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Патент USA US3076097

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Jan. 29, 1963
Filed May 24, 1960
3 Sheets-Sheet l
John R. Mahoney
Jan. 29, 1963
Filed May 24.‘ 1960
3 Sheets-Sheet 2
John R. Mahoney
v Jan. 29, 1963
Filed May 24.- 1960
5 Sheets-Sheet 3
John R. Mahoney
" 1C6
United States Patent
patented .lan. 29, teen
provision of simpli?ed circuitry for indicating abundance
ratios and/ or mol percent by the simultaneous measure
ment of the components of a mixture in a single opera
MEASURING Cllit'lll?‘
John R. Mahoney, (lair Ridge, Tenn, assignor to The
Applicant has as another object of his invention the
United §tates of America as represented lay the ‘United
States Atomic Energy Commission
provision of a measuring circuit for a mass spectrometer
that may be shifted from measurement of abundance
ratios to mol percentage.
Other objects and advantages of my invention will ap
Filed May 24., 1960, Ser. No. 31,561
5 Claims. (Ql. 250-413)
This invention relates to measuring circuits and more
particularly to a circuit especially adapted to and useful 10 pear from the following speci?cation and accompanying
drawings and the novel features thereof will be particularly
for the determination of isotope abundance ratios, mol
pointed out in the annexed claims.
percentages, and/or weight percentages of a mass spec
In the drawings, FIG. 1 is a simpli?ed circuit diagram
of applicant’s improved measurinu circuit positioned to
It has generally been the practice in arriving at abun—
dance ratios such as" "
15 read mol ratio. FIG. 2 is a schematic of the same circuit
" ‘
positioned to read mol percent on the XI scale or con
tact. FIG. 2A is an equivalent circuit for the system of
FIG. 2. FIG. 3 is a modi?cation of the circuit of FIG. 1
positioned to read mol percent on the X-lO scale or con
to measure the U235 content and the U238 content in sep
arate measurements and by calculation from these meas
urements, arrive at the ratio. if these measurements are
undertaken on a process stream, both the pressure and
tact. PlG. 3A is the equivalent circuit for the system of
FIG. 3.
Applicants circuit is of the direct-reading ratio recorder
type and is best suited for the measurement of either
isotope abundance ratios or isotope mol percentages, as
the composition may change between measurements of
U235 and U238. Under these conditions, the measurements
would not be made on the same sample, and in addition, 25 desired. in addition, the circuit is adaptable for the
measurement of isotope weight percentages. The circuit
the ratio is not directly measurable but requires the time
is characterized by the advantage that it provides a signal
cons-uming step of calculation. The same is true in mak
output which is proportional to the total of the ion cur
ing determinations of mol percent, such as
rents reaching the collectors. The inventor has discovered
30 that improved spectrometer operation can be obtained by
In an attempt to overcome some of these inaccuracies
in measurement and those resulting from changes in com
position and pressure, it has been the practice to isolate
a sample from a process stream and take a series of meas
employing this “summation” signal to control the gas
input to the spectrometer ion source, thus maintaining the
total ion current at a constant value.
Referring to the drawings in detail, and particularly to
projection, arrive at values for these peaks that are ad
FIG. 1, the system is shown connected to the collector
plates of a mass spectrometer. The spectrometer tube is
of conventional design except that it is provided with three
collector plates. Collectors 1, 2 and 3 may be used for
justed for drift, Hanson, et 211., Serial No. 17,442, new
the collection, respectively, of the isotopes U238F5't,
Patent No. 3,012,139.
U235F5+ and U234F5i’. The collectors i, 3 are joined to
urements which include a plurality of measurements of
the U235 peak and the U238 peak, and by plotting and
The circuits involved are com
plicated, and the procedure followed is time consuming,
gether to combine the U238F5+ and the U234F5+ signals
and requires additional steps of calculation to arrive at
abundance ratios and mol percent.
Electrical ratio measuring systems were known to the
for feeding into the input of a high current ampli?er 4.
In the alternative, a single collector may be employed for
these latter isotopes instead of joining two collectors. A
prior art. In one form, a current proportional to one of 45
conventional Consolidated Electrodynamics Corporation
two input voltages is fed through an adjustable impedance
Spectrometer Model No. 2 1~320 is suitable for this pur
pose. The collector 2 for the U235F5+ beam is connected
to feed into the input of a low current ampli?er 5. Both
high current ampli?er 4 and low current ampli?er 5 are
to develop a voltage which in part is led into a balancing
circuit. A current proportional to the other input voltage
is fed through a second and independent adjustable im
pedance to develop a voltage which in part is fed into the 50 of the high gain-negative feed back type, and preferably
balancing circuit. The two are combined subtractively
should have similar time constants. Each ampli?er ter
in the balancing circuit and one of the impedance contacts
minates in a ?nal cathode follower stage 6, 7. The cath~
is set to measure the mol fraction of said currents. No
ode follower stage for each ampli?er conducts at a maxi
effort is made to combine the currents so that summation
mum when the input to the ampli?er is zero. One form
would occur so as to permit accurate determination of 55 of ampli?er that will conduct at a maximum when the
mol fraction but instead, a signal for improving the regu
input is zero would be an ampli?er having an odd number
lation of sample gas flow is derived. Nor is there any
of plate ‘ampli?er stages. As shown, bucking batteries
provision for increasing sensitivity without impairing the
8, 9 are connected across the cathode followers 6, '7 to
measurement of mol fractions nor is such a system readily
reduce the ampli?er outputs to Zero when there is no in
adaptable for automatic zeroing. An example of such a 60 put. As the input signal increases the ampli?er output
system may be found in the patent to Williams, 2,522,976.
ecreases and the bucking voltage of the battery predomie
Applicant with a knowledge of these problems of the
nates. Therefore during operation the output voltage
prior art, has for an object of his invention the provision
from each ampli?er-battery combination is equal to the
of a system for use, with a mass spectrometer that will per~
difference between the greater bucking battery voltage
mit measurements that are simultaneous in time on the 65 and the lesser voltage developed across the cathode fol
same sample that will yield abundance and/ or mol per
Applicant has as another object of his invention the
The high current ampli?er 4 is preferably provided
with 100 percent feed back to get output voltage which
provision of a measuring system for a mass spectrometer
is proportional to ion current. Any high impedance input
that will permit the direct measurement of abundance 70 current ampli?er that is capable of measuring spectrom
ratios and mol percent.
Applicant has as another object of his invention the - eter current and which will provide low output imped
ance will be suitable. Its output voltage, Earl-E238 is
dropped across the ground resistor R3 bridged across the
output circuit. Since the voltage of source 8 predomi
nates, the polarity of the voltage developed across resistor
R3 is as indicated in FIG. 1. The low-current ampli?er
properly with the polarity of the potential across resis_
tor R3 and provide a potential across potentiometer 10
equal to Emirm plus synthetic E235. The equivalent
circuit for this arrangement is shown in FIG. 2A where
the potentials across resistors R3, R5 are treated as
5 should be provided with 50 percent feed back in order
to give a simple Way of obtaining the synthetic E235. Its
output, which is equal to twice the input to the ampli?er,
is coupled into and dropped across two series-connected
sources. As indicated, the voltage now impressed across
potentiometer 10 is the sum of the voltages developed
. aCrOSS R3 and R5_1-.e., E234+233+E235
Normally, the
resistors R4 and R5. The junction point of these resistors 10 recorder slidewire 11 is positioned automatically by'the
recorder 12 to null out the voltage across resistor R5
is grounded. Resistors R4 and R5 are of equal value so
with a fraction kERD of the decade or potentiometer volt
that E235 and synthetic E235 are of equal value, and con
age. With the circuit at balance,
sequently a voltage equal to the input voltage of the am
pli?er is developed across each of these ‘resistors. As
shown, the voltages across resistors R4 and R5 are opposite 15
ERD E2a4+2as+E2a5
in sign with respect to ground. The voltage across resis
tor R‘; istermed “E235,” whereas that across resistor R5
is termed “synthetic E235.”
'kERD_____ I225
Series resistors R4 and R5 are tapped, and ganged
ERD Iz34+238+1235
switch S6 is adapted to move across spaced contacts of 20 The recorder thus can be calibrated in terms of mol per
the tapped resistors for coupling selected portions to a
potentiometer or decade 10.
cent (mol ratio><l00).
If it takes the form of a
potentiometer, .it may be made in two parts 10a, 10b
having ganged sliders that are manually operated to put
in as much resistance from one part as is taken out by 25
the .other in their common circuit. Resistor R3 is cou~
pled to the upper end of potentiometer or decade 10
and the slide 11 is a part of a conventional self-balanc
ing recorder 12, such as a Leeds & Northrup potentiom
As previously indicated, this measuring circuit is
adapted to measure mol percent by establishing the fol
lowing relationship at circuit balance:
This equation assumes equal inputresistors for the am
pli?ers. It is desirable, however, for the low-current
eter recorder, to switch S6. Gan'ged switch S6 has 30 ampli?erto be provided with a selection of input resis
‘fratio” and “mol percent”>positi'ons. In the “ratio” po
tors, thus providing a choicej-ofcircuit sensitivities.
sition, shown in FIG. 1, switch S6 connects the decade
For example, a selected inputr'esistor might be switched
or potentiometer 10 through self-balancing recorder 12
into the circuit to increase theiinput voltage to the am
directly across the resistor R3. The voltage ERD, across
pli?er 5 by a factor of ten. 'Thé’desired circuit sensi
the decade or potentiometer, thus is equal to E234+E238. 35 tivity then would be obtained only if the voltage 104E235
In the “ratio” position, the switch S6 also connects re
is balanced against kERD. With regard to the voltage
sistor R4 into the balancing circuit of the recorder, as
across the potentiometer 10, however, it is necessary
that the unmultiplied voltage (E235) be added to the
In a normal balancing operation the recorder slide
voltage E234+238. Application of the unmultiplied volt
wire is positioned automatically by servo motor in re
corder 12 to null out the voltage across R, with a frac
tion of the voltage ERD, the drop across the lower part
of “put andtake” ganged potentiometer 10v is designated
With the circuit at balance, the following relationship
age E235 across the decade avoids the necessity of ap
plying complex correction factors to the recorder-read
ing, and thus makes possible the direct reading of mo]
The modification of FIG. 3 is a broad range measur
ing circuit intended to measure very low ‘concentrations
of a particular isotope or substance in a mixture, as well
as the normal ratios. This is accomplished by chang
ing the sensitivity of one of the ampli?er stages, such
as low current ampli?er 5' which feed resistors R'4
Assuming that the ‘input resistors R235 and R234+238
and R'5. At the same time, it is desired to impress
are equal, and that the ampli?ers have like character 50 across
potentiometer 10' a voltage that will represent
istics, then
the true (unmultiplied) concentration of the isotope or
kERD___ E235
@HL I235
Em) _-[234+238
substance to be measured.
To increase sensitivity of the low current ampli?er
and the recorder is read directly in terms of abundance 55 5' grid resistors 13', 14' and 15’, of different value may
ratio. In the event that the low-current ampli?er 5 is
provided witha selection of input resistors, the operator
applies a scale factor
be selectively inserted in the input circuit of the am
pli?er by switch v16'. When switch 16’ is on contact
X-1, a resistor of normal resistance is inserted in the
grid circuit of ampli?er 5’ and a normal output voltage
60 is obtained at the output of the ampli?er. If switch
v16' is moved to contact X-lO, a resistor ten times as
to obtain the correct abundance ratio.
FIG. "_2'is the same circuit as shown ‘in vFIG. '1, 'but
great is inserted in the 'grid circuit of the ampli?er and
the sensitivity for low ion currents is correspondingly in
switch 8,; ‘has been shifted ‘to the'positionfor measuring
mol ‘percent, that is
X 100
1 2a4+2as + 1 235
creased. In like .manner, if switch 16' is moved to “con
65 tact X400 position, a resistor ‘one ‘hundred times as
great is placed in the input circuit of ampli?er 5', and
sensitivity thereof is further increased.
The result of this is to increase the voltage output of
ampli?er 5' as the switch ‘16' is progressively moved
The circuit is for the condition illustrated R235+238='R235.
In this position, the switchSs ‘serves to ‘connect resistor 70 from contact X-l to contact X-lOO. This is desired
R5 in series with decade 110. Voltage from the low
'currentrampli?er ‘source Sis placed across “resistor R5
in order to get ‘a polarity which’is opposite in relation
toground'to that across-"resistor R3 so-itcan be summed
for the circuit of resistors RC; and R's between the con-
tacts ‘of ganged “switch S's ‘where increased voltage is
needed'to'm'easure the'lower-ratios. However, ‘it is not
necessary to correspondingly increase the ‘voltage im
75 ‘pressed across potentiometer ‘10' from ‘ampli?er ‘5' as
It is apparent the resistors R4 and R5 and/ or the decade
switch 16' selectively passes across contacts X-lO and
X-lOO from X-l, but instead it is desired to have it
resistors can be selected to weight E-235 as described. If
the amount of U-234 in the sample is not negligible, but
correspond to the normal voltage output of ampli?er
5' regardless of the position of the switch 16'.
In the arrangement for carrying this out, resistor
R'q, and R15 are so tapped that .9 of the voltage is
dropped across the lower sections, 0.9 across the inter
mediate sections, and .01 across the upper sections, and
the ganged sliders of switch 8'6 are so spaced that the
rap across the section of resistance of R'r and R's 10
can be assumed to vary in direct proportion with the
amount of U-235, accurate determinations can be made
by building a correction factor into the circuit. This
could be done by altering the values of RC, and R’5 and/ or
the resistors of decade 10’ as required. If the proportion
of U-234 to U-235 is not constant from sample to sam
ple, the resistors could be preselected to provide at least
a partial correction.
bridged by them is always unity regardless of the posi
When the isotopic composition of a gas sample is de
tion of the switch.
by comparison with a standard gas sample, the
Referring more in detail to FIG. 3, when switch 16’
operating conditions normally are such that the only
is set on contact X-IO, gang switch S's is positioned
as shown in that ?gure. In this position, exactly 0.1 15 variable is the gas pressure in the spectrometer source.
Ordinari y, this pressure is controlled by noting the value
of the voltage dropped across R';, is applied to the po
of the higher signal (e. g., E238) when the standard sample
tentiometer Hi. It will be noted that .9 of the voltage
is run, and adjusting the gas input to the source so that
dropped across R's is not applied to the potentiometer
the same signal (B238) is produced when the unknown
1t}, and that .9 of the voltage dropped across the re
sample is run. However, since the composition of the
sistor R’4 is applied to the balancing circuit of the re
standard and the unknown will not be identical, this pro
corder 12'. This causes the slide to move up the slide
fails to maintain exactly the same source pressure
wire 11’ to balance the circuit. The resulting reading
or gas flow from run to run. It was discovered that closer
of recorder 12' is then read as .1 of the indicated read
control will be obtained by regulating the gas ?ow with
ing to get the correct value. As best illustrated in the
signal E234+238+E235—that is, with the total ion cur
equivalent circuit of FIG. 3A, the voltage ERD across
rent. roceeding in this manner also improves operation
the decade 10 now is E234+238+E235. The voltage bal
by eliminating the effects of any non-linearity in the ion
anced by kERD, however, is the full input voltage l0E235.
ization, beam formation, beam separation, and beam col
Thus, the voltage-multiplication obtained with the X-10
lecting mechanisms since the total ion current and the
input resistor is applied in the balancing circuit to achieve
ionization chamber pressure will be maintained at con
increased sensitivity, but is not applied across the decade.
stant values.
Under these conditions (see FlG. 3A), the following
The inventor has found that regulation of gas flow into
relationship obtains at circuit balance:
the source with the total ion current will reduce “mem
ory effect.” There is evidence that memory effect is
kERD__ 10E235
caused in part by the physical interchange of material
present on the walls of the source inlet system. This phy
To obtain the proper mol ratio, it is necessary for the
operator only to apply the scale factor
sical interchange will be minimized if the pressure and
?ow conditions in the inlet system are standardized from
run to run; and this can be accomplished effectively by
40 regulating the pressure and ?ow with the total ion cur
rent. As pointed out above, it is conventional to regulate
i.e., 1/ 10. The circuit operates in an analogous man—
gas ?ow into the source by maintaining the larger output
ner on the X’—l00 scale. To summarize: The deter
signal constant. With that type of control, however, the
mination of mol percent on any scale is as follows:
?ow varies in such a direction as to allow any physical
interchange of samples to have an effect proportional to
R’ 344-238 [RI2“4+5"”8X
vol ercen‘ u-zss: “
1 p
R'iss X
the difference in sample concentration.
Having thus described my invention, I claim:
where the resistance ratio outside the brackets is the
1. A measuring circuit for a plural collector mass spec
are proportional to the input currents, the recorder can
input coupled to at least one of the collectors of said spec
trometer, a bucking battery in the output of said high
current ampli?er to normally overcome the output there
from, a low current ampli?er having an odd number of
plate ampli?er stages set for maximum output on mini
aforementioned scale factor applied by the operator, and 50 trometer comprising a high current feed back ampli?er
having an odd number of plate ampli?er stages set for
the resistance ratio inside the brackets is the reduction
maximum output on minimum input signal having its
factor applied by the circuit. Since the output voltages
be calibrated in mol percent U—235, and the operator
need only apply the proper scale factor to the recorder 55
ince the subject circuit is adapted for the measure
ment of mol percentages, it is readily adaptable for the
measurement of weight percentage. Assume, for ex
ample, that the subject circuit is switched to “M01 Per iii)
cent” and to the X—l contact. Then,
M 0 l per c ent. U—235=—————~——
E2s4+2as+E2s5X 100
Weighting each of the voltages in terms of the weight
of 11-238, then Weight Percent=U~235=
mum input signal having its input coupled to another of
said collectors, a bucking battery in the output of said
low current ampli?er to normally overcome the output
therefrom, means for changing the input impedance of
said low current ampli?er to alter its sensitivity, a volt
age divider coupled to the outputs of said high cur
rent ampli?er and said low current ampli?er to combine
their signals, a second voltage divider coupled to the out
put of the low current ampli?er to receive signals there
from, a recording potentiometer for bridging said ?rst
named voltage divider and said second named voltage
divider to measure abundance ratio and mol percent, and
adjustable means for altering the coupling between the
70 recording potentiometer and the said second voltage di
vider and between said ?rst and said second voltage di
Assuming the amount of 11-234 to be negligible, then
viders for increasing the accuracy of measurement of low
Weight Percent U-235=
abundance ratios and mol percentages.
Ezaigéé-l- 1572352375 + Eras
miTnggx 100
2. In a mass spectrometer provided with a plurality of
75 collector electrodes, a circuit arrangement associated with
said electrodes comprisinga point of reference. potential;
at ?rst ampli?er circuitprovided ‘with a cathode follower
output, an input connected to a ?rst and a third of said
electrodes, and characterized by 7100 percent feed back;
a second ampli?er circuit having a cathode follower out
put stage including a pair of cathode resistors, an input
coupled to a second of saidelectrodes, and characterized
of said collectors to receive a selected portion of the mass
spectrometer ion current and to generate an output volt
age proportional thereto, an impedance connected to re
ceive the .output voltage from said 100 percent feedback
ampli?er, a direct current 50 percent feedback ampli?er
coupledto at least one other of said collectors to re
ceive another selected'portion of the mass spectrometer
by-substantially 50 percent feed back; ?rst and second
current and to generate an output voltage proportional
load impedances and bucking potential sources connect
a pair of impedances of equal value joined in
ing said outputs to said reference point; a self-balancing 10 series combination
and connected to receive the output
recording potentiometer provided with moveable .input
voltage from said 50 percent feedback ampli?er, an end
and output arms;.a second arm; ganged to saidinput arm;
of the ?rst named impedance being connected to the junc
means coupling one end of said potentiometerto said sec~
tion of said pair of impedances to maintain the voltage
0nd armuneans coupling the othertend of.said'potentiom—
drops acrosssaid ?rst named impedance and one of said
eter to the junctionof'said?rst impedance and source; a
of impedances'in additive relation, a slidewire in~
plurality-of taps onrsaid secondimpedance and one of
said cathode resistors to contact 'said moveable :arms to
provide said-recording vpotentiometer with ‘a .signal pro
portional to'the total ioncurrentzreceived on said elec
cluding a-voltage divider connected across said ?rst im
pedance ‘and a selected part of said series combination of
impedances to'receive signals solely therefrom, and a
voltage~balancing vdevice connected between said slide
20 wire and an intermediate point of said series combina
3. A measuring circuit for'aplural collector mass spec
tion of impedances to measure mol ratio.
trometer ‘comprising a high current ampli?er fed from
the high ion collector, a lowcurrent ampli?er fed from
the -low ‘ion collector, a voltage divider comprised of a
5. The combination according to claim 4, wherein the
individual impedances ‘of said pair of impedances are
tapped to form in combination a put-and-take
pair of ‘series connected equal impedances similarly'tap 25 identically
impedance, and switching means to make the said con
ped to provide va progressively ‘greater impedance gra~
dient,,a voltage divider 'network'including ~serially.con~
nected resistors-and the slide .wire of a self-balancing re
nections between said voltage divider and said series com
bination and .between ,said balancing device and said
series combination, whereby said voltage divider and said
balancing device are connected to corresponding taps on
cording potentiometer'having onerend coupled toithe out
put :of the-high currentampli?er, ganged contacts bridg v30 said pair of said impedances.
ing corresponding taps on said impedances for movement
"over-“the tapped Jirnpedances while maintaining constant
References Cited in the ?le. of this patent
impedance between them, one 'of said contacts being cou
pled tolthe opposite end of said'voltage divider network,
and .the :arms of said recording potentiometer being
bridged acrosspotentiometer resistance element and the
other of said movable contacts 'to receive ‘Signals solely
from said high current ampli?er and said low current
“ampli?er to'provide a reading of relative ion abundance at
\said high ion collector andtsaid low ion collector.
- , 4. ,A measuring circuit for a mass spectrometer having
a plurality .of ion collectors comprising, ‘a direct current
{100 percent feedback ampli?er coupled to at least one
Sink _________________ .... Aug. 4, 1953
Robinson et al. ________ __ Dec. 8, 1953
Clark _______________ .. Nov. 17, 1959
Journal of Scienti?c Instruments, vol. 30, Nov. 1953,
article by R. ‘K. Wanless and H. G. Thode, entitled A
Mass Spectrometerfor High Precision Isotope Ratio De~
terminations, pages 395-398.
Patent No. 3,076,091
January 29-. 1963
John R‘. Mahoney
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 5, line 7, for "0.9" read ~— .09 ——°
Signed and sealed this 3rd day of December-1963.
fEggisgT W
Attesting Officer
AC ‘Ling
Commissioner of Pan-ms
Watent No“ 3,076,091
January 29‘, 1963
John R., Mahoney
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 5,
line 7, for I‘0.9" read -—- .09 ——=
Signed and sealed this 3rd day of December-1963.
lmmesting Officer
Commissioner of Pmmm
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