A Compton-suppressed coincidence gamma-ray scintillation spectrometer with large NaI(Tl) crystals

Abstract

A Compton-suppressed coincidence gamma-ray scintillation spectrometer has been assembled to obtain high sensitivity and selectivity in gamma-ray spectral measurements. The spectrometer is intended to satisfy requirements for the identification and estimation of low levels of gamma-ray radioactivity in a broad range of sample sizes and masses containing moderately complex mixtures of gamma-emitting radionuclides. The spectrometer detector is made of NaI(Tl) and consists of two identical subassemblies, each of which contains a central crystal integrally packaged with, and surrounded by, an annular Compton- suppression crystal. Each central crystal is nominally 6 sol1 2 ″ in dia. by 4″ thick, and is optically coupled to a 2″ thick crystal of unactivated sodium iodide (NaI). The annular crystal is 13 1 2 ″ in dia. by 6 1 2 ″ in length, with a nominal 6 1 2 ″ dia. hole through the center. For each subassembly, the exact diameters of the central crystal and of the annular hole are such that a 3 3 2 ″ space exists between the outer surface of the central crystal and the adjacent surface of the annulus. This space is filled with magnesium oxide to optically decouple the central crystal from the annulus. A 9″ dia. by 1 3 4 ″ thick NaI(Tl) backscatter-suppression detector can be optionally included as part of this detection system. Other components of the spectrometer are the following: (1) the photomultiplier assembly; (2) the detector mount and shield; (3) the amplification and gating circuitry; and (4) the pulse height analyzer. The spectrometer detector response in the following modes of operation is presented: (1) gamma sum; (2) gamma-gamma coincidence; (3) gamma-gamma coincidence sum; (4) beta-gated gamma sum. The detector qualities evaluated were resolution, background, degree of Compton-suppression, efficiency, and relative sensitivity.