Activation Analysis of ~ i c k e l s Jack C. Nonnan1 and David S. Blinstmb2 University of Wisconsin-Green Bay, Green Bay, WI 54301 We have found that students are more interested in, and learn more from, laboratory exercises that use familiar materials that are relevant to their lives. Nickels, United States five-cent coins, are an excellent example of familiar ohiects that may be used as the "unknown" sample in a radiochemistry or instrumental analysis experiment. The analysis of nickels for copper by the neutronactivation method using a small neutron source is easily done in a three-hour lahoratory period, is interesting to students because it uses a "real" sample, and is open-ended in that students may he stimulated to explore the analvsis of other tvnes .. of coins or other metals in coins. The principles and techniaues of neutron activation analysis have heen described by many authors (for example, see references 1-9). We use a 5-Ci 239Pu-Beneutron source. hut other (a,n),(y,n),252Cfsources (I), a tritium-deuterium Leutron generator, or a nuclear reactor could he used as the source of neutrons. Twenty-five nickels (about 125 g) are irradiated for 20 min. Starting 1min after the end of irradiation, either the total sample gimma-ray activity or the 1.039 MeV gamma ray from "Cu (10) is monitored for about an hour hv takine a series of 4-min counts with a NaI(T1) , . detect& and a scaler, single-channel analyzer, or multichannel analvzer. I t is helpful if students have studied the eamma-ray Bpectrum of ;radiated copper in a previous lacorators exercise. An analvtical-made cooner-shot standard (about 125 g) with a p p r i x i m a k y the same geometry as the nickels is irradiated in the same position for the same time. The sample size and irradiation- time may he varied, but, because the neutron flux from most small neutron sources is very inhomogeneous, it is important that the sample and standard he nearly the same in size and shape and that they he irradiated and counted in the same way. A series of 4-min counts, starting 1 min after the end of irradiation and extending over an hour, is also taken for the standard. Semilog plots of the counts for both the nickels and standard are made. After ahout 30 min the activitv becomes nearly constant. This background activity, due to"the detector background and long half-life activities produced in the irradiation, is subtracted from the total activities over the first half hour, and the net activities due t o WUare plotted. The 5.10-min half-life of WU(11) is verified from this plot,
' A h o r to whom correspondence should be addressed.
Present address: Physics Department, University of NebraskaLincoln.
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Journal of Chemical Education
and the net activities of the nickels and the standard a t the end of irradiation are determined. The mass of copper in the nickels is then found by simple proportion: (Net %u activitv in nickel8 at the end of irradiation) (Mass Cu in = (Mass Cu in X (Net "CU activity in standard nickels) standard) at the end of irradiation) We have usually found a lower percent copper than is given in A Guide Book of United States Coins (12). The Eomposition of nickels minted since 1946 is listed as 7590 Cu and 25% Ni. We tvpirallv find about :ODi comer and a small amount of manga&se. The identification anddetermination of manganese may he incorporated directly into the experimental~procedureor may be suggested as a problem to he solved by the students. A similar procedure may be used to determine the copper content of other coins. Some care must he taken to insure that all the coins of agiven sample had the same comoosition when thev were minted. esneciallv if nennies (US one-cent pieces) are to he used. 1" 1 k 2 , the-composition of pennies was chaneed from 95% Cu with 5% Zu to 97.6% ~ n k i t 2.4% h Cu (13c After a t least a week of "cooline". the nickels mav he reused in this experiment. ~ l t h o u g h t h iprocedure s is nlndestructive, t h e coins used should not be returned to circulation f o r a t least two weeks a n d unless careful monitoring indicates t h a t a l l induced radioactivity, iucludiug 12.7-h half-life W u , h a s decayed t o below hackground. Llterature Clted 1. Bowen, H. J. J.Chem. Educ. 19'15.32.682. 2. Chase, G. D.: Rabinowitl, J. L. Plineiples of Radioisotope Methodology, 3rd ed.; Burg-: Minneapalis, 1967; pp 427432. 3. Choppin, G.R.; Rydberg.J. Nvrlenr Chemistry: Perprnon: New York, 1980:pp 411414. 4. Downey.D. M.:Ssndy,T.A.J.Chem Edur 1986,63,178 5. Meinke. W. W. A d . Chem. 1960,32,104R. 6. Meinke. W.W.: Anderson. R. E. A n d Chem. 1953.25.778
389-398.
1970:p
10. Adarns, F.;Darns, R. Applied Gommo-Rqv Spselromfryi Pergamon: Oxford, 346. Chart of the N u c l i d q 13th ed.; General Electric: San Jose. CA, 1983. 12. Yeoman, R. S. A Ouida Book o/ United Stoles Coins. 39th ed.;Westom: Rscine.
11.
1986:p 96. 13. Miller, J. M. J.Chem. Edue. 1983.60, 142..
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