Activation Analysis Using low level Neutron Sources W. WtlYNE MEINKE AND RICHARD E. ANDERSON Chemistry Department, University of Michigan, Ann Arbor, Mich. Activation analysis using neutrons from a nuclear reactor has developed into a major analytical tool within the past few years. It was the purpose of this work to explore the possibility of using low level neutron sources for precise activation analysis. A 25-mg. radium-beryllium neutron source was used to develop a method for the activation analysis of rhodium, silver, or indium in the presence of all other elements. Analysis for rhodium can be completed in 5 minutes, for silver in 12 minutes, and for indium in several hours. The accuracy of the method is limited by the statistical error in the radioactivity counting determinations. Typical probable errors for an assay of 1% rhodium is 4.8%; for 1% silver is 6.7%; and for 1%‘indium is 3.6%~. The method is applicable to industrial laboratory work and is competitive in cost with other instrumental methods of analysis for these elements. The plot of all neutron activation cross section values versus half life of the resulting daughter radioisotopes presented herein is applicable to thermal neutron activation analysis w-ith all types of neutron sources and can indicate the most probable contaminants in the analysis.
T
not the only source of neutrons for activation analysis. Table I (10) lists a number of possible neutron sources 13-ith the usable flus of neutrons available from each. (The value for the 25-mg. radium-beryllium source was determined ey~erimentallgin this laboratory.)
HE method of analysis using slow neutron activation was early suggested by Hevesy and Levi ( 4 ) who used a 200- to 300-me. radium-emanation beryllium source in their determination of certain rare earth elements possessing high neutron activation cross sections. They were able to measure without chemical separations as little as 0.1% dysprosium in yttrium mixtures with this method. ?\lore recently Boyd ( 1 ) has summarized the progress, the limitations, and the problems of activation analysis and the feasibility of using this method in certain analyses. Taylor and Havens ( I O ) , Senftle and Leavitt ( 9 ) , Leddicotte and Reynolds ( 7 ) , and others have reported on activation analysis techniques and have presented tables of activation cross sections, analysis sensitivities, etc., in the literature. Recently the announcement was made by the Oak Ridge National Laboratories and reported in this journal ( 8 ) ,that activation analysis service nil1 be offered by the laboratory to industrial, scientific, and medical groups. Thus activation analysis is now avail rble to everyone and gives the individual scientist another poneiful analytical method. With the increasing interest in activation anal>-& it is natural to inquire as to whether small, cheaper, more portable neutron sources could not be used for certain analytical applications.
Table I.
Neutron Sources and Usable Flux of Neutrons Available from Each (10)
Neutron Source Nuclear reactor Cyclotron Low voltage D D neutron generator Ra-Be, 1 gram Sb-Be, 1 curie Ra-Be, 25 mg.
Usable Flux ( n Sq. Cm./Sec.) 1010
10s 105 104
10s -102
- 1012 - 109 - 10s - 104
- 108
(thermal neutrons only)
The amount of a reaction product formed is directly proportional to the neutron flux in which it is irradiated. Hence a hile the smaller sources do not give such a profusion of reaction products as the nuclear reactors, they are useful in analyzing for certain high yield reactions. Mention should be made of the cost of the above sources. The reactors, cyclotrons, and neutron generators are in general so costly that they are out of the realm of use for an industrial laboratory for analytical purposes. A 1-gram radium-beryllium source costs =1525,000 and. while it is compact it is too eypensive for individual analyses. ilntimony-beryllium sources are non available from the Isotopes Division of the Atomic Energy Commission (11) a t a cost of about $150. The activity of these sources, however, decays n i t h a 60-day half-life and the source must be reactivated from time to time a t an additional cost, to maintain the high neutron flu..;. The 25-mg. radium-beryllium source used in this work was obtained for about $600 and gave a constant neutron flux. There are certain basic principles which should be mentioned that are common to all activation analysis work. Analysis by activation is possible because every isotope has a certain probability of absorbing a low energy neutron to become the isotope of next higher mass number. This probability is called the “cross section” for the nuclear reaction and has the units of square centisq. cm. meters; nuclear cross sections are of the order of (one barn). Since the cross sections for neutron activation reactions may vary with the energy of the neutrons it is important that analysis data be obtained a t one definite energy. Therefore it is customary for most neutron activation analyses to be carried out using thermal neutrons. These thermal neutrons are produced when neutrons from any source are slowed down by collisions with moderators such as paraffin, water, or graphite,
GENERAL ASPECTS OF NEUTRON ACTIVATION ANALYSIS
Activation analysis is a method of determining the constituents of a sample by utilizing certain nuclear properties of the isotopes of the elements in the sample. Radioactive isotopes are formed by activation of the nuclei of the sample elements using nuclear particles. These radioisotopes formed can then be detected and measured by their nuclear radiations. Thus an exact knoL5ledge of their nuclear characteristics allows a determination of the amount of element present. Activation analysis has proved very helpful in certain specific analytical problems which previously had been difficult to solve. The analysis of very small amounts of hafnium in zirconium and the detection of submicrogram amounts of arsenic are only two of a large number of examples. Neutron activation analysis requires two main types of equipment: a neutron activation source and radioactivity detection equipment. Unfortunately the availability of neutron sources has not kept pace with the general commercial availability of radiation detection equipment. As a consequence, activation analysis has been generally thought of as the special tool of the -4tomic Energy Commission National Laboratories and of some universities, and has not been considered readily adaptable to industrial production analysis. It should be borne in mind, however, that nuclear reactors are
778
V O L U M E 25, NO. 5, M A Y 1 9 5 3
779
until they eventually reach thermal velocities a t which time they possess only the constant thermal energy of -0.023 e.v. (-571 cal. per mole). .\ plot of the thermal neutron activation cross section (6) of the isotopes against the half-life (6) of the daughter radioisotopes resulting from the neutron reaction is shown in Figure 1. The croqs section values are "atomic cross sections"-i.e., the iso-
'1
I
I
topic cross section multiplied by the natural abundance of the isotope. This plot suggests that with judicious selection of irradiation time and source flux it is possible to detect with this method only rhodium, iridium, indium, silver, and dysprosium of all the elements in the periodic table. For a given neutron source and detection apparatus the amount of activity induced in a sample containing material of high neu151
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.P 1651
16'
Figure 1.
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IO0
IO'
130
I I I IO2 lo4 HALF-LIFE OF DAUGHTER (MINUTES)
Id
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lo5
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Atomic Cross Section for Thermal Neutron Activation against Half-Life of the Daughter Radioisotope Produced Underlined isotopes are activated t o a metastable daughter activity
.
A N A L Y T I C A L CHEMISTRY
780 tton activation cross section is directly proportional to the size of the sample-assuming there are no effects of neutron saturation in the source. Figure 1 shows the probability for formation of radioactive isotopes by neutron activation with no consideration given to the detection of the radiations from these isotopes. The counting arrangement used, however, has a definite effect on the sensitivity and accuracy obtainable in activation analysis since detection of these nuclear radiations depends upon their characteristics-i.e., the range and energy of the beta particles or the half thickness and energy of the gamma rays. I031
I
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1
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W
Figure 2 introduces another variable-the range of the beta particles of the reaction products-into the pertinent data of Figure 1. The term u/TI/2 is an expression which indicates the relative importance of a particular element in short-term irradiations, From this graph it is readily apparent ho\T thick a sample can be tolerated in the activation analysis of these elements. The plot illustrates that with proper selection of source strength and sample thickness only rhodium, silver, and indium could be detected in irradiations of thick samples for short intervals of time. Since Geiger-Muller counters detect beta particles many times more efficiently than gamma rays, only the range of the beta particles is plotted in Figure 2. A similar graph could be constructed to include additional isotopes formed by a more intense neutron source or for gamma-emitting isotopes if required. From Figures 1 and 2 it is possible to ascertain how stringent a chemical separation will be required in activation analyses of mixed samples. They can also be used as an easy reference guide to the most probable contaminants in thermal neutron activation analysis. I n activation analysis with sources of ION flux levels the statistical error occurring because of the randomness of radioactive decay is one of the principal errors. The probable error in a counting determination giving C counts is PE = 0.67 Hence the more counts that can be obtained for a given sample the more certain the value of sample activity. If a sample of irradiated rhodium could be followed for the first half-life immediately after irradiation, 50% of the original total counts in the sample would be detected; if followed for two half-lives, 75% would be detected; if followed for three half-lives, 87% would be detected, etc. Hence i t is important in activations with low neutron flu^ levels to begin counting an activated sample as soon after the end of the irradiation as possible, and to continue the count for several half-lives for the best statistics. Khen a count is taken over several half-lives it is helpful to use a relationship for the instantaneous rate of decay a t the end of irradiation. A4brief derivation of this relationship (Equation 1) is presented below. Radioactive decay is a first order reaction and can be represented by the following differentid equation:
6
?Io7
c
$ 1
Ado7
l
cn
I
I I I I I I I 1 200 400 600 000 1000 1200 1400 1600 RANGE OF BETA PARTICLES (Mg.lCrn2 ALUMINUM) Figure 2. Ratio of Atomic Cross Section for Thermal Neutron Activation of Parent to Half-Life of Daughter against Range in Aluminum of Beta Particles Emitted by Daughter
1001
0
dS/dt = - A S whrre S equals the numhrr of atoms prrsrnt Integrating we get
S
Underlined isotopes are activated to a metastable daughter activity
The total amount of activity induced in the sample is directly proportional to the sample thickness if one standard sample holder is used for the irradiation and counting. Since, hox-ever, both beta and gamma radiations are absorbed in matter, the thickness of the sample to be counted is limited by the self-absorption of the radiations in the sample, regardless of counter design. Therefore, some maximum sample thickness must be designated for each radioisotope counted. This thickness must be chosen such that the accuracy and sensitivity of the activation method are optimal. Several groups of investigators have made extensive studies of the problem of self absorption ( 2 ) . Plots of self-absorption corrections for the weak carbon-14 beta particles in barium carbonate show a sensitive dependence on sample thickness in the initial portion of the range. As the samplr thickness approaches a sizable fraction of the range of the beta particle, however, the corrertions become less sensitive to small changes in sample thickness until finally a t sample thicknesses equal to or greater than the range of the beta particle there is no change a t all in the correction. Since this effect is a general one for beta particles of all energies it is felt that in activation analysis the thickness of the sample should not exceed one fifth the range of the beta particles for maximum sensitivity and accuracy. For lox activity samples it may be necessary to go beyond these limits but it should be understood that this is done with a resultant loss in accuracy.
=
.YO(e-Xt)
Let
- S?
A.1- = A',
where A$-l
= number of atoms a t time
SZ= number of atoms at time
tl
t?
Then
- e-htz)
A.v = J*o(e-Xti
or
ATo =
= activity at time
t
=
0
In this equation A N is the measured value of the number of atoms that have decayed or the number of counts obtained in the interval from t l to t~ The total probable error of these counting determinations is a function of the gross count and the background count and is defined as: PE = d ( P E I ) 2 (PE,)?,where PE, = probableerror in the gross count and PE, = probable error in the background count.
+
V O L U M E 25,.NO.
5, M A Y 1 9 5 3
781
OU'I'LINE OF METHOD
the source for safety purposes. The shielded source must he kept a t a considerable distance from the counting apparatus to avoid increasing the background. Some samples were irradiated to saturation (the point a t which the atoms being formed by activation and the atoms bring lost by decay are equal-approached in irradiations of six or more half-lives). In certain instances, the time of irradiation was required to he short in order to discriminate against longer halflived materials. I n such c&msirradiations were made for one, two, or three half-lives or some other convenient short interval. When comparing the activity of a sample with that of 8. standard i t is important that the sample and standard he irradiated for exactly the same amount of time. Furthermore 8, second irradiation of a sample cannot follow another irradiation before the activity has decayed out completely or an error will be intraduced into the determination. After irradiation, samples were counted internally in a Model D46A lead-shielded "Nuclear" Q-gas counter. This GeigerMiiller flow counter used a mixture of 2% isohntane with helium as a counting gas. A scale of 128 scaling circuit, also manufactured by the Nuclear Instrument and Chemical Co.. Chicwo, Ill., was used in conjunction with the counter. I n t h e most recent experiments two "pancake-type," end window, chlorine-quenched, argon-filled Geiger-Miiller tubes with a window thickness of 3 me. Der so. em. were used. These tubes were placed one above ana one helow the sample, in conjunction with an Anton lead sample changer to give abaut 95% counting geometry. The tubes and sample chsnqer were manufactured by Anton Electronic Laboratories, Inc., Brooklyn, N. Y. Counts from both Anton tubes were recorded on one scale of 128 counting unit
The purpose of the work reported here wa8 to expiore the possibility of using readily available portable radium-beryllium neutron sources for routine activation analysis of certain elements in an industrial laboratory. The work was directed toward the exploration of methods that could be used with commercially available equipment. The cost of the entire analysis unit (including neutron Source and detection equipment) was limited to around $lOO&competitive with other instrumental methods of analysis. Thermal neutron activation with a small radium-beryllium source-e.g., 25 mg.-has one major advantage over activation with larger sourcesnamely, that only those isotopes with a very high activation cross section and a short half-life are detected after a short irradiation. The profusion of neutron-induced activities obtained when 8 mixture of elements is irradiated in a nuclear reactor is eliminated. Thus it is possible to perform activation analysis on I-gram samples without subsequent chemical separation. Moreover the amount of activity formed in the irradiation is so small and decays out so rapidly that the sample used for the irradiation is unchanged st the end of the analysis. Furthermore, analyses far the most favorable elements can be completed rapidly. Figure 3 shows the activation analysi3 source apparatus use(i for these experiments. The apparatu,s was similar to tha t described by Hamill, Williams, and Schul,er (3').
The source consisted of a mixture of 25 mg. of radium with 250 mg. of beryllium, prepared and sealed h a small Monel metal capsule by Eldorado Mining and Refining LM., Ottawa, Ontario, Canada, and costing about $600. Tlie source was surrounded by a cylinder of paraffin to moderate the fast neutrons produced to thermal velocities. The sample was ground into a fine powder in an agate mortar, evenly compressed into a polystyrene sample holder, and weighed. The sample holders were 32 mm. in diameter and 4 mm. thick with a circular hole 26 mm. in diameter and 1.5 mm. deep cut out of the center
was obtained h$ compa&on with a pure Ample of the elemeht: If a mixture of two elements was present the decay curves were resolved into the two half-life components and the activity of each component was compared with the standard. The length of irradiation was so selected that only rhodium, silver, indium, iridium, and dysprosium could possibly he detected. Nuclear data (5, l $ ) for these elements are summarized in Table 11. The conversion electrons from iridium-l92m 'and dysprosium-165m are very weak. In fact the range of the iridium particles is lcss than 0.4% of the range of the rhodium beta particles, while the range of the dysprosium particles is about 1.1% (Table 11). The factor between these particles and the silver beta particles is ahout the same. (The 13-second indium-116 emits more energetic beta. particles than the silver-109 but the short half-life of this isotope makes i t very difficult to work with.) The iridium-192m and dysprosium-165m conversion electrons are completely absorbed in several mils of aluminum. Therefore, these elements do not interfere in the rhodium and silver analyses when the samples are covered with 4 mils of aluminum foil. In the rhodium analyses, conversion electrons of the 4.2-minute rhodium isomer are also completely absorbed ont by the 4mil aluminum foil. One-gram rhodium ssmples were irradiated for 2 minutes and covered with the foil before insertion into the counter. These samples decayed from an activity of about 2800 count8 per minute in the internal counter down to background with a 44-second half life. All samples oontaining rhodium were counted in this manner. Figure 4 shows a typical decay curve of pure rhodium. Silver is more difficult to determine than rhodium. Because of the close similarity in the energy of the beta particlos emitted by the two d v e r isotopes (Tablo 11) differentiation m u t he
avohed in the syitem & reduce 10% of neutrons r y absorption by the boron in the glass. Similarly the system must be free of metals which can absorb neutrons and thus reduce the effective neutron flux of the source. The polystyrene sample holders served as additional moderating material. The sample holder was placed in a slot out in the paraffin moderator a t a distance of 2.5 om. from the source where the flux of thermal neutrons was a t a maximum. This position was determined experimentally for the 25-mg. rsdiam-beryllium source by measuring the yield of the thermal neutron activation reaction of rhodium a t different positions in the moderator. At distances closer than 2.5 om. many of the neutrons of the source have not yet been slowed to thermal velocities and hence do not readily activate the rhodium. Beyond 2.5 em. the inverse s uare law causes diminution of the neutron flux. Four of these j o t s were available in the moderator used. A lead wall 1 inch thick surrounded the paraffin moderator and was sufficient shielding for
Duringthe %minute decay period, ne& all (-97%) of the 22second silver isotope decayed out while only -46% of the 2.3minute silver isotope had decayed. A typical decay curve of a silver sample (covered with 4 mila of aluminum foil) is shown in Figure 5. A few analyses were made with the 54-minute indium-116m. Indium samples were irradiated for 1 hour and counted afterian interval of several minutes. This decay interval was necessary to eliminate the effects of the 13-second indium-116. The resulting decay curve of the 54-minute indium is shown in Figure 6. The time required far these analyses is dependent upon the half-life of the activation products formed. In the most favorable case, that of rhodium, the complete analysis (excluding the grinding of the sample) can be completed within 5 minutes. Silver requires ahout 12 minutes, while indium requires several hours. Often i t is necessary to determine whether both silver and
Figure 3.
Aotivation Analysis Source ADDaratus
ANALYTICAL CHEMISTRY
182
-Table 11.
Abundance,
%
Atomic Cross Section, Barns 12 140 22.8
100
100 61.35
Kcight of elemcnt in unknown \Veight - oi element in m n d a r d - activity in unknown 12) activity i n staridatcl
Summary of Nuclear Data (5, 12) Daughter Radioisotope Produced RhlO4m RhI@4 Aglos
of
Half-Life 4 . 4 minutes 44 seconds 2 . 3 3 minutes
48.65
52.9
Agllo
24.5seconds
96.77
138.9
In1Mn
64.31minutes
95.77 28.18 28.18
49.8 733.2 282
In118 Dylesm
Dyl6i
13 seconds 1 . 2 5 minutes 2 . 4 2 hours
38.5
100.1
Irl92m
1 . 4 2 minutes
38.5
269.
Irlg2
7 4 . 3 7 days
Radiation
1000
I"
2
: s
I-
0 a
100
-
Range of Particles in AI, Mg./Sq.
Energy Particles, IV1.e.v.
Crn. 10.5
This ratio can be determined precisely if irradiations of the 1280 8-, Y known and unknown are made @-, y 680 under identical conditions, ,~L?~ 8-, Y thus eliminating corrections 1390 for geometry, counting effi@-,y 410 ciencies, scattering, absorption, 340 200 etc. 81440 Pure 1-gram rhodium foils e-, y 14 p-,y El50 and 1-gram samples contain345 ing rhodium powder and lead 120 monoxide in varying come-, y 3.5 positions were irradiated and 5.0 counted in the internal count e r , L e a d m o n o x i d e was 8-, Y 230 select2d as a filler since it has about the same mass absorption as rhodium. The principal error in analyzing these rhodium samples of known composition is the statistical error of the counting determination. Figure 7 shows a plot of the per cent composition versus the probable error in per cent for representative known mixtures of rhodium and lead monoxide. Each experimental point on the graph represents the average of several determinations. This graph represents the degree of accuracy that can be expected from the method. I n evaluating an unknown sample the probable error of counting the standard must also be considered. Curves for the per cent composition versus the probable error in the activation analysis of silver and indium are also shown in Figure 7. As in the case of the rhodium, samples were made up for the analysis using lead monoxide as a filler. The total amount of element present in unknown samples can be determined by comparison with a known sample of the pure element. When the Anton tubes were used for the determination of rhodium and silver, absorption due to the window thickness of the tubes was only a minor disadvantage because of the high energy beta particles involved. The main advantage to these tubes was to increase the counting geometry of the samples. Using this counting apparatus, thicker samples could be tolerated since both sides of the sample were being counted. Experimental data obtained with the 1-gram samples previously used in the internal counter show a definite increase in geometry. This increase in detection sensitivity for rhodium and silver is shown in Figure T I)? the dotted lines. 6-,
0.087 2.6 1 5(60%) 0 . 8 3 (40%) 2 . 2( 6 0 % ) 2 . 8 (40%) 1.00(51%) 0 . 8 7 (28%) o.60(21yo) 2.95 0.10 1.25 0.88 0 42 0 044 (-60%) 0 046 (-30%) 0 . 0 5 4 (-16%) 0 . 0 5 6 (-4%) 0 67
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-
1000 lo
I
2
Figure 4.
3
4
5 6 7 8 9 DECAY TIME (MIN)
IO
I1
12
13
1415
Decay Curve of Activated Pure Rhodium
rhodium are present in detectable amounts. In a rhodium analysis, for example, a differential decay curve of the irradiated sample can prove the absence of silver. If half-minute counts of the sample (covered with 4 mils of aluminum foil) show a decay into background with a half life of about 44 seconds it is established that no detectable amounts of contaminating materials are present. I t is also possible to analyze some rhodium samples that are known to contain silver. Rhodium can be favored by short irradiation times. When gross decay curves are resolved, the intercepts of the resolved lines give the activity a t the end of irradiation. This activity can then be compared with that of a standard to give the amount of rhodium present.
\\
EXPERIMENTAL RESULTS
The percentage of an element in an unknown sample can be determined by using the relationship between the activity induced in a sample of known weight, and the unknown. The direct ratio between the amount of element and the activity of the sample is shown in Equation 2 .
'00
I
Figure 5.
:
6
:
b
DECAY TIME (MINI
b
I
b
I
I
Decay Curve of Activated Pure Silver
V O L U M E 25, NO. 5, M A Y 1 9 5 3
783
1 ’ ” ’ ’ ’ ’ ’ ’ ’ ’ 1
IO
L-+udLd 2
3
4
5
Because of the recent availability of reliable scintillation counters this activationanalysis technique couldalso be readily adapted to elements yielding ‘predominantly gamma-ray emitting isotopes. Data similar to those presented in Table I1 and Figure 2 could be compiled (6, 6) for these gamma-ray emitters and used to develop an analysis scheme.
6
DECAY TIME (HOURS)
Figure 6. Decay Curve of Activated Pure Indium (After Decay of 13-Second Indium-116) DISCUSSION
One major uncertainty in this method of activation analysis using the comparison between sample and standard is the statistical uncertainty in the counting rate. These probable errors are shown in Figure 7 for one particular irradiation source and one or two sets of counting apparatus. It can be seen that as the sensitivity of the counting apparatus is increased the error of the measurements is decreased. Furthermore if larger sources producing more activity are used the probable error is made even smaller. Antimony-beryllium sources are available from the Atomic Energy Commission a t a nominal fee to provide a higher neutron flus. One main disadvantage of these sources is that they decay with the 60-day halflife of the antimony isotope. It is possible to use them for these analyses if the standard counts are run consecutively with the saniplr to avoid error due to the decay of the source. One word of caution should be raised a t this point about increasing the sensitivity of counting equipment and raising the fluies of the neutron sources. One main objective in using the 25-nig. radium-beryllium source and commercially available chaunting equipment was to escape the detection of elements that had somewhat lower neutron activation cross sections than silver, rhodium, and indium. Care should be exercised so that as more activity is detected because of improved methods of irradiation or detection, contributions to the observed activity are not made by other elements such as vanadium, bromine, and iodine. Several other possible errors in the method should be mentioned. The sample cannot be so thick and contain material of such high cross section that the front surface of the sample removes an appreciable amount of the neutron flux. None of the samples used in this work exhibited this effect. In addition, the mass absorption of the “filler material” in an unknown sample can produce errors if this filler material is of a verydifferent atomic number than the metal activated and if the composition of the standard is of a different type. This error is usually rather small, however, since the mass absorption coefficient is a slowly varying function of the atomic number. Further errors could be introduced by contaminating activities. K i t h the procedure as outlined above only rhodium, silver, and indium could contaminate the analyses. Of these the interference of rhodium and silver could cause the most trouble. Silver rarely occurs with rhodium in nature, however, and solid solutions of silver and rhodium are not formed.
\
0.10
2
3
4
5
6
7
8
PER CENT PROBABLE ERROR
Figure 7.
Statistical Error in Activation Analysis Determinations Solid lines, flowcounter Dashed lines, Anton counter
The activation analysis technique using low level neutron sources can be particularly useful for rapid assay of rhodium and silver ores as well as identification of alloys cont’aining these elements. The method may also prove useful to laboratories having analysis problems of this type involving other high cross section elements. ACKNOWLEDGMENT
The aut’hors wish to thank the Michigan Memorial Phoenix Project for its generous support of this research and continued encouragement of work in the nuclear field. LITERATURE CITED
(1) Boyd, G. E., U. S. Atomic Energy Comm., AECD 2507 (March 1949); AINAL. CHEM.,21,335 (1949). (2) Calvin, hI., et al., “Isotopic Carbon,” p. 27, New York, John Wiley 8: Sons, 1949. (3) Hamill, W. H., Williams, R . R., Jr., and Schuler, R . H., J . Chem. Educ., 26,310 (1949). (4) Hevesy, G. V., and Levi, H., Kgl. Danslie T’idenskab. Selskab, Matfys. Medd., 1 4 , 5 (1936); 15, 11 (1938).
(5) Hollander, J. M., Perlman, I., and Seaborg, G. T., Table of Isotopes, Cniversity of California Radiation Laboratory Report, UCRL-1928, Revised (December 1952). (6) Hughes, D. J . , et al., Xeutron Cross Sections, G.S. Atomic Energy Comm., AECU, 2040, (May 15, 1952). (7) Leddicotte, G. IT., and Reynolds, S. A , , .l’ucleonics. 8, S o . 3, 63 (1951). (8) hIurphy, W. J., AXAL.CHEM.,24, 1235 (1952). (9) Senftle, F. E., and Leavitt, W. Z., Sucleonics, 6 , Yo. 5, 54 (1950). (10) Taylor, T. I., and Havens, W.W., Jr., Ibid., 6 , No. 4, 54 (1950). (11) L-. S. Atomic Energy Comm., Isotopes Division, Oak Ridge,
Tenn., “Isotopes Catalog and Price List KO. 4,” March 1951. (12) Way, K., et al., S a t l . BUT.Standards Circ. 499 (1950).
RECEIVED f o r review h-ovember 10, 1952. Accepted February 7, 1953. Presented before t h e Division of Analytical Chemistry a t t h e 123rd Meeting of the AMERICASCHEMICAL SOCIETY, Los Angeles, Calif.
