Simplified modeling of “fission products / converting material & filling gas” interaction in a miniature fission chamber — Comparison with experimental data

The main objective of this study was to make a modeling of a miniature fission chamber behavior regarding the charge creation in order to understand the impact on the measurements of some physical parameters such as the fissile deposit thickness. To do so, and in order to compare the model with experimental results, it was decided to focus on the modeling of an output pulse height spectrum (also known as PHA spectrum). The physical modeling of the fission chamber comes in two parts in order to finally obtain proper equations describing the PHA spectrum. In a first phase, we have considered that fission reactions occurred at a random area in the fissile deposit. From there, the first step was to calculate, for each initial energy value of the fission fragment, its distribution in energy when released in the filling gas of the detector. To achieve this, a few assumptions were made: the equiprobability of the location of the fission in the deposit, the absence of imperfections in the deposit which is supposed to be perfectly flat (this point is probably the most open to discussions) and the isotropy of the fission fragment emission angle distribution. The energy deposition mechanism in the deposit was simulated using the SRIM software. At the end, comparisons with experimental data were made. Results were encouraging although they revealed differences in the high energy area. In order to solve this problem, the mechanism of the energy deposition in the gas must be taken into account. This second phase used the energy distribution of fission fragments at the surface of the deposit calculated in the first phase, in order to determine the energy distribution of fragments available for gas ionization. Again, it necessitated assumptions such as the equiprobability of the fission fragment emission angle from the deposit. In addition, several different filling gases were considered. One more, all calculations were achieved thanks to the SRIM software. This work gave so far very promising results and also provided some perspectives which are detailed in the paper.