A system of high-purity germanium detectors for the detection and measurement of inhaled radionuclides

A system of eight high-purity germanium detectors mounted, four each, as arrays on two downward-looking cryostats is described. This system is designed to measure photons emitted by inhaled radionuclides and is efficient for photon energies between 10 keV and 200 keV. In routine operation since October 1976, the system is used mainly to detect and measure inhaled plutonium and americium. Each of the eight detectors has a surface area of 10 cm2 and a resolution of 650 eV full-width half-maximum or better at 60 keV. This excellent resolution means that the photopeak can be placed in a narrow portion (about 1 keV) of the background energy spectrum. Consequences of this feature are: (1) The subject background count rate for a given photopeak is low (between 0.025 and 0.035 counts/min per square centimetre of detector surface area for a 1 keV band in the range of 15 to 100 keV; (2) The subject background can be well estimated from counts in the energy spectrum a few keV above the photopeak of interest; (3) Baseline resolution can be obtained for photons separated by as little as 1.5 keV in energy; and (4) Identification of unknown or unexpected radionuclides can be accomplished with good accuracy. Application and capabilities of the system are discussed. The current application is for the measurement of plutonium via the measurement of the 59.54 keV gamma from 241Am. The measurement of L X-rays from the decay of plutonium is also discussed. (author)