DEVELOPMENT OF A PROMPT GAMMA NEUTRON ACTIVATION ANALYSIS FACILITY AT THE UNIVERSITY OF MISSOURI RESEARCH REACTOR AND OBSIDIAN IDENTIFICATION.

摘要

A facility has been developed at the University of Missouri Research Reactor (MURR) so that the unique features of Prompt Gamma-Ray Neutron Activation Analysis (PGNAA) can be used to measure trace and major elements in samples. This facility consists of a radial beamport, external sample position with shielding, and multi-mode counting system. Beamport A was developed to bring a well-collimated thermal neutron beam out of the reactor to interact with the samples. A single crystal silicon neutron filter was installed along the path of the neutron beam to reduce fast neutrons and gamma radiation associated with the beam. Using thin gold foils, the beam was measured at the sample position to be 5 x 10('8) n/cm('2)/sec with a cadmium ratio of 42. The radiation background outside the shielding is one-third of the allowable dose, and the thermal and fast neutrons at the Ge(Li) detector 0.1 n/cm('2)/sec and 2 n/cm('2)/sec, respectively. The prompt gamma-ray counting system consists of a Ge(Li) detector surrounded by a cylindrical NaI detector. Three modes of operation are possible: single, Compton-suppression, and pair spectrum. Prompt gamma-ray spectra were collected in the energy range from 50 keV to 9 MeV. Irradiation of pure element or simple compound standards were performed to identify the prompt gamma ray energies from each element, their count rates, and their interferences, if they exist, as well as the magnitude of these interferences.; The PGNAA technique was applied to the analysis of 16 Meso-american obsidian samples. The following elements were detected in each sample: H, B, Na, Mg, Al, Si, Cl, K, Ca, Ti, Mn, Fe, Sm, and Gd. From these measurements, it was found that B, Cl, Sm, and Gd can be used to determine inter- and intra-source obsidian variations. Accordingly, characterization of obsidian samples resulted into five groups of different origins. Four rock standards were analyzed to confirm the method, and the results agreed with previous measurements. The analytical usefulness of the PGNAA technique has been realized.