Mic roac tiva tio n An a lysis for Oxygen in the Actinide Metals ANDRE C. DEMILDT' Lawrence Radiation Laboratory, University of California, Berkeley, Calif.
b He3-activation analysis has been found ideal for oxygen determination. By extrapolating from present data, we estimate that oxygen can b e determined in as low a concentration as 0.001% in a few hundred micrograms of matrix material. Interference by activation of other elements can b e partly restricted b y control of the bombarding energy of the He3 particles. This makes the method applicable to the analysis of fissionable actinides, since no nuclear reactions will occur between the target matrix and the bombarding particles if the kinetic energy is essentially lower than the coulomb barrier.
T
HE
DIRECT
DETERMINATION
Ne's
1.6 seconds +
FlS + 8 + + V
and 110 minutes "18
___.
+
0'8
+ B+ +
Y
A count of the F18 (109.7 niinutci) (28) can be made either by @+ counting or by y detection of the annihilation radiation.
Beam I
EXPERIMENTAL
Of
microamounts of oxygen remains a difficult analytical problem. Various methods have been used more or less successfully under various experimental conditions; among these are vacuum fusion (1, 17, 23, S6), isotopic (19-22, S2), carrier-gas (SS), and other methods (12,16).
Figure 1.
Platinum seal
( 1 ) Anodized AI beam monitor of 1.59 mm. = 0.0625-inch diameter (2) Plain AI absorber (some size as monitor) (3) Thick-target sample (different conflguration and smaller than monitor) (4) Platinum seal
Since activation analysis offers a sensitive and nondestructive means of The approxiate cross section of both analyzing many elements, as shown in reactions together is about 400 mb. at several review articles (2, 14, 18, S?), an energy of 7.5 m.e.v., as determined a number of activation methods have by Markowita and hlahony. They been proposed for oxygen. Those methdetermined the over a wide range of ods employing neutrons (4, 5-10, 24, 25, energy, thus giving the excitation func89, 31, 34, 58) are impractical in our tion. case, because of fission, which conipliOxygen analyses performed by theye cates the detection and counting probauthors, and further analyses of T h lems. foils by hlahony and Hingorani ( 2 6 ) , Charge-particle reactions induced by were done on large samples of previprotons ( I S ) , deuterons (36, 39), LY ously prepared foils, with Mylar sheets particles (SO), or y actilation (3, 6, 7 , used as beam monitors. Furthermore, 16) are either inapplicable or of low the metallic foils were thin enough to be sensitivity. used in thin-target irradiation. Activation by He3. A suitable acFor a more general application to the tivation analysis was proposed by actinide metals, the experimental setup Markowita and llahony (28). They had to be changed extensively, since used He3 bombardments for the determetallic foils of the heavier transuranmination of oxygen in A1 or Be. RIylar ides are not available and samples of 1 sheets were used as monitoring foils. mg. represent an upper limit. To The nuclear reactions involL ed are maintain adequate sensitivity, beam either exothermic or slightly endointensities must be increased and Mylar thermic, and have a rather large cross eliminated because of its poor thermal section. The following are reactions of stability. practical use: p , p + F** 2.0 m.e.v. 0 1 6 + HeS-+ ["el9]
+
+
Y' n Ke18 - 3.0 m.e.v., Present address, In-tituut voor Iiernwetenschappen of Belgium, 11 Egmontstraat. Brussels.
1
1228
ANALYTICAL CHEMISTRY
Moreover, the heavier inetds were prepared essentially as "thick-target" samples, requiring an increase in beam energy to obtain adequate subsurface activation; this had the disadvantage of increasing the possibility of activating nonoxygen impurities. In these cascs, the Hilac He3-beam of 31.2 1n.e.v. was degraded to 11.5 1n.e.v. This implies a coulomb barrier restriction for elements from A > 75 (Z > 36), following Marion (27). The heavier elements do not react significantly under these conditions and charged-particle fission is excluded in the actinide elements.
Preliminary Experiments. We measured microscopically the surface area of Amz4' and Pus39 t,hat had been flattened between pieces of tool steel. As Mylar could not be used as beam monitors, quartz disks of k n o m oxygen content were employed. By placing several monitors in the immediate neighborhood of the sample, it was shown that enormous beam inhomogeneities could appear. Flux differences up to ratios of 1 to 100 were noted within a 2.25-sq. em. area of beam cross section in a beam of 7/s-inch diameter. This showed t,lieneceasity of beam monitoring near the sample. Encapsulation of the very alpha-active samples proved difficult. Previous workers (26, 28) irradiated the T h foils in plastic bags but, when we used the smaller samples and higher beam intensities, we found it necessary to change to Pt capsules. After irradiation, the sample TVSS counted in a y spectrometer; to avoid alpha contamination, the Pt seal was not opened. The y spectra were very complicated owing to many impurities which had been activated in the Pt. The estimated oxygen content was also too high because of oxygen in the 1%. The decay of the activities induced in the platinum foils was followed serernl times with a gainma-scintillatioIi niultichannel spectrometer. In addition to P, both S a z 2and SaZ4 were detected. Alany other peaks were found in smaller amounts, but could not be identified as the half lives were too long and the si)ectra too complicated for the resolving power of tlie equipment. For this reason, t'he Pt seal must always be removed in a gloved box after irradiatioii. 'I'licn, t o :ivuid coiituriiiiiation, tlie active sample is sealed in a plastic sheet before it is counted.
From the experim3ntal results, we can conclude that very few Bf emitting isotopes were formed in the act,inide !natris. Except for IC1*, shortliverl isotopes such as C11 (20.5 minutes) and E13 (10.0 minutes) could be detected in only small amounts, but these do not intcrfcrc: as the coiint>ing can he started 2 hours after irradiation. The resulting decay c u r v s always had a slope of 110 minutes for a t least seven half lives. Sometimes a residual Na" activity was formed biit this activity was always less than 2% of the original activity. The activit,ies, registered in t,he photopeak, which result from impurities, are essentially dependent on the type of target matrix. The main contribution is from Compton edges coming from higher energy gamma I-adiationsemitted by the different long-1. ved actinides isotopes. These activities, however, do not really interfere SI that the serisitivity is not affected. Another difficulty may arise because of overloading the y s,pectrometer with the low-energy y rays from Amzd1, which may overload the electronic equipment. Overloading can be avoided by use of thick Sn absorbers mil. or 3.56 grams per s~q. em. *0.3%). The intensity of the annihilation radiation is reduced by only a factor of 1.36. Final Experimental Setup. BEAM AND S BMPLE SEALING. MONITORING Since the beam must be monitored in the sample area for accurate analyses, thin sheets of material of known oxygen content were required. Anodized A1 of 1-mil thickness (6.74 mg. per sq. cm. ily,) proved suitable. We used anodized foil of unknown oxygen content. The oxygen content was determined by activation analysis against quartz-foil monitors. The thickness of these monitors was of a few milligrams per smquare centimeter (determined by weight and surface-area measurement in each case) and inch in diameter. However, they were brittle and difficult t o handle and were of no direct use for further acitivation analysis. The calibration of the anodized A1 gave an oxygen conterit of 0.49 mg. per sq. cm. within 10%. 'This figure agrees fairly closely with thtvt calculated from i 1 I 2 0 3 from microscopic measurement of the thickness of the oxide layer. Circular flakes of 62.5-mil. (1.59 mm. i 1%) diameter were cut from this material and put on top of the sample to be analyzed. A thin A1 absorber of the same diameter wa's placed between the sample and the monitoring foil to avoid radioactive contamination. The three pieces were sexled together in a Pt seal (Figure 1). The asscrnbly is about l / g inch in diam&r. TARGETLk3SERIBLIIASAI.. CHEhf. 33, 1056 (1961).
2,280 (1957). (14) Gibbons, D., Mapper, D., Millett,
M. J., Simpson, H., Radioactavation Analysis-A Biblzography, AERE-I/R-
2208 First Supplement, Oct. 1960. (15) Gilman, A. R., Isserow, S., Iiuclear Metals, Inc., Concord, Mass., NMI 1234/10F1 (May 1960). (16) Grieser, D. R., Cocks, G. G., Hall,
E. H., Henry, SV. A., C‘. S. dt. Energy C o m m Battelle Memorial Institute, Columbus, Ohio, BMI, 1538, 38 (1961). ( 1 T ) Horton, W, S., Brady, J., ~ A L . CHEM.25, 1891 (1953). (18) Hoste, H., Bouten, F., Adams, F., Nucleonzcs 19, (3) 118 (1961). i19) Kirshenbaum, A. D., Grosse, -4.V., ANAL. CHEV. 26. 1955 (1954). ( 2 0 ) Kirshenbaum, A. D., Grosse, A. V., d n a l . Chim. Acta 16, 225 (1957). (21) Kirshenbaum, A. D., Grosse, 8.V., Trans. Am. SOC.Metals 45, 758 (1953). (22) Kirshenbaum, 9.D., Mossman, R. A,, Grosse, A. V., Trans. Am. Soc. Metals 46, 525 (1954). (23) Kopa, L., Unzv. Calif. Rad. Lab.Trans., Determination in Oxygen and Aluminum by Melting in Vacuum, i i 4 (L) (1961). (24) Leddicotte, G. TV., Oak Ridge Xational Laboratorv, Oak Ridge, Tenn , ORNL 60-1 1-124 (Iiovember 1960). (35) Leddicotte. G. IT.. Mullins. W. T.. Bate, L. C., Emery, J. F., Oak Ridgk National I,aboratory, Oak Ridge, Tenn , ORNL TID-7555, Sovember 1957. (26) hlahony, J. D., Hingorani, S. B., Chem. Dept., Lawrence Radiation Laboratory, Universit,y of California, Berkeley, Calif., private communication, January, 1962. (27) Marjpn, J. B , “Xuclear Data Tables, U. S . At. Energy Comm., Part 3, 119, Washington 25, D. C. (28) Markowitz, S. S., Mahony, J. D., A s a ~ CHEM. . 34, 329 (1962); Mahony, J. D., Markowitz, S. S., Lawrence Radiation Laboratory Rept. UCRL-10512 I1 962). (29) Meinke, \ - - - -
W. W.,Shideler, R. W.,
n’ucleonics 20, (3) 60 (1962). (30) Xozaki, T., Tanaka, S., Furukawa, F.. Saito., K.., Nature 190. (4770), 39 (1961). (31) Osmond, R. H., Smales, A. A., Anal. Chim.Acta 10, 117 (1954).
(32) Pearce, M. L., Mason, C. R., Iron Steel Inst. (London), Spec. Rept. 68, 121 (1960). (33) Shanahan, C. E. A,, Rev. Metals 5 8 , 55 (1961). (34) Steele, E. L., Meinke, W.W., Axar,. CHEM.34. 185 f 1962). (35) Sue, hi. P.,‘Compt. Rend. 242, 770 (1956). (36) Taylor, R. E., Aiial. Chetn. Acta 21, 549 (1959). (37) Thomson, B. A,, ANAL. CHEW 33, 583 (1961). (38) Veal, D. T., Cook, C. F., Ibid., 34, 178 (1962). (39) Winchester, J. W., Bottino, hl. L., Ibid., 33, 472 (1961).
RECEIVEDfor review August 31, 1962. .4ccepted ilpril 22, 1963. Work financially supported by the “Interuniversitair Instituut voor Kernwetenschappen” of Belgium and by the University of Ghent in Belgium. Work also supported in part by the 17. S.Atomic Energy Commission under Contracat No. W-7405-eng-48.
