Prompt Activation Ana lysis for Lithium-6 W. A. Sedlacekl and V. A . Ryan Department of Chemistry, University of Wyoming, Laramie, W y o .
Activation analysis in which sample, detector, (and flux monitor) are placed in a particle flux, and in which prompt events are counted and stored while bombardment continues has been called “promptlactivation analysis.” In the present work, the reaction $Li(n,a)BH was so used to determine lithium-6 in minerals and laboratory chemicals. A thin, uniform, reproducible film containing the lithium, exposed to a slow neutron flux, produced alpha particles and tritons. The latter were counted using a solid state detector mounted to the film, and using multichannel pulse height analysis. Mounted nearby in a “flat” region of the flux was a similar film and solid state detector. Triton counts from this were related to the neutron flux intensity. By calibration against separated isotopes, results within 1% may be rapidly obtained.
THEABUNDANCE
RATIO of lithium-7 t o lithium-6 is not constant from source to source (1-3), and the gram molecular weights of compounds of the element change rather markedly with this ratio. A likelihood of errors in stoichiometry involving lithium compounds is indicated. The most obvious source of variation arises from the defense industry needs for separated lithium-6. Lithium isotope fractionation in some minerals might also take place because of cation exchange in minerals such as zeolite near the surface in the range of the hydrosphere. The presence of radioactive materials in lithium-bearing minerals may also change the abundance ratio over long periods of time because of the high cross section for the !Li(n,a):H reaction (4). Cameron (1) found that the ratio 7Li/6Li varied from 12.44 to 12.93 in minerals and laboratory chemicals of a wide variety of origins. Lithium compounds are usually light; therefore, lithium isotopes are more likely t o undergo separation due to diffusion processes than are oxygen-16 and oxygen-18. The latter pair have been studied (5) with a view to establishing a geological thermometer. The more favorable 1/Mz/Ml
(1.060804 for oxygen, 1.079997 for lithium) and much greater abundance of the rarer isotope should make the lithium isotope ratio more sensitive for such work. Several mass spectrometry methods have been reported for lithium isotope abundances (1, 3, 4, 6-11), as have a number of photospectral methods, including atomic absorption spectrophotometry (2), high resolution emission spectrometry (12), and interferometry (13). The reaction !Li(n,a):H has been used previously for lithium-6 determinations (14-26) and for total lithium, assuming the tabulated value for the natural ratio of abundance of lithium-6 and lithium-7. Most of these methods utilize secondary transformations having small cross sections, or count radioactive decay products, in order to eliminate interference and background effects. We accomplish the same end by utilizing the high resolution of the charged particle spectrum obtained with solid state detectors. MacFarlane and Almodovar (27) and Cheifetz et al. (28) used this technique for a neutron flux calibration as did Finston, Wellwart, and Bishop (18) for absolute lithium-6 and boron-10 analysis of thin films. Finston et at., suggest a potential analytical procedure. Their work contains a fine discussion of the very real advantages of the prompt activation technique. This technique can afford greatly increased sensitivity over conventional activation methods. In counting tritons from the reaction !Li(n,a)?H in a reactor neutron flux of low intensity we have sought an accurate, quick method for lithium-6. Our present scheme does not determine total lithium. We must depend upon
N. M.
(11) R. E. Sladkey, U . S. At. Energy Comm. Y-1143 (1956). (12) R. E. Sladkey, R. F. Fenske, and 0. E. Schow 111, U. S . A t . Energy Comm. Y-1091 (1955). (13) Y. Urano, S. Nakajima, Y. Ueda, K. Kosasa, Y. Maruyama, S . Katsube, and M. Iwata, Osaka K6gy6 Gijutsu Shikenjo KihS, 14, 260 (1963). (14) S . Amiel and Y. Wellwart, ANAL.CHEM., 35, 566 (1963). (15) L. P. Bilibin, A. A. Lbov, and I. I. Naumova, Atomnaya Energ., 7, 528 (1961); In English, p 522. (16) L. Clark, Jr., and N. C. Rasmussen, Air Force Cambridge Research Labs, Bedford, Mass., AFCRL-63-575 (1964). (17) R. F. Coleman, Analyst, 85,285 (1960). (18) H. L. Finston, Y. Wellwart, and W. Bishop, U. S . A t . Energy Comm. BNL 7146 (1962). (Unpublished. Copy available at
(1) A. E, Cameron, J . Am. Chem. SOC.,77,2731 (1955). (2) J. A. Goleb and Y . Yokoyama, Anal. Chim. Acta., 30, 213 (1964). (3) H. J. Svec, U . S. At. Energy Comm. IS-500, C-42 (1962). (4) . . V. S . Venkatasubramanian, U. S. A t . Energy Comm. “-9166, 222 (1959). ( 5 ) S. Eustein and H. P. Taylor, Jr.. Res. Geochem., 11, 29 (1967). (6j H. 0. Finley, J. F. Been, R:E. Sladkey, and C. R. Fultz, U. At. Energy Comm. Y-1142 (1956). (7) H. 0. Finley and J. J. Pucilowski, Jr., U. S.At. Energy Comm. NBL-205 (1963). ( 8 ) I. Omura and N. Morito, J. Phys. SOC.Japan, 13, 659 (1958). (9) G . H. Palmer, J. E. Johnson, D. B. McCulloch, and T. F. Johns, Atomic Energy Research Establ. (Gt. Brit.) AERE-GPlR.1572 (1954). (10) J. Pupezin, M. kiri6, and D. Lazarevii., Bull. Boris Kidrich Inst. Nucl. Sci., 13, 77 (1962).
(19) L. Kaplan and K. E. Wilzbach, ANAL.CHEM., 26,1797 (1954). (20) C. A, Kienberger, R. E. Greene, and F. S . Voss, U. S. A t . Energy Comm. K-1042 (1953). (21) E. Picciotto and M. van Styvandael, Compf. Rend., 232, 855 (1951). (22) D. C. Von Aumann and H. J. Born, Radiochim. Acta, 3, 62 (1964). (23) H. Wanke and E. U. Monse, U. S. A t . Energy Comm. AECtr-2535 (1955). (24) J. W. Winchester, L. C. Bate, and G. W. Leddicotte, U . S . A t . Energy Comm. CF-59-7-127 (1959). (25) E. I. Zaitsev and V. Yu. Zalesskii, Zavodsk. Lab., 27, 553 (1961). (26) A. L. Yakubovich and E. I. Zaitsev, Zbid., 28, 819 (1962). (27) R. D. Macfarlane and I. Almodovar, Phys. Rec., 127, 1665 (1962). (28) E. Cheifetz, J. Gilat, A. I. Yavin, and S . G. Cohen, Phys. Letters, 1 (7), 289 (1962).
1
Present address, Los Alamos Scientific Laboratory, Los Alamos,
BNL Publications.)
s.
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chemical o r spectrometric methods for this. Currently, gravimetric determination of total lithium is used t o provide a measure of the lithium-7 content by difference, as suggested by Winchester, Bate, and Leddicotte (24). EXPERIMENTAL I n order to observe the !H particles emitted from the reaction ZLi ;nth :H (2.74 MeV) ;He (2.05 MeV)
+
-
+
directly and quantitatively it is necessary to have the lithium spread out as a thin, uniform, reproducible film-10 pg per cm2 of lithium-6 or less. Samples were prepared by an adaptation of the method called “molecular plating” by Parker, Bildstein, and Getoff (29) which gives an excellent deposit of activity suitable for alpha counting. We deposited the carbonate onto 1 X j/16-inch stainless-steel planchets and obtained excellent, uniform plates that differed very little in appearance from a n unplated planchet. The plating is carried out in an electroplating cell having a Teflon tube with a bore the same size as the sensitive area of the detector. The sample is deposited electrophoretically as lithium carbonate from a colloidal suspension in acetone. The sample material is prepared as purified lithium carbonate by standard procedures (SO, 31) and made up as an aqueous stock solution containing 50 mg of the carbonate per 10 ml of solution. A 10-pl aliquot of the stock solution was then blown from a self-filling micropipet, held with the tip just below the surface, into 5 ml of acetone in the plating cell, producing a colloidal suspension of Li2C03. A straight wire platinum electrode was positioned in the cell 0.5 inch from the planchet and a potential of +400 volts was applied. The plating solution was not stirred. After several trials, it was found that a standard plating time of 2 hours deposited more than 99.75z of the lithium. The thin films so produced were sufficiently uniform not t o produce diffraction patterns from reflected light. Typical plating currents were 0.1 to 0.2 mA. After plating, the planchets were heated t o 500” C in a furnace. The assembled solid state detectors with lithium planchets were inserted in a reproducible manner into one of the beam ports of the Atomics International Reactor. The reactor was operated t o give a flux of about 108 neutrons cm-* sec-1. The exact value of the flux is unimportant, as the integrated flux was monitored. The charged particle spectrum was observed with a gold-silicon surface barrier detector having a 100-mm2 sensitive area and recorded in a multichannel pulse height analyzer. A second detector was placed beside the sample detector to count a reference planchet. The output of this second detector was counted into the memory of a single channel pulse height analyzer tuned to the triton peak. This count served as a monitor of the integrated flux. Counts accumulated were sufficient for a 1% error limit at a 9.5z confidence level. The live timer of the multichannel pulse height analyzer was used to time the counts, and simultaneously a clock time was measured for use with the flux monitor. After the background had been subtracted, the number of counts in the central 15 pulse height analyzer channels of the triton peak were summed as a measure of the peak area. (29) W. Parker, H. Bildstein, and N. Getoff, Nucl. Insrr. Met/zods, 26, 55 (1964). (30) J. W. Mellor, “A Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. 2, Longmans, Green and Co., New York, 1937, p 725. (31) W. W. Scott, “Standard Methods of Chemical Analysis,” 5th Ed., N. H. Furman, Ed., D. Van Nostrand Co., New York, 1939, pp 882-3 and 888-9.
Channel
Channel
Figure 1. a. $Li(n,a)tH in a vacuum b. !Li(n,cu);H spectrum in air Gamma-ray background shown dotted
These sums were normalized to the value expected for a flux monitor count of 3000 counts per second and adjusted for variations in live-time counting periods used for different sized samples (Equation 1). Normalized count = sample counts
monitor counts
) (-
live time 3000 cpsclock time-)
(1)
A standard curve was constructed and used to determine the lithium-6 content of the samples. RESULTS Enriched lithium-6 and lithium-7 carbonate samples were obtained from Oak Ridge National Laboratory for standardization. The results of the mass spectral analysis of these samples by that laboratory is given in Table I.
Table I. Isotopic Analysis of Lithium Carbonate Samples Obtained from Oak Ridge National Laboratory Sample number Atom 6Li Atom Z’Li SS 5(HCR) 95.44 =t0 . l a 4.56 i 0 . 1 ~ H D 885(Z) 99.98 0.021 f 0.W5Q SS 7(CB) Not reported 99.9924 a The limits quoted above are an expression of the precision of these measurements only. The error is estimated at less than 1 from known sources of systematic errors.
VOL. 40, NO. 4 , APRIL 1968
679
~
~~
~
Table 11. Standard Samples Stock solution
Wt 95.44% 6LiK03 g f 5 pg
Wt 99.99z ’Li2C03g f 5 pg
V
0.040519 0.033111 0.23559 0.010031 0,008044 0.001931 0,994148
L M N
Y 0 P
Q
K
Wt 99.98% 6Li2C03g =k 5 pg 0,050364
0.015288 0.014788 0,032785 0.067981 0.03212 0.057526 0.065386
Total wt in g of Li2C03per 10-ml
... ... ... ...
solution 0,050364 0.040519 0.048399 0.038347 0.042754 0.076025 0.033943 0.61674 0.065386
Table 111. Analysis Results of Standard Samples at (=lo-’ n Stock solution
1st plate 257,149 198,455 164,727 113,988 39,227 20,692 9,748 - 38
V
L M N 0
Q
P K Av a
b
Normalized peak area counts 2nd plate 3rd plate 261,618 263,131 197,526 200,059 163,205 152, 672a 114,068 116,375 40,176 38,762 20,853 20,641 9,666 9,489 - 13 ...
Average 260,633 198,680 163,966 114,810 39,388 20,729 9,634 Ob
Li that is 6Li 99.98 95.44 65.29 58.64 23.51 10.10 5.43 6.42
...
sec-’)
Divergence from average +% - % 0.96 1.34 0.69 0.58 0.46 0.46 1.36 0.72 2.00 1.59 0.60 0.42 1.18 1.51
...
...
1.03
0.95
6Li per plate, fig 8.4091 6.4102 5.2399 3.7284 1.2737 0.6572 0.3060 0.2
1.76 0.5
... 5400 5400
0.047 MeV 0.21 MeV 0.58 MeV 1.42 MeV Continuous spectrum of energies 0.57 MeV 0.19 MeV
neutrons (33)],self shielding effects, even for thin films, make the use of a standard curve advisable (34). The precision of the method appears to be of the order of 1% for different analyses of the same material. Known sources of random variations in plating procedures and counting times account for about OSZ. The overall accuracy of the method, when several plates are analyzed for each sample, is considered t o be better than 1%. The atom per cents of two lithium-6 samples determined are more than 1% higher than the generally accepted natural
(33) D. J. Hughes and R. B. Schwartz, U.S. At. Energy Comm., BNL-325, 2nd ed., p 3. (34) H. Bowen and D. G. Gibbons, “Radioactive Analysis,” Oxford Press, London, 1963, pp 95-9. VOL. 40, NO. 4, APRIL 1968
681
abundance, 7.42%, but are not out of line with results reported by Pupezin, eeric, and LozareviC. (IO). An obvious improvement of the method would be t h e counting of the alphas and tritons in coincidence to reduce the background. This would require a modified arrangement for holding the sample, perhaps on a “weightless“ zapon (35) film, mounted between the two required solid state detectors, all in a vacuum. The accuracy of the present method would further be improved in a reactor with a thermal column, as room would be available for needed gamma shielding to reduce the background spectrum. It is pleasant to consider a method which would determine both the lithium-6 and lithium-7. The lithium-7 absorbs a neutron and becomes lithium-8. This has a 0.85-second beta activity and becomes beryllium-8. The beryllium-8 disintegrates with a t1I2of 3 X 10-16 second to produce two alpha particles. Alpha, alpha coincidences would be counted for the lithium-7 and of the alpha, triton coincidences for lithium6. The low energy (47 keV) and the low cross section for formation of the lithium-8 present difficulties in our low flux, and the rather short useful life of existing solid state detectors in a higher flux would mitigate against working in such. With regard to analysis of the same sample by mass spectrometry and by the present method, such samples are listed in Tables I and 11, and the results are compiled in Table 111. If SS 5 is assumed correct, then HD 885 is so analyzed and is (35) L. Yaffe, Ann. Rec. Nucl. Sci., 12, 154 (1962).
in error by +0.67% ; or, if you assume HD 885, then SS 5 is 0.67x low. We believe that our present method requires very little in the way of special equipment, and simple wet chemistry. Our method provides adequate accuracy to sense reported geologic eLi/’Li variations of from 2 to 4.5%. It can easily be used for checking the atom ratios of depleted laboratory lithium compounds providing accurate molecular weights, A neutron flux suitable for this work may be found in many laboratories. The results are directly useful to the chemist utilizing lithium salts and presumably useful to students of the chemistry of the earth and of meteorites. Nuclear experimenters find the variation in the lithium-6 to lithium-7 ratios especially important because of the high thermal neutron cross section of lit hi um-6. ACKNOWLEDGMENT
We thank Ronald Macfarlane for suggesting that we study n, alpha reactions; Scott Smithson and Robert Houston with whom we discussed the geological possibilities of the reaction; and Jere Green and Jere Knight who read the manuscript most diligently and helpfully. RECEIVED for review August 7, 1967. Accepted January 6, 1968. Abstracted from Ph.D. Thesis, W. A. Sedlacek, University of Wyoming, 1965. Presented in part at the 153rd Meeting ACS, Miami Beach, April 1967. Work supported in part from the National Defense Education Act funds.
Remote Analysis of Radioactive Alloys for Carbon, Oxygen, Nitrogen, and Hydrogen Harvey T. Goodspeed, Ben D, Holt, John H . Marsh, Jr., and John E. Stoessel Chemistry Dicision, Argonne National Laboratory, Argonne, Ill. Combustion and inert as fusion methods are applied to the determination o carbon, oxygen, nitrogen, and hydrogen in alloys of high alpha, beta, and gamma radioactivity. The combustion and fusion furnaces are designed such that either can be used with a common shielding facility, energy supply, and analytical train. Sample addition, crucible replacement, and reaction tube exchange can be performed remotely using masterslave manipulators. Provisions are made to strip the carrier gas streams of radioactive and/or interfering gases before entering the measurement components of the analytical train. Advantages of the new furnace tubes, other than permitting remote handling, are noted.
9
VARIATIONS in the physical properties of the fuel and structural alloys that make up the cores of atomic energy reactors have led scientists t o look for correlations with changes in the concentration and distribution of impurities: carbon, oxygen, nitrogen, and hydrogen. Shifts in concentrations during power production have been suspected as being relatable to swelling of fuel materials and/or t o embrittlement, cracking, and dimensional changes in cladding alloys. This report deals with the determinatior? of these impurities in alloys characterized by strong alpha, beta, and gamma radioactivity. Samples were handled remotely in a n isolation box enclosed in a dense-walled cave cell. The combustion-
682
ANALYTICAL CHEMISTRY
manometric method was used for the microdetermination of carbon and the inert gas fusion method for oxygen, nitrogen, and hydrogen. The furnaces used in the combustion and fusion apparatus were chosen to meet the following requirements: a reaction chamber that could be opened remotely by mechanical hands (“master-slave” manipulators) for loading and disassembling; a replaceable crucible; a means of reaching the operating temperature rapidly without releasing excessive heat inside the box; a procedure for reducing the blank t o a low constant level while the crucible was heated in situ, prior t o sample addition and without exposure of the crucible to the containment atmosphere; provision that each sample should enter the crucible only after the blank was established, and several samples should be accumulable in each crucible; the same shielded box, induction heater, cooling water lines, and carrier gas lines should be usable for either the combustion or the fusion analyses; the portion of the analytical train confined to the hot cell should be minimal. EXPERIMENTAL
Carbon Analysis. APPARATUS.The combustion furnace used in the determination of carbon is shown in Figure 1. The envelope, crucible, and liners are made of fused quartz and are arranged such that the oxygen stream flows by the
