Activation Analysis with Californium-252 Enzo Ricci and T. H. Handley Analytical Chemistry Diaisiori, Oak Ridge Natiorial Laboratory, Oak Ridge, Teirii. We show that a W f spontaneous-fission neutron source placed in the center of a 1-foot cube of moderator can be considered a portable, maintenance-free, quasi reactor by the activation analyst. One such source was calibrated by activation to have a 4n neutron emission intensity of 8.6 x l o 8 sec-’--i.e., 0.37 mg 25ZCfthe thermal, epithermal, and fast neutron fluxes in the moderating cube were 9.7 X lo6, 9.4 x lo4, and -108 cm-2sec-1 per mg 252Cf,respectively, and the cadmium ratio 7.6. The average cross section for the reaction 27Al(n,cu)24Na was calculated for the IZ2Cf neutron spectrum to be 1.35 mb; use of analogous values, corresponding to the 2aW fission spectrum in 2%f calculations is shown to result in serious error. A pneumatic activation-analysis system was built; interference-free sensitivities and detection limits, determined for 13 elements of geological and biomedical interest, were found quite satisfactory for activation analysis. The system was also reliable and had good irradiation and counting-position reproducibilities. As a consequence, a mobile, power-free, maintenance-free, sensitive and reliable activation analysis unit is suggested for field work and discussed. Increasing demand is also predicted for more and larger *j2Cfsources for use both in American biomedical centers and small universities, and in research institutes of other countries, where maintenance of a nuclear reactor is not economically feasible.
ACTIVATION ANALYSTS have always been attracted by isotopic neutron sources (Ref. I , p 44) because of their simplicity and maintenance-free operation. However, the relatively low flux, large volume, and/or short half-life of these sources have severely limited their use so far. The increasing availability of 252Cf,first discovered in 1950 (2), has clearly changed this situation. Seaborg (3)predicted the successful application of the 25Cf “intense, concentrated source of neutrons” in activation analysis. Indeed, the nuclear properties of this nuclide are ideal for a neutron source with all the advantages and none of the drawbacks of the conventional isotopic sources ( 4 ) . It has a comfortably long half-life of 2.646 yrs., decaying both by a-particle emission (96.9 %) and by spontaneous fission (3.1 %); the average yield of 3.802 neutrons per fission (5) results in a substantial neutron output of 2.34 X 1012sec-1per gram 25*Cf. Figure 1 shows that the maximum intensity in the *;*Cf fission spectrum corresponds to -0.7 MeV neutrons. Upon moderation these fast neutrons yield a respectable thermal flux. Further, the shape of the neutron spectrum near a 25*Cf source placed in the center of a mass of moderator resembles that near the core of a nuclear reactor (Ref. I , p 15), as Figure 2 shows. In short, the system could be considered a portable, maintenance-free, quasi reactor for activation analysis. Our study was aimed at finding a useful setup and condi(1) “Guide to Activation Analysis,” W. S. Lyon, Jr., Ed., D. Van Nostrand Co., Inc., Princeton, N. J., 1964. ( 2 ) S. G. Thompson, K. Street, Jr., A. Ghiorso, and G. T. Seaborg, Phys. Rec., 78, 298 (1950); ibid.,80, 790 (1950). ( 3 ) G. T. Seaborg, “Man-Made Transuranium Elements,” PrenticeHall, Inc., Englewood Cliffs, N. J., 1963, p 67. (4) W. C. Reinig, Nucl. Appl., 5 , 24 (1968). ( 5 ) E. K. Hyde, “The Nuclear Properties of the Heavy Elements111 Fission Phenomena,” Prentice-Hall. Inc., Englewood Cliffs, N. J., 1964, p 217. 378
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
tions of operation for a system of this kind, establishing its analytical reliability and sensitivity, and determining the areas where its application can be most efficient and successful. This work was a joint project of the Analytical Chemistry Division and the Transuranium Research Laboratory of O.R.N.L. EXPERIMENTAL Instrumentation. Because of the experimental nature of our program, we require considerably more flexibility and versatility in our equipment over facilities specifically designed for routine operation. Figure 3 shows our setup schematically. An approximately cubic dry box, 107 cm o n the side, contains two irradiation devices. One is a 30.5 cm high-density polyethylene cube capable of producing an optimum thermal neutron flux when the 252Cfsource is at its center (Figure 2 and Ref. 6 ) ; the other is a conventional Kaman Nuclear dual-axis rotator for irradiations in the nonhomogeneous, fast, fission-neutron flux. Two samples (one of them may be a comparator or standard) can be placed into the loading station and sent through the pneumatic system to be simultaneously irradiated in either of these devices; a switch directs the pair of samples to the chosen terminal. The total “rabbit” path of 34 m was traveled in 8-10 sec by the capsules; it can be easily shortened by relocating the loading-counting station if short-lived radionuclides are sought. After bombardment the samples are counted in succession in the counting station. The irradiation and counting times can be preset and their sequence programmed for automatic operation. To shield the gamma rays and neutrons emitted by 252Cf, the dry box is surrounded by 91-cm cubic concrete blocks on all sides and 31-cm blocks above. The cell features a zinc bromide window, two master-slave manipulators, and an access glove-box to take equipment into or out of the dry box. Figure 4 is an internal view of the cell, taken through the zinc bromide window when the ?;*Cf source holder [Teflon (Du Pont) parallelepiped] was being held by one manipulator. The rotator and its “rabbit” terminals are shown in the foreground, with the stainless steel sheath where the source (i.e., the Teflon block) is inserted for the irradiations. The high-density polyethylene cube and its “rabbit” terminals are shown in the back. The pipes stop at the center of the cube providing one of the inner walls of the slot (see Figure 5) into which the Teflon block slides for the irradiations; thus, the operation is similar to that at the rotator. As Figure 5 shows in detail, during bombardments the source is at the center of either the rotator (fast neutrons) or the cube (thermal, epithermal, and fast neutrons), closest to the samples; the latter are contained in 2-cm3 polyethylene vials. Figure 6 is a close-up of the source and holder; the 25ZCf is located at the nonthreaded tip of the source capsule which, after insertion in the Teflon parallelepiped, appears on its back side as shown in Figure 4. The counting system (Figure 3) consists of two 12.7 cm X 12.7 cm NaI(T1) integral-line detectors, located face to face (3 cm apart) to enhance sensitivity, and a multichannel analyzer. The detectors are housed in a conventional twodoor lead shield 7.6 cm thick, lined with copper and cadmium sheets on the inside. The resolution of this system was found to be 9.4% for the 0.662 MeV peak of laiCs. The counting efficiency is 0.44 photopeak-area counts per photon (0.412 (6) J. P. Nichols. Niicl. Appl., 4, 382 (1968).
iot,
I
,
I
,
I ,105
CI
E
5
-
z 0 z J
a
s "
'' N o I l T h l
DETECTORS
0
z
0
u 2 4 6
8
io
NFUTRON . . IIN E R G Y ,
12
i
MeV LOADING-RECEIVING STATION,
Figure 1. Spontaneous-fission neutron spectrum VL -Lf, thermal-fission neutron spectrum of T J , and excitation function for the reaction 2'Al(n,a)s4Na from Refs. (7,s)
Figure 3. Schematic view of the x9Cf neutron activation facility
IO"
doIO*
10' 10'.
10
Figure 4. Interior of the 252Cfcell
NEUTRON ENERGY, MeV
Figure 2. Neutron suectrum at 0.86 cm from a 1cm3 spherical zszCfsource centered in a 20-cm radius polyethylene sphere (density 0.961 g/cm3). Histogram calculated from Ref.
(a,
MeV y-ray of loSAu),ie., about 3 times that of a standard 7.62 cm X 7.62 cm NaI(T1) detector. The natural background shows peaks at 1.46 and 2.62 MeV, attributable to "OK and 2aT1p S T h decay chain), respectively, and was subtracted from all the measurements. Determinations. Two main objectives were pursued in these experiments. First, as the two samples follow independent paths through the rabbit system, both should take geometrically and physically equivalent positions for irradiation and for counting. Thus, reproducibility of results in each of the rabbit stations was studied and comparisons of results from the two pipelines were made. Second, parameters characteristic of the 2SZCfsource and of the facilities were determined: (a) source strength; (b) cadmium ratio, and thermal, epither-
mal, and fast neutron fluxes; (c) activation analvtical sensitivities for a number of elements. NEUTFION ACTIVATION WITH CALIFORNIUM-252 "
.
.
"~"-"
. ,.
. -,.
As the shape 01tne "'LI neurron spectrum at m e center 01 the high-density polyethylene cube should be quite similar t o that of a nuclear reactor core (see above), we can define for the cube a thermal, a n epithermal, and a fast neutron flux analogous to those of a reactor. Naturally, the neutron energy distribution near the source at the rotator, or anywhere else outside the plastic cube, should approach the unperturbed 252Cf fission-neutron spectrum (Figure 1). The total activity A induced by (n, y) reaction and the cadmium ratio CdR are (Ref. I, pp 17-20), respectively, A = Am
+ A,
PrNALYTlCAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
(9 379
TOP
SIDE VIEW
VIEW
1.3cm
'"RABBIT" TERMINALS
Figure 6. 2 T f source and Teflon (DuPont) holder
H 2.0cm
:es
$(E) = 0.63589 E'lz exp (-E/1.466). Serious error may be introduced if any of the numerous b data, readily foundin theliterature for the 235Ufission-neutron spectrum, is used for simplicity in calculations related to 25Cf experiments. Figure 1 shows the zasUspectrum, for which the accepted 8 for AI + *dNa is 0.57 mb, Le., less than half of the 1.35 mb value calculated for the s52Cf spectrum; this comparison and the obvious differences between the two spectra eloquently support the above statement. To calibrate the 25zCfsource, the absolute 24Naactivity induced in a thin, pure aluminum disk (0.4 g) was measured. During the irradiation the source was in the Teflon block, outside the plastic cube, and the disk was facing the 2S2Cfside of the source capsule (Figure 4) at 10 cm from it. By Equation 4 the total (4r) neutron output, Le., the neutron intensity (njsec or sec-1) of the source is I
=
4nr2$, =
4arZA, ~
$A-
+,
(7)
where r is the distance source-disk (cm), and N are, respectively, the integrated fast-neutron flux and the number of 2'Al nuclei at the disk, and 8 is 1.35 mb (see above). Using Equations 4 and?, we found our 4 s C f source to have a n intensity of (8.6 0.9) X lo8 sec-', i.e., 0.37 f 0.04 mg z5zCf,on March 10,1968. The analytical sensitivity data were also determined near this date. The error is mainly due to uncertainties in the geometrical location of the * W f within the capsule (i1 mm) and to the absolute activity calculation. Flux Measurements. In view of the above uncertainties, no accurate measurement of fluxes was attempted. However, approximate values were obtained to correlate with the sensitivity determinations, T o measure qh,$, and CdR the reaction " ' A ~ ( n , y ) ' ~ ~ A u was induced in bare and cadmium-shielded gold foils in the plasticcube. The activities.4 a n d & respectively, wereso ob-
*
380
*
ANALYTICAL CHEMISTRY, VOL. 42. NO. 3, MARCH 1970
Table I. Reliability and Reproducibility Tests of 25zCfFacility for Activation Analysis Reaction used and/or nuclide counted 137cs
Average total counts ... 77,856 B 77,232 Moderated 27Al(n,y)BAl A 5 31,104 B 5 31,250 Fast z7Al(n,p)27Mg A 4 11,330 B 4 13,892 Fast 27A1(n, p) z7Mg A 5 20,502 B 5 20,289 A and B correspond to the positions for sample and standard, respectively, in each of the facilities.
Facility Counting station Plastic cube Plastic cube Rotator
Neutrons
Position" A
tained, and Equations 1-3 solved with the known data of N , u t h (98.8 barn), and I, (1550 barn) for the foils. The results for @ l h and & are (9.7 f 1.0) X lo6cm-2sec-1 and (9.4 f 0.9) x l o * cm-2sec-1, respectively, for a 1 mg *j2Cfsource, with CdK = 7.6. The errors are largely due t o uncertainties in the calibration of the ze2Cfsource. Aluminum-28 is induced in 2iA1 at the rotator, but with a yield 59 times smaller than at the plastic cube. Neutron moderation in the Teflon block is believed responsible for this small (n,r)activation effect, and considered insufficient to perturb significantly the fission-neutron spectrum (Figure 1) near the source assembly. Because of the rotator's manufacture, geometrical reasons (Figure 5) cause fast neutron activation measured by the reaction 2 7 A l ( n , ~ ) ~ ~ M t o gbe, 2.1 times more effective in the plastic cube. The integrated fast neutron fluxes, 95, were not determined experimentally but simply calculated by Equation 7 for the calibrated 252Cfsource; they were approximately 8 and 4 x lo7 cm-2 sec-1 per mg *j2Cffor the plastic cube and the rotator, respectively. RESULTS AND DISCUSSION
Reliability and Reproducibility Tests. Flux uniformity was expected to be quite limited in both irradiation facilities. Thus, sample position reproducibility was checked for each of the rabbit lines at the counting station and a t the two irradiation sites. I n these experiments, either a I 3 C s source was sent through the pneumatic system, recovered, and counted (counting-position tests) or an aluminum sample was delivered, irradiated, and counted (irradiation-position tests). Determinations were repeated a number of times for each of the rabbit lines at each of the facilities. Standard deviations for sets of similar determinations were obtained as a measure of reproducibility; also, comparisons of result averages from equivalent positions in both rabbit lines were made t o measure discrepancies. The aluminum samples subjected to activation occupied the entire volume of the rabbit capsules; thus, they were able to detect flux nonuniformities within this full volume. Table I lists details and results of these experiments. The number of counts was always sufficiently large t o make statistical variations negligible. In the irradiation-position tests, the activated samples were counted always in the same position of the counting station regardless of the rabbit line used for the bombardment, to avoid error compounding. The counting results show high reproducibility for both rabbit lines and insignificant discrepancy between them; in fact, discrepancies can be minimized by manually varying the location of the counting station between the crystals (Figure 3). Though only limited efforts were devoted t o guarantee flux spatial
No. of tests 7 I
Std dev, 0.24 0.29 1.2 1.5 4.0 5.1 3.3
x
Discrepancy between averages of A and B, % 0.80 0.46 20.3 1.0
5.5
homogeneity at the plastic cube facility, results for moderated neutrons are also quite satisfactory. This performance may be attributed to a homogeneous, relatively large cloud of neutrons of constant energy distribution, caused by the moderator near the source. As expected, this is not the case for fast neutrons in the same facility because of the critical geometry dependence of the fast-neutron flux near the 252Cfsource. However, this flux discrepancy can be easily determined in each case and results corrected in accordance; sample and standard can also be irradiated successively in the same position t o circumvent this problem. Finally, the performance of the rotator is rather disappointing. Though the discrepancy between its two positions is quite acceptable, the reproducibility is not better than that for fast neutrons in the plastic cube, where the fast flux is greater. Sensitivities. To assess in general the usefulness of 262Cf neutrons in activation analysis, a survey of interference-free sensitivities and detection limits was made at our facilities for 13 elements of interest in geology and biomedicine, The results are listed in Table 11. Net photopeak areas in cpm were obtained by first subtracting the natural background, and then subtracting the counts under the line of the valleys from the total peak counts. Detection limit is defined as the amount of element capable of yielding a net-peak count equal t o the natural background integrated within the peak channels. Data calculated for a 10-mg z5zCfsource are given to facilitate conversions. The results show that 262Cfis a very satisfactory source for activation analysis. Even with a 1-mg source, most elements can be detected at the O . l - l % level in 1- t o 10-g samples; particularly outstanding are gold and maganese which could be detected at the p . p m range in these conditions. Some elements not determinable by moderatedneutron activation display reasonably good sensitivities for fast neutrons, thus showing the versatility of the 252Cfsource. CONCLUSIONS
These experiments demonstrated that 252Cfis an ideal isotopic source for neutron activation analysis. Its long half-life, high neutron output, and small size make it much more efficient than conventional neutron sources, while retaining the great advantages of maintenance-free operation and portability. *>2Cfis expensive ($1000 per microgram) and thus, its real present usefulness is for determinations of elemental constituents at the per cent level. Fortunately, this is precisely the level required for field applications (e&., geology and mining) where portability is mandatory and freedom from maintenance highly desirable. Approximate calculations, based o n the performance of our shielding, indicate that a 4-
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
381
~~~
Element 0
Na (NaCl)
~~
~
~
Table 11. Interference-Free Sensitivities and Detection Limits for 252Cf-Neutron Activation Analysis Sensitivity,a Detection limit,* (cpm/mg element) (mg element) Reaction ?-Ray 0.37 mg 10 mg 0.37 mg 10 mg Half-life measured, MeV 252Cf Z59Cf 252Cf 25 2Cf and product ... . . . ... ... Nil ... ... 15.0 hr 1.37 3.4 x 103 3.2 0.12 23Na(n,y)z4Na 124
Fast n ... ... Nil ... ... ... 27Al(n,y)28A1 97.7 2.31 min 1.78 2.6 x 103 1.9 0.068 27A1(n ,p) *’Mg 24.8 670 9.46 0.84 1.01 35 1.3 1.37 27Al(n,a)24Na 15.0 hr 49 1.83 220 8.0 Nil Moderated n Si ... ... ... 2.31 min 1.78 Z8Si(n,p)2sAl 770 28.5 6.4 0.23 2.58 hr 55Mn(n,y)56Mn 0.21 Mn 2,570 0.847 0.0075 7.0 x 104 ... ... ... ... ... Nil Fast n Fe Nil ... ... ... ... Moderated n 2.85 2.58 hr 77 6.8 KGFe(n,p)56Mn 0.847 190 1.48 -500c ~ 0 . 9 ~ -2oc 2.56 hr -2oc 4Ni(n,y)6aNi Ni 0.511 3.1 cu 12.8 hr 0. I 1 63C~(n,y)W~ 4 . 5 x 103 167 13.8 hr e8Zn(n,y)6grnZn 0.439 230 Zn 150 8.3 5.56 1.11 G4Zn(n,y)eKZn 240d 1. 67d 8.8d 245 d 45d 0.511 2.77 6.8 12.8 hr 64Zn(n,p)64Cu 190 75 2.42 min lo7Ag(n,y)lo*Ag 0.632 1 . 3 x 103 15 48.4 0.54 Ag ... Nil ... ... ... ... Fast n lTt(n,y)199Pt 6.32 Pt 31 rnin 8.6 0.540 170 240 Au 0.412 0.11 1g7A~(n,y)198A~ 2.70 d 2.6 x 105 0.0042 9,650 ... ... ... ... ... Nil Pb Thermal n za4Pb(n,n’)zo 4mPb 0.90 3700 140 3.1 0.113 66.9 min a Net photopeak counts obtained between two 12.7 cm X 12.7 cm NaI(T1) detectors, 3 crn apart, at end of one-half-life bombardment. All results determined at the plastic-cube facility. Detection limit = (cpm natural bckgd. in peak channels)/(sensitivity). Uncertainty due to 0.24% Mn found in Ni sample by 252Cfneutron activation analysis. 7-day bombardment. A1
+
.
.
I
.
.
. . I
I
ton truck can carry enough concrete to shield a 3-mg 2jTf source. If more efficient shielding were used (e.g., lithiated polyethylene or water, steel, etc.), the amount of 2S2Cfon the truck could be substantially increased, with proportional enhancement of activation-analysis sensitivities. Detection limits for a IO-mg source (only a factor of 3.3 increase) in Table I1 are generally very attractive. In view of the excellent moderated-neutron flux homogeneity found in the high-density polyethylene cube, a similar twocapsule (or multi-capsule) irradiation setup is suggested for future fixed and mobile facilities for routine work. Naturally, the shielding should be placed immediately outside the plastic cube, t o reduce weight, and simple gravity would be preferred t o compressed air for operation of the rabbit. A rotator is not advisable, particularly in a mobile 2j2Cf activation analysis unit, because of the inherent power and space requirements and relatively low flux. We have shown that the za2Cfsource provides a reactor-like neutron flux distribution when used in the center of a mass of moderator. The vast experience already accumulated o n reactor-neutron activation analysis can thus be directly applied t o 252Cfneutron bombardments. The convenience of the steady state operation of reactors for long (overnight or weekend) irradiations, us. the intermittent performance of neutron generators, is also retained and even enhanced by 252Cfsince, unlike reactors, it needs neither power nor attention. At present, however, the neutron fluxes attainable with 252Cfare several orders of magnitude smaller than those furnished by nuclear reactors. Yet, features of the 252Cfsource (added t o predictable price reductions) may prove invaluable for small universities and for most hospitals and biomedical centers where maintenance and qualified attendance problems weigh against the purchase of a nuclear reactor for activation analy382
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ANALYTICAL CHEMISTRY, VOL. 42,
NO. 3, MARCH 1970
sis. The great interest expressed in less developed countries for 2ezCfmay arise from similar reasons. Because of these foreseeable uses, the demand for more 2j2Cfsources of increasing size is expected to rise sharply. Meanwhile, it is hoped that 2j2Cf will become more available and less expensive by progressive improvements in its manufacture. ACKNOWLEDGMENT
We thank 0. L. Keller and C. E. Bemis for meaningful discussions and for their help and permanent, affable encouragement. Thanks are also due to E. L. Earley and his crew for efficient installation and modifications of the facilities, and to C. E. Haynes for health physics measurements. RECEIVED for review September 11, 1969. Accepted January 12, 1970. Research sponsored by the U. S. Atomic Energy Commission under contract with Union Carbide Corp.
Correct ion Levelhg Effect of Lithium(1) on the Polarographic Reduction of Pyridinium Species in Pyridine In this article by Keiichi Tsuji and P. J. Elving [ANAL. CHEM.,41, 1571 (1969)] there is a n error o n page 1578, line 3 of the second paragraph under “Potentiometry.” The molarity of the benzoic acid solution should be 0.01M and not 0.1M.
