Monte Carlo Simulations of the Current Obtained with Ionisation Chamber Detectors in Mixed Fields of Neutrons and Gammas

It needs no introduction that good measurements regarding BNCT dosimetry are of vital interest. Above all, calculations for patient treatment planning are initially based on these measurements. Closely related, well understood dosimetry of mixed neutron and gamma fields is necessary to explain the outcomes of the many experiments performed. It is believed that the sometimes confusing and incomprehensible outcomes in BNCT research are due to incorrect dosimetry, i.e. misleading measurements. A popular detector used to describe the absorbed neutron and gamma doses is the ionisation chamber. To understand better the behaviour and intricacies of this detector, the collected and measured current is directly simulated with MCNPX. This Monte Carlo code is able to track neutrons, gammas and electrons all around and in the ionisation chamber. The calculated dose deposited by the electrons in the gas is proportional to the current measured. Protons and alphas emanating from the wall and/or gas materials due to nuclear reactions can also cause ionisations and thus add to the current. A custom-made program has been written to simulate this contribution. The issue in this study is that a disagreement between simulated and measured current can be caused by the computer code and/or measurement set-up and/or unknown influences of source and/or materials. Therefore, the model of the ionisation chamber as well as the neutron and gamma source descriptions are validated step-by-step. After having obtained enough confidence in the model it can be concluded that ionisation chamber measurements can be significantly affected by neutron interactions (this is energy dependent). Neutrons can increase the measured current due to unknown and unconsidered beta-, proton- and/or alpha-producers in the wall material and gas; this dose component does not exist without the presence of the ionisation chamber.JRC.F.4-Safety of future nuclear reactor