P. J. Daly, K. J. Hofstetter, and F. Schmidt-Bleek Purdue University Lafayette, Indiana 47907
Determination of Copper in Alloys by Fast Neutron Activation
Several general papers dealing with neutron sources and their use in activation analysis have appeared in THIS JOURNAL.' We have developed a specitir experiment which illustrates the basic principles of radioactive decay and of the activation technique and is offered with varying degrees of sophistication a t all levels of undergraduate education. The following features, which make such an experiment very suitable for laboratory courses involving large numbers of students, are emphasized. While the experiment is simple to perform, it is formally correct and accurate results can he obtained in a 3-hr laboratory period. Health hazards and risks of contaminating laboratory or counting equipment are entirely negligible. As a nondestructive technique is involved, the same calibrated "unknowns" can be used in successive laboratory periods. In order to perform the experiment a source providing a 14-MeV neutron flux exceeding 5 X 108neutrons/cm2 sec must be available; such sources are now installed in many large educational institutions. Principles
The spont,aneous decay of a radioisotope is a firstorder process, i.e., the number of disintegrations per minute (dpm) is directly proportional to the number of radioactive atoms present:
where N is the numher of radioactive atoms, - (dN/dt) is the decay rate, and Xis the decay constant characteristic of that particular isotope. It follows that
When the decay rate is plotted versus t on semilog paper a straight line results; the half life of the isotope can readily be obtained from such a plot. In the case where two radioisotopes are present in a sample, the resulting decay curve can be resolved into two straight 1'me components. When a sample containing X atoms (stable) of an element E is bombarded with suitable nuclear proL
V
~K. S., ~ J.~CHEM. ~ E~ ~ ~,c . 3 7 . 3 9(1960); 1 WISE, E. N.,
J. CEEM.EDCC.39, A771 (1962); and Nnss, H. W., MADDOCK, R. S., AND MEINKE, W. W., J. CAEM.EDUC.41,127 (1964). IC 2 Model 111P, Technical Measurements Cop., 441 Washington Ave., Xorth Haven, Cam. (now available through Picker Nuclear Corp., 1275 Mamaroneck Ave., White Plains, N. Y. 10605). a GM Scaler Model 124, Baird Atomic Inc., Cambridge, Mass. 'Timer Model ET-79A, Atomic Accessories Inc., Valley Stream, K. Y.
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jectiles, a tiny percentage (depending on the nuclear reaction cross section and on the projectile flux) of the X atoms are converted into radioactive atoms. If two samples containing XI and X2atoms of E are bombarded under identical conditions producing activities A1 and Az respectively, and it is known that elements other than E present in the samples give rise to negligible levels of activity, the following simple relationship holds
When natural copper (a mixture of WUand =%CU) is bombarded with 14 MeV neutrons the most important nuclear reactions which occur are:
+n Wn + n
65Cn
--
2n 2n
+ WU
+ WU
"Cu has a half life of 12.9 hr, but WUis short-lived (tl,2 = 9.9 min) and in the course of the present experiment, the student determinesits half life. In bombardments of commercial brass foil with fast neutrons, "Cu and "Cu are again the principal activities induced; in addition, 38 min 63Znactivity results from theB4Zn(n, 2n) 63Znreaction but in such low concentration that its presence can be neglected in applying the activation principle to the determination of the percentage copper in brasses. The Experiment
Apparatus. A TMC Neutron Generator2providinga fast neutron flux of approximately lo8neutron/cm2 see is used to irradiate brass discs of 200 mg/cme thickness and 2.5 em diam for 10 min. Copper discs of identical dimensions (monitors) are irradiated along with the unknowns. Baird Atomic scalersS with built-in high voltage supplies and attached clocks4 are used in conjunction with Geiger-Miiller counters to measure the induced activities. Procedure. The experiment is performed by pairs or groups of three students. Initially, standard R a D-E sources are used to check that the counting equipment is in good operating condition. (Upperclassmen are also required to determine the plateau characteristics of the G-M counter.) The counter background is established by counting for a 10-min period with no source present. Background is subtracted from all subsequent counts. Each group is given a pair of recently irradiated copper and brass discs, which are then counted alternately until the end of the laboratory period. It is recommended that in order to achieve statistical precision (*3yo) a t least 10a counts should be accumulated in each counting period. The data are tabulated, and the count rates (corrected for background) are plotted
versus time. Virtually all (>997J of the 'WU decays within 100 min of irradiation, and therefore the points on the decay curves after this time represent 04Cu activity only. The 12.9-hr decay component is obtained by drawing a straight line through these points. To ensure that thisline is drawn correctly, the student is informed that 04Cuactivity decreases by about 10% in a 2-hr period. By subtracting the 12.9-hr component from the gross counts, a new set of points representing the contribution of the WUactivity a t various times is obtained (see the figure). A straight line is drawn through these new points and the half life of 6 2 Cis~ evaluated. In order to determine the percentage of Cu in the brass, each WUdecay line is extrapolated to time zero. As the samples have the same thickness, 0 absorption corrections are unnecessary, and the following simple relationship can be used: % Cu in Brass
= =
0 A
Long lived r rho,,
lived
Short l i d
weight of Cu (brass) X 100 weight of Cu (Cu) count rate of WUin brass X 100 count rate of "Cu in capper at the same time
For brass unknowns containing about 65% copper, a freshman student can be expected to determine the copper content within an accuracy of 57& using this method. In advanced undergraduate courses, a detailed statistical error analysis can be performed, and a higher degree of accuracy can he attained.
Semilogarithmic plot of decay doto and ih rssolution into components.
Volume
44, Number 7, July 1967
and 04Cu
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