Investigation and use of dual (twin) low pressure proportional counters for active neutron-gamma mixed field dosimetry
This thesis tackles accurate neutron-gamma mixed field dosimetry in radiation protection and radiation biology. While passive dosimeters are suitable for measuring low radiation levels, active instruments are necessary in high radiation environments, such as found in nuclear power plants and particle accelerator facilities and it is highly desirable to develop a single detector capable of discriminating between neutrons and gamma rays, providing real-time and independent dose-rate measurements. Tissue equivalent proportional counters (TEPCs) have been used, but their accuracy is limited. In this study, a custom-built Cylindrical Graphite Proportional Counter (Cy-GPC) along with a twin Cylindrical TEPC (Cy-TEPC) has been extensively investigated for n-γ mixed field dosimetry. Following a series of experiments to confirm the identical nature of both counters for photon dosimetry, various mixed-radiation field measurements were conducted to explore the operation of the dual counters over a wide range of neutron and photon energies and health physics operational environments. Monte-Carlo modeling was employed to assist in interpreting the experimental data and determining the neutron sensitivity of the graphite-walled counter. The study demonstrates that utilizing dual counters and the proposed methods improves neutron dose rate precision by approximately 5% to 20% compared to the standard TEPC method. This improvement is particularly significant in radiation biology and medical neutron applications, but of lesser importance in radiation protection where stringent accuracy requirements are not as crucial. The GPC's graphite wall exhibits limited sensitivity to neutrons, while the tissue equivalent gas inside the counter contributes to neutron sensitivity at specific energies. However, within the framework of radiation protection, it is acceptable to assume that energy deposition events above 10 keV/μm in a TEPC are attributable to neutrons, and events below 20 keV/μm recorded by a graphite-walled counter are solely due to photons. The agreement between measured and simulated data validates the use of simulations for predicting counter performance, particularly in scenarios where actual measurements are impractical, such as space exploration or future particle beam radiotherapy facilities. The study provides suggestions for counter geometry and manufacturing for facilitating the design of a single device that effectively addresses the challenges of neutron-gamma mixed-field dosimetry.
Radiation dosimetry, Neutron, Gamma, Proportional counters
Networked Digital Library of Theses and Dissertations
University of Ontario Institute of Technology