation[’441has been accepted-he
has no idea of Frisch’s intermittent experiments-and after having seen Hahn and Strassmann’s publiwhich describes
cation at Princeton he submits a
the fission process in the framework of the liquid drop model. Not until January 22 does he receive news from Frisch.
Bohr departs to attend a theory conference in Washington;
on January 25 he stops by in New York and talks to H. L.
Anderson, a student of Fermi, who immediately starts preparing the experiment furnishing Figure 25. On January 26
the conference is opened with Bohr and Fermi reporting the
spectacular news, the main event of the conference.[’ 751 A
number of groups immediately start to carry out experiments
similar to the one by Frisch and thus, on January 28 the
conference is closed with positive results being presented;[’55.
1641 the local group can even demonstrate to
NewspaBohr and Fermi the fission phenomenon.[158*
pers report on it.[172,1761
Bohr has some trouble defending
the priority of Meitner and Frisch with regard to the physical
Literally a flood of studies on fission and fission products
is set in motion. The rare uranium isotope 235, with its
0.72% abundance only discovered a few years before,[’781is
recognized by B ~ h r [ ’to~ be
~ ]that component of uranium
which is fissile with thermal neutrons. A quantitative formulation“ 1 1 , 1 7 9 , ”01 of the concept of a fissioning liquid drop
1 6 3 3
is developed; it forms the basis of Bohr and Wheeler’s[’81]
pioneering theoretical study, which appears at the beginning
of the second world war. As first step towards a chain reaction the neutrons are found that are being released in the
course of fission.“82*
is the first to clearly
announce the feasibility of an energy producing “uranium
machine” and of a nuclear explosion. About one year after
the discovery, L. A . Turner[441completes the first review article which is based on close to one hundred publications.
Almost all important aspects are already mentioned. As early as December 2, 1942, Enrico Fermi and his team[’851
succeed in Chicago in carrying out the first chain reaction.
Only much later is the public informed about this result
because very soon almost everything that deals with fission
is kept secret.
Not the slightest doubt is raised against the phenomenon,
yet there is some trouble for the discoverers which is expressed in the letters between Hahn and M e i t n ~ r .981
A number of publications-for
example Bohr’s first
notes[”4. ‘ 7 7 1 - s ~ ~ n as
d if it was not Hahn and Strassmann
who had realized the significance of the process but only
Meitner and Frisch. Elsewhere,“ 821 this realization is even
ascribed to Joliot as well as Meitner and Frisch without even
mentioning Hahn and Strassmann; interestingly enough, the
respective publication is immediately followed by another
one11861which starts with the names of Hahn and Strassmann. At tirnesf167*1871
the story is told in such a way as if
Hahn and Strassmann had simply confirmed results by I.
Curie and Savitch; in one case, upon intervention by Hahn,
a correction[’881is made. Quite likely the peculiar style of
Hahn and Strassmann’s “first” publication has to some
extent provoked this confusion : Upon taking a brief look at
the article, hardly anybody would anticipate the discovery of
nuclear fission hiding behind the “alkaline earth elements”[105]mentioned in the title-Figure 19. Even a reader
who has become more inquisitive will, after reading a few
paragraphs, no doubt put aside the detailed radiochemical
report about radium from uranium. And those who finally
make their way through and even realize the significance of
the chemical fractionations will again be made helpless by
thefamousambivalentparagraph“Als Chemiker-als Kernchemiker” which Strassrnann would have formulated not as
In contrast, the subsequent physical artic l e ~ [ ’l ~
’ with their imaginative titles are much more
S 4~
, 1671
readable. Today, however, some reader may discover a particular attraction just in the style of the “first” publication.
3.2. Missed Opportunities
Before moving on to some of the later results we want to
look back wondering why nuclear fission had not been discovered earlier, since it is so simple to observe fission fragments with an ionization chamber. Experiments similar to
the one by F r i ~ c h [ ’are
~ ~ at
] that time being performed in a
number of laboratories. They are designed to search for energetic a-particles from an (n, a)-process [cf. Reaction (8,9)],
which may occur when uranium or thorium are irradiated
with neutrons, and thus to corroborate the production of
“radium” activities from uranium or thorium. Such experiments are being carried out at Berkeley,[189]Berlin,[821Cambridge,[’”] Heidelberg,[”’] Rome,[38.1921 Zu rich,[”]
Angew. Chem. Inr. Ed. Engl. 29 (1990) 481-508
presumably elsewhere, too. To this purpose the disturbing
background of natural a-particles, visible in Figure 25, is
suppressed by absorber foils and not by electronics, as in
Frisch’s experiment; in this way, however, the fission fragments are suppressed, too. As soon as fission is discovered,
the foils have only to be removed in order to see the fragments. “But who can say whether even Fermi would have
recognized them for what they were if he had seen
them?” [”’I Although perceived at times, they are ignored as
being electrical noise;[’901 this is von Droste’s opinion in
Berlin[”*] and presumably also that of researchers in
Zurich, since an announced[*’] detailed article on prompt
a-particles is never published.
P. Abelson, a graduate student at Berkeley, has also come
very close. Using a strong sample he measures the energies of
the characteristic X-rays of 66-h eka-iridium-Figure S i n
a crystal spectrometer; while the exposure is taken, Abelreads the news from Berlin in a newspaper. He immediately modifies the experiment to measure the absorption of the radiation, which works faster, and finds the
characteristic K-X-rays of iodine emitted after the a--decay
of its mother substance 66-h eka-iridium. The latter is thus
identified as an isotope of tellurium, as is reported in a
note[’941of nine lines length; later A b e Z ~ o n [ ’presents
details. Nuclear fission is thus again corroborated by detection
of a fission product.
The Berlin group, too, could have arrived at fission as
early as 1936 had only some night-time playful chemistry by
Fritz S t r a ~ s r n a n n [ ’been
~ ~ ] taken more seriously. In order to
find a fast chemical separation for 16-min eka-rhenium he
attempts to coprecipitate it with barium sulfate. Potassium
permanganate gets incorporated into crystals of barium sulfate. The homologous eka-rhenium might behave the same
way if present in a modification corresponding to perrhenate. The precipitate is active; however, the activities are not
those of the 16-min species. Strassrnann has to admit to Lise
Meitner that adsorption effects cannot be ruled out. In turn,
she rejects the results, in a friendly manner but quite firmly.[1961
It is difficult to judge from I. Curie’s short remark[”] how
close she came to seeing the lighter elements. In the fall of
1939 she tells[34”511G. von Hevesy that at times she thinks
the whole periodic table is in the irradiated uranium samples.
In spite of intensive discussions during seminars no one finds
a solution to the puzzle, as a participant recalls.11971
Also in
Berlin lighter elements are being considered but this idea is
not pursued because of the seemingly insurmountable Coulomb barrier.” ‘*I
In contrast to a recent suggestion,[431element 43, the masurium of Walter and Ida Noddack,[21.”] should not be considered as one of the missed opportunities. All isotopes of
this element are short-lived having already decayed since the
heavy elements were synthesized in the interior of stars. On
the other hand, element 43 is continuously being produced
on the Earth by the spontaneous nuclear fission of uranium,
which we will discuss below. Its quantity, however, is minute.
In order to detect this amount and thus to-unconsciouslyobserve fission the Noddack‘s X-ray fluorescence spectroscopy would have had to be five orders of magnitude more
sensitive than according to their own claim; there are no
arguments in favor of this.[’981
Angew. Chem In[. Ed. Engl. 29 (1990) 4x1-508
3.3. Further Fission Products
What is left now of the uranium-transuranium decay
chains of Figure 6 and what is the story behind the
“transuranic elements”? It has just been mentioned that
A b e l ~ o n [ ’1951
~ ~identifies
the 66-h eka-indium as a tellurium
isotope and thus the daughter substance 2.5-h ekaplatinum as iodine. His result is confirmed[’991by the same
method, namely X-ray absorption, and by Hahn and Strassmann[Zool
who investigate in detail the chemical properties of
this eka-iridium. In these experiments they find a second
activity with a quite similar half-Iife which they correctly
identify as molybdenum. They discuss how the 2.5-h iodine
could have been taken for a platinum metal: The crystallization of ammonium hexachlor~platinate[~~]
was thought to
be a selective reaction for platinum. The possibility that
iodine would precipitate as the analogous ammonium hexaiodoplatinate was not taken into account since fission was
not envisaged. In the platinum sulfide precipitate 2.5-h
iodine appears, because it is formed from the coprecipitating
66-h tellurium. Additional experimental studies concerning
the transuranic elements are not published, even though they
were announced[2001by Hahn and Strassmann.
Instead of looking at individual activities of “transuranic
elements”, Meitner and Frisch[1861examine the complete
mixture coprecipitated with platinum sulfide as a whole. To
this purpose recoiling fission fragments emitted from irradiated uranium are collected in a water layer from where they
are coprecipitated with platinum sulfide. The decay curve is
compared with a precipitate of platinum sulfide which is
directly isolated from irradiated uranium without exploiting
the recoil effect. The two decay curves match perfectly.
Therefore, the “transuranic elements” are fission products,
which are the only species having such a high recoil energy.
In the same way fission products of thorium are identified;12”] thus, similar products result from the fission of
uranium and thorium. Using such recoil experiments, other
authors[”’. 2031 reach similar conclusions.
In his autobiography12041Hahn reflects, on the basis of
comments by S t r a ~ s r n a n n , [ ’on
~ ~what
~ might have simulated
the other “transuranic elements”. Later, this problem is also
approached experimentally[206]with high-resolution y-ray
spectroscopy by examining precipitates of platinum sulfide
and other transuranic fractions produced according to
Hahn, Meitner and Strassmann. These components are
found to be complex, too. One half of the historically most
important activity, the 16-min eka-rhenium, consists of technetium-101, the remaining half is a mixture of molybdenum101, antimony-l30g, antimony-131, and tellurium-13lg. The
55-min activity is dominated by iodine-134, with some tellurium-134 and tin-128, and the 5.5-h activity by ruthenium105, with some antimony-129 and iodine-133. The 16-min
activity precipitating with manganese dioxide, which was
considered by Fermi et al. to be element 93, is also complex;
mainly molybdenum-101 is precipitated, from which technetium-101 is formed.[207]
With respect to the 3.5-h activity of I . Curie and Savitch it
is soon realized[441that it is related to 3.5-h yttrium-92; its
3.6 MeV p--radiation is much more energetic than that of
natural protactinium-2341x1with 2.3 MeV. In addition, contributions are to be expected by 3.90-h lanthanum-141, 9350 1
min lanthanum-142, and, in the chemically unprocessed mixnucleus is more difficult to establish experimentally beture, by 76-min krypton-87. Now it is understandable, with
cause-as can be seen from some examples in Figure 20-the
yttrium-92 as the main constituent, that the 3.5-h activity can
fission products at the beginning of the decay chains are
be fractionated from lanthanum; thus, this finding turns out
usually very short-lived. In order to establish the initial
to be correct. Since I. Curie and Savitch isolate this activity
abundance of individual elements the yields of all the respecdirectly from irradiated uranium, their lanthanoid fraction
tive isotopes have to be determined and added. Data of this
kind have only been obtained in more recent times. The
always contains yttrium-92. Hahn and Strassrnann, on the
other hand, do not obtain the lanthanoid elements directly
abundance of elements formed by fission of uranium235f2141is shown in Figure 27. The lighter and the heavier
but only as decay products of “radium”-barium; this way
they exclude yttrium because its mother substance strontium
fragment from the asymmetric fission process can be seen
is not contained in their barium fraction.
with five elements each created predominantly, krypton to
zirconium and tellurium to barium. Hahn and Strassmann’s
The fission products prove to be an extremely complicated
mixture of elements with several isotopes each. They form
barium isotopes are mainly formed indirectly via p--decay of
long P--decay chains, often branching out into different isoshort-lived xenon and cesium nuclei, which are close to the
mers. Some of them are depicted in Figure 20. During the
center of the heavy fragment wing.
subsequent years the small group at Berlin, which has been
joined by H. J. Born, H. Gotte, and W: Seelrnann-Eggebert, is
busy unravelling these chains, at least to the extent it is
possible in a war-time situation that is becoming more and
4. Transuranic Elements Again
more difficult. These experiments are all published. As Hahn
reports in his Nobel Prize lecture,[461in the beginning of 1945
4.1. Element 93, Neptunium
about 100 products of 25 elements have been found.[208]
Around this time the corresponding American results are
We are left with one member of the decay chains of Figure
released which have been obtained by Charles C 0 r y e 1 1 at
~ ~ ~ ~ ~6, the 23-min uranium-239, which is ~ h e m i c a l l y [ ~
“Met Lab” under superior conditions; they first appear in
to be uranium and whose assignment to mass number 239 is
later in more detail.[” ‘I Here, 170 prodestablished physically[611in uranium-238. As a P--emitter it
must decay into the isotope 239 of element 93. The detection
ucts of 37 elements are listed. Very few of the products obof this decay product, however, proves to be very difficult. E.
served in Berlin are not contained in these American
tables; focusing on some of the unsettled cases, work is reMcMillan[’ ’I1 takes a first decisive step. When measuring
sumed by Fritz Strassrnann and his students as soon as it is
the range of fission products in cigarette paper he examines
possible after the war.[2121
the thin uranium layer as well and notices two strong activities: one with a 23-min half-life, which he ascribes to
In Berlin, only the first steps[2131towards quantitative
uranium-239, and one with about a 2 d half-life, which is
measurements of the abundance of individual fission prodstrong compared to the activities of fission products of
ucts are made which would indicate into what kind of fragsimilar half-life. Even the natural activity of uranium is negments the uranium nucleus tends to fission predominantly.
For this aspect the American groups can already present a
ligible compared to this. This hint [1711 causes a reader-was
it Hahn himself?-at
the Berlin institute to put a large
complete distribution as a function of the mass number.[210.2’ The mass distribution is chosen, because the
exclamation mark at the edge of the publication which is still
mass number does not change during p--decay; if one waits
nicely visible today; the much superior means of the American competitors start to take effect.
for p--decay to reach the long-lived final members of the
Is this the sought after element 93, whose recoil energy is
decay chains, the total yield of all members of a chain is
expected to be so small not to recoil out of the uranium
summed up in quite a natural way. The element distribution,
target? Based on decay curves, E. SegrP[2151
first draws the
however, continuously changes with time due to P--decay.
conclusion that the 2.3-d activity does not emerge from 23The initial distribution just after the breakup of the uranium
min uranium-239; at this point he is misled12161by its low
P-energy. He then attempts to chemically identify the longlived activity again assuming element 93 to have the properties of eka-rhenium. As one of the discoverers[2171of element
43 he already had experience with chemical studies in the 7th
group of transition elements. SegrC?finds, however, that the
2.3-d activity does not coprecipitate with rhenium sulfide
but rather behaves like a lanthanoid element: it coprecipitates with lanthanum fluoride and oxalate, with potassium
lanthanum sulfate and other typical precipitates and it can
be separated from actinium by fractionation of lanthanum
oxalate. Segr$[2151
ascribes the 2.3-d activity to a lanthanoid
fission product and concludes that 23-min uranium should
decay into a long-lived element 93 which has not yet been
Fig. 27. EIement distribution in the fission of uranium-235 by thermal neutrons: primary abundance in percent as a function of the atomic number. The
where element 93 was to
found. Also other
asymmetry of fission into a smaller and a larger fragment is evident; xenon,
be isolated with rhenium sulfide from highly enriched
cesium, and barium, key elements for the discovery, are located in the heavy
samples of 23-min uranium end with negative results.
branch. After [214].
Angew. Chem. In!. Ed. Engl. 29 (1990) 481-508
After one year E. McMillan resumes his studies["11 applying a larger neutron flux which allows the use of a thinner
uranium layer. Even then the 2.3-d activity remains inside
uranium and can be completely separated from fission products this way. It is obtained with resonance neutrons just as
well as the 23-min uranium-239. P. Abelson, who is now at
Washington, observes that the 2.3-d activity does not always
behave like a lanthanoid. The two start working together at
Berkeley and within five
they find the s ~ l u t i o n : [ ' ' ~ ~
In a strongly oxidizing medium, bromate in strong acid, the
2.3-d activity does not coprecipitate with cerium fluoride;
however, in a reducing medium, sulfite, it precipitates quantitatively. After oxidation the activity co-precipitates with
sodium uranyl acetate, and after reduction it accompanies
thorium iodate and peroxide. Properties typical of rhenium,
like the sulfide precipitation, are not observed. Obviously,
like uranium the finally discovered element 93 has two main
oxidation states, four and six, the lower state being the more
stable one, in contrast to uranium. McMillan and Abelson12191presume a new series of related elements starting
with uranium. Uranium is also the basis for the "astronomical" name of element 93: neptunium.
With these chemical results McMiIlan and Abelson'2191are
able to prove that the 2.3-d activity is the daughter of 23-min
uranium. This experiment is depicted in Figure 28. Every 20
crease with a half-life of 23 minutes if displayed as a function
of the time of precipitation, as is shown in Figure 28. According to the laws of radioactive decay, this proves that the 2.3-d
activity arises from 23-min uranium and thus must be neptunium-239. Neptunium-239 has soft P--rays of only
0.47 MeV energy. A decay product, element 94, is sought
after but not f o ~ n d . [ ~ ' ~ ]
In Berlin, following McMiZlan's first publication," ''I one
does not succeed in observing the 2.3-d activity, even in
samples which have been more strongly irradiated[2201
at the
Copenhagen high voltage facility. The explanation is simple:
the activity is not contained in a platinum sulfide precipitate;
in the uranium fraction it is only weakly produced from
23-min uranium. In addition, the activity is difficult to detect
due to the strong absorption of its low energy radiation.
Therefore, in Berlin special enrichment procedures have to
be developed for 23-min uranium. This is achieved by K.
2221based on a Szilard-Chalmer~-separation.[~~~~
This technique exploits the fact that atoms having become
radioactive as a result of nuclear reactions may be transformed into a chemical state different from the one of the
irradiated species. If uranium is irradiated as uranyl benzoyl
acetonate, 23-min uranium-239 becomes partly converted
into an inorganic species and can thus be concentrated from
a large bulk of uranium. In strongly enriched uranium-239
samples of this kind StarkeIZz4.2 2 5 1 finds the 2.3-d activity
even before the paper of McMillan and A b e l s ~ n [ ~arrives
at Berlin[2261at the end of August 1940. Due to the war,
however, Starke's results are published later.[221.'",224*2 2 5 1
Hahn and Strassmann also start working on element 93;
again they encounter some difficulties in finding the 2.3-d
and therefore construct a counter with a thin
entrance window. They develop a chemical procedure to
isolate element 93 directly from large amounts of uranium
and perform a detailed study [228] of the chemistry of element
93. Properties already described['"*
are partly confirmed and new ones are added. They all point at a discontinuity in the periodic table: Element 93 has nothing in
common with eka-rhenium.
4.2. Eka-Rhenium, Element 107
Fig. 28. Discovery of element 93, neptunium, by E. McMillan and P. Abelson
at Berkeley [219]: Proof of the genetic relationship between the 23-min uranium-239 and its 2.3-d decay product. The initial count rates of the 2.3-d activity
are measured in cerium fluoride precipitates obtained upon successive precipitations from the same solution of uranium-239. When depicted logarithmically
as a function of time these count rates decrease with the half-life of uranium239.23 min Thus, uranium-239 decays into the 2.3-d activity which is therefore
identified as neptunium-239.
minutes cerium fluoride is precipitated from a reducing solution of the 23-min activity; thus, each time the 2.3-d activity
continuously formed from 23-min uranium-239 is "milked"
from its parent. The initial intensities of these samples deAngew. Chem. Int. Ed. Engl. 29 (1990) 481-508
Now starts the era of nuclear chemical element syntheses.
The periodic table extends more and more and takes a course
different from the one originally expected: The series of actinoids is recognized, first by S e a b ~ r g . [ 'It~ extends
as far as
lawrencium, 103, and then returns to the transition elements
with element 104, presently reaching up to element 109.[2301
Today's periodic table is shown in Figure 29.
Element 107 is placed below rhenium, and it takes almost
four decades until this eka-rhenium is finally produced by
Gottfried Miinzenberg, Peter Armbruster, and the SHIPgroupIZ3']at the heavy-ion accelerator UNILAC in Darmstadt by nuclear fusion of chromium-54 with bismuth-209
[Reaction (1I)].
+ "ZBi
262107+ An
Its detection is again based on the genetic connection with
decay products: The isotope 262 decays with 8.2 msec half503
Fig. 29. The periodic table of the elements today. The atomic numbers of
unknown elements are given in brackets.
life via a-emission into the known isotope 258 of element 105
and its well-known decay chain; an example for the decay
chain of an individual atomic nucleus is depicted in Figure
9176 keV
n d 3 6
and estimate the half-life of uranium-238 with respect to
spontaneous fission to be in the range of 1OI6 to IOl7
already the right order of magnitude. For the
transuranic elements spontaneous fission becomes an important decay mechanism which in the end limits the periodic
But also the chain reaction has occurred in nature. The
possibility of a natural nuclear reactor is discussed earl y [ 1 8 4 . 2 4 2 1 but IS
. first dismissed with the argument that in
such a case uranium deposits should not exist; later this
question is raised again and it is realized[z431that a natural
chain reaction is possible. A natural nuclear reactor is indeed
discovered due to minute deviations of the uranium-235 concentration of some uranium delivered to France, namely[2431
0.7171 % as compared to the usual value of 0.7202%. This
deviation was caused by a uranium ore from Oklo, Gabon;
at the respective mining site samples are found with uranium-235 depleted down to a concentration of 0.29%. About
1.8 billion years ago the reactor was operating for about
600 000 years and has released roughly 10 000 megawattyears of energy, the amount delivered by a large power reactor within about three years; upon operation 4 tons of uranium-235 were consumed.[245.2461 It is interesting to note that
the fission products formed have not migrated from the site
during this long time interval.r245.2461
6. Conclusion
8 L39 ke$
2.1 s
7.L57 keV
Fig. 30. Discovery of element 107, eka-rhenium, by the SHIP-group a t Darmstadt [231]: one of five observed decay chains of a short-lived a-emitter which
decays into the known nuclide 105-258 and its known decay products; the
%-emitter therefore has to be assigned to the isotope 262 of element 107. This
element 107 is produced by nuclear fusion of chromium-54, element 24, with
bismuth-209, element 83
Nothing is known as yet about the chemistry of eka-rhenium; thus, it is not clear whether the extrapolations of chemical properties will eventually be correct. Due to short halflives and extremely low production rates, studies of this kind
are not feasible for the time being. Chemical experiments
with transactinoids have been performed u p to element 105
and are described in review articles[z3z- 2341 and several
more recent publications[236- 2381.
5. Nuclear Fission in Nature
The fission of uranium is a natural process; uranium already fissions spontaneously without an additional energy
input. This spontaneous fission is discovered in 1940 by
C. N . Flerov and K. A .
who use a large
ionization chamber where about 10 g of uranium is spread to
a thin layer. They observe about six fission events per hour
The discovery of nuclear fission has given access to the
energy hidden in atomic nuclei intensively discussed for several decades." "I Not only was the discovery itself exciting
but also the way in which it came about: The breakup of the
uranium nucleus was not observed directly, but only some
resulting secondary products. Looking back, however, this
was probably the most straightforward path towards this
discovery; there was no way to circumvent the barium in the
Berlin beakers: "It was good solid chemistry that got things
on the right track."['471 Otto Hahn and Fritz Strassmann and
their predecessors had intended to pursue a different goal.
This, however, does not make the discovery of nuclear fission a chance happening; they did not simply stumble on it,
instead it was the fruit of many years of intensive work in
radiochemical analysis. To think of chances may be more
appropriate when imagining how strongly the events described here depend on minor fluctuations of the nuclear
binding energy, which is, however, governed by subtle quantum mechanical effects. These fluctuations cause the asymmetry of the fission process with a heavy fragment just in the
region that Hahn and Strassmann studied in their search for
radium. On the other hand, such local fluctuations cause the
p--decay of neptunium-239 to be of an unusually low energy; for this reason, Otto Hahn, Lise Meitner, and Fritz
Strassmann could not find element 93, even though it is
formed from uranium with an abundance one order of magnitude greater than that of the strongest fission products.
Thus, to Huhn's disappointment,[2471whereas protactinium,
the neighbor to the left of uranium in the periodic table, was
found in Berlin, the corresponding neighbor to the right
escaped discovery.
Angew. Chem. Int. Ed. Engl. 29 (1990) 481-SO8
The history of nuclear fission tells us of how science often
evolves, not by successive logical steps but rather through
strange detours ; how for many years the most experienced
groups miss the central point in their experiments and in
their thinking because they remain attached to seemingly
well founded but erroneous concepts; how in the end they
intuitively find the right track; and how after decades a scientist returns to the kind of work he pursued as a beginner.
The author thanks Peter Brix, Hans Otto Denschlag, Helmut Folger, Jens- Volker Kratz, Klaus Liitzenkirchen,
Chaturvedula Sastri. Matthias Schadel and Norbert Trautmann who have critically read this article, and Karl-Heinz
Glasel, Ursula Othmer and Annette Zauner, who have helped
to prepare the manuscript.
Received: September 21, 1989;
Supplemented: December 5, 1990 [A 760 IE]
German version: Angew. Chem. 102 (1990) 469
Translated by Dr. Klaus Lutzenkirchen, Maim (FRG)
Note: In the case of some works the date received for publication is quoted.
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