Numerical evaluation of micro-pocket fission detectors

Micro-pocket fission detectors (MPFDs) are miniature fission chambers suitable for in-core neutron measurement that have been under development at Kansas State University for over one decade. Current-generation devices have been used at a number of university reactors (Kansas State, Wisconsin, and MIT) and as part of the first experiments performed during the recent restart of TREAT. Ongoing research aims to improve understanding of the existing MPFDs and to optimize designs for future deployment. To aid in this development, the dynamic response of a prototypic MPFD was evaluated using Garfield++, Elmer, Gmsh, and Stopping and Range of Ions in Matter (SRIM). Specifically, the finite-element code Elmer was used to calculate the electric field on a mesh generated by Gmsh. SRIM was used to compute the energy loss tables of the fission fragments in the gas. With output from Elmer and SRIM, Garfield++ was used to simulate the ionization process, the resulting electron drift, and the induced signal. This particular Garfield++ application was developed with hybrid parallelization based MPI and OpenMP. The performance of the MPFDs subjected to different temperatures and applied voltages was evaluated. The preliminary results indicate the fission fragment deposits a few MeV of energy in the gas, consistent with previous estimates. The pulses in the MPFDs can be formed in the nanosecond scale, thus accommodating high count rates and, hence, high neutron-flux levels. Ongoing work aims to extend this model and validate it against existing and planned experimental data. (author)