Ion Counting and Accumulation System for Mass Spectrometry of Very Small Samples Application to Uranium a n d Plutonium G. W. BARTON, Jr., L. E. GIBSON, and L. F. TOLMAN Lawrence Radiation laboratory, University of California, livermare, Calif.
F A system has been devised which permits rapid and precise isotopic analysis of 1 O-8-gram samples of uranium and plutonium, and of other elements and compounds for which beams of 1 0-l8i o 1 0-l2ampere can b e achieved and kept for several minutes. The system is insensitive to growth or decay of sample emission by factors of 2 or more in a 1-minute period. The ion beam is detected by an electron multiplier, and individual ion pulses are accumulated in a modified 256-channel pulse analyzer. The spectrum is scanned by sweeping the accelerating voltage in synchronism with a scan of the analyzer channels, so that pulses in each mass increment are assigned a cor1 esponding channel. The normal sweep ; a t e is 8 complete spectra per second, which are accumulated in two groups, the up-mass sweep and the down-mass sweep. The data stored in the analyzer at the end of a run are printed out in decimal form on a five-line per second printer. Reduction of the data consists only of correcting for background and coincidence loss and normalizing to a convenient parameter.
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numerous problems in the rapid measurement of isotope ratios on submicrogram samples of heavy elements. These problems become acute if more than 4 analyses per day are needed. The system to be described is capable of analyzing 18 samples ranging from 2 X lo-* to 2 X l o + gram in one 8-hour shift. In the heavy element region, surface ionization sources are inescapable in the present state of the art. Surface ionization, however, is notoriously unstable, and the combination of this with other fluctuations poses an exceedingly difficult problem. With electrometer detection of the beam the fluctuations are conventionally averaged by use of a long time-constant circuit, but then one cannot sweep rapidly and still resolve small isotopes adjacent to large isotopes. Furthermore, to obtain precision one must scan the spectrum many times and the reading of the charts HERE ARE
and the reduction of the data become laborious. Pulse counting (4) improves the signal-to-noise ratio in small abundance isotopes, but if it is done with enough measurements to define the shapes of the peaks and valleys, and with enough full scans of the spectrum to be able to correct for growth or decay of gross ion emission, the reduction of the data again becomes a chore. If instead of printing out data digitally at discrete intervals, the data are fed to a counting rate meter, the problems associated with time constant smoothing are again present. In conventional systems the attention given to trivial details such as range changing, control of sample emission, control of the ends of the spectrum, and so forth, over the period of a run lasting up to an hour, leads to operator fatigue and loss of data, sometimes even the loss of the whole of an irreplaceable sample. It was felt that any new system must perform as many of these operations as possible automatically, so that the operator's attention could be focused on things where judgment is necessary. It was also felt that the total duration of a run should be minimized to reduce fatigue as well as to permit more samples to be run per unit time. The system to be described meets most of these objections to conventional systems. The problem has been divided into three distinct operations: the mounting and installation of the sample, the attainment of the operating vacuum and the taking of data, and the reduction of data. It is desirable to minimize each of these times, and to separate the operations so that they can be carried on simultaneously on three different samples. The mass spectrometer proper is a symmetrical 60' magnetic sector, 12-inch-radius spectrometer designed originally by F. L. Reynolds of the Berkeley Lawrence Radiation Laboratory and is similar to instruments which can now be purchased from several sources. This instrument solves the problem of mounting and introducing the sam-
ple by the use of a sample lock similar to the one described by Stevens (3). The time betneen finishinq one sample and the time the next sample is ready to run is about 5 minutes. The useful volume in the carriage is 2 inches in diameter by 33/4 inches long, which has proved adequate for all sources tried so far. For this work, primarily with uranium and plutonium, the source is operated with two parallel filaments in a fashion similar to the three-filament source of Inghram and Chupka (I). Both filaments are mounted edge-on to the beam, and the only focusing is obtained by optimizing the size of the hole in a cover plate and the distances from the plane of the cover plate to the filaments, and the cover plate to the analyzer entrance slit. By doing this it was possible to remove all the other focusing slits with only a small loss in analyzed current and great gain in ease of adjustment and simplification of power supplies. Measurable, but very reproducible, mass disci imination is introduced by sweeping the high voltage instead of the magnetic field. This correction has been measured by use of isotope dilution techniques, and is made in the reduction of data to final reported numbers. Figure 1 is a block diagram of the system in the counting mode. By sweeping the accelerating voltage rather than the magnet, it becomes possible to scan the spectrum in an almost linear fashion a t a rate of 40 spectra per second, without encountering any difficult electronic problems. The spectrum is usually scanned in a triangle fashion a t a rate of 4 cycles per second, which is 480 full spectra each minute. The problem then becomes one of superimposing and accumulating these spectra, in register, so that the accumulation of all 480 spectra is printed out as one histogram. The ions arc analyzed in the 60' magnetic field in the usual fashion. It has been found convenient to tabulate mass number us. frequency for a proton nuclear resonance gaussmeter a t the normal accelerating voltage A high quality calibrated communications VOL. 32,
NO. 12, NOVEMBER 1960
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Figure 3. Upper-synchronization pulse, lower-clock pulse, superimposed-scanning voltage
Figure 1.
Block
diagram of counting system
receiver used as a frequency meter makes it simple to identify unknown peaks or to set the magnetic field for expected peaks. Ions are detected in the electron multiplier mounted after the second focus slit. The multiplier is currently a DuMont S P 172. This has 14 stages of the same silver-magnesium box structure used in the common 6292 photomultiplier. It is purchased activated, without photocathode or tube base, but sealed in the glass envelope with the leads brought through the standard glass press base to keep proper spacmg. It is mounted so that the ribbon of ions is perpendicular to the axis of curvature of the first dynode. This eliminates a source of variable masa discrimination. Each of the dynodes is brought out through a separate Kovar seal and the voltage divider is mounted outside the vacuum system. The multiplier is electrically connected as shown in Figure 2. The vibrating reed electrometer is used routinely only for measurement of the gross beam stability, but its independence from the counting circuitry makes it simpler to locate and identify troubles. Its input is taken off in the middle of the multiplier partly to isolate it from the counting circuit, and partly because more current is available from the multiplier than is needed by the reed. The input to the Hewlett Packard Model 460 wide-band amplifiers is taken off through a pulse shaping network, which serves to put the pulse energy into the amplifier band width. The pulse output when a new multiplier is installed is large, but drops rapidly to a stable value of around 50 pa. into 200 ohms with a pulse length less than 10-8 second. This gain of 106 is stable from 4 to 6 months in our operation. The pulse counting system with memory and continuous display of both the information arriving a t a given time and the information accumulated in the memory makes it possible to achieve quasi-simultaneous ion collection, minimizing fluctuation effects of the ion beam. The instrument used is a modified production model of the 256-channel pulse analyzer described by Schumann and McMahon (2). This instrument has a magnetic core memory, a 1-megacycle binary arithmetic scaler which can be written into or read out of the memory a t any one of 256 addresses selected by the address scaler, 1600
0
ANALYTICAL CHEMISTRY
TO-3 k v
T O VlBPATlNG REED ELECTROMETER
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i
TO+ 2 k v
Figure 2. Electrical connection of electron multiplier
and logical circuitry to accomplish this. In the print position it provides a train of uniformly spaced pulses equal in number to the number of counts stored a t each address. In the count mode used here, the address scaler is cycled through each of the 256 addresses by an external clock pulse. This is usually and most usefully 1 millisecond per address, although circuitry has been incorporated to permit anything from none to 10,000 addresses per second. After each clock pulse the input ion pulses are gated off, and the count in the arithmetic scaler is written into the appropriate memory address. The scaler is reset, the address stepped to the next larger address, the previously accumulated count a t the new address read out of the .memory into the arithmetic scaler, and new counts fed into the arithmetic scaler until the next clock pulse. Each time the address scaler fills up and overflows, a synchronization pulse is sent to the function generator, controlling one corner on the triangle. Thus the period of the function generator is 256 times the clock period. The other triangle corner is controlled by the usual voltage comparator, so that the spectrum cannot drift with respect to the addresses. Figure 3 shows the synchronization pulses, clock pulses, and one corner of the triangle. The output pulses from the multiplier are amplified, discriminated, shaped, gated, and fed to the input of the arithmetic scaler. Figure 4 shows the
analyzer accumulating data. The top line shows the sweep voltage and hence the mass position. The bottom line shows the analyzer address. The second line is the output of a counting rate meter connected to the multiplier output. The peak is Relm and shows both statistical fluctuation and the fluctuations characteristic of surface ionization. The third line shows data accumulating during the first four sweeps. The count switch operates a gate which allows data accumulation to start or stop only a t the beginning of a sweep. After enough time to accumulate a significant number of counts in the least abundant isotope of interest, the count is stopped and the analyzer switched to the print mode. The usual analyzer counts the pulse train produced by the analyzer a t each address in a printing decimal scaler. We have interposed a device called a “running summer” in the print cycle. This has the same function as the integration time constant in an electrometer. Most spectra of interest have perhaps 6 interesting isotopes displayed in 128 channels. The requirements of optimization of time between peaks and background set the peak width approximately equal to the valley width. Thus each peak will cover approximately ten addresses. If the data are printed out directly, there is the problem of summing a t least the largest six address positions to make good use of the information. The running summer does this electronically for all groups of n adjacent addresses and prints the sums out in proper order. In Figure 5 the solid line is the ion intensity as a continuous function of mass. The histogram is a plot of the counts actually accumulated in the memory. Because of statistical fluctuations the three largest addresses are equally good measures of the peak intensity, but their sum is better. By printing out the sums of groups of channels in order it is easy to locate the best value for the peak intensity, and the most annoying and time-consuming part of the computation has been done. Conceptually, this running summation is easy to perform, although the hardware gets complex. In the usual system, after the pulse train equal in number to the binary number at a
Figure 4. Analyzer accumulating data TOP, *weep *oltoge Second, output of counting rate meter Third, doto accumulating in memory Bottom, analog of malyrw
Figure 5. Continuous probability distribution of counts vs. mass position and histogram resulting from this mode of accumulation
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siven addresv has k e n counted in the decimal scaler, a print command is initiated. The printer prints, the decimal scaler is reset, and the analyzer proceeds to decode the next address. If the print command is inhibited, and the decimal scaler not reset, after the pulse train at the next address the decimal scaler will contain the sum of the counts stored at the first and second memory positions. This can he continned for n addresses, at which time a print command is given. The printer now prints the sum of the counts stored at all n memory positions. Fortunately, t,he analyzer does not forget the information it has decoded, so that it can be mused in the next summation. After the print, the decimal scaler is reset. and the analyzer address cycled through 257-n addresses without decoding. This process leaves the analyzer now a t the second address of the first summation, and the process is repeated for all 256 groups of n adjacent channels. Thp n addresses of the summation and the 257-n addresses of the readdress are kept track of on one or more variable scalers for flexibility in the number of addresses in the summation. Figure 6 is a spectrum of rhenium photographed from the analog output of the pulse analyzer at the end of a 1.3minute run. These data are printed out on the high speed printer in a form directly available for computation. The largest m s and representative background values are transcribed into columns 3 and 6 of Figure 7 . Column 4 is the correction for counts lost hemuse of the finite resolving time of the discrimination, pulse shaping, and scaling circuitry. Comiderahle effort has been taken to design the circuitry so that one and only one circuit controls the resolving time and this circuit has been designed SO that it is as iudependent of vacunm tube parameters a8 the state of the art will permit. This circnit has the delightful feature that it follows a simple algebraic law '
N. NT = 1 -
(NJN~
where NT = the true counting rate N. = the ohsenred ounting rate N. = the saturation counting rate-the reciprocal of the resolving time T
This enahles us to use a 1-megacycle scaler up to at least 5 X IO6 random counts per second and correct the observed countig rate to within 1% of the true counting rate. The form of this coincidence correction is particularly convenient for calculation if counting times are used for which N. is a simple multiple of a power of 10. The true counts are the observed counts corrected according to the above
.
=:
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. L
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-
-
- -. .
1
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Figure 6. Rhienium spectrum
rules. As long as the coincidenGG ulyluuL is not too small, and the running summation covers only the relatively flat peak top, Little error is made hy correcting the summation in a straightforward fashion. Background is subtracted to give net counts. Atom ratio is given by normalizing to the count of a conveu~. ient reference isotope, and making the fractionation correction. The productivity of an instrument has been improved enormously hy 6he combination of a sample lock, a fast scan, ion counting, and puke analyzer accumulation. Current quotations for the additional electronics a r e about one third t.he cost of another comparable spectrometer. For the full productivity of the system to he realired a crew of three is needed, one preparing samples, one operating the instrument, and one reducing d a b . A full crew can run 18 samples of uranium and plutonium, all smaller than 0.2 fig. in 8 hours. Two persons find 12 samples a good day's work.
Among those who have made important contributions to the development of this system are A. J. Stripeika, J. M. Moore, R. M. Rodriques, and R. E. Roulette of the Electronics Enpineering Department. Chemistry Division personnel who participated include R. P. Burns (now a t the University of Chicago), H. R. Bowman, and G. H. Higgins of the Livermore Laboratory, and A. Ghiorso and M. C. Michel of the Berkeley Laboratory. LITERATURE CITED
(1) Inghram, M. G., Chupka, W. A,, Rev. SCi. Instr. 24,518 (1953). (2) Schumann, R. W., McMahon, J. P., I b d . , 27,675 (1956). (3) Stevens, C. M . , l b i d . , 24,148(1953). (4) White. R. A.. Collins. T. L.. A d .
RECEIVEDfor review January 8, 1960. Accepted June 28,1960. Work performed under auspices of the TI. 5. Atomic Energy Commission.
VOL 32, NO. 12, NOVEMBER 1960
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