Anticoincidence-Shielded Ge(Li) Gamma-Ray Spectrometer for Marine Environmental Radionuclide Analysis

An anti-Compton γ-ray spectrometer has been developed, which possesses a number of characteristics particularly adapted to direct measurement of low-level complex mixtures of radionuclides in marine environmental samples.

The detector assembly consists of a 42.7 cm³ coaxial Ge(Li) detector and a well-type 6″diam. × 4″ Nal(Tl) detector for the anticoincidence shield. The sample to be measured is placed on the Ge(Li) detector inside the well of the Nal(Tl) crystal.

Suppression of the Compton background is particularly marked at the Compton edge when the assembly is operated in anticoincidence mode. The anticoincidence shield reduces the ¹³⁷Cs Compton edge by a factor of 6.6 to provide a peak-to-Compton edge ratio of 210: 1 without reducing the full-energy peak efficiency by more than 2%. Similarly, for the 1,332.5 keV ⁶⁰Co γ-rays the Compton edge is reduced by a factor of 7 to give a peak-to-Compton ratio of 85:1, though in this case the peak efficiency is reduced by a factor of 1.8 due to the cascade decay.

This spectrometer has permitted the detection of a few pCi of fallout nuclides such as ¹³⁷Cs and ⁹⁵Zr, contained in 30 g environmental wet samples counted at intervals of 1,000 min.