Design and characterization of next-generation tissue equivalent proportional counters for use in low energy neutron fields

Abstract

Tissue Equivalent Proportional Counters (TEPCs) are devices that can measure the radiation dose levels for the mixed neutron and gamma ray fields that are encountered in nuclear facilities. TEPC designs typically consist of a low density gas sensitive volume surrounded by a solid wall, each of which is composed of tissue-equivalent material. The response of a TEPC in a radiation field is characterized by two quantities: the sensitivity, defined as the ratio of the counts recorded to the measured dose equivalent, and the dose equivalent energy response, defined as the ratio of the measured dose equivalent to the ambient dose equivalent for the radiation field of a given energy. TEPC technology has great potential for use in low energy neutron fields, such as those in nuclear power plants, however, what hinders its use as a portable area monitor in such fields is its large physical size. For any given neutron field, an optimized TEPC design is one that can provide the same or better sensitivity and dose equivalent energy response relative to that offered by a standard spherical TEPC design of five inch diameter albeit with a much smaller physical size to allow for portability. Two optimized TEPC instruments have been designed: the Compact Multi-Element Tissue Equivalent Proportional Counter (CMETEPC), consisting of 113 cylindrical gas sensitive volumes, and the one-quarter inch TEPC-based system (QITEPC), consisting of 392 spherical TEPCs. These instruments are approximately 90% smaller than the standard counter design. To verify that these optimized counters offer the same or better sensitivity and dose equivalent energy response relative to the standard design for realistic nuclear power plant neutron fields, the performance of these instruments when irradiated by the 252Cf – D2O moderated, 252Cf, and 241Am-Be neutron energy spectra were simulated using the Particle and Heavy Ion Transport Code System (PHITS) three-dimensional radiation transport code. For these fields, it is found that the two instruments do offer the same or better sensitivity and dose equivalent response than the standard counter design. The process of designing the optimized instruments as well as the directional dependence of these instruments’ sensitivity and dose equivalent energy response for each of the three above mentioned neutron fields will be presented. This thesis presents two novel compact designs for TEPCs that may be suitable for use in area monitoring applications.