A Solid State X-Ray-Alpha Coincidence Counter

Abstract

X rays emitted in the decay of transuranic elements (particularly plutonium) are often used for radiochemical and health physics measurements of these materials. For any given source the X ray to alpha ratio (X/?) varies with isotopic composition (variations of a factor of seven are possible) due to the unique X/? ratios and specific activities for the various isotopes. To make accurate assessments of the amount of material present from X-ray measurements, it is necessary to know the X/? ratio for the material under consideration. By knowing the isotopic composition and the isotopic X/? ratios, the material X/? ratio can be calculated or it can be measured directly from a sample of the material. At Battelle-Northwest we developed a system using solid state detectors to measure these X/? ratios. This paper describes the solid state system and some of its applications. The main developmental criteria were to maintain high efficiency and accuracy at low levels of activity. To meet these criteria, we built a coincidence system with high geometrical efficiency. The system uses a coaxial geometry with the alpha detector sandwiched between the source and X-ray detector. A 3 mm by 110 mm2 lithium-drifted-silicon detector cooled to liquid nitrogen temperature is used for X-ray detection. The alpha detector is a 40 V by 100 mm2 totally depleted surface barrier detector. Sources are either 0.635 or 1.58 cm diam electrodepositions of the material on stainless steel planchets which are placed on a rotatable disc 0.