Theoretical performance analysis of a novel hemispherical tissue equivalent proportional counter for neutron monitoring and dosimetry

Abstract

Neutron dosimetry in reactor fields is currently conducted using thermal neutron flux monitors. It has been demonstrated that Tissue Equivalent Proportional Counters (TEPCs) have the potential to improve the accuracy of neutron dosimetry in these reactor fields, and Multi-Element Tissue Equivalent Proportional Counters (METEPCs) could reduce the size of instrumentation required to do so. The current METEPC designs are prohibitively complex and are sensitive to motion and audible noise. This work proposes a novel hemispherical element with a wire-less anode ball as a solution. The hemispherical METEPC element was analyzed as a single TEPC to first demonstrate the potential of this new design. Its performance was evaluated relative to current cylindrical and spherical counter designs that have been demonstrated experimentally to perform very well. The performance analysis first used equipotential distributions generated with ANSYS Maxwell (V. 14.0) in order to solve the Townsend equation and calculate radial gas gain distributions for each counter. Through this it was found that the hemispherical counter exhibits completely uniform gas gain for electrons approaching the anode from all directions and its avalanche region occupies only 0.0004% of the entire gas cavity volume, whereas in the cylindrical and spherical counters the avalanche occupies 0.6% and 0.12% of the total gas cavity volume, respectively. Energy deposition simulations were conducted using the Monte Carlo code PHITS (V. 2.24) with both mono-energetic 2.5 MeV neutrons and the neutron spectrum of 252Cf-D2O moderated. In these neutron fields the hemispherical counter appears to be a good alternative to the reference spherical geometry, whereas the cylindrical counter tends to under-respond to H*(10) for the lower neutron energies of the 252Cf-D2O moderated field. These theoretical results are promising, and if the recommended follow-up experimental work demonstrates the hemispherical counter works as anticipated it will be ready to be incorporated into an METEPC design.