A novel approach to the determination of iridium via Ge-coincidence/NaI(Tl)-anticoincidence gamma-ray spectrometry

Abstract

Iridium has proven to be a useful fingerprint of extraterrestrial material, and neutron activation analysis is the method of choice for its determination because of the high thermal cross section (910 b) of 191Ir (37.3% natural abundance) and the favorable decay characteristics of the product nuclide. However, radiochemical separations are frequently required for the determination of iridium at low concentrations (in the ppb range). The present work describes a precise, nondestructive neutron-activation procedure that is capable of analyzing samples of both high (ppm) and low (ppb) Ir content. Advantage is taken of the fact that the principal gamma lines of 73.8-d 192Ir are in various coincidences with one another. Instead of conventional gamma-gamma coincidence spectrometry with two Ge detectors, we utilize the 784.6- and 920.9-keV sum peaks which are generated when an iridium containing sample is placed in the well of a highly-efficient HPGe detector. The well-type detector is positioned centrally in a large annular NaI(Tl) detector which is operated in an anticoincidence mode. This improves the quality of the gamma spectra by suppressing the Compton backgrounds due to nuclides such as 60Co by a factor of 4–6 in the regions of interest without affecting the intensities of the two sum peaks. The sensitivity of this method under our experimental conditions (20-mg sample irradiated at 1.5 × 10 14 n cm −2 s −1 for 5 min and counted 3–4 weeks after irradiation) is ∼5 ppb Ir in a relatively unfavorable matrix such as peridotite.