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Патент USA US3068188

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Dec. 11, 1962
Filed Oct. 14, 1959
7 Sheets-Sheet 1
F1‘ 51.7
E 80
I00 gm/l
.2 r
o E 70
.- 2
"E < 60
U ‘D
40gm/l X
a 2 5<>~
5 40-‘
lOgm/l r
I6 20 24
Percentage Distilled Wm‘er of
Total Solvent Volume
Gamma-ray fluorescence of
2,5-diphenyloxclzole (‘IO gm/l) in solu
tions of p-dioxcme plus water
Dec. 11, 1962
Filed Oct. 14, 1959
'7 Sheets-Sheet 2
Percent Hexane -mass
. Gamma-ray induced ?uorescence in
hexane-xylene mixtures.
1m 7
Perz'enl ?-Buty/plvosbhute -mass
. Gamma-ray induced ?uorescence in mixtures of n-Butyl~
phosphate and xylene or phenylcyclohexanc.
Dec. 11, 1962
Filed Oct. 14, 1959
'7 Sheets-Sheet 3
JFlue/"an thene
2A5 dlpberlgli \
am 3 ale 2 9/1
. Gamma-ray induced ?uorescence in ethyl alcohol
(95 percent)-xylene mixtures.
Percem! CCL, — mass
. Gamma-ray and light induced ?uorescence in
CCh-xylene mixtures, solute: ?uoranthene.
Dec. 11, 1962
Filed Oct. 14, 1959
7 Sheets-Sheet 4
. Gamma-ray induced ?uorescence in
paraffin oil-xylene mlxtures.
. Florescence of 2,5-diphenyloxazole in mixtures
with n-butylphosphate.
6/ 5
(5%46/7/ém/W ATTORNEYS
Dec. 11, 1962
Filed Oct. 14, 1959
‘7 Sheets-Sheet 5
EPfects of additional “solvents” on n-butylphosphate
+0.10-diphenylanthracene (0.5 g/D.
2, $- D/phe nq/aXago/e (on re
Dec, 11, 1962
Filed Oct. 14, 1959
'7 Sheets-Sheet '7
86 04
Percen’r by Weigh’! Tri-n-umylborofe
Gamma-ray fluorescence of
2,5-diphenyloxozole (5 gm/l) in solu
Hons of iri-n-umylborcfe plus naphthalene
or xylene
M470” [(24357
guy //5eon//v
ie Stats
Patented Dec. 11, 1952
rescence e?iciencies that can be obtained with the sub
stance external to the scintillator. However, such a tech
Hartmut P. Kallmann and Milton Furst, Bronx, and Felix
H. Brown, Kew Garden Hills, N.Y., assignors to Leon“
artl E. Ravich, Brookline, Mass.
Filed Oct. 14, 1959, .ier. No. 849,703.
15 Claims. ((Ji. 252-3012)
nique is often limited because of the decrease of light out—
put of the scintillator solution by the desired substance
whose radio-activity is to be measured, as in the case where
the substance to be tested contains elements of medium
or heavy atomic weights, i.e., having an atomic number
of 10 or greater.
The ?uorescence ef?ciencies of con
ventional scintillator solutions are also reduced by the
This invention relates to liquid scintillators wherein
?uorescent emission is induced by high energy excitation 10 introduction of additional material to make the desired
substance soluble in the scintillating solution.
and more particularly to techniques for increasing the
Heretofore, the scintillation solution frequently used
?uorescence where the transfer of energy from the solvent
employed a material such as 2,5-diphenyloxazole as the
to the solute is poor or inhibited by quenching materials
solute with toluene, dioxane, or xylene as the major sol
present in the solution.
This application is a continuation-in-part of our appli 15 vents. Certain organiccompounds caused a quenching
effect in the system and water soluble materials could
cation Serial No. 561,208 (now abandoned), ?led Janu
not be tested. If a water soluble material were to be
ary 25, 1956.
examined it was necessary to add ethanol to the solution so
Scintillator solutions are composed primarily of a sol~
that a small amount of water could be tolerated. This
vent and a solute which has the property of exhibiting
?uorescence induced by high energy as far down and in 20 procedure was quite unsatisfactory and a substantial de
crease in e?iciency was noted due to quenching etlect of
cluding the ultra-violet~light excitation. There are a large
the water which, for all practical purposes, could be con
number of materials which exhibit ?uorescence or light
emission. These materials when dissolved in a suitable
solvent usually provide a fast response to radiation, are
sidered a contaminant.
Prior efforts to provide scintillating solutions contain
of great value in studies involving very short resolving 25 ing water are reported in articles by F. N. Hayes and
R. Gordon Gould, published in Science, volume 117, pp.
times, can be fabricated into a variety of shapes and have
480-481, May 1, 1953, and Earle C. Farmer and Irving
almost unlimited volume.
A. Berstein, published in Science, volume 115, pp. 460
Since the solute is the material which exhibits the
461, April 25, 1952. In the former article it is stated
?uorescence, larger concentrations of solute will, in gen
that the principal advantage of using dioxane as the sol
eral, produce a greater amount of light emission for a
vent was that it was then not necessary to add ethanol
given level of radiation incident on the scintillator liquid
to produce miscibility of the water sample. The article
until self quenching eifects become important. The maxi
continued, however, to state that the ei?ciency of dioxane
as a scintillation solvent is su?iciently inferior to toluene
the usual scintillating solution contains only a few grams, 35 and xylene that the latter solvents provide a greater sen
mum amount of solute that can be used in scintillator
solutions is limited by its solvency in the solvent, and
usually less than 20, per liter of solvent.
amounts are readily known or easily determined by those
skilled in this art.
Many of the scintillator solutions have similar relative
sitivity, although dioxane permits counting of larger sam
pics. The latter article similarly describes a liquid scin~
tillator suitable for both organic and water soluble mate
e?iciencies for emitting light under both high energy and
rials and an experimental arrangement that permits high
counting efficiency. The solution employed was p-di
ultraviolet-light excitation and many other solutions ex
hibit a considerable variation between the di?erent types
oxane saturated with p-terphenyl, the ?uorescence output
of which is very poor relative to the non-water containing
of excitation as discussed in detail ‘below. Thus some
solvents, hereinafter referred to as “poor,” are unfavor
scintillating solutions.
In accordance with our present
invention the water content can be increased by a factor
able for producing ?uorescence from high energy excita
of at least ten with sufficiently high e?iciency for effective
tion such as gamma ray radiation, or from alpha or beta
it is accordingly the primary object of this invention
to provide a technique for enhancing the fluorescence in
stance, hereinafter referred to as an “intermediate sol 50 duced by high-energy radiation either directly in or out
side of the solutions where ?uorescence is relatively low
vent,” to a conventional scintillator solution which has
due to either use of a “poor” solvent or due to quenching
“poor” or “weak” light emitting properties to thereby in
materials present in the solution.
crease the e?iciency of the solution for the production of
In accordance with this invention, it has been found
light. The present invention does not involve the dis
covery of new scintillator solutions, but simply a means 55 that substances exist which, when put into poorly ?uores
cent solutions in sizable amounts, enhance the ?uorescence
for increasing the light emission e?iciency of those solu
to a greater extent than does the introduction of the
tions which are weak light emitters.
known most efficient solvents. The enhancing substances
Fluorescence ‘from known good scintillator solutions
are not solvents normally used in scintillating solutions
induced by high-energy radiation is often strongly reduced
by the presence of certain substances which are placed in 60 which have maximum ?uorescence and may be substances
which are solid at room temperature. These substances
the scintillator solution. This condition has been found
which enhance the ?uorescence of the scintillator will be
to exist, for example, in solutions of xylene as a solvent
referred to as “intermediate solvents.”
and terphcnyl as the light emitting solute when oxygen
The present invention relates to the addition of a sub
It is another object of the invention to provide as an
is added to the solution and also in a solution where tetra
lin is used as the solvent and 9,10-diphenylanthracene is 65 “intermediate solvent” a material which undergoes smaller
acid molecules into the scintillator solution which pro
quenching by surrounding molecules than do the excited
molecules of other substances normally used in the liquid
scintillator solution.
duces strong
of xylene or
tillators, for
A maior effect of many useful substances appears to be
a quenching of the energy present in the excited solvent
molecule. As explained in detail below, it appears that
energy transfer takes place to the solute from the solvent
used as the solute when the tetralin is exposed to air.
Another example is presented by the introduction of nitric
quenching of the high-energy ?uorescence
phenylcyclohexane solutions.
of radio-active shubstances in liquid scin
example, offers in many cases higher ?uo
molecules with the probability of energy transfer being
and greater than the energy level of excitation of the
smaller for the “poor” solvent than for an “e?icient”
solvent. The probability of energy transfer is propor
tional to the concentration of the molecules to which the
energy is transferred, the life time of the excitation of the
solute. Thus, it is possible to ?nd an “intermediate
solvent” e?ective to enhance ‘the ?uorescence output of
solvent molecule, and the “collision” cross section asso—
ciated with the transfer of energy from the excited solvent
molecule to the solute molecule.
Based on the experi
nearly every weakly ?uorescing scintillator solution where
fluorescence is relatively low due to either use of a
solvent or due to quenching material present
solution. In View of the large number of known
lator solutions, the same “intermediate solvent”
in the
is not
amounts (greater than 20 g./l.) has property of taking
etfective for every solution. While naphthalene is an
effective “intermediate solvent” for many scintillator solu
tions, it has been found that solutions employing p
terphenyl as a light emitting solute cannot be enhanced
with naphthalene apparently because of lack of energ
transfer. This is explainable because the energy level of
excitation of naphthalene is very close to the energy level
of excitation of p-terphenyl. However, an “intermediate
over much or most of the energy originally in the bulk
solvent” such ‘as biphenyl which has a lower energy level
ments described below, it is believed that the probability
of energy transfer is effected quite largely by the life
time of excitation of the solvent molecule which factor
is in turn determined mostly by the probabilities of in
ternal and external quenching processes.
“Intermediate solvent” is a substance (it may be solid
at room temperature) which when present in moderate
of excitation than naphthalene can be used to enhance‘
material (major solvent) and transferring it more effi
solutions containing p-terphenyl.
ciently to the emitting solute (?uorescing material) than
In addition to many physical and chemical applications
does the bulk material. The lowest excited electronic 20
it is important in biological (medical, biochemical) re
state of the “intermediate solvent” lies between that of
search for example, to measure the activity of physiologi~
the bulk material and that of the emitter. (The lowest
cal compounds labelled with radioactivity. Since the
state has an energy greater than 3.4 e.v.) The transfer
activity is generally small, the best method is to dissolve
to and from the “intermediate solvent” does not take
the compound in the scintillator solution of a liquid
place primarily via radiation. The time constant of the
scintillator counter. This has been successfully used in
“intermediate solvent” is longer than that of the bulk
many cases, for example with labelled steroids. Many
important compounds, however, are not soluble in the
The effectiveness of the substance as an “intermediate
usual scintillator solutions because of solvent polarity.
solvent” is believed to be due to two processes. First, a
Another object of the invention is to provide an im
more efficient energy transfer occurs from the “poor”
proved scintillation solution containing water, 5% or
solvent to the “intermediate solvent” than, for example,
more, whereby certain water soluble substances such
to a known “e?icient” solvent such as xylene. Secondly,
for example as metallic salts, water soluble organic solids
the excited “intermediate solvent” molecules undergo a
important for biological use and the like may be dis~
smaller quenching by surrounding molecules than do the
solved in the solution. Water has been found to reduce‘
excited molecules of other substances. The quenching
the ?uorescence e?iciency to a degree making the scintil
of the excited solvent molecule by the addition of special
lation solution essentially useless for detection and measa
molecules, such as those containing oxygen or elements
urement. The greater the water content, the less the light
of medium or heavy atomic weights, is often more promi
output. However, for a large number of biological,
nent than that of the solute molecule.
medical and physical applications, large amounts of
A further primary object of the invention is to provide
water are essential for solubility. These solutions may
for ‘a weakly ?uorescing scintillator solution an additive
such as naphthalene and other compounds which can be
similarly employed to enhance the ?uorescence of such
scintillator solutions. While the additives have been
be made far more c?icient ‘by the use of the present
Solutions containing alcohols such as ethanol and
named by us as “intermediate solvents," they are not an 45 methanol are “poor” primary solvents of special impor
tance in work with biological materials. The addition
“efficient” solvent capable of being used as the bulk
of naphthalene to such solutions produces a moderate
solvent. Instead, the usual bulk solvent is used, a few
increase in ?uorescence. Accordingly, another object of
grams per liter (usually less than 20) of the conventional
the invention is to provide an improved method of pre~
solute is added to make up the conventional scintillator
solution. If this solution is a relatively weakly ?uorescing 50 paring a scintillating solution containing alcohols and
an “intermediate solvent” such as naphthalene.
solution, then addition of the “intermediate solvent” in
A further object of the invention is to provide effi
accordance with the present invention produces increased
cient scintillator solutions embodying certain compounds
light emission to thereby enhance the ef?ciency of the
insoluble in water or in dioxane, but soluble in a mixed
scintillator solution.
The “intermediate solvent” in accordance with the 55 solvent, for example, in a mixture of suitable proportions
of dioxane and water to provide the necessary polarity
present invention utilizes entirely different phenomena
of the solvent.
than are present in the use of mixtures of two light
‘ When one or more substances have been placed in the’
emitting solutes in the solvent for shifting the wave length
liquid scintillator, for example compounds containing
or spectrum of the emitted light as was accomplished by
us earlier and/or as is disclosed in U.S. Patent No. 60 heavy metals or boron for neutron detection, we have
found that the ?uorescence e?iciency of the liquid scintil
2,755,253 to Muelhause et al. T0 shift the wave length
lating solution can be increased by the use of an appro~
or spectrum of the emitted light, only small quantities,
e.g. a few milligrams per liter, of the second light emitting
priate “intermediate solvent" such for example as naph
thalene, \biphenyl, phenol and the like, with a known
solute such as diphenylhexatriene, are used whereas in
the present invention, several grams, e.g. at least 10 and 65 “ef?cient" solvent such as xylene.
It is accordingly a further object of the invention to»
usually more than 20 grams per liter, of the “inter
provide a novel liquid scintillating solution wherein the:
mediate solvent” are added to the bulk solvent and light
solvent contains desired elements for example of inter»
emitting solute. Thus, the “intermediate solvent” addi
mediate or heavy atomic weight and naphthalene or a
tive of the present invention is usually present in the
scintillator solution in a quantity greater than the quantity 70 similar compound. Scintillator solutions containing
compounds of elements of atomic number greater than
of the light emitting solute.
10 are poor light emitters since in most cases such ele
It has been further found that the “intermediate sol
ments quench the ?uorescence of a scintillator. Experi
vents” which are eifective as enhancing substances have
a lowest energy level of excitation that is smaller than
ments and applications especially in nuclear physics,
the lowest energy level of excitation of the primary solvent 75 often use materials containing elements of large atomic
The invention makes detection and measure
FIGURE 11 is a table of ?uorescent solutions contain
ment feasible under the adverse conditions which prevail
when high atomic number materials are in the scintillator.
Solutions containing boron are especially useful for
ing various substances;
neutron detection. The most effective boron containing
solution known to us prior to our invention employed
tri-n-amylborate plus naphthalene or plus xylene; and
mixtures of phenylcyclohexane and trimethylborate as
containing scintillating solution.
a solvent as reported in an article by Muehlhause and
Basic investigations of various solvent combinations
were made usually keeping the solute concentration
Thomas published in Physical Review, vol. 85, p. 926
FIGURE 12 is a graph showing the relative ?uores—
cence intensity of the scintillator solution in solvents of
FTGURE 13 is a graph showing ?uorescence of a water
(grams/liter of solution) unchanged and with apparatus
(1952). In accordance with our invention, solutions in
of the general type as illustrated in FIGURE 1. The
alkyl borate esters have been found to have greatly in
scintillation solution 8 was either placed in a beaker 10
creased ?uorescence upon adding considerable amounts
of material “non-re?ecting” to the radiation to be de
of an “intermediate solvent” such as naphthalene. This
tected and excited by a one millicurie gamma-ray source
solution is additionally desirable because it is less subject
to hydrolysis when exposed to the atmosphere. It is 15 12 located on support 13, or was placed in beaker 10 of
porcelain or glass and excited by a. light from a similarly
accordingly a further object of the invention to provide
an enhanced liquid scintillation solution containing boron.
placed source 12, the light energy being directly incident
While a slight impurity content in the solvent sub
upon the solution and having a wavelength which is
absorbed only by the solute. The ?uorescence output
lating solution in many cases, it has ‘been found that 20 was measured by means of an arrangement using photo
slight impurities in certain solvents have a marked effect
multiplier 14- of a IP28 type supported in cap 15 and the
on the light emission of the solution as the concentration
integrated ?uorescence output was provided by indicator
of the impurity containing solvent is increased. It is a
16. The background light level was kept low so that the
further object of this invention to provide for such
only light energy received by photomultiplier 14 was
solvents a novel method of detecting the presence of 25 from the liquid scintillating solution 8. For this pur
stances does not affect the ?uorescence of the scintil—
certain impurities.
It is a further important object of this invention to
provide for the enhancement of liquid scintillators of
high viscosity, an important ‘group of such scintillators
pose, container 18 as shown in FIGURE 1 was used,
though a darkened room may be used. Cap 15 and con
tainer 18 were threaded at mating ends 20 and shutter
22 was provided for controlling the light intensity on the
being rigid plastic scintillators. Plastic scintillators are 30 photomultiplier tube.
essentially of two types in relation to the invention.
_ Pairs of solvents were used such that one solvent could
One type, such as scintillators made with polystyrene or
usually be described as the “effective” solvent, and the
polyvinyl toluene, are made by the “intermediate solvent”
other as the “poor” solvent. In an “effective” solvent
of the present invention to ?uoresce efficiently at much
rather strong ?uorescence is produced under gamma-ray
lower concentration of the emitting materials than with 35 excitation when suitable solutes are dissolved in it, where
out the invention. This is of ‘great signi?cance when
as in a “poor” solvent the same solutes produce only a
emission through long lengths of scintillators are re
small light output. Typical results are depicted in FIG
quired as in use with ‘a grid of rods. Absorption by large
URES 2 to 5; in these graphs the ?uorescence with 100
concentrations of emitting materials is eliminated. This
percent “effective” solvent is always shown on the left.
application is irnportant in present day nuclear work.
There are essentially two different types of curves in
these ?gures. Type I shown in FIGURES 2 and 3 does
not show a marked decrease in ?uorescent output until
arbitrary units
large amounts of the “poor” solvent are added; only as
[Polystyrene]+2,5-diphenyloxazole (0.023 M)__ l
100 percent “poor” solvent is approached does the ?uo
[Polystyrene-{-naphthalene, 0.2 M] +2,5-diphenyl
oxazole (0.0045 M) ____________________ __ 1 45 rescent output dip sharply.
Curves of type II, shown in FIGURES 4 and 5 on the
The other type, such as scintillators made with poly
methylmethacrylate (PMMA), is made by the invention
other hand, show a sharp initial drop when only small
amounts of the “poor” solvent have been added. This
behavior is clearly shown, for example, when carbon
to ?uoresce much more e?iciently at all practical con—
50 tetrachloride is added to xylene solutions of ?uoranthene
(FIGURE 5). Curves lying between these extremes are
found in some of the experiments.
arbitrary units
The curves of FIGURES 2 to 5 depict results for
[PMMA]+2,5-diphenyloxazole (0.045 M)____ l
[PMMA-t-naphthalene 0.8 M]+2,5-diphenyl
oxazole (0.045 M) ___________________ __ 2.4 55
Another important object of this invention resides in
providing an improved radiation detecting and measur
gamma-ray excitation unless otherwise indicated; under
ultraviolet light excitation the shapes of the curves are
often very different, as can be seen for example from
FIGURE 5. With many solvent combinations under
ultraviolet excitation some solutes (e.g., ?uoranthene),
ing device for sources which may or may not be dissolved
have almost the same ?uorescent yield with 100 percent
in the solution. Such a device consists of a light detect 60 “poor” solvent as is found with 100 percent “effective”
ing and measuring unit such as a multiplier phototube
with associated electronic equipment, and the scintillat
solvent. These differences for the two types of excita
tion stem from the necessity for energy transfer from
ing solution, containing the “intermediate solvent” which
solvent to solute under gamma-ray excitation to produce
enhances the energy transfer from the bulk material to
strong ?uorescence. Under ultraviolet light excitation
the emitter.
65 the light-emitting solute molecule is directly excited, so
These and other objects of the invention will become
that if a change because of variation of the solvent is
more fully apparent from the claims, and as the descrip
found, it is to be attributed to a quenching of the excite?
‘tion proceeds in connection with the drawings wherein:
tation of the solute before the light-emission process
FIGURE 1 is a diagrammatic illustration of the appa
occurs. A comparison between the results for light and
ratus used in carrying out the tests which form the basis 70 gamma-rays then makes it possible to differentiate be
tween the effects due to solute quenching and those de
of the present invention;
FIGURES 2 through 6 are graphs showing the results
pending upon energy transfer. The results obtained with
light excitation show that the light-emission e?iciency
of the ?rst series of tests;
FIGURES 7 through 10 are graphs showing the results
generally is not impaired by the presence of the “poor”
of a second series of further tests;
75 solvent. It can thus be de?nitely stated that in these
experiments no new impurities which markedly affect the
published in Physical Review, vol. 85, p. 816 (1952),
light output of the solute are introduced into a solution.
when the “poor” solvent is added.
In Table I solvents of high purity at the time of the
tests are arranged according to their relative capabilities
of energy transfer. This table may be interpreted by
which article is incorporated by reference in this applica
tion to more fully explain background material leading
up to the present invention. The parameter Q is pro
portional to the reciprocal of the energy transfer prob
ability per unit concentration [i.e., to (at)-1]. It has
assuming that the solvents marked “ef?cient” in the table
have the longest actual lifetimes of excitation and those
been found that this parameter varies only slightly for a
given efficient solvent (same t) with many different el?'
marked “poor” the shortest. Most of the better solvents
cient solutes, and furthermore, the concentration for effi
contain double bonds. p-Dioxane is the only very good ill cient energy transfer is found to be of the same order of
magnitude for most of these solutes. These ?ndings imply
solvent known which contains no double bonds, although
there are many such with moderate transfer ability (e.g.,
cyclohexane and Decalin). There are, of course, numer
that the cross section does not vary greatly for most
ous solvents which do contain double bonds and have
times of the solute light ?ashes, one can also infer that the
15 actual lifetime of the excited solvent molecules must be
of the order of 10*9 second or smaller.
inferior transfer properties.
Qualitative Solvent Ratings for Gamma-Ray Induced
Benzyl alcohol.
Benzyl ether.
Phenyl ether.
Parathn Oil.
Methyl alcohol.
Ethyl alcohol (95
e?cient solutes. From the experimentally known decay
With reference to the above questions, the “poor” sol
vent is thought to be one in which the actual lifetime, I,
of the excited solvent molecule is smaller than is the case
120 for an “effective” solvent, which means a large Q in Eq. 2
for the “poor” solvent. One could also assume that a’
the transfer cross section, is strongly reduced in a “poor”
solvent, but then one would also have to assume that
such a cross-section reduction occurs for all solutes. The
' expectation would then be a considerable variation in Q
among the different solutes, but less variation than ex
pected under such an assumption is found. Furthermore,
it will be shown below the the shape of the curve can be
understood more easily if the assumption is made that
30 it is the lifetime of the excited molecule that is reduced.
With a reduced t at a given concentration of solute mole
cules, the number of “collisions” which are accompanied
by energy transfer are insu?icient to produce sizable trans
proved results secured:
fer during the comparatively short actual lifetime of
Consider ?rst the behavior under gamma rays of a 35 these excited solvent molecules. This idea is borne out
solution made with a “poor” solvent, that is, one in which
by the fact that in “poor” solvents a considerable increase
a low ?uorescent output is obtained for all solutes, even
in ?uorescence is obtained under gamma-ray excitation if
We now present our theory for explanation of the im-
for those which are efficient under ultraviolet excitation.
The low output which occurs under high-energy irradiation
greater concentrations of the solute are present than are
needed in an “effective” solvent (see FIGURE 5). These
in such solutions is then to be attributed to poor transfer
of energy from the solvent to the solute. The assumption
might be made that no transfer at all takes place from
the “poor” solvent to the solute. This is, however, cer
output is no longer obtained in an “effective” solvent but
are still small enough for direct-excitation effects of the
solute to be negligible. It is of interest to note that in
tainly not the case in many solutions made up with a
solutions made with “poor” solvents the in?uence of self
single “poor” solvent rather than a combination of sol
vents; in these the light emission is found to increase con
siderably with increasing solute concentration (at con
centrations suf?ciently small to make direct excitation
effects negligible). In addition, under the assumption of
concentrations are in a range where an increase of light
quenching on the shape of the ?uorescence vs. concen
tration curve seems to be less evident since the usual
decrease after reaching a maximum is almost completely
eliminated. This comes about because of the poorer
energy transfer which shows up in Eq. 2 as a relatively
no transfer, the extended ?at portions of some of the 50 large value for the coe?icient Q. Nevertheless the effect
curves which are obtained using various solvent combina
of self-quenching is still present and is even more im
tions cannot be explained. It is consequently assumed
portant because of the higher solute concentrations neces
that energy transfer can take place to the solute from both
sary for obtaining maximum light outputs. The concen
types of solvent molecules of a combination, the prob
ability of energy transfer being smaller for the “poor”
The probability of energy transfer, w, is approximately
proportional to the concentration 0, of the molecules to
which the energy is transferred, to the lifetime, t, of the
excited solvent molecule (this is determined mostly by the
probabilities of internal and external quenching proc
esses); and to a “collision” of cross section, 0', associated
with the transfer of energy frm the excited solvent mol
ecule to the solute molecule. Thus approximately
( 1)
tration cm: (QRW‘, at which the maximum light output
occurs according to Eq. 2, is shifted to higher concentra
tions in “poor” solvents because of their higher Q, and
the maximum intensity Im=P/(Q‘/"-i-R'/=)2 is reduced for
the same reason.
If the ?uorescence of a solute under ultraviolet light
excitation so that its internal quenching can be determined
and if its self-quenching coefficient [associated with factors
P and R in Formula 2] is available, the ?uorescence ob
tained under gamma-ray excitation can be adjusted for
equal internal quenching and zero self-quenching of the
solute, and then the effects due to energy transfer can be
The probability for energy transfer enters the expression
When small amounts of an “effective” solvent are added
for light emission under high-energy excitation in the fol
to a solution in a “poor” solvent or vice versa, one might
lowing way: The equation
except a mixture curve to be followed since the high
energy radiation is absorbed in these organic solvents es
I = ——~———————
( )
gives the intensity of ?uorescent light output, I, as a func
tion of solute concentration, 0, and parameters P, Q, and
sentially in the ratio of the masses of the two solvents.
Such a curve would be a straight line if the ultraviolet light
induced ?uorescence in the “poor" and “effective” solvents
were of equal magnitude. With equal light ?uorescence
R. ‘The parameters P and R are de?ned in our article 75 any chemical interaction in?uencing ?uorescence would be
ruled out or be equal. In most instances a straight line
does not result as can be seen from the curves of FIG
URES 2 to 5.
The addition of relatively small amounts of “effective”
is ful?lled. In numerous cases, however, this does not
occur (e.g., curves of FIGURE 4), indicating that
other effects must be taken into account. This is particu
larly true for those curves which display a sharp
drop in intensity when small amounts of “poor” solvent
solvent to a solution made in a “poor” solvent often pro
duces a rapid increase in the gamma-ray induced ?uores
cence. This means that with small amounts of an ef~
are added.
fective solvent a considerable amount of energy absorbed
clear up the situation. It is found that when substances
like xylene, durene and p-terphenyl are used as solutes,
in the “poor” solvent reaches the solute.
In order to
The behavior of solutions under light excitation helps
account for this We assume that energy transfer occurs 10 their light-induced fluorescence (the solute molecules are
from excited molecules of the “poor” solvent to (unex
cited) “effective” solvent molecules and then to the solute.
This assumption is made because: (a) it is known that
in these solutions with a given fraction of “effective” sol
vent present no more than about this fraction of in
cident energy is directly absorbed in the “effective” sol
vent, and (b) a large portion of the available energy in
directly excited) is tremendously decreased (quenched)
upon the addition of even small amounts of “poor” sol
vents like carbon tetrachloride which exhibit a type II
behavior. This quenching means that the actual lifetime
15 of the excited state of these solutes is shortened by
the addition of such a “poor” solvent. Solvents display
ing a type I curve quench only to a slight extent if at all.
These results provide experimental evidence that some
“poor” solvents quench materials some of which are used
trations used, whereas practically no energy transfer oc
curs from the “poor” solvent to the solute as shown by 20 as “effective" solvents, e.g., xylene. This supports the
contention made above that the change in lifetime t rather
the experiments with only a single solvent present. Such
than that of cross section or is of greater signi?cance for
transfer is possible if the lowest excitation energy of the
the change of energy transfer.
“effective” solvent molecule is smaller than that of the
Therefore, one must expect that a similar‘ shortening
“poor” solvent, and it takes place if a sufficient number
of “effective” solvent molecules are present in the solu 25 of lifetime of excited molecules of the “effective” solvent
is produced by the presence of even small amounts of
tion. (Ionization effects are not completely ruled out in
the “effective” solvent goes to the solute at the concen
these experiments with “poor” solvents.)
Under such
circumstances a considerable transfer may occur with only
a suitable “poor” solvent. Most of the curves can then
be explained as follows: In a solution made with a com
bination of an “effective” and a “poor” solvent, all or
about 10 percent of “effective” solvent, despite the rela—
tively short lifetime, t, of the excited “poor” solvent mo 30 practically all of the excitation energy initially induced
in the “poor” solvent is transferred to the “effective” one
lecule. In effective solvents energy transfer from the
(if the excited energy level of the “poor” solvent is above
solvent occurs at solute concentrations of several grams
that of the “effective” solvent), so that most of the ex
per liter. Here the energy transfer from the “poor” sol
citation energy eventually resides in the “effective” sol
vent to the “effective” one occurs at concentrations of
about 100 grams per liter. This indicates that the actual 35 vent. At the same time, however, the actual lifetime of
the excited molecules of “effective” solvent (independent
lifetime of the excited “poor” solvent is less than one tenth
ly of whether their excitation is induced directly or by
of that of the “effective” solvent. This is in agreement
energy transfer from the “poor” solvent) can be de
with the behavior that at a given solute concentration,
creased by the presence of the “poor” solvent (markedly
the ?uorescent yield under high-energy excitation is re
duced by more than a factor of ten in a “poor” solvent. 40 with type II solvents) and in such cases the energy trans
fer to the solute is diminished. Consequently the ?uo
Once the energy resides in the “effective” solvent mole
rescent-light output declines with increasing concentration
cule, its actual lifetime is increased (now tis the lifetime
of such “poor” solvents. If this quenching effect of “poor”
of the excited molecule of good solvent). Now the en
solvents on excited molecules of “effective” solvents is very
ergy can easily be transferred again, this time from the
“effective” solvent to the solute, with a corresponding in 45 strong, a sharp decrease of ?uorescence is observed with
small amounts of added “poor" solvents. These consid
crease in ?uorescence.
erations provide an explanation for the type If, sharply
It should be here noted that in the process of energy
decreasing ?uorescent-light output curves.
transfer in solution only the lowest excited states need
This idea of quenching of the excited sovent molecule
be considered since these states have the longest lifetimes.
The higher excitation levels have comparatively short 50 is given further credence by the effects of increasing solute
concentration upon these curves. It is found that at
lifetimes on account of the many possibilities of degra
greater solute concentrations, the degree of the decrease
dation of energy to the lowest levels because of strong
in light output resulting from the addition of “poor” sol
interaction with phonons. The lowest excited states of
vent is diminished. This shows that the excitation energy
the “poor" solvent will consequently be reached very
quickly, and effects connected with the higher states are 55 must not necessarily be lost; at larger solute concentra
tions the number of “collisions,” with accompanying
unimportant. The energy will then be transferred to the
lowest excited state of the “effective” solvent if this state
has a smaller excitation energy than that of the “poor”
solvent and if the “effective” solvent concentration is
energy transfer, per unit time is increased so that more
energy goes to the solute before the excited solvent mole
cule is quenched.
in other cases the in?uence of the
great enough. In such a case no return of the energy to 60 “poor” solvent on the “effective” solvent may be slight,
and curves which lie above the mixture curve are ob
the “poor” solvent is possible. it may be remarked that
energy transfer has never been observed where the lowest
A pertinent example bearing out the above ideas is that
in FIGURE 5, where the ?uorescence of ?uoranthene in
the lowest level.
65 carbon tetrachloride-xylene combinations is shown. Un
der ultraviolet light excitation the ?uorescence of ?uoran
Thus an energy transfer from the “poor” to the “effec
thene is only moderately decreased as one proceeds from
tive” solvent is assumed, similar to that which has been
pure xylene to pure carbon tetrachloride as solvent. Un
successful in explaining the high-energy induced ?uores
cence in dilute solutions in a single “effective” solvent.
der gamma-ray excitation, however, the addition of only
If energy transfer from the “poor” solvent to the “effec 70 5 percent carbon tetrachloride causes a reduction in ?uo
excitation energy of the solvent is below that of the
The emission of the solute is always that of
tive” solvent and then to the solute were the only perti
rescence by more than a factor of 2.
It is also readily
observed in this figure that at large concentrations of
?uoranthene less decrease in light emission occurs. The
more quickly than a mixture curve in practically all cases
strong decrease in ?uorescence under gamma-ray excita
in which an “effective” solvent is added to a solution
made in a “poor” one and the necessary energy condition 75 tion this reduction in ?uorescence is less in spite of the
nent mechanism, the ?uorescent intensity would increase
1 l.
tive” solvent, a decrease in light output by the addition
of the excited xylene molecules produced by carbon tetra
of “poor” solvent could be attributed to an energy trans
chloride molecules. At greater ?uoranthene concentra
tion this reduction in ?uorescence is less in spite of the
decreased lifetime of excitation of xylene molecules be
cause a greater number of “collisions” with the solute
molecules occur within this lifetime period.
Study of one solvent combination with different solutes
fer from the “effective” to the “poor” solvent. In the
cases of the addition of carbon tetrachloride or alcohol
where the lowest excitation energies are above those of
xylene, this certainly is not the reason for the decreased
emission. In acetone, however, which has a lower ex
citation energy than xylene, the type II behavior found
is very likely at least partially because of such an energy
shows that the shape of the light-output curve (whether
type I or II) is generally unchanged; but this is not strictly
true in all cases. For example, pyrene and m-terphenyl 10 transfer. When energy transfer occurs from a molecule
with shorter lifetime to one with a longer lifetime, the
show anomalies in xylene-hexane and xylene-trimethyl
experiments described show that the ?uorescent output
borate mixtures. These may be associated with the poorer
is enhanced. It has been found that such a process can
energy transfer to these solutes when observed in xylene
successfully be employed to make rather efficient solu
(this is deduced from the larger Q values obtained for
these solutes). Actually, hexane and trimethylborate pro 15 tions containing considerable amounts of substances
which ordinarily exhibit inferior high-energy induced
duce only slight decreases in the actual lifetime of the
?uorescence properties.
excited solvent molecule (xylene) as is shown by the
It is thus shown that the high-energy-induced ?uores~
many type I curves obtained with these solvents. Because
cence in scintillation solutions which have reduced light
of the generally poor energy transfer to pyrene and m
terphenyl, however, the e?ect of this small decrease in 20 emission due to either a “poor” solvent or contaminating
substances which cause quenching of the solvent mole
cules is caused by lack of energy transfer from the sol
vent to the ?uorescent solute molecule. There is pre~
sented strong evidence to indicate that the lack of
1 is more easily noticeable in the range of concentra
tions here employed. Thus, in Eq. 2, if c is large com
pared to Q, relatively large changes in Q are necessary
to produce sizeable changes in the light output; but if Q
and c are of the same order, then a much smaller relative 25 energy transfer is a consequence of the very short life
change in Q produces the same change in emission. In
the solutions of pyrene and rn-terphenyl the concentra
tions, 0, at which considerable light emission occurs and
the Q were of the same order of magnitude so that the
effect of the comparatively small changes in Q produced
by hexane and trimethyl borate were more easily ob
time of the excited solvent molecule.
It is also shown that another effect frequently im
peding the high-energy-induced ?uorescence is that some
molecules, especially those of a “poor” solvent, often
have the property of quenching other excited molecules,
primarily of the solvent and to some extent possibly of
the solute, thereby reducing the high-energy-(and light)
induced ?uorescence.
The next group of tests to be described was performed
?uoranthene a type I curve was obtained, whereas a type 35 with the same general arrangement of equipment as
shown in FIGURE 1. The purpose of these tests was to
Ii curve was exhibited with m-terphenyl (FIGURE 6).
establish that “intermediate solvents” were available
Such a difference indicated the presence of impurities in
which would increase the high-energy-induced ?uores
the solvent since in no other cases were two different types
cence in solutions where there is a lack of energy trans
of behavior found in the same solvent combination.
Measurements with light excitation did indeed reveal 40 fer from the solvent to the ?uorescent molecule.
The particular effect of naphthalene is demonstrated
the presence of an impurity affecting m-terphenyl but
in FIGURES 7 to 11. N-butylphosphate which is a
scarcely in?uencing fluoranthene (because of their differ
“poor” solvent was used as the basic solvent; it is very
ent absorption spectra). As can be seen from FIG
stable and little deterioration of the ?uorescene of suit
URE 6, when a purer para?in oil is used the result with
able solutes with time is found. Considerable quantities
m-terphenyl is also of type I if the eifect of the high Q
of many organic substances of interest for high-energy
of m-terphenyl is taken into account. This experiment
?uorescence can be dissolved in it. This solvent by it
shows that such mixed solvent measurements can reveal
self, however, displays only small high-energy ?uores
the presence of impurities.
cence with most of the well-known ?uorescent solutes in
In most of the experiments herein reported the role of
impurities as causes of the large effects discussed in the 50 the usual moderate concentrations.
FIGURE 7 presents the gamma-ray-induced ?uores
earlier sections can be ruled out by theoretical considera
cence of solutions in n-butylphosphate with two different
tions and previous studies of the importance of impurities.
concentrations of 2,5-diphenyloxazole as a function of
Nevertheless many of the materials (both solvents and
additional xylene and naphthalene. It is seen that for
solutes) were additionally puri?ed as by continuous chro
matography. These further puriiications (including the 55 both solute concentrations the naphthalene curve is above
the xylene curve by a factor 1.5. This is interpreted as
removal of oxygen) produced only minor changes and
being due to a faster energy transfer from the n~butyl
did not alter the type I or type II behaviors. This shows
phosphate to naphthalene than to xylene when equal
that a “poor” solvent under high-energy induced ?uores
masses of naphthalene and xylene are added, since there
cence is not inferior because of the presence of impuri
ties, ‘but rather because of properties intrinsic to the sol— 60 is no difference in energy transfer from naphthalene or
xylene respectively to the solute as shown below. If the
?uorescent intensities are compared when equal numbers
The experiment described above with mixed solvents
using high-energy shows that two processes are mainly
of molecules are added to the original solvent (instead
responsible for the observed c?ects: (l) the decrease in
of equal masses), the difference is even more favorable
the lifetime of the excited molecule of “effective” solvent 65 for naphthalene. It is unfortunate that the naphthalene
curve cannot be obtained for high concentrations; only
in the presence of a suitable “poor” one, and (2) the oc
about 23 percent of the solution can consist of naphtha
currcnce of energy transfer from “poor” to “effective”
solvent. The process responsible for quenching of the
lene at room temperature.
“effective” solvent by the “poor” is as yet unknown. The
If the ?uorescence of 2,5-diphenyloxazole in pure
following observation, however, provides a clue. It is 70 naphthalene could be determined, the intensity would
probably be the same and not higher than that in pure
quite often found that molecules having greater excitation
xylene. This conclusion can be deduced from the result
energies are more easily quenched by other molecules
that the addition of naphthalene to xylene does not no
than those with smaller excitation energies.
ticeably alter the ?uorescence although it is known that
If the “poor” solvent has a lowest (singlet) excitation
with su?icient naphthalene the energy is ?rst transferred
evel of smaller energy than the lowest level of the “e?ec
A particularly unusual behavior was found in some
experiments with xylene-paraffin oil combinations. With
from xylene to naphthalene and then ?nally to the solute.
If this conclusion is correct, the curves for xylene and
naphthalene of FIGURE 1 would merge for 100 percent
xylene and naphthalene.
FIGURE 8 describes similar experiments, this time
with a ?xed concentration of 9,10-diphenylanthracene
serving as the light emitting solute and various amounts
of di?erent additional “solvents.”
Again it can be seen
internal quenching of such a solute, and since the curves
of ?uorescence vs. concentration for naphthalene and
Xylene respectively in n-butylphosphate have the same
These curves display shifts of the optimum con
centration toward higher concentrations when compared
to the curve of 2,5-diphenyloxazole in xylene. This can
be interpreted as stemming from a shorter lifetime (due
to a stronger quenching) of the excited naphthalene or
xylene molecules by surrounding n-butylphosphate mole~
that added naphthalene is more effective than xylene.
Acenaphthene behaves very much the same as naphtha 10 cules than without this substance being present. Such
an increase of quenching reduces the true lifetimes of.
lene; its own ?uorescence is larger than that of naphtha
these excited molecules, thereby decreasing the energy
lene, but it is still small when compared to that produced
transfer probability to the solute; the result of the shorten
by the 9,10-diphenylanthracene, 1,1’-binaphthyl gives a
ing of lifetime is a shift in the maximum of the ?uores
much larger ?uorescence than naphthalene; in this case
the energy is transferred to diphenylanthracene by ab 15 cence vs. concentration curve. Such stronger quenching
by the addition of n-butylphosphate to xylene can also be
sorption of radiation as well as by collision, and this
seen from the xylene curve of FIGURE 7. There the
makes the ?uorescence curve higher. The effect of ab
drop in fluorescence occurring when small amounts of n
sorption is noticeable because of the small concentration
butylphosphate are added to xylene is probably due to
of diphenylanthracene, which is only slightly soluble.
These measurements show that the energy transfer to 20 this quenching.
One must also consider that some energy transfer still
naphthalene and to related compounds is very similar.
The energy transfer to m-diethoxybenzene has also been
measured in order to study the behavior of a substance
which is a moderately effective solvent (as a pure solvent
takes place directly from n-butylphosphate to the solute;
this increases with greater 2,5-diphenyloxazole concen
tration. Its contribution, however, is still too small in
it is about half as effective as xylene). In n-butylphos 25 the case of 300 g./l. of naphthalene or xylene to account
for the observed shift in the curve; with only 50 g./l. of
phate it is again about half as good as xylene, indicating
naphthalene present this process does become important
that the energy transfer from n-butylphosphate to this
and partially accounts for the different shape of the corre
substance is of the same order as to xylene.
sponding curve. The forms of the 300 g./l. naphthalene
Comparison of these results with those obtained from
the addition of naphthalene to a solution of p-terphenyl 30 curve and that of the respectivexylene curve, however,
are so similar to each other that no difference in lifetime
in xylene reveals that the lifetime of the excited solvent
between the excited xylene and excited naphthalene mole
molecule is shorter in n-butylphopshate than it is in xy
cules (i.e., in energy transfer from these molecules to
lene; this is seen from the better energy transfer to naph
the solute) is indicated under these conditions where the
thalene in solutions made with xylene than in those with
n~butylphosphate. Thus when 10 g./l. of naphthalene 35 n-butylphosphate quenching is relatively small.
Such a difference is, however, indicated in other cases
are added to a terphenyl-xylene solution a decrease in
in which strong quenching by the surrounding molecules
occurs and becomes quite evident from the experiments
a solution of 2,5-diphenyloxazole in n-butylphosphate,
described in connection with FIGURE 10. Here the
however, the addition of 10 g./l. of naphthalene which
reduced the n-butylp‘hosphate concentration to 90% pro 40 quenching in?uence of chloroform on gamma-ray-induced
?uorescence is studied in solutions of 2,5-diphenyloxazole
duces only a small increase as can be seen in FIGURE
in pure xylene and in xylene plus 300 g./l. naphthalene;
7. In the terphenyl-xylene solutions most of the energy
?uorescence by a factor of more than three is found. In
originally in the xylene proceeds to the naphthalene with
in the latter case the energy transfer to the ?uorescent
solute takes place from the naphthalene molecules. The
only 10 g./l.; but it cannot then go to the terphenyl be
cause of the close proximity of the energy levels of 45 in?uence of chloroform on the diphenyloxazole molecule
does not differ in the two solutions.
naphthalene and terphenyl; thus the observed large de
It is thus shown that the quenching in the xylene plus
crease in light output occurs. In n-butylphosphate, how
naphthalene solution is smaller than in the solution with
ever, with the same amount of naphthalene only a small
xylene alone throughout the entire range of chloroform
?uorescence change occurs because of the poorer transfer
of energy to the naphthalene due to the shorter lifetime 50 concentrations including small concentrations. Energy
transfer from the “poor” solvent (chloroform), especially
of the excited butylphosphate molecule. Only when
at low chloroform concentrations, produces little inter
larger amounts of naphthalene are present can this effect
ference and therefore does not conceal the actual quench
of shorter lifetime be overcome. It is interesting to note
ing process. The difference between the two “solvents”
that the ?uorescence of the highest available naphthalene
concentration is only about 25 percent below the maxi 55 15 due to the difference in quenching of the excited mole
cules of xylene and naphthalene respectively; the naph
mum ?uorescence in solutions with pure xylene. This
thalene molecule is therefore considered to be quenched
shows that the energy transfer process from n-butylphos
less by chloroform than the xylene molecule in agreement
phate to naphthalene does not involve much loss of
with the previous results.
energy, which is in agreement with the contention made
Results with naphthalene similar to those described
above in n-butylphosphate are also obtained with other
FIGURE 9 presents the ?uorescence light output as a
“poor” solvents and quenchers; some examples can be
function of the 2,5-diphenyloxazole concentration for
seen in FIGURE 11.
various amounts of xylene and naphthalene in n-butyl
phosphate. For the maximum naphthalene concentra
tion (23 percent of solution by mass), the intensity is 65 clear physics, it is desirable for speci?c elements (or
more complicated substances) to be present in a scintil
already 75 percent of that of a 100 percent xylene solu
lating material rather than outside of the scintilltaing
tion. Since 25 percent of the primary energy is directly
material. Frequently, however, the substance by itself
absorbed in the naphthalene, it is calculated from the
has only poor ?uorescent properties under high-energy
preceding result that about 70 percent of the energy di
rectly absorbed in n-butylphosphate is transferred to the 70 radiation, and when it is put into an e?icient ?uorescent
solution a considerable decrease in light output results
naphthalene at these concentrations whereas with xylene
because of quenching. Such behavior is often found
only about 50 percent is transferred. This conclusion
when materials contain elements of medium or heavy
is drawn since measurements with light-induced ?uores
atomic weights. By applying the results obtained with
cence have shown that the addition of naphthalene or
xylene does not change the light output and thus the 75 added naphthalene or its compounds, solutions which
exhibit considerable ?uorescence have been made with
such quenching molecules present.
A list of substances containing different elements is
presented in FIGURE 11 with which at least moderate
high-energy ?uorescence e?iciencies can be obtained in
organic liquid solutions. (The common elements in
organic substances such as hydrogen, carbon, nitrogen,
over those used previously in liquid~scintillation neutron
counter work is that the alkyl esters have less hydrolysis
in air.
p-Dioxane is one solvent known to have excellent
transfer properties that is completely miscible with water.
FLJURE 13 shows the effect of the quenching caused by
the addition of distilled water of the ?uorescence of a
In most cases, as can be
2,5~diaphenyloxazole solution in p-dioxane. With 25%
seen from the table, the addition of large amounts of
water, a quenching of more than 70% of the original light
naphthalene produces sizeable enhancement of the light
output although considerable amounts of the quenching
intensity occurs in a solution of 2,5-diphenyloxazole in
puri?ed dioxane. We have found that addition of naph
and oxygen are not included.)
The various
thalcne to such a solution results in a large increase in
?uorescent solutes shown in the table give comparable
?uorescence to about 65% of the original ?uorescence in
the pure p-dioxane; with 20% water, 100 g./l. of naph
thalene gives 85% of the pure dioxane ?uorescence.
(The ?uorescence in pure p‘dioxane is about 55% of that
of a. solution of 5 gm./l. of p-terphenyl in xylene.)
Only slightly more naphthalene or water added to such
material may be present in the solution.
results, and in many cases l,1’-binaphthyl may also be
p-terphenyl cannot be utilized with naphthalene as the
“intermediate solvent” because of the insufficient energy
transfer found with naphthalene. p-Terphenyl can, how
a solution causes it to separate into two phases, which
ever, be used with biphenyl as the “intermediate solvent.”
The heaviest element that has been successfully used to .20 is undesirable for many applications. If more water is
require , the amount of naphthalene must be decreased,
the present has been bismuth. The list of FIGURE ll
resulting in a considerable decrease in ?uores ence. Such
is representative of the initial results of a search for suc
solutions may still be acceptable for certain counting
cessful substances.
Finding substances with desirable properties presents
certain problems. One of the major dif?culties is the
lack of solubility in suitable organic solvents. Once a
soluble material is found, it must generally be such that
it does not quench the solution too strongly. 'Ihe naph
thalene or its compounds, as discussed above, acts as a
Experiments with phenyl-biphenyloxadiazole, which is
the most e?icient solute ‘known for liquid scintillators, did
not produce substantially more ?uorescence than 2,51
diphenyloxazole, partially because of its more limited
The mere addition of water in certain quantities will
“solvent” in which less quenching occurs and provides a 30
change the polarity of the solution suf?ciently to permit
medium to and from which more energy is transferred.
materials to be dissolved which are not otherwise soluble
It has also been found that ?uorescence enhancing of
scientillation counters is possible in solutions containing
considerable amounts of water. Water ordinarily pro
duces a sizable quenching of ?uorescence and moreover
has only very poor capabilities of transferring energy
to suitable solutes. Also, most of the ?uorescent solutes
that are appropriate for high~energy-induced ?uorescence
are at best only slightly soluble in water, and most sol
vents with good energy transfer properties are not misci
ble with water.
Because metallic salts, as well as many other materials
are soluble in water, a scintillation solution which will
provide satisfactory ?uorescence output when Water is
added is highly desired.
In boron-rich solutions alkyl borate esters have been
found to have a greatly increased ?uorescence upon add
ing considerable amounts of an “intermediate solvent”
such as naphthalene. Thus as shown in FIGURE 12 a
solution consisting of 26% naphthalene by mass and tri-n
amylborate as solvent and 2,5~diphenyloxazone as solute
gives a ?uorescence intensity 93 % as great as the same
amount of solute in pure xylene. Almost identical re
sults were obtained with the higher alkyl esters tri~n~
hexylborate and tri~n-octylborate.
Naphthalene also increases the high-energy-induced
?uorescence of trimethylborate solutions with suitable
solutes with considerably greater ef?ciencies than the same
percentage of xylene. ‘bus, in a solution of 2,5-diphen
yloxazole in 75% trirnethylborate and 25% naphthalene,
about 80% of the ?uorescence of the same solute in
100% xylene is obtained; however, in a solution con~
taining 2,5-diphenyloxazole with the same amount of
in scintillating solutions.
An example of a typical water-containing scintillator
solution of our improved e?iciency due to the addition of
a ?uorescent enhancer such as naphthalene is:
p-Dioxane (puri?ed) __. 80% (by volume),
Distilled water ______ _. 201% (by volume).
Naphthalene ________ __ 100 grams per liters of solvent.
2,5-diphenyloxazole ___. 5 grams per liter of solvent.
It is preferable for highest ?uorescent output that the
p-dioxane be puri?ed for use in the scintillator liquid. A
satisfactory method of p-dioxane puri?cation is described
in “Laboratory Experiments in Organic Chemistry” by
L. Fieser.
In biological work it is common to work with solu
tions which contain alcohols, such for example as ethanol
(95%) or methanol. The addition of alcohols to scintil
lating solutions causes a decrease in the ?uorescence in
tensity in much the same manner as does water.
naphthalene is added to the solution, the ?uorescence
intensity increases ‘but only a relatively small amount
and the maximum e?iciency obtainable is still less than
15% as ef?cient as p-terphenyl (5 gm./l.) in xylene. This
is in comparison to the approximately 50% efliciency
when naphthalene is added to a solvent formed of p-di
oxane and water.
We have found the ?uorescence intensity of solutions
containing alcohol can be greatly increased if the alcohols
are ?rst mixed with considerable amounts of xylene as
the primary solvent, then larger amounts of naphthalene
as an “intermediate solvent” can be dissolved and the re~
sulting solution exhibits a much higher ?uorescence out
65 put. The ?nal solvent then comprises the alcohol con
taining substance, a substantial quantity of “effective”
thalene that is soluble in trimethyl~borate at room tem
solvent such as‘ xylene which is preferably in excess of
perature. If greater ?uorescence is desired, it is best to
50% (by volume) of the total solvent and as much “in
keep the naphthalene concentration as high as possible
termediate solvent,” such as naphthalene as can be dis
and add xylene or phenylcyclohexane while removing 70 solved.
xylene instead of naphthalene, only 53% ?uorescence is
obtained; 25% is nearly the maximum amount of naph
In special cases, the boron content may be of prime
interest; trimethylborate is then superior to triamylboratc
because the boron content is twice as great. However, an
advantage of tri-n-amyl'corate and the higher alkyl esters
It is thus apparent that the use of an “intermediate
solvent” in addition to the “primary” solvent has broad
application in a large number of types of scintillating
solutions. Based on the above experiments, the “in
termediate solvent” may be characterized in that its lowest
absorbing the ?uorescing light of'the solute and the,»
energy level‘ of" excitation must be smaller than the lowest
energy level of excitation of the major or primary solvent
presence of the “intermediate solvent” makes practical‘v
and greater than the lowest energy level of excitation
the use of as little as from 50 down- to 20% of the light
emitting solute with no decrease in intensity of light out
of I the ?uorescent solute.
“Intermediate solvent” is a
substance (it may be solid at room temperature) which
when present in moderate amounts (greaterthan 20 g./l.)
Example: '
‘Intensity, arbitrary'units,
has the property of taking over muchor most of the
[Polystyrene]l+2,5-diphenyloxazole (0.023 M)‘ __ 1*
energyoriginally in the bulk material (major solvent)
[Polystyrene+naphthalene, 0.2 M]'+2,5-dip"nenyl
and transferring it more efficiently to the emitting solute
oxazole (0.0045 M)- ____ ____ ______________ __~_ 1‘
(?uorescing material) thandoes the bulk material. The 10'
(Lucite) has mechanical ‘and
lowest excited electronic state of the “intermediate sol
optical, advantages over the plastics. commonlyvused as
vent” lies between that of the bulk material and that of
the major solvent in plastic scintillator-s. However, itis
the emitter. (The lowest state has an energy greater
poor for energy transfer and has therefore not found
:than 3.4 e.v.) The transfer to and from the “intermedi
:ate solvent” does not take place primarily via radiation. 15 practical use. The incorporation of an “intermediate
solvent” such as naphthalene improves the ?uorescence
‘The time constant of the “intermediate solvent” is longer
e?iciency so that Lucite scintillators become useful. For
zthan that of the bulk material.
lby the property that certain materials have less quench
example: if a scintillator comprising Lucite as major sol
vent and 0.2 M 2,5-diphenyloxazole as ?uorescing solute
primary solvent; or stated in other words, the lifetime of
corporation of 0.8 M naphthalene raises it to approximate,
The “intermediate solvent” may also be characterized
:ing effect on its molecules than on the molecules of the 20 has a ?uorescence efficiency of 100, the additional ‘in;
ly 250.
(“excitation of the “intermediate solvent” molecule is not
‘In summary, the present invention has a number of im
reduced as much as the time of excitation of the primary
portant practical applications. First, it permits the en
.solvent molecule by the quenching action of said ma
:terials. Another property of an “intermediate solvent” 25 hancement of scintillation solutions containing water (5%
or more). Water has been found to reduce the ?uo
i-is that it enhances the ?uorescence of solutions made in
rescehce efficiency to such a degree that the conventional
'“poor” solvents. While naphthalene has been found to
the one of the best enhancing materials, naphthalene deriv
prior scintillation solution is essentially useless for detec
:atives including ethoxynaphthalenes, methylna-phthalenes,
)methoxynaphthalenes, benzylnaphthalenes and naphthyl
tion and/ or measurement.
The greater the water content
;amines have been found to have valuable enhancing prop
the less'the light ‘output. However, for a large number
of biological, medical and physical applications, large
erties. Other “intermediate solvents” include acenaph
:thene and derivatives, binaphthyl and derivatives, bi
--phenyl and derivatives, ?uorene, and derivatives, anisole,
terial being tested. By the use of the “intermediate sol
vent” in accordance with the present invention, these
.durene, dimethoxystilbene, cyclohexylbenzenes and bi 35
solutions can be made e?icient scintillators.
.cyclohexyl. The phrase “intermediate solvent” as used
;in the claims means the foregoing materials or equivalent
amounts of Water are essential for solubility of the ma
' '
Second, this invention provides enhancement of scintil—
;materials having the foregoing properties.
lation solutions containing compounds of elements of
atomic number greater than ‘10. These compounds in
i(polymethylmethacrylate). In such solutions, the rigid
search experimentation and practical applications especial
most cases quench the ?uorescence of a scintillator and
This invention is also very useful with rigid scintillator
:solutions such as polystyrene, polyvinyl toluene and Lucite 40 themselves make poor light emitting materials. Re
.ly in nuclear physics, often use materials containing ele
ments of large atomic numbers. The use of the “inter
mediate solvent” makes detection and measurement feas
:such as 2,5-diphenyloxazole may be incorporated. As
_; pointed out above, the heavy metal compounds quench the 45 i-b-le under the adverse conditions which prevail when high
atomic number materials are in the scintillator. _ In ad
:?uorescence of the scintillator solution to such an extent
:polymerized material serves as the bulk solvent in which
;a compound of a heavy metal and a light emitting solute
-. that it is no longer useful. Addition of an “intermediate
dition to the examples given above, the following speci?c
examples are given:
:solvent” such as naphthalene vovercomes this quenching
. I
Intensity, arbitrary units a large extent. For example, if the fluorescence ef
j?ciency of a scintillator comprising polystyrene as major 50 (a) [Tetraethyl lead 10%+xylene 90%]+9,10-,di
phenylanthracene (10 g./l.) ______ __g_,____ 1.0
:solvent and 0.045 M 2,5-diphenyloxazole as ?uorescing
:solute'is called 100,- the incorporation of 0.14 M diphenyl
rmercury reduces it to 24. If 0.2 M naphthalene is incor
; porated as well, the e?iciency rises to 50.
The foregoing example illustrates a system wherein 55
the scintillator solution becomes a weakly emitting body
‘.becauseof the quenching effect of the added heavy metal
.compounds and wherein the emitting properties are im
_;vproved by simply adding an amount of “intermediate
[Tetraethyl lead 9% +xylene 81% +naph—
thalene 10%]+9,10-diphenylanthracene (.10
g. l.)
g./l.)_+2,5-diphenyloxazole (5 g./l.) _____ __
[p-dioxane-j-water 20% +naphthalene 60 g./l]
+cupric nitrate (15 g./l.)+2,5-diphenyl
[Polystyrene+naphthalenej 0.8 M] +diphenyl
the effect of the quench caused by the compounds of th
m e r c u r y (0.14 M) +2,5-diphenyloxazole
heavy metal elements.
(0.045 M)
oxazole (5 g./l.) _.___, ________________ __ 2.8
tsolvent” not greater than that which prevents polymeriza 60 (c) [Polystyrene]+diphenylmercury (0.14 M)+2,5
diphenyloxazole (0.045 M) _____ __; _____ __
tion or solidi?cation of the rigid plastic material. The
“intermediate solvent” in this instance serves to reduce
(b) [p-dioxane-l-water 20%]+cupric nitrate (15
With the (same rigid plastic material (polystyrene) and 65 Third, the present invention is useful for the enhance
lightemitting solute (2,5-diphenyloxazole, 0.023 M) but
ment of scintillators of high viscosity, an important sub
without the heavy metal compound, use of “intermediate
class being-the rigid plastic scintillator. Plastic scintil
solvent” (naphthalene) is desirable because it decreases
lators are essentially of two types in relation to the in
the reabsorption of its own ?uorescence by the solute
vention. One type, such as scintillators made with poly
molecules. It is thus advantageous to use as low a con 70 styrene or polyvinyl toluene, is made by the “intermediate
centration of the ?uorescing solute as possible in long
solvent’? of the present invention to. ?uoresce et‘?ciently
rods to thereby make it possible to, transmit the ?uoresc
ing light through the rigid scintillator rods to be detect
at much lower concentration of vthe emitting materials
than without the invention. This is of great signi?cance
able at; the end of the rod. This is possible because the
when emission through long lengths of scintillators are
“intermediate solvent” does not havev the property of 75 required as in use with‘a grid of rods. Reabsorption due
solvent, but less than that quantity which changes the
physical or chemical state of the solution, to substantially
increase the ?uorescence output of the improved solution
over that of the weakly ?uorescing solution.
to the large concentration of the light emitting solute is
greatly reduced. This application is important in pres
ent day nuclear work.
Intensity, arbitrary units
2. The improved scintillator solution as de?ned in claim'
1 where in the light emitting solute is present in a quan
tity greater than one-half gram per liter and less than
20 grams per liter and said “intermediate solvent” is pres
ent in a quantity greater than 20 grams per liter.
[Polystyrene+2,5-diphenyloxazole (0.023 M)___ 1.0
M]+2,5 - di
phenyloxazole (0.0045 M) ______ _; _____ “a 1.0
The other type, such as scintillators made with poly 10
3. An improved scintillator solution comprising a
methacrylate (PMMA), is made by the invention to
weakly ?uorescing scintillating solution composed of a
?uoresce much more efficiently at all practical concentra
bulk solvent; an e?icient light emitting solute; and a
further substance in said solution for aiding in the detec
tion of radiation, which causes quenching of an excited
[PMMA]+2,5-diphenyloxazole (0.045 M) ____ 0.1 15 solvent molecule; and an enhancing element other than
said further substance having a lowest energy level of
([PMMA-l-naphthalene, 0.8 M]+2,5-diphenyl
excitation less than the lowest energy level of excitation
oxazole (0.045 M) Y______,__ _____________ __ 2.4
of said bulk solvent and greater than the lowest energy
Fourth, scintillator solutions made with solvents hav
level of excitation of said solute, said enhancing element
ing poor ability to transfer energy to the solute can be 20
made to yield markedly improved results by the addition
of an “intermediate solvent” in accordance with the pres
ent invention.
[n-Butylphosphate] +2,5 - diphenyloxazole
70% +naphthalene
25 tion.
eing present in a quantity in excess of 10 grams per liter
of the bulk solvent, but less thanthat quantity which
changes the physical or chemical state of the solution,
to substantially increase the ?uorescene output of the im
proved solution over that of the weakly ?uorescing solu-.
30 % ]
+2,5-diphenyloxazole (1‘0 ‘g./l.) _________ __ 8.0
4. The improved scintillator solution as de?ned in claim
3 wherein said further substance comprises a compound
containing an element having an atomic number greater
Other. examples of solvents having poor energy trans 30 than
5. The improved scintillator solution as de?ned in claim
,fer properties with which the invention is capable of
4 wherein the bulk solvent is polystyrene, the solute is
enhancing include:
2,5-diphenyloxazole and said enhancing element is naph
. Cyclohexane
6. An improved scintillator solution comprising a
Butyl borate
35 weakly ?uorescing scintillator solution composed of a
Para?in oil
Amyl borate
From the foregoing, it is apparent that the use of the
bulk solvent having a poor ability to transfer energy se
lected from the group consisting of polymethylrnethacry
late, n-butylphosphate, dimethylformamide, tricresyl
phosphate, parai?n oil, hexane, cyclohexane, butyl borate,
“intermediate solvent” of the present invention is for
the purpose of increasing ?uorescence of the solution 40 amyl borate and tetraethylorthosilicate to an efficient light
where the ?uorescence is relatively low due to either the
emitting solute; and an enhancing element other than said
use of a “poor” solvent or due to the presence of quench
solvent or light emitting solute having a lowest energy
ing materials in the solution. Our invention cannot be
level of excitation less than the lowest energy level of ex
'used to increase the ?uorescence where the low ?uo~
citation of said'bulk solvent and greater than the lowest
rescence is due to use of a poor or weak light emitting
energy level of excitation of said solute, said enhancing
solutes. By ei?cient light emitting solutes, we mean‘a
element being present in a quantity in excess of 10 grams
solute which when dissolved in an “e?icient” solvent
per liter of said solvent but less than that quantity which
such as those enumerated in Table 1 above, in an optimum
changes the physical or chemical state of the solution,
‘concentration has a scintillating e?iciency which is at
to substantially increase the ?uorescence output of the
least 15% of that of a solution of p-terphenyl (5 grams
improved solution over that of the weakly ?uorescing
per liter) in xylene.
It will be understood [that each component of our im
proved solutions may consist of more than one compound,
depending upon the intended use, and that the invention
7. An improved scintillator solution comprising: a
weakly ?uorescing scintillator solution composed of an
e?icient light emitting solute and a bulk solvent; and an
enhancing element other than said ?uorescent solute and
may be embodied in other speci?c forms without depart
ing from the spirit or essential characteristics thereof.
said solvent selected from at least one of the group con
The present embodiments are therefore to be considered
in all respects as illustrative and not restrictive, the scope
sisting of naphthalene, acenaphthene, binaphthyl, bi
phenyl, ?uorene, anisole, durene, dimethoxystilbene,
of the invention being indicated by the appended claims
cyclohexylbenzenes, and bicyclohexyl; the enhancing ele
rather than by the foregoing description, and all changes 60 ment having a lowest energy level of excitation less than
which come within the meaning and range of equivalency
of the claims are therefore intended to be embraced
the lowest energy level of excitation of said bulk solvent
and greater than the lowest energy level of excitation of
said solute and being present in excess of 10 grams per
What is claimed and desired to be secured by United
liter of said solvent, but less than that quantity which
States Letters Patent is:
65 changes the physical or chemical state of the solution,
1. An improved scintillator solution comprising a weakly
to substantially increase the ?uorescence output of the
?uorescing scintillating solution containing a bulk solvent
improved solution over that of the weakly ?uorescing
and an e?icient light emitting solute, the energy trans
fer from said solvent to said solute being less than that
8. In a rigid plastic scintillator solution comprising a
which is theoretically possible, and an “intermediate sol~ 70 rigid body of material selected from the class consisting
vent” having a lowest energy level of excitation less than
of polystyrene, polyvinyl toluene and polymethylmeth
the lowest energy level of excitation of said bulk solvent
acrylate having mixed therewith as a solution an e?icient
and greater than the lowest energy level of excitation of
light emitting solute, the improvement comprising an en—
said solute, the “intermediate solvent” being present in
aquantity in excess of 10 grams per literof the bulk .
hancing element other than said body of material and
solute, said enhancing element being an “intermediate
solvent” having a lowest energy level of excitation less
than the lowest energy level of excitation of said rigid
body material and greater than the lowest energy level
of excitation of said solute, and being present in an
amount to substantially increase the ?uorescent output
from said improved scintillator over that of the scintil
lator solution without said “intermediate solvent.”
13. An improved scintillator solution comprising: an ef
ficient light emitting solute in p-dioxane to provide a con
ventional scintillating solution, an improvement to said
conventional solution comprising water present in an
amount of at least 5% by volume of the p-dioxane, and
naphthalene in excess of 10 grams per liter of the bulk
. solvent but less than that quantity which changes the
physical or chemical state of the solution to substantially
9. The scintillator solution as de?ned in claim 8 where
increase the ?uorescence output of the improved solution
in said “intermediate solvent” is naphthalene and said
light emitting solute is 2,5-diphenyloxazole.
10 over that of the solution without naphthalene.
14. An improved scintillator solution comprising: an
10. The scintillator solution as defined in claim 9 where
et?cient light emitting solute in p-dioxane to provide a
in the amount of 2,5-diphenyloxazole is less than 0.01 M,
conventional scintillating solution, an improvement to
said solution is in the form of a rod and said solvent is
said conventional solution comprising water present in
11. A scintillator solution composed of a light emitting 15 an amount suf?cient to change the polarity of the solu
tion sufficiently to permit materials to be dissolved in the
solute and a solvent, a radiating substance soluble in
water mixed in said solution which reduces the ?uorescing
and light emitting ef?ciency of said solution, and means
scintillating solution which are not otherwise soluble in
p-dioxane, and naphthalene in excess of 10 grams per
liter of the bulk solvent but less than that quantity which
solvent” having the property of taking over a substantial 20 changes the physical or chemical state of the solution
to substantially increase the ?uorescence output of the im
portion of the energy from said bulk scdvent resulting
proved solution over that of the solution without naph
from absorption of radiation emanating from said radiat
ing substance and transferring said energy to the light
for increasing said e?iciency comprising an “intermediate
emitting solute, said “intermediate solvent” being pres
15. The improved scintillating solution as de?ned in
ent in excess of 10 grams per liter of the solvent but 25 claim 1 wherein the “intermediate solvent” is naphthalene.
less than that quantity which changes the physical or
chemical state of the solution to substantially increase
the ?uorescence e?iciency of said solution.
12. The scintillator solution as de?ned in claim 11
wherein the light emittng solute is 2,5-diphenyloxazole, 30
the solvent is p-dioxane and the “intermediate solvent”
is naphthalene.
References Cited in the ?le of this patent
Muehlhause et al _______ __ July 17, 1956
Buck et al ____________ .._. Feb. 25, 1958
Patent No. 3,,068, 178
December l1‘I 1962
Hartmut P, Kallmann et al0
It is hereby certified that error appears in the above ‘numbered pat
. ent requiring correction and that the said Letters Patent should read as
corrected below.
Column 7, line 63,, for "frm" read -- from --; column 8u
line 28v for "the‘"\, first occurrenceq read =m that =—;; column
l0‘z line 75‘, for "tion this reduction in fluorescence is less
in spite of the" read =--= tion is due to the reduction in the
lifetime (quenching) —-—;‘, column 19;, line l5u for "001" read
Signed and-sealed this 25th day of June 1963o
Attest: '
Attesting Officer
Commissioner of Patents
3,068,178.—Ha7'tmut P. Kallmcmn and Milton Furst, Bronx, and Felz'w H.
ENHANCERS. Patent dated Dec. 11, 1962. Disclaimer ?led May 1,
1969, by the assignee, Balm Om‘pomtz'on.
Hereby enters this disclaimer to claims 1, 2, 6 and 7 of said patent.
[O?icz'al Gazette Octobew 14, 1.969.]
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