to assume K was constant, the binary counts fraction us. composition curves drawn through the calibration points a t 25, 50, and 75% (Figure 12, a, b, c) allow for a limited amount of “adjustment” and it is this fact which enables a skew grid to be drawn, the grid having its greatest accuracy in the region of the grid lines passing through these points. To see how far we would deviate from the binary calibration curves by making the respective values of R strictly constant over the whole compositional range, values of K were computed for each system for the experimentally determined points a t 25, 50, and 75%, respectively. These values are given in Table I. -1s can be seen from Table I, the value of K varies remarkably slowly for the systems Ge-Bi and Te-Ge, while for Bi-Te it is almost constant. Consequently, as shown in Figure 12, a, b, and c, the idealized points with constant K coincide over wide ranges of composition with the adjusted curves actually employed in drawing the skew grid. Because, of this, the worst deviations of the grid lines from the ideal compositional values are only of
the order of 1 or 2 atomic %, the greatest accuracy being in the immediate vicinity of the 0,25,50,75, and 100% positions on the binary edges. Thus the grid may be considered to be good enough for approximate analyses. In the binary system Ge-Te, the intermetallic compound “GeTe” does not occur a t the stoichiometric 1 to 1 value, but lies a t Gem.STe48.5. Consequently, four calibration samples were actually used in this system-at 25, 49.5, 50.0, and 75 atomic % Te. Similai!y, advantage was also taken of the intermetallic compound located a t BizTea. Because of the variations in K , the grid lines drawn through the “adjusted” binary calibration curves do not pass accurately through foci located on the extended sides of the teroarq composition triangle. If they had done so-i.e., if K were truly constant for each system-only one calibration point would have been required on each of the traingles’ sides in order to establish the grid. As i t is, a minimum of nine is requiredthree alloys to each side-for a satisfactory grid to be obtained, although, of course, any extra calibration points,
such as a t BiZTes, assist in increasing
its over-all reliability. ACKNOWLEDGMENT
The authors are indebted to W. H. Tiller for supplying the alloy samples and to Eugene H. Rosehoom for his very helpful suggestions in connection with ternary projection theory. LITERATURE CITED
J., Brissie, R. &I., A N A L . CHEW26,980 (1954). 1~, 2 ) Darken. L. S.. Gurrv. R. W.. “Phvsical Chemistry’ of Metals,” ’ p. 236, (1) Beatty, H.
McGraw-Hill, Xew York, 1953. (3) Gibbs, J. W., “Collected Works,” Vol. 1, Longmans, Green, Kew York, 1928.
(4) Hamos,
L. v., *4rkiv Matematik Astronomi och Fysik 31A, 1 (1945). (5) Korehinskil, S., “Physicochemical Basis of the Analysis of the Paragenesis of Minerals” (Translation, Consultants Bureau, New York), 1959. (6) Roozeboom, H. W., 2. physik. Chem. 15. 147 (1894). (7) Stokes, C. G., Proc. Roy. SOC.(London) 49A,171 (1891).
RECEIVED for review February 13, 1961. -4ccepted August 29, 1961.
Use of Very-Short-Lived Isotopes in Activation Analysis OSWALD U. ANDERS Radiochemistry Laboratory, The Dow Chemical Co., Midland, Mich.
b Until recently activation analysis worked mostly with long-lived induced activity, requiring hours or days for activation. A new type of activation analysis is reported here for tungsten, gold, silver, selenium, hafnium, fluorine, and oxygen, making use of their 5to 24-second isotopes for counting. The concept of the y-ray difference spectrum is applied to suppress longerlived components in the gamma spectra of the activated samples. Analyses require less than I/$ hour. As little as 1 p.p.m. of hafnium can be detected.
possible and the procedure has to be entirely instrumental. Multiple irradiation runs as proposed previously (1) can be used for increased sensitivity and statistical accuracy. Identification and analysis will then be carried out by y-ray spectrometry and may for multiple-component systems, be followed by the peeling procedure set forth earlier (3). The purpose of the present report is to discuss the procedure and demonstrate its applicability for samples of several elements.
multichannel analyzer after the return of the rabbit to the counting position. Multichannel Analyzer. The RIDL-34 analyzer permits subgrouping of the memory into 1 X 200, 2 X 100, or 4 x 50 channels. Our model also includes a fast-switching subgroup selector, so that it is possible to collect a spectrum into the first subgroup for a preset amount of time, then switch, and within 0.2 second start collecting a spectrum into the second Rubgroup.
INSTRUMENTATION
The samples were weighed, packaged into small polyethylene rabbits prepared from 2-inch pieces of l/rinch i.d. polyethylene tubing, and closed with two oversized polyethylene plugs to provide the antistreamline feature to the finished rabbit. When only small samples were analyzed, they were centered inside the rabbit with two pieces of polyethylene foam. For data collection a rabbit is introduced into the tube system via the air lock a t the counter terminal and the rabbit-control timer set for eight runs a t 2-minute cycles. The rabbit is then automatically transferred into the 2-
EXPERIMENTAL PROCEDURE
U
recently, neutron activation analysis procedures were restricted almost exclusively to the counting of activation products with half lives longer than 1 or 2 minutes. A recent survey of the short-irradiation properties of the majority of the elements (2) made it apparent that accelerator-based activation analysis might profitably exploit inducible short-lived activities with half lives below 1 minute. For the type of analysis a t issue chemical separation steps are probably not NTIL
1706
ANALYTICAL CHEMISTRY
Rabbit System. The shuttle-rabbit system employed for this work was an improved version of the one previously reported (1). The timer controls the solenoid air valves individually to assure reproducible positioning of the rabbit in the irradiation and counting positions and prevent back-pressure build-up. It is possible to select 1- or 2- minute cycling as well as the number of cycles (up to 9) to be carried out. The timer will also reset and trigger either a scaler or
106
-
IO6
I V 0)
,
N
E
105
i m
P
105
\ a W \
f
I io4
~
0
10
20
30
40
50
60
0
10
CHANNEL NUMBER
Figure 1. Normalized gamma spectra of neutronactivated selenium Cumulated spectra taken 2 seconds after 30-second irradiations Cumulated difference spectrum representing amount of activity that decayed in first half minute after irradiation
10
8
d‘
20
40
50
60
irradiation is added to the content of the first 100 channels, containing the cumulative spectra fioni the previous runs. Similarly, the spectra taken 32 seconds after the elid of the irradiations are cumulated in the second half of the memory. ilt the end of eight cycles, tlie content of the memory is printed out and punched on paper tape in a code compatible i\ith the LGP-30 Royal XcBec computer. Immediately following the sample runs, a rabbit containing a known amount of the element is irradiated and counted in the same manner as the unknonn to serve as comparison standard. Finally, cumulative spectra are collected with a n empty rabbit to serve as “background” correction to the spectra.
0
O5Mev
30
Figure 2. Normalized gamma spectra of neutronactivated silver
0
into the first 100 channels of the memory. The dead time is read from the dead-time meter of the analyzer a t the half time of the count. At the end of this time, the analyzer switches and starts collection of a second 30-second spectrum into the second subgroup. Again, the dead time is read a t the half time of the count. At the end of the data collection the step switch of the “number-of-cycles” selector is advanced by one to end the cycle. The sequenre is repeated once every 2 minutes until the eighth cycle is completed. After e7w-y 30-second irradiation the spectium taken 2 seconds after the eiid of the
foot-cube paraffin moderator to the irradiation position - J / 4 inch in front of the Be target of the 2-mv. Van de Graaff accelerator. The neutron flus there is approGinately 2 X 108 nlsq. em. /see of tlwrnializ~dneutrons. After ewctly 30 seconds the rabbit is brought back to the counting position by the reversed air stream. T n o seconds later the tiiner resets and triggers the 200channel ana1yLer t o collect the y-spectruin. The delay allows for the transfer of eyen the heaviest rabbits and assures reproducibility of the counting data irideperidmt of rabbit eight. The spwtruni is tnkcii for esactly 30 seconds
20
CHANNEL NUMBER
2 2 Mev
30
40
50
CHANNEL NUMBER
Figure 3. Normalized gamma spectra of neutronactivated hafnium
0
IO
20
30
40
50
60
CHANNEL NUMBER
Figure 4. Normalized gamma spectra of neutronactivated gold VOL. 33, NO. 12, NOVEMBER 1961
e
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'EN L
0
10
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30
40
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60
0
10
CHANNEL NUMBER
Figure 5. Normalized gamma spectra of neutronactivated tungsten
DATA TREATMENT
The punched paper tapes are fed into the computer, which is programmed to smooth the data and reduce it to half the experimental points. The computer will then convert the data to counts per minute, correct for the dead time, and subtract out the smoothed background spectrum. Finally, the corrected spectra taken 32 seconds after the irradiations are subtracted from the spectra taken 2 seconds after the irradiation. The computer then prints out this difference spectrum as well as the corrected first spectrum (taken 2 seconds after the irradiations). It also plots both as four-cycle semilog plots for easier inspection. DISCUSSION
Il'hile there might be a marked contribution to thc spectra by longerlived activation products, the short irradiation-counting cycles strongly favor activities n i t h half lives less than 1 minute. By subtraction of the spectrum taken 32 seconds after the irradiations froin the spectrum taken 2 seconds after the irradiations, most of the longer lived contributions are suppressed. This brings out clearly the spectra of those activities which decayed rapidly during the first half minute. This elimination of interferences might well outweight the possible "loss" of useful activity of isotopes with half lives of 30 seconds and up to even 2 minutes, where only half, or 8 , of the total available activity is used for the analysis. The method is, of course, most efficient for activities with half lives between 5 and 15 seconds. The interpretation of the data is made by comparing the number of 1708
ANALYTICAL CHEMISTRY
20
30
50
40
60
CHANNEL NUMBER
Figure 6. Normalized gamma spectra of neutronactivated fluorine
counts in the photopeak ranges of the difference spectrum with the equivalents in the spectrum of the standard. If a collection of standards is available for all elements giving rise to shortlived activation products, they may be normalized t o I-gram sample weight and a standard flux and a peeling technique (3) employed for the case of dij-
Table I.
Sample Se, std.
No. 1 No. 2
Ag 1 std No. 3 A g 3 std. so. 4 Hg std. KO.5 Au-71 std. No.
No.
2 l
IT std. No. 8
No. 9 F std. s o . 10 s o . 11 s o . 12
0 std.b s o . 13
Element Se
Se Se
Hf Hf AU
-2U -iu
IT I!\\-
F F F ti'
ference spectra containing components from several elements. EXAMPLES
OF
ANALYSES
A preliminary experimental investigation of the very-short-lived neutron activation properties of some 60 elements indicated the usefulness of the above method for the determination
Analytical Results for Seven Elements
Content, hlg. 57.0 61 1 35 8
1.36 1,io 15,s 16.0 15,l 145 0 191 . u
5i.S
18 15 11 21
15 39 76 17
Found, xg.
% Deviation
61.3 36.5
0.3 1.9
14.5
1.3
6.5
2.9
1.G9
0.00
15.3 15.6
1.3 3.3
185.0 64.9
3.1 13.
15.34 11.58 21.20
0.3 1.5 0.1
Peak Count Rate" 1,261,165 77,308 46,044 830 730 12 044 903,391 5,868 9,458,000 16,005 77,881 1,217 1,190 53,916 9 987 3,503 103,743 1,591 1$201 2,199 5,370 18,09i 22,199 3,603 2,373 17,243 13,000 ~
3370 0 .5 4134 1.2 s o . 15 671 2.i So. 16 4-12 "5 No. 17 3211 0.5 2121 KO. 18 2.1 a All count rates normalized t o flux of approximately 1 x 108 nlsq. cni./sec. Count rates of standards are further normalized to 1-g. weight of element. * Oq*gen determinations employed 14-1n.e.v. neutrons obtained by irradiation of tritiated titanium foil containing 1 curie of Ti per square inch. .260-pa. beam of 400k.e.v. deuterons and deuterium ions was used. KO. 14
of oxygen, fluorine, sodium, scandium, germanium, selenium, bromine, yttrium, rubidium, rhodium, silver, indium, erbium, ytterbium, hafnium, tungstrn, iridium, and gold (Figures 1 to 6). The examples given in Table I mere chosen as typical to demonstrate the method and indicate the sensitivities obtainable for seven elements. To facilitate comparisons, the counting data from the thermal neutron activations were normalized to a flux of approximately 1 x IO8njsq. cm./sec. The data are expressed as counts per minute, obtained by dividing the deadtime corrected cumulated 30-second spectra from eight irradiation-counting cycles by 4. This is a somewhat arbitrary procedure, since the count rates vary markedly during the time of each of the counts. For the illustrations the data of the standards were further normalized to 1-gram sample size. For the determinations of fluorine and oxygen the fast neutron reactions
F19 (nJo)XI6and 0 I 6 (n,p)N16 were used. I n the fluorine case the same flux was used as for the other elements, but a Cd foil was wrapped around the rabbit tube in the irradiation position to exclude the thermal neutrons inside the paraffin cube. The 14-m.e.v. neutrons for the oxygen determinations were obtained by irradiating a tritiated titanium target with 60 pa. of 400-k.e.v. deuterons and deuterium ions. I n both cases the 6.1-m.e.v. photopeaks and pair peaks of the N16 were used for the analysis. Because of the high Q value of the oxygen reaction, no interference has been observed in the fluorine determinations. The oxygen samples used were free of fluorine. It is important that the standards are of approximately the same weight and geometry as the unknown, whenever materials of high cross section are involved. This is seen from the cases of silver and tungsten. The self-shielding effect otherwise experi-
enced can cause errors of more than 100%. Most of the error encountered is due to statistics, This is seen in the case of gold, where only relatively few counts were collected for the samples employed. ACKNOWLEDGMENT
The author thanks R. Wayne Miller for his assistance with the operation of the Van de Graaff accelerator. LITERATURE CITED
(1) Anders, 0. U., ANAL.CHEAT. 52, 1368
(1960).
(2) Anders, 0. U., “Gamma-Ray f3pectra of Neutron-Activated Elements, Dow Chemical Co., May 1960. (3) Anders, 0. U., Beamer, W. H., ANAL.CHEM.33,226 (1961).
RECEIVEDfor review June 9, 1961. Accepted July 3, 1961. Division of Analytical Chemistry, 140th Meeting, ACS, Chicago, Ill., September 1961.
X-Ray Spectrographic Determination of Thorium in Uranium O r e Concentrates W. C. STOECKER and C.
H. McBRlDE
Uranium Division, Mallinckrodt Chemical Works, Saint Charles, Mo.
b
Thorium is determined rapidly and precisely in uranium ore concentrates by an x-ray spectrographic measurement of the intensity at the Thla peak. The intensity is compared to that of the U 11 peak as an internal standard. Background interference due to the nearby U l a peak is compensated by a simple mathematical correction applied to the intensity ratio. The variable (but accurately determined) internal standard compensates for differences in composition of ore concentrates from various sources, and the effects of the common contaminants are eliminated. No elements interfere at concentration levels ordinarily encountered.
T
was determined in rock samples by Adler and Axelrod ( I ) , using thallium as an internal standard on a two-channel spectrograph. They used either the Lpl or the La peak of thorium, depending on interferences, and compared x-ray intensity with that of the thallium I&/32 peaks. Pish and Huffman (8) determined thorium and uranium in aqueous solutions and in a nonaqueous solution by measuring the La peaks of both elements. Strontium was used as the internal standard for HORITJM
aqueous solutions, and bromine (as bromobenzene) for the nonaqueous. The K a peaks of both internal standard elements occur in the desired region. Campbell and Carl (4) combined x-ray and radiographic measurements to determine small quantities of thorium and uranium in ores. The weight ratio of the two elements, together with a total radioactivity measurement, enabled a determination of the concentration of both elements. Uranium, in particular, is often determined on the xray spectrograph, usually through a measurement of La peak intensity. The authors have determined minor amounts of uranium in several materials using strontium as an internal standard and measuring the U La/Sr K a x-ray intensity ratio. Although in uranium ore concentrates, the thorium La peak intensity is a function of thorium concentration, it is also dependent on other factors such as instrumental variables and difference in sample composition. An internal standard is therefore required for the necessary precision arid accuracy. The use of uranium ( 7 ) as the internal standard eliminates the need for sample preparation except for the reduction of coarse lumps. Figure 1 shows a spectrum of a typical ore concentrate, and
indicates the need for a correction of background due to the U La peak shoulder. EXPERIMENTAL
Apparatus. The work was done on a Philips Electronics x-ray spectrograph, Type 52260, operated with the usual power supply and scaling units. A scintillation counter detector and a lithium fluoride analyzer crystal were used throughout the investigation. The sample boat used was somewhat smaller than that supplied with the instrument. It has a depth of l:g inch and a capacity of 3 ml. and was machined from a 3/*-inch sheet of Plesiglas. Mising operations were done on a Spex Industries KO. 8000 ?vlixer/hIill using the No. 6135 polystyrene vial and four Plexiglas balls. Some grinding tests were also made with the steel vial. Hand grinding in a glass mortar was used in most cases for preparing samples for packing into the sample boat. Analytical Procedure. Grind 5 to 7 grams of sample in a glass mortar until a smooth surface can be formed by pressing with a spatula. The Mixer/Mill may be used if desired. Pack the sample into the Plexiglas sample boat and form a smooth level surface with a spatula or glass slide. VOL 33, NO. 12, NOVEMBER 1961
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