High Sensitivity Low Background Gamma Spectrometry System-Design Through Initial Results

Abstract

The article describes the construction and initial operation at the Robert A. Taft Sanitary Engineering Center of a multipurpose steel room, 12 x 8 x 8 ft (LWH) and constructed of 5-in. plate. The facility can be used to count a wide variety of objects exposed to environmental radioactivity, including large quantities of environmental samples, animals such as guinea pigs and cats, and human beings.The room is divided into two compartments by a movable inner wall, so that comparative studies on crystal arrangement, sample geometry, and background may be performed, Pertinent design aspects of the room are described and specifications relative to radiological and magnetic field effects on fixed or scanning detector systems are presented in detail. The problem of obtaining steel suitable to meet the stringent specifications is discussed, as are the various aspects in which this facility differs from other similar systems. The main detection system, including the crystal, is described in detail.Initial results of studies of background as a function of crystal location and orientation are discussed and sample counting reproducibility data are given. The crystal light pipe, radioactively clean steel, and the shield location make the background one of the lowest known,0.221 cpm/cm3 of detector over the energy range 0.1-2.0 MeV.