Determination of Americium in Plutonium by Gamma Counting JOSEPH BUBERNAK, MARION S. LEW, and GEORGE M. MATLACK University o f California, 10s Alarnos Scientific laboratory, los Alamos, N. M.
peaks in a multichannel analyzer, thus obtaining a measure of the amount present. Because the main alpha energy peak of plutonium-238 ( E = 5.50 m.e.17.) is too close to that of americium-241 ( E = 5.48 m.e.v.) to be separated by the analyzer, a correction must be made from other measurements on the sample. Another difficulty with alpha pulse analysis occurs, when the samples have a high salt content. Although the difference in alpha energies of americium-241 and plutonium-239 is 0.33 m.e.v. and can be resolved readily in pure samples, considerable tailing of the higher energy americium peak into the plutonium peak occurs when there is much salt present, owing to the decrease in energy of some of the N THE PRODCCTIOK of plutonium americium alpha particles on passing in a reactor, using normal uranium through the salts. as the raw material, the main isotope I n addition to alpha-particle pulse formed is plutonium-239. This ocanalysis, there are several chemical curs by the ITell known series of reacseparation techniques which might be tions used to determine americium in plutonium. I n one method, the plutofn -8-8e2U238 ----f s2T;239 +s3Np239 +scPuZ3@ nium is oxidized to the sexivalent state; the trivalent americium is then carried I n addition, other isotopes of plutonon ceric fluoride (6), or lanthanum ium and americium-241 are produced fluoride ( I ) , and the plutonium remains in solution. The fluoride precipitate is fn f n then dried on a plate and alpha counted. 94Pu23@ @4Pu240 @,PU"' The procedure is lengthy, sometimes requiring a double precipitation, and -0yields results inferior to other methods. -f,,&4m241 A second technique involves extracSn -8f n tion of plutonium(1V) into a xylene @2u238 @JJ23' + + solution of 2-thenoyl trifluoroacetone -2n (TTA) from 1-If nitric acid, similar to -893Np238- s4Pu238 a recently described method for the determination of plutonium (6). ilmerProcessing of plutonium from uraiciuni remains in the aqueous phase nium slugs, therefore, gives a product and can be determined by conventional which, in addition to plutonium-239, alpha-counting methods. The greatest contains small amounts of plutoniumdifficulty is that the extraction may not 238, plutonium-240, and plutonium-241, remove plutonium completely enough to plus a variable amount of americiumobtain pure americium. Low results 231, which will depend on its degree may also be due to alpha-particle abof depletion or enrichment by the sorption losses caused by the salts chemical separation procedures. used to stabilize the plutonium(1V). Radiometric procedures for determinThe various plutonium isotopes and ing americium in the presence of pluamericium-241 are known to emit tonium have been based on its alpha gamma rays as well as alpha particles. activity. Data for alpha energies and Preliminary studies in this laboratory alpha specific activities of the common showed that the gamma specific activity isotopes of plutonium and of americiumof americium is much greater than that 241 have been tabulated by Metz (4). of any of the plutonium isotopes. One method for determining americiumTherefore a study was conducted to 241 involves separating the alpha energy examine the possibility of utilizing this
,The gamma specific activity of americium-24 1, a common impurity in reactor plutonium, is 30,000 times greater than that of plutonium, when gamma rays above 30 k.e.v. are counted. This i s the basis of a procedure for determining as little as 5 p.p.rn. of americium in plutonium. When counted in a well-type scintillation detector, americium-241 shows a specific activity of 2.2 X lo6 c.p.m. per y , while that of plutonium2 3 9 i s only 75 c.p.m. per y. The average error i s 2% standard deviay of americium tion for 5 X or greater in the presence of up to 10 y of plutonium.
I
- -
fact for determining americium in the presence of plutonium. EXPERIMENTAL
Stock Solutions. A solution of americium, originally containing about 7% plutonium alpha activity. was purified by adsorbing the plutonium on Dowex-1 anion resin and eluting the americium with 10M nitric acid. A fraction of eluate containing the first large activity peak was collected as the americium fraction. Alpha pulse analysis on a small aliquot showed no detectable plutonium peak. d further test for plutonium was made by extracting a 1-ml. aliquot n4th 2-thenoyl trifluoroacetone (TTA) in xylene. Of the 500,000 alpha c.p.m. activity taken, only 150 c.p.m. were extracted, indicating the virtual absence of plutonium. Four samples of plutonium, each containing a different amount of plutonium-240, were extracted from 111f nitric acid into 0.5M 2-then01 trifluoroacetone in xylene to remove traces of americium. The extracts were washed twice with 1M nitric acid, and the plutonium was back-extracted into 11M nitric acid. The plutonium-240 content of these samples varied from 0 to 6% of the total plutonium by weight. A similar 2-then01 trifluoroacetonetreatment was made on a sample highly enriched in plutonium-238. This sample also contained some plutonium239, plutonium-240, and plutonium-241, but pulse analysis showed that more than 99% of its alpha activity was due to plutonium-238. Counting Equipment. Alpha activities were measured by evaporating small aliquots on I-inch square glass plates (micro cover slips) and counting in an internal sample, methane proportional chamber with a counting efficiency of 51%. Aliquots evaporated on glass plates were gamma-counter1 using a Harshan- sodium iodide (T1) crystal, l-lj2 inches in diameter and 1 inch thick. Solutions in test tube. were gamma-counted by using a Harshaw well-type sodium iodide (Tl) crystal. Both crystals were coupled to Dumont 6292 photomultiplier tubes. Gamma pulse height analyses were made with a 100-channel analyzer, equipped with a magnetic core rnemor!. and auxiliary oscilloscope data presentation. RESULTS AND DISCUSSION
Gamma and X-Ray Spectra. Data VOL. 30, NO. 1 1 , NOVEMBER 1958
1759
Table I. Principal X-Rays and Gamma Rays Emitted b y Some Plutonium Isotopes and Americium-24 1
X-Ray and Gamma Ray Energies, Isotope K.E .v. Pu238 L x-ray, y's at 43.5, 99.8, 153.1 Pu239 L x-ray, yJs at 12.5, 38.3, 50.8, 117 Puza0 L x-ray, y at 44.6 PuZ41 None AmZ41 L x-ray, y ' s at 26.4, 43.4, 59.6, 98.8
Reference (7 ) (8)
13 \
glass plates. The plates \wre placed 2 nim. below the sodium iodide crystal. The other method involved counting 2.0ml. aliquots added to 13 x 100-mm. test tubes in the well counter. The solution reached a height of 2 em. in the 1.7 cm. diameter X 4-cm. deep hole of the crystal. The discriminator was set so that only gamma rays with energies abol-e 30 k.e.v. were detected and counted.
I
(3)
L X-RAY
-
. .
Pu 238
Table 11. Gamma-to-AlphaActivity Ratio of Plutonium Isotopes and of Americium-241 for Gamma Rays above 30 K.E.V.
Ratio, Gamma Counts Der lo6AlDha Counts Plate Solution Species counting counting Pu230 37 108 Put39 (2% Pu240) 35 108 Pu239 (4% PU240) 39 120 (6% P U Z ~ O ~
~ ~ 2 3 9
Pu238
Am2"
43
17
20,800
.".
L X-RAY Pu
.
I'
. .
49 60 ,600
'
26 K,EY
1760
ANALYTICAL CHEMISTRY
51,KEV
.'.u-.-
123
for energies of x-rays and gamma rays emitted by the isotopes under consideration have been reported in the literature (Table I). T o determine the relative intensities of these rays, a study was made of the spectra using the 100-channel gamma pulse analyzer. The solutions studied were plutonium239 plus 6% plutonium-240, plutonium238 (enriched), and americium-241. Two milliliters of solution were placed in 13 X 100 mm. test tubes and gammacounted in the well counter, Fvhich was connected to the pulse analyzer. Gamma-ray spectra of the samples are s h o m in Figure 1. The plutonium solutions had an alpha count of l o 7 per minute and the americium solution had 5 X lo* per minute. The spectra include the range of energies from 6 to 121 k.e.v. A surplus counter connected to the analyzer showed very little activity for energies above 121 k.e.v. Gamma Activities. Figure 1 shows t h a t except for the L x-rays, very little gamma activity is shown by the various plutonium isotopes. The main gamma peak of americium is a t 59.6 k.e.v. This method for determining americium in the presence of plutonium uses a gamma counter which discriminates between the 59.6-k.e.v. peak of americiuni and the L x-rays of plutonium. Thus a minimum contribution by plutonium to the total gamma activity is obtained. Two types of gamma counting were used in this study. I n one type, aliquots of solutions were evaporated on
- 239
,'
'.
59.6
KEY Am-241
Figure 1. Gamma ray spectra of plutonium and americium
A list of gamma to alpha activity ratios for several samples is given in Table 11. The high value for americium shows that gamma counting compares favorably with alpha counting (alpha specific activity = 3.65 X 106 c.p.m. per y a t 51% geometry, gamma specific activity = 7.59 x l o 5 and 2.21 x lo6 c.p.ni. per y for plate counting, and solution counting, respectively). The low values for the plutonium samples indicate that americium can be gamma-counted in the presence of these isotopes with little correction for the gamma contribution by them. Thus, the gamma specific activity of plutonium-239 in well counting is 75 c.p.m. per y or only 1/30,000 as great as americium. When plutoniuni240 is present the correction is slightly greater; if plutonium-238 is present it is slightly smaller. Because reactor plutonium is composed mainly of the 239-isotope, there would be little error in using its value for determining the plutonium contribution to total gamma activity. The error can be made even less by evaluating the correction factor using an average plutonium-e.g., containing 47, plutonium-240. Comparison with Other Methods. An evaluation of the plate gammacounting procedure was made using solutions containing known amounts of
aim1 iciuiii and plutonium.
Each solution except one was made to contain 10,600 americium alpha counts per minute per 200 pl., and the concentration of plutonium alpha activity was varied from 0 to loo'% of the total alpha activity. The americium alpha content was then determined in each solution by gamma-counting, \T here the alpha-to-gamma ratios from Table I1 nere used to correct for the plutonium contribution to total gamma activity and to convert the gamma activity of americium to alpha activity. The results are shon n in Table 111,with those obtained by the three methods mentioned previously. The data shon that the gamma-counting method gives good results for all ranges of americium content. Alpha pulse analysis gil es good results when the americium alpha content is high, but for low values the error is greater than by the gammacounting method. The lanthanum fluoride method gives consistently low results with large errors. The 2-thenoj-1 trifluoroacetone-extraction method gives poor results for higher ranges of americium content. Precision of Method. An indication of t h e precision obtained by the gamma-counting technique is given in Table IV. Duplicate determinations were made on actual plutonium samples, containing a wide range of americium alpha activity content, by gammacounting 2-nil. aliquots of the samples after suitable dilution to bring the count rate within the range of the counter. The n-ell-type crystal was used. Separate small aliquots of the dilutions ryere evaporated on plates and alpha-counted. Assuming that all of the alpha activity was plutonium, a corresponding plutonium gammacount rate was calculated, and then subtracted from the measured gamma-count rate, giving the net gamma-count rate caused by the americium. Table IV lists the equivalent per cent americium alpha count rather than gamma count, in order that possible comparisons with other published data nil1 be more ohvious. The per cent range is a measure of the difference between the duplicate determinations of the americium concentration divided by the average of the duplicates. It includes the error inherent in both alpha and ganinia counting. Table IV shows the precision of the gamma-counting method to be independent of the americium alpha-activity content in mixtures with plutonium, provided sufficient aliquots are taken to give reasonable rates for alpha and gamma counting. The lower limit for solution counting may be considered to represent a sample in which 2.0 nil. gives 100 gamma c.p.m. Such a sample would contain 5 x 10-6 y of americium, and the counting error for a count time
of 10 minutes would be 57c. The samples in Table IV which represent the lowest americium content (0.03 to 0.05% americium alpha), are in the range of about 10 p.p.m. americium in plutonium. The accuracy of the gamma-counting method depends on several factors. The americium gamma specific activity as determined from alpha-actiuity-togamma-activity ratios involves errors in alpha and gamma counting of standards. This error can be kept low by calibration n ith several highly purified standards. Similarly, each instrument used in counting should be calibrated with pure plutonium standards t o determine the plutonium gamma correction factor. The error involved in assuming that all the alpha activity is due to plutonium, none being contributed by americium, is constant and equal in magnitude to 0.2y0 of the americium alpha activity present. This is below the counting error in most cases. Another error is involved if there is a variation in the isotopic composition of the plutonium in samples, from that used to obtain the gamma correction factor. For this reason, it is best to take a typical plutonium sample-Le., average aiiiounts of plutonium-240 and plutonium-238-to measure the correction factor. Referring to Table 11, suppose the correction factor were measured from a standard containing 4yc plutonium-240 and a sample were analyzed in n hich the plutonium mas present only as the 239-isotope. The error in the correction factor would be 10%. This would represent an error of 10% in the americium determination for the 40-p.p.m. range and 1% for the 400-p.p.m. range. This is an extreme case. Routine samples usually differ in isotopic content by much less than this. The change in correction factor with changing plutonium-238 content is even less than for a corresponding change in plutonium-240 content. For accurate determination of americium, when it is present in small amounts (parts per million in plutonium), the correction factor can be adjusted from a knowledge of the isotopic composition of the plutonium present. Absorption of americium gamma rays by the salts present in samples is much less than the corresponding absorption of alpha particles. Up to 10 mg. of such heavy elements as silver or mercury can be present in 2 ml. of solution counted in a well-type crystal before absorption of gamma rays amounts to 1%. When more than this amount is present, a correction can easily be applied by the addition of a known amount of americium to the solution as a “spike,” and recounting the gamma activity to determine the amount of
Table 111. Results of Four Methods for Determining Americium in Presence of Plutonium Found. 97, Am CY Alpha Added, Gamma pulse
%
Am a
count-
anal-
ing
100.0 100.7 83.7 81.2 67.2 68.9 50.6 51.8 33.9 34.4 20.4 20.0 9.30 9.45 4.88 4.97 2.50 2.55 0.85 0.88 0.00 0.03
ysis
pptn.
LaFS
TTA-
97.7 84.6 70.6 50.1 36.0 23.3 11.8 7.9 5.9 2.6 1.2
82.1 74.0 56.9 41.1 27.8 14.8 6.2 3.5 1.98 0.76 0.20
87.5 74.5 63.6 47.4 33.4 20.3 9.3 4.81 2.85 0.98 0.42
extn.
absorption. Although this addition of a spike increases the volume of solution, experience has shonn that addition of 50 11. of spike to 2.0 ml. of sample results in a change of the gamma count of the original sample by only 0.2%. A convenient permanent spike can be made in which a n americium source is contained in a small glass capillary a t the end of a glass rod. The glass rod containing the probe or americium sample of known gamma activity is placed in the center of the sample solution. The amount of absorption of americium gamma rays is obtained from the difference between the activity of the sample alone and of the sample plus probe, and comparing this with the original activity of the probe. A standard curre must be used to shorn the relation between gamma ray absorption from the probe and from the sample. Determination of americium in plutonium by gamma counting is rapid. I n this laboratory, alpha and gamma counting are done routinely for 10minute periods. Duplicates are run on all samples. The total elapsed time
Table IV.
for making a determination of americium in plutonium is about 1 hour. RECOMMENDED PROCEDURE
The following procedure has been used in this investigation and is recommended for the rapid routine determination of americium in mixtures with plutonium. Calibration of Gamma Counter. Solutions of americium-241, purified by ion exchange, and of plutonium purified by extraction with 2-thenoyl trifluoroacetone are prepared. The alpha activity of each solution is determined by evaporation of suitable aliquots on glass plates and counting in a counter of known geometry. Another suitable aliquot of each solution is evaporated on a glass plate and counted for gamma activity with the discriminator set a t 30 k.e.v., or slightly higher. Alternatively, a 2-ml. aliquot containing an amount of americium or plutonium, sufficient to give a suitable count rate, is placed in a test tube and counted in a well-type counter. Again, counting is done with the discriminator set a t 30 k.e.v. or slightly higher. The americium gamma specific activity for each counter used is obtained from the alpha count rate-to-gamma count rate-ratio for similar aliquots after correction for background. The plutonium correction factor is obtained from the gamma-count-to-alpha count ratio for similar aliquots of plutonium. A stardard americium source may be used t o check the performance of each counter a t frequent intervals. Routine Analysis. A suitable aliquot of sample is counted for alpha actirity, and another aliquot IS counted for gamma activity in the manner explained above. The total alpha activity equivalent to the aliquot used for gamma counting is assumed to be due to plutonium only, and, using the plutonium factor, the gamma activity contribution by plutonium is obtained. This value subtracted from the total gamma activity
Precision of Gamma Counting Method
(In per cent Am 0 to 1%-
1 to 10%
CY)
10 to 50%
Content
Range
Content
Range
Content
Range
0.034 0.038 0.035 0.064 0.091 0.090 0.175 0 201 0.436 0.493 0.806 0.821 0.827 0.927 0.989
0.9 0.6 0.6 2.4 0.8 0.5 5.8 0.4 0.2 0.3 2.1 0.5 1. 0 1.5 0.6
1.043 1.103 2.01 2.26 2.39 3.13 3.44 3.67 4.59 4.78 6.60 7.68 7.77 8.74 9.72
0.2 0.9 0.2 0.5 0.9 0.1 0.8 1.3 0.4 0.6 0.2 1 .o 0.9 0.2 0.6
10.20 10.97 14.87 18.15 19.77 21.75 22.3 24.4 25.3 27.2 27.3 29.5 37.6 39.5 43.7
0.2 0.3 0.4 0.2 4.3 0.9 0.1 0.9 0.0 1.7 0.8 0.4 0.6 0.2 0.5
VOL. 30, NO.
50 to 100% Content Range 51.6 53.5 56.1 57.2 59.9 68.0 70.7 75.9 94.1 96.9 97.2 97.8 97.9 98.9 100.0
11, NOVEMBER 1958
0.3 0.6 0.8 0.6 0.7 1.5 0.3 0.2 0.5 2.9 0.5 0.5 0.2 0.1 0.2
1761
gires the americium gamma activity, and the amount of americium present in the sample can then be calculated. LITERATURE CITED
( I ) Clifford, J. H., Koshland, D. E., Jr.,
U. S. Atomic Energy Commission, Rept. CN-2040 (1944) secret. (2) Freedman, M. S., Wagner, F., Jr.,
Engelkemeir, D. W., Phys. Rev. 88, 1155 (1952). (3) hfagnusson, L. B., Ibid., 107, 161 (1957). (4) MetZ, c. F.,ANAL. CHEX 29, 1748 (1957). (5) Miller, H. W., U.S. Atomic Energy Commission Rept. HIV-22267 (October 1951)unclassified. (6) Moore, F. L., Hudgens, J. E., Jr., AKAL.CHEV.29, 1767 (1957).
(7) Xewton, J. O., Rose, B., Milsted, J., Phil. Mag. 11, 981 (1956). (8) Shliagin, K. N., Zhur. Eksptl. i Teoret. Fiz. 30, 817 (1956); Soviet Phys. JETP 3, 663 (1956). RECEIVEDfor review A4pril 28, 1958. Accepted July 18, 1958. Work performed under the auspices of the U.S. Atomic Energy Commission.
Development and Preparation of a Set of Gamma Spectrometer Standards L. J. BEAUFAIT, Jr., E. E. ANDERSON1, and J. PAUL PETERSON Western Division, Tracerlab, Inc., 2030 Wright Ave., Richmond 3, Calif. b Remarkably few gamma emitters have decay schemes sufficiently well established to make good standards for accurate efficiency calibration. The absolute gamma-emission rate for any isotope is not generally probable known better than &5% error. From the extensive list of available gamma-emitters, eight isotopes were selected on the basis of their established decay scheme, availability, half life, and convenience of radiation characteristics: cadmium109, barium-1 33, tin-1 13, cesium1 37, sodium-22, manganese-54, zinc65, and cobalt-60. The method of calibrating each of these isotopes and the resulting data obtained on a standard [Nal(TI)] crystal scintillation counter setup are described.
P
uses of radioactive materials from nuclear reactors, cyclotrons, and accelerators have increased enormously in the past few years. Vitally important from the industrial standpoint are radioactive isotopes which emit gamma radiation, and their identification and quantitative measurement. Therefore, it is necessary to have accurately calibrated gamma spectrometer standard sources for calibrating both the energy of y-rays emitted and the total gamma emission rate. EACETIME
ESTABLISHING A METHOD OF CALIBRATION
A choice had first to be made between alternatives: whether to calibrate the standard sources in terms of absolute disintegration rates of the pure radioisotopes, or to calibrate in terms of the absolute number of y-rays of one specific energy-Le., one photopeak-each prepared source would emit 1 Present address, General Atomic, LaJolla, Calif.
1762
ANALYTICAL CHEMISTRY
per unit length of time. As it would be of more practical value to eliminate the errors involved in specific decay schemes of the isotopes, it was decided to calibrate each source in terms of its y-ray emission rate. This method was also more efficient, as the worker would not have to look up each decay scheme and make corrections in his data each time he used the source. Counting Arrangement. Figure 1 illustrates the geometrical consideration of the gamma spectrometer detector head for the initial calculation of the disk-type primary standards (Figure 3).
-4, Dumont Type 6292 multiplier phototube, 2 inches in diameter, was used, standard, but especially selected from among 30 to 40 identical tubes for its high signal to noise ratio. A standard sodium iodide (thallium-activated) crystal [NaI(Tl)] 11/2 inches in diameter and 1 inch long was o p tically coupled to the phototube with high viscosity Dov-Corning 200 silicone oil. Rigid planchet mounts using spring tension were located a t distances from the crystal as shown, to ensure reproducible geometry: Samples with or without backing can be used with this support. The entire detector assembly was surrounded with 4 inches of lead shielding and connected to a conventional 50-channel pulse height analyzer. The resolution for cesium137 y-rays under these conditions is about 9%. Resolution here is defined as the width of the 661-k.e.v. cesium137 y-ray peak a t half-height in thousand electron volts, divided by the energy of the y-ray (661 k.e.v.). Basic Calibration Curve. I n Figure 2 is seen the original calibration curve obtained using the geometrical setup shown in Figure 1 for a large number of radioactive isotopes which were used to calibrate the spectrometer. The abscissa is the gamma energy in million electron volts and the ordinate
Photomultiplier Glass
-
source I I/ 1.7-2.1 c m ' \ diameter
' ;g 1 Top Shelf
j
i
'
-
7
1
2
Bottom Shelf
Figure 1. Geometry of detector head for initial calibration of standards Photopeak counting efficiency = gamma counting rate in photopeak gamma emission rate Bottom shelf efficiency = (0.207) times top shelf efficiency
is the photopeak counting efficiency. The absolute disintegration rates of the isotopes shown were obtained by various methods. A few were primary standards obtained from the National Bureau of Standards. Some were determined by 4-pi beta counting in this laboratory. The K-electron capture isotopes of chromium-51, beryllium-7, and manganese44 were obtained from the Oak Ridge Xational Laboratory. The disintegration rates used were also determined by that organization. The smooth curve obtained is in escellent agreement with the shape of the curves found in the general literature for isotopes in the range of 0.2 to 1.4 m.e.v. At energies less than 0.2 m.e.v. the data still give a satisfactory working curve. CHOOSING ISOTOPES FOR STANDARD SOURCES
The preparation of a set of r-ray
