Gamma-Ray Spectrometers for the Assay of Complex Mixtures of Low Concentration of Radionuclides in Environmental and Biological Materials

Abstract

Solid state detectors, especially Ge(Li) drifted diodes, have given the biologist a new and powerful tool for determining the identity of radioactive species in biological samples. Often these samples contain very low quantities of radioactivity in a complex mixture of radionuclides. High resolution, maximum counting efficiency, suppression of background and Compton radiation and enhancement of photopeaks are required. A series of gamma-ray spectrometers has been developed to assay radionuclides in biological materials. These spectrometers have incorporated features that facilitate achieving these requirements. Samples up to 8.5 cm in diameter and 2.5 cm thick are now routinely analyzed. A range of gamma and x-ray energies from a few kilovolts to 2.5 Mev can be analyzed. Since the probability becomes rather high above 1 Mev that the favored gamma interaction within the Ge(Li) detector will be a Compton event with the resultant photoelectron interfering with the spectrum below the initial gamma-ray energy, it is necessary to suppress this type of interaction. This has been achieved by surrounding the Ge(Li) detectors with a scintillator for detecting scattered Compton photons. A method is described to enhance both anticoincidence and coincidence events. Both NaI(Tl) and plastic scintillators are used as anticoincidence shields. The optimum anticoincidence shield may not be a simple annulus. Some arguments and preliminary results are given to show the advantages of totally surrounding the Ge(Li) with a scintillator for 4 π Compton suppression. The Compton shields vary from 9 inches in diameter for NaI(Tl) to 30 inches for plastic.