Direct Nondestructive Method for Determination of Ca I ifo r nium-252 Application of Prompt G a m m a Rays from Spontaneous Fission FLETCHER L. MOORE and JAMES S. ELDRIDGE Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn.
b A new direct nondestructive method for the determination of californiu,m252 is described. The method is based on gamma scintillation spectrometry involving the measurement of prompt y-rays coincident with the spontaneous fission of californium-252. The technique may be used either for the rapid qualitative or quantitative determination of californium-252 in the presence of associated fission products and actinide elements. As little as nanogram of californium-252 can be quantitatively measured. Several useful analytical and process applications of the method are discussed.
C
ALIFORNIUM-252 will be the major product of transuranium processes currently under development a t Oak Ridge Xational Laboratory. This nuclide will be produced by the intense slow-neutron irradiation of plutonium239 (?). I n the following nuclear reaction sequences, neutron capture reactions are represented by horizontal arrows and beta decays by vertical arrows : 100
I
99
98 97 96
240
242
244
246 248 250
252
254
MASS NUMBER A
Virtually no information has been published regarding the analytical radiochemistry of californium. Previous methods (6) for californium-252 are based on classical ion exchange separation techniques followed by a-particle or fission-fragment counting. While such methods are highly sensitive, they are tedious, time-consuming, and not quantitative because of the stringent requirement to produce solid-free plates for counting. This is of particular concern a t this laboratory, inasmuch as the processes employ highly concentrated 808
ANALYTICAL CHEMISTRY
salt solutions. I t occurred to the writers that the prompt y-rays accompanying the spontaneous fission of californium-252 offered the interesting possibility of a direct nondestructive method for its determination. This paper reports some recent work in this area. Californium-252, discovered in 1952 (S), has a half life of 2.55 + 0.15 y and emits a-particles of 6.11 and 6.07 m.e.v. I n addition to the a decay mode, there is -3.1% spontaneous fission branch in this nuclide’s decay. The y-ray spectrum in coincidence with spontaneous fission has been reported (1) to extend to about 8 m.e.v. with an average energy of 1.2 m.e.v. Each fission event yields an average of 10.3 (8) to 12 ( I ) y-photons. EXPERIMENTAL
Californium-252 Stock Solution. Californium-252 was purified by conventional ion exchange and solvent extraction methods described adequately elsewhere (6). The stock solution was contained in 1M nitric acid and contained 6.9 X lo2 a-counts per second per ml. One milliliter of this solution was used as a standard to calibrate the y-spectrometric equipment described below. The y-ray spectrum of the stock solution is shown in Figure 1. Counting Equipment. a-Counting was done using an internal sample, methane proportional chamber. Suitable aliquots were evaporated on 115/16-inchdiameter stainless steel plates, flamed to a dull red heat, cooled to room temperature, and a-counted a t 50,7y0 geometry. A voltage setting of 2100 volts was used for a-counting. Fission-fragment counting was done on the same proportional counter a t a voltage setting of 1400 volts to discriminate against a-particles. y-Scintillation pulse height analyses were performed with a 3 X 3 inch NaI(T1) detector and 512-channel pulseheight analyzer. One-milliliter samples were placed directly on a 1.23 gram per sq. cm. beta absorber covering the top of the detector for all standard spectrometric determinations. The beta absorber was 0.7-cm. thick and the liquid height was approximately 2.2 cm. The effective distance from the face of the
NaI(T1) detector was therefore -1.8 cm. All y-ray spectra were determined with a group size of 256 channels and an energy calibration of 20 k.e.v. per channel. It was possible, therefore, to cover the energy range from -0.02 to 5.2 m.e.v. in one counting interval. For the determination of californium252 with the spectrometer, the y-ray spectrum was examined on the oscilloscope display, and an integral count was taken over a region of the spectrum above any possible interference from all other y-emitting nuclides. This integral was typically 2.8 to 5.1 m.e.v. for fission product mixtures and 0.8 to 5.1 m.e.v. for actinide fractions. Recommended Procedure. Pipet a 1-ml. aliquot of the sample solution into a 10 x 75 mm. culture tube. Insert cork and place directly on a 1.23 grams per sq. cm. beta absorber covering the top of the sodium iodide detector. Set the discriminator so that only y-rays with energies above 2.8 m.e.v. are counted. Count for 1000 seconds or until at least 1 X l o 4 net y-counts have been counted. From this californium-252 y-radioactivity, use the calibration factor to calculate the californium-252 content of the sample. RESULTS AND DISCUSSION
A typical y-ray spectrum of californium-252 is shown in Figure 2. One milliliter containing 1.38 X lo3 disintegrationsper second of californium252 (6.7 X 10-5pg.) was measured using the 512-channel pulse-height analyzer. The prompt y-rays accompanying the spontaneous fission of californium-252 are seen to be continuous in nature and decrease nearly exponentially a t energies greater than about 1 m.e.v. Figure 3 shows the y-ray spectra of a typical fission product mixture and californium-252. It is clear that one may count the y-rays from californium252 with no interference from fission products or transuranium elements by using integral counting above 2.8 m.e.v. Figure 4 shows the y-ray spectra of californium-252 with cerium-praseodymium-144-a typical long-lived fission product pair with high energy y-rays. Figure 5 shows the 7-ray spectra of
3 i r . x 3 in. N a I ( T I ) Ablorber, 4230 mg/cmZ 25 ml in 2in.diamefer bottle on absorber Energy scale =20 Kev/channel
40’
too
CHANNEL NUMBER
40’ CHANNEL NUMBER
Figure 1. y-Ray spectrum of Cf252stock solution (3.5 X disintegrations per second)
1 O4
Figure 2. ?-Ray spectrum of 1.38 X per second Cf252calibration standard
lo3 disintegrations
EVALUATION O F THE PROCEDURE
californium-252 with lanthanum-140a typical short-lived fission product with high-energy y-rays. Figure 6 shows the y-ray spectra of californium-252 with a typical transuranium mixture. Figure 7 shows the y-ray spectra of a composite of califomium-252 with a typical fission product and transuranium mixture. The spectra shown in Figures 3 through 7 indicate that there is a region in all these spectra in which californium252 is present with no interference from y-rays of other nucli3es. By examination of Figure 1, ii; is obvious that californium-252 may be determined with a higher sensitivity by using a lower threshold for the integral count. The recommended procedure described above specifies l-ml. aliquots because usually this is a convenient volume. Obviously, one may use various sample sizes or containers if he is careful to calibrate similarly with a californium-252 standard.
The proposed y-spectrometric method has certain advantages over other possible methods for the determination of californium-252. Alpha and fission-fragment counting methods require the tedious preparation of solidfree plates. Neutron counting (another nondestructive assay method) is not specific for spontaneous fission events in the presence of high concentrations of a-emitters. a-Neutron reactions on light elements, such as Li, Be, B, etc., produce counts indistinguishable from spontaneous fission neutrons. In addition, most neutron counting systems are sensitive to 7-radiation as well as neutrons. The sensitivity of many neutron counting systems is somewhat less than appropriate y-counting systems. Another advantage in the use of direct y-ray spectrometry for the determination of californium-252 is that the technique allows the simultaneous inspection of fission products and other y-emitting nuclides in mixtures.
Solutions containing known amounts of californium-252 mixed with typical fission products and actinide elements were analyzed by the direct y-counting technique (see Recommended Procedure). Compositions of the solutions are given in Figures 3 through 7 . The values listed in Table I give the counting rates and standard deviations due to counting statistics. INTERFERENCES
The method is virtually specific for californium-252. Potential interferents are those nuclides of appreciable halflives which emit y-rays greater than 2.8 m.e.1’. The only known interferent is californium-254, which decays almost solely by spontaneous fission ( 2 ) . In all productions to date, the yield of californium-254 is relatively low. Its short half life (61.5 days) combined with low yield does not constitute a problem currently. However, it has been calculated (4) that appreciable amounts of
Io3F -
CALlFORNlUM-Z52+ C e - P r - ( 4 4 ~.
I
1
I
-
Absorber, 4230 rng/cm2 Source disfonce, I 8 c m Contribution due lo Ce-PI-144
CHANNEL NUMBER
lo3
Figure 3. ?-Ray spectrum of 1.38 X disintegrations per second of Cf252arid 1 X 1 O4 disintegrations per second of mixed fission products Contribution due to the mixed fission products is shown in the dashed curve
CHANNEL N U M B E R
Figure 4. ?-Ray spectrum of source containing 1.38 X 1 O3 disintegrations per second of Cf252and 4 X 1 O4 disintegrations per second of Ce-Pr144 Contribution due to Ce-Pr144is shown in the dashed curve
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CALIFORNIUM 2 6 2 + L a -440 31n.x 3 1 n N a I ( T I ) Absorber, 1 2 3 0 mg/cmz Source distance, - 1 S c m Energy scale = 20 Kev/channel Contribution due to L a - 4 4 0 ~
3 i n . x 3in. N a I ( T I 1 Absorber, 1 2 3 0 mg/cm2 Source distance, - 4 . 8 c m Energy scale = 2 0 Kev channel Contribution due to transuranium mixture
40
0
80
CHANNEL NUMBER
Figure 5. y R a y spectrum of 1.38 X lo3 disintegrations per second of CfZ5'-and 3 X 1 O3 disintegrations per second of La'40 Contribution due to
is shown in the dashed curve
californium-254 will be present in future production methods. Depending on the cooling time, one will have to consider this possibility. In many instances for process control, the determination of the total californium fission rate will suffice. When future developments do produce appreciable amounts of californium-254, its presence can be detected in shortcooled samples by fission decay studies and careful measurements of fission to a-ratios ( 4 ) . All measurements were made with the sample on top of the detector. Use of a well-type 7-scintillation counter may occasionally be desirable by various investigators. While this technique has not been evaluated, one would predict a somewhat higher bias would be necessary because of coincidence summing effects of cerium-praseodymium144 and barium-lanthanum-140 nuclides. The radioactivity level of the unknown sample should be below the level which produces appreciable random summing. Typically, the dead time of the analyzer system should not exceed 20 to 30%. A visual observation of the oscilloscope display will indicate the occurrence of summing effects. The proper bias level required for comparison of the standard and unknown samples can be ascertained from the oscilloscope. Table I.
+
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ANALYTICAL CHEMISTRY
200
240
Figure 6. y R a y spectrum of 1.38 X 1 O3 disintegrations per second of Cf252and 4 X 1 O4 disintegrations per second of mixed actinides (PU'-~~, P U ' - ~ Am241, ~, and Cm244) Contribution due to the mixed actinides i s shown by the dashed curve
!02
Source distance, -1.Ecm
CHANNEL NUMBER
Figure 7. y R a y spectrum of 1.2 X 1 O3 disintegrations per second of Cf252and 4 X 1 O4 disintegrations per second of a composite made from a mixture of actinide and fission product nu:lides Contribution due to the composite is shown b y the dashed curve
It should be pointed out that several years from now relatively large amounts of some heavy element nuclides, such as curium-244246-248, which have a small spontaneous fission branch will be produced. While these spontaneous fission half lives are very low, in certain cases these spontaneous fission gamma rays may interfere with the californium-
Direct Determination of Californium-252 in Presence of Other Radioelements
Solution Cerium-144-praseodymium-144 Lanthanum-I40 Mixed fission products Mixed actinide elements Composite (fission products actinide elements)
!2 0 460 CHANNEL NUMBER
Cf252Added, 7-Cps. 1.52 f 0.05 1.56 rt 0.05 1.56 f 0.05 18.42 f 0.15
Cf252Found, 7-Cps. 1.47 & 0.04 1.61 f 0.05 1.56 & 0.05 18.62 + 0.15
1.41 + 0.04
1.40 f 0.04
252 determination. The radiochemist should be aware of this future situation, which can be evaluated only when large amounts of these nuclides are manufactured. A word of caution is in order regarding the future uses of strong sources of californium-252 in which the sodium iodide detector may be activated to produce iodine-128 and sodium-24. A background determination will indicate any potential interference of this type. Actually, such high strength californium-252 samples will be diluted prior to analysis because of the neutron health hazard. APPLICATIONS
The technique described offers a variety of useful applications both to the analytical radiochemist and process chemist.
RAPID QUALITATIVE TEST
Californium-252 mrty be readily detected qualitatively by observation of its continuous heterogeneous spectrum out to about 5 m.e.v. The typical spectrum (Figure 1) is quite obvious in the presence of all the fission products, particularly a t energies greater than 1 m.e.v. QUANTITATIVE DETEIMINATION O F CALIFORNIUM-252
The direct nondestructive quantitative determination o ‘ californium-252 is easily performed by calibration of the counter with a suitable standard. The following three categories demonstrate the wide applicability of the method. 1. Determination of Californium252 in Presence of IFission Products and Actinide Elements. This method should find considerable application by the analytical rac iochemist, process chemist, and engineer in transplutonium work. Sensitivity depends o n the situation (see 1)iscussion) but is at least nanogram of californium252. For the analyt cal chemist, the method allows the rapid analysis of large numbers of samples and eliminates much handling of h ghly radioactive
materials. For the processor, future applications doubtless will be found in monitoring for californium-252 in various production processes. One of the most practical applications of the technique may be found in monitoring for californium in salt deposits or man-made ores resulting from underground nuclear explosions (6). y-Scintillation spectrometry will be particularly valuable for following the fate of californium because absorption of the high energy y-rays is negligible. Thus, it may readily be detected in various salts, vessels, pipes, precipitates, solutions and resins-a technique which can be predicted to have growing uses as the chemist moves into this new area. 2. Determination of Californium252 in Actinide Group. The actinide group separation from other elements is a key step in purification schemes. After this group separation, the californium-252 may be readily determined by the method described. In this case the sensitivity is increased considerably because of the lower bias voltage which may be used. The sensitivity (radioactivity equivalent to 0.1 background counting rate) in this instance is a t least 7 X nanogram. 3. Californium-252 as Convenient Tracer in Physical, Chemical, and Engineering Studies. One of the most valuable immediate applications
of the method is found in the use of pure californium-252 in various research studies. The simple measurement of the gross y-radioactivity using a well-type counter greatly facilitates such research by eliminating the conventional tedious preparation of solid-free plates so necessary for a-particle or fissionfragment counting. LITERATURE CITED
(1) Bowman, H. R., Mann, L. G., Phys. Rev. 9 8 , 2 7 7 ( a ) (1955). (2) Chart of the Kuclides, General
Electric Co., Schenectady, N. Y., sixth
ed. (19611. ( 3 ) Fiklds, P. R., etal., Phys. Rev. 102, 180 (19.56) \ - - - - I .
(4) Fields, P. R., Diamond, H., U . S. A.E.C. Rept. TID-17527(1962). (. 5.) Hiarrins, G. H.. “The Radiochemistrv of tK< Transcurium ElementP,” NAsXS-3031 (1960). Available from Office of Technical Services, Department of Commerce, Washington 25, D. C. (6) Hoff, R. W., Dorn, 13. W., University of California,Unclassified Rept., UCRL7347 (1963).
( 7 ) Seaborg, G. T., Physics Today 15, 19 (1962). (8) Smith, A. B., Fields, P. R., Friedman, A. M., Phys. Rev. 104, 699 (1956).
RECEIVED for review October 23, 1963. Accepted December 20, 1963. Oak Ridge Sational Laboratory is operated by Gnion Carbide Corp., Nuclear Division, for U.S. Atomic Energy Commission.
Combined Radioc hemica I-N eut ron Ac tiva ti o n An a lysis Method for the Determination of Sulfur and Phosphorus in High-Purity Paper and Beer ANTHONY G. SOULlOTlS Chemistry Department, Nuclear Research Center Democritus, Athens, Greece
b Sulfur and phosphorus were evaluated by a mathematical treatment of the specific activities, expressed as P32, of analyzed materials and of standards after a dc’uble irradiation with and without cadmium protection of the targets. After demineralization of the sample by treatment with acid and evaporation of the whole solution to dryness, lphosphorus was extracted with ether containing 5% sulfuric acid, and then precipitated as ammonium magnesium phosphate followed by a beta coilnt of P32. Numerical data were obtained for American and Greek papers and for beers of different types and origin. The sensitivity of this method reaches the value of 10-3 p.p.m. for sulfur and phosphorus determinations.
materials: phosphorus was determined in iron (8); sulfur and phosphorus were determined in high purity alumilium (1, IS); sulfur was determined in chromium and arsenic using a double irradiation technique (7) ; phosphorus was determined in biological material ( 4 ) ; sulfur and phosphorus were determined in meat (11); sulfur and phosphorus were determined in steel ( 3 ) ; and phosphorus was determined in aluminum-silicon alloys (2). As an extension of these studies, and also using neutron activation analysis, the above mentioned elements were determined in high purity paper and beer using double irradiation techniques and radiochemical separations.
N
Nuclear Reactions. On irradiating any material with neutrons, various nuclear reactions take place, those in Table I being of interest for sulfur and phosphorus determination:
analysis has been used by several investigators for the quantitative determination of sulfur and phosphorus in a variety of EUTRON ACTIVATI~N
EXPERIMENTAL
All of the reactions in Table I result in fi- radiation. Although the activation cross section of S34 was higher than that of S32, reaction (11) was used because the abundance of S3*was higher than that of S34. Reaction (I) can be eliminated to a great extent by cutting off the thermal neutrons by means of 1-mm. cadmium foil. Reaction (111) interferes (6) but this interference is negligible and does not exceed 1% even though the quantity of chlorine is one third that of the sulfur (3, 7). Reagents. P3* RADIOTRACER. Radiotracer techniques were used to investigate the different analytical steps of this experiment and to evaluate the chemical yields. Johnson-hlatthey spectrographically pure diammonium phosphate was irradiated and a 50 mc./ml. solution was prepared; Johnson-Matthey spectrographically pure ammonium sulfate was also used. Procedure for High Purity Paper. DEMINERALIZATION OF PAPER. The paper was heated with nitric acid in a flask equipped with reflux conVOL. 36,
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