Further Development of the Two-Stage Mass Spectrometer for Isotopic Analysis of Uranium L. A. DIETZ, C. F. PACHUCKI, J. C. SHEFFIELD, A. B. HANCE, and L. R. HANRAHAN Knolls Atomic Power laboratory, General Nectric Co., Schenectady, N. Y.
P Precise, absolute abundance measurements for nuclear reactor problems have been accomplished through improved ion source optics and development of a fast pulse-counting technique for electron multiplier detection of positive ions. Voltage discrimination for the isotopic ratio U23g+/U235+ is approximately 0.1% for 15-kv. ions. Counting loss for random events is 15% a t an observed rate of l o s c.p.m. Use of getter-ion pumps and CI commercially available electron multiplier has simplified operation and maintenance.
T
emission mass spectrometers are beconling increasingly useful in measuring neutron capture cross sections of fission products, trace quantities of fuel contamination on nuclear reactor components, and long irradiation isotopic burnup. Isotopic analysis of small samples associated with reactor development is becoming more critical in respect to the evaluation and design of reactors with improved performance. Permissible radioactivity levels and the available amount of a given isotope after neutron irradiation geiierally limit sample size to less than a microgram. Even though present surface-ionization techniques possess high sensitivity and freedom from sample memory, mass spectrometers with greatly improved sensitivity am1 precision are required for this work. A mass spectrometer consisting of two sector magnets in tandem n-as first constructed by Inghram and He(6). This paper describes further deveIopment of the two-stage mass epectrometer designed by TVhite and Collins ( 7 ) . HERMAL
INSTRUMENT DESIGN
Because tests have shown that Cand S-shaped trajectories h a r e the same abundance sensitivity ( 7 ) , the C-shaped configuration for the tn-ostage magnetic analyzer was chosen for its compactness. K i t h defining slit widths of 0.016 inch a t the source, 0.020 inch between the magnets, and 0.050 inch a t the detector, abundance sensitivity is IOfi in the uranium mass
1276
ANALYTICAL CHEMISTRY
region Although abundance eensitivity cannot be improved significantly by narroning the slits, i t can be improved by addition of an electrostatic analyzer ( 9 ) Ion-optical discrimination from samplr to sample is not constant, since surface ionizatioii filaments emit nonuniformlj . Emission geometry changes nith filament temperature, e ~ e nfor the qame physical geometry of the sample filament. For this reason, high transmission through the entire optical system is a necessary condition for high precision in surface emission mass spectrometry. An ion-optical in1 estigation ( 1 ) has led to the development of simultaneous, electrostatic z focus an? tilt. Orientation of tllp t focus half-plates is shon-n in Figure 1, and electrical connections of the souice are shonn in Figure 2. At 15-kv., only a fen per cent of the total accelerating voltage is required for positive z focus n hich conrerges the beam a i d produccs up to a tnofold iiicreasc in detected ion current. Potential distribution and focusing in the z leng are similar to those betneen the 0.05- and 0.52-cquipotential lines of Figure 1 in reference ( 1 ) 'Thc tilting rang? of & l ois ini1t.p 'ndent of accelerating voltage. Getter-ion pumps are very vitisfactory for use on surface emission mass yiectrometers. Three Varian AssociateS T'acIon pump^ of 5 liters per second pumping speed are positioned on the 4ource chamber, first mass spectrometer tube, and detector chamber, respe~tirely. The courne chiimber can he isolated from the a,iectrometer tube-. and from its YacIon pump by a clottedrod v a h r and an O-ring valve, respectirely. Both type- of high ~ a c u u n i valve. seal nith Teflon. During an analysis a cold finger in the source cap is filled n i t h liquid nitrogen to pump condensable g a v - e l olved from the filament. Operating pressure indicated 11v pump cur:ent is 2 X 10-7 nim. of mercury in thc spectrometer tubes and 4 X 10-7 mm. of mercurj in the source chamber. The tn o punipr on the spectiometer tubes have been in operatioil for 16 month' n itliout shon ing tletelloration. The first pump on the cource cliamhei was removed for vacuum bakeout after 6 nioiith- of operation, and its replacemmt has been in operation for 10 month-. Small wii1~1eq
have been analyzed a t the rate of four per 8-hour period. The pumping action of getter-ion pumps does not meet t'he conditions of free molecular flon~. This is uniniportant in a surface emission mass spectrometer, but must be considered in a gas mass spectrometer (6). Theory and design of getter-ion punips are discussed by Hall and Holland (2-4). Electronic instrumentation was chosen from commercially available types wherever possible. A scshematic diagram of the mass spectrometer circuits is shown in Figure 3. The nuclear magnetic resonance field control system is Model FC-501, made hy Harvey-Kells Electronics, Inc. The 15-kv, accelrrating voltage pon-er supply with 0.005% volt'age regulation is 3Ioclel 315 CF, made by Beva Laboratory, Iiic. Fast pulse-counting equipment allow more data to be taken per unit time with lon-er per cent corrections for random counting loss and machine background. Output pulses from a n electron-multiplier detector are fed into three Hen-lett Pnckard lloiiel 460;1, 120-JIc.. n-ide-band amplifiers in series. follon-ed by two llodel 460I3, 120-1\Ic., fast pulse amplifiers, the first operated in linear, the second in pulse position. This arrangement produces undistorted negative pulses with amplitudes up to 40 volts a t the output of the 1:ist amplifier. The collector lead from the multiplier to the input of the first amplifier is kept as short a3 pos;.ible and is grounded through a resistor of several hundred ohms. The aiiiplified pulses are counted in a Beckman/ Berkeley Model 7370, 10-Ale. scalrr, which has been modified so that its \vide-band amplifier is bypassed ant1 its discriminator accepts negative pukes.. The output of the last pulse amplifier also feeds into a parallel circuit in nhich pulies are inverted by a Gudeman Co. pulse transformer, Model 1GT 0.05-11, t,hen pass into a Hewlebt Packard Model 520AiA, 10-hIc. scaler, then into a Baird Atomic Model 112 count-rate meter. and finally into a Lee& & Sortlirup SpeeLloiiias H, lO-niv., strip-chart recorder. The recording system is useful for focusing t,he ini;trument and for ' emission. observing t.rend-i in The ioii detect,or is a Radio C o r p of America Type C7187E multiplier of 14 stages. The entrance slit is elongatecl to 3/4 inch, and the mult'iplier h ; e is \;-axed into a bellows-mounted flange,
I FOCUS PLA'ES
3
-.
5-ECTROMETER TUBE
:ON SOURCE 2 1
-v
-
.
-
"V
'c,
UIGNET
;& Figure 1 .
1
M
i
I I
I
--
}
Orientation of ion source and magnet
c
as-
3-b
- 2
3
+ 2
:a;-:-
v4->a
1 =' 23
' Y
1 -
1 C-FDC
The y direction is into the poper. The z focus plates ore 0.43 inch deep X 1.62 inches long, and are separoted by 0.25 inch. Separation between collimator slits i s 0.69 inch
1
with the dynode axes parallel to the plane of the beam. While monitoring an ion beam, the moiint.ing flange is rotated about its z axis until the most sensitive portion of the first dynode intercepts the beam. Gain is niaximized by adjusting the multiplier focus voltage t'o obtain maximum counting rate. llagnetic shielding is used on tlie multiplier vacuum chamber. Dynode voltage is provided by a Beva Laboratory 1\Iodcl 301I3, h 5 . 1 - k v . supply with 0.1% voltage regulation. At -4kv. tlie current, gain exceeds lo7, average background i. 10 c.p.m., antl efficiency of tlie detection 7,570. The niiiltiplicr h:i tion for 9 months. Counting losses m r e detmiiined by the method of White and Collins ( 8 ) . From thcir nork it can be shon-n that tlie error from corinting loss in measuring an isotopic ratio of two beams of positive ions which are formed by random rniission from a surface-ionization filanicnt may he expressed as (f? - R o : , ' J 2
=
(1 - l/X)rrz
where R antl Ro,rrspec.tively, are actual and obacrvccl isotonic ratios, T is the effective resolving time of thc mtire detection systcm, and T? is the ohscmwl counting rate of tlw larger isotope. At an otiscrvctl rate of 1.0 x lo9 c.p.m. for 1 , ~ " ~ the ' fractional decrease in blie linon-11 17.5 '176 isotopic ratio is 0.15, tli~rcforeT is 0.092 bser. :ifter T is caali1)rateti o1jwi.vCd isotopic ratios are correctctl for counting loss by us(' of tlic above c>qtIatioil.wlvecl for R.
Figure 2.
Table
I.
Analysis of Uranium Samples
Size, Sample
Argonne std. 0.710 i 0.004 Argonne stcl. 0.698 1 0.001
Xatural U
pg.
+\tom yo I;"""
0.7111 i 0.0014 0.i420i0.0016 0.002 0.74TO i 0 . 0 0 2 3 0,2 0,8984 i 0.0016 0 . 0 2 0.6979 i 0,0017 0.002 0 . 7 0 1 9 ~ 0 . 0 0 2 4 10 0.7223 +cOo.0009 0.2 0.7214i0.0013 0.02 0.724-l10.0011 0.002 0 7219 1 0 . 0 0 2 1 0.2 0.02
INSTRUMENT PERFORMANCE
a Each C235 ahundance is average of three analyses with different, filaments. Precisions are standard deviations.
;lwurnte analysis of small samples is possible only if contamination is minimized during chemical preparation. For this reason, the ultrapure chemical solutions which must be used are prepared from purified reagents which have been mass spectrometrically analyzed for specific contamination.
single analysis consisted of 40 pairs of 238 '235 ratios which were obtained by stepping the accelerating voltage to count tlie P8+ and V 3 5 + beams for 1 and 10 seconds, respectively. -4 standard deviation of c = 0.317, for
- 7 . .
2 YL-,?
-
Y
r c r = 5
-
Voltage divider for z focus source
All resistors ore wire wound, and quired infrequently
Before sample loading, filaments are outgassed a t temperatures exceeding 2000" C. for several hours in an auxiliary vacuum system. Sample solutions are pipetted into tlie fold of the V-filanient as nitrates in solution IT-ith 0.05W nitric acid and are evaporated by passing current through the filament. The mass spectrometer Kas tested n-ith uranium standards of varying sizes and isotopic composition so that niachine bias could be determined. The results (Table I) indicate that no significant differences exist b e t w e n our data arid the standards, except for a slight bias a t the 0.002-pug. size.
s-c:\>
M
Figure 3.
is megohm.
Negative z focus i s re-
Schematic diagram of mass spectrometer circuits
a single determination \vas calculated from 22 analyses of iiatural uranium. Data n-ere corrected for multiplier hackgroiincl, rounting losses, and voltage discrimination. -innpparrnt tlisc~riniination of 0.3'; for tlLr isotopic. ratio - ~ v a ?nic:isurc~ti1)). oh;tsi,viiig ity of :I L?" bcani \yliile varying bcitli voltage and niagncxtic fipld. Bccausc* it \VAS slioim that the changing stray magnetic field affwted iiiost of the multiplier gain a i ~ l observed effect. volt,age discrimination for the L-2%'- I---?S5- ratio is approximately 0.1%. From a coniirleration of tlie mac.liine errors and the d a h in Table I it is concluded tliat the V-filament docs not preferentialiy emit uranium ions acACKNOWLEDGMENT
The authori are indPbted to IT. D. Davis for his investigations of a cold finger and a C7187E multiplier, to C. IT. Bechtoldt for design drafting, VOL. 32,
NO. 10, SEPTEMBER 1960
1277
and to G. E. Xartin‘s mechanical equipment development group. Tlw uranium standards were obtained from 11.H. Studier a t the Argonne National Laboratory. All samplrs were aliquoted a t this laboratory by A h . H. C. Hendrickson. Many of the analytical data were obtained by G. -4.Lumnianik and G. R. Hertel. Coiisultations with Alas Halperin aided statistical interpretation of the data.
No. 14, p. 24, (1954).
LITERATURE CITED
(1) Dietz, 1,. h.,Rtc. Sei. Inslr. 30, 235 (1959). (2jHali, L. I)., Ibid., 29, 367 (1958). (3) Hall, L. I)., Science 128, 279 (19 ttrium or rare earths prior to and during the cation exchange step would r a i v questions as to the interpretation of tliv function of the cation exchange resin These methods are evaluated below. I n attempts to apply the Carter and Dean procedure ( I ) to the dettvmination of the rare earths in inipii’e uranium oside. a sizable precipitate clvveloped in the fluoride solution, which n a s found to contain over 95% of the added yttrium carrier. An analysii identified the precipitate as mainly calcium fluoride with a minor amount of magnesium fluoride. T h r exact mechanism of the collection of the rare earth fluorides by a mixture of calcium and magnesium fluorides is not yet u n d w
