Krishnaswamy Rengan Eastern Michigan University Ypsilanti, Michigan 48197
An Elegant Neutron Activation Analysis An undergraduate experiment
T h r intwduction of the lithium-drifted germanium detertor by Tavendaleand Ewan ( I ) ha3 revolutionalized wmma-ray spectroscopy. T h e use of this detector as a powerful tool for neutron activation analysis has been demonstrated by Prussin, et al. (2).Nondestructive and destructive neutron activation analyses are findimg applications in archeological, biomedical, cosmochemical, environmental, and geochemical sciences as well as in industry (3).Also, Bernstein (4) has summarized the agricultural and food industry applications of neutron activation analysis. Since this technique is finding applications in routine quality control in industry as well as in advanced research in a wide variety of fields, it is very important to teach undereraduate science maiors, es~eciallychemistry maiors, the fu;;diimental uf nt:utrc;tl arti\ation &alysis. 'l'hii author i i of the u ~ i n i u nthnt wherever possihle a neutron activation analysis experiment should he introduced as part of an upper level laboratory course. Bowen (5)described the use of a californium-252 neutron source for simple neutron activation analysis. T h e current article describes an elegant neutron activation experiment that could he done in a regular lahoratary period. This experiment introduces the student to hoth aunlitative and auantitative asoects of this techniane. Since the experiment is based on short-lived radionuclides it must, however, he performed in a laboratory adjacent to a nuclear reactor. Theory In general when a sample is irradiated by neutrons the activated products are produced hy an (n,y)reaction of the isotopes of the elements present in the sample. The radioactive products, which form, decay by emission of negatrons and gamma rays. The energy of the gamma rays emitted by a given sample after neutron irradiation depends on the elements that are ;resent in the samole. The measurement of the enerav of a gakma ray and the half-life with which the intensity ofthe gamma ray is decreasing allows qualitative identification of an element. For example, identification of a 1369-keVgamma ray decaying with a 15.0-hr half-life indicates the presence of sodium. Quite often, measurement of the energy of a gamma rav is sufficient to identifv the presence of an element. There are several publications whichiist gamma-ray energies in increasing order and identify the corresponding radionuclide (68). Dams and Adams (6) listing is the most convenient one for teaching purposes. The amount of activity formed from a specific isotope of an element is given by the equation (9) where A = activitv in disinteaations Der second. N = number of atoms of the specific isotope which is being activated, o = interaction probability, cm2, f = neutron flux, neutrons cm-% s-1, A = decay constant of the radionuclide produced, s-', and t = irradiation time, s. For quantitative analysis of an element, a known amount of that element (standard) is irradiated along with a known weiaht of the sample and the intensities of a specific gamma ray in the sampleand the standard are compared. 'The author is willing to supply small quantities of the alloy to those who want to introduce this experiment in their curricula.
gamma ray],,,,l. - _- A rample Ilnl~ns~ls
