Simulation of MG-AR and TE-TE Ionisation Chambers with MCNPX in a Straightforward Gamma and Beta Irradiation Field

New methods are required for a better interpretation of the response of ionisation chambers in order to improve further the determination of the absorbed doses in a mixed neutron-gamma BNCT field. Simulations might help to understand in particular the behaviour of the sensitivity factors of an ionisation chamber in the mixed field. The study presented here is a continuation of previous work with the final aim to obtain validated computer models of the Mg-Ar and TE-TE ionisation chambers. With these two chambers the so-called paired ionisation chamber technique is performed by which the neutron and gamma dose components can be separated and determined. By knowing exactly the neutron and gamma source in a simulation, the chamber response can be investigated. However, the validation of a simulated ionisation chamber set-up, starting directly with the mixed neutron/gamma fields is too complicated regarding the number of possibilities that could cause a discrepancy between measurements and simulations. Therefore, two simplified and well-known irradiation fields are considered first and will be discussed in this paper: an existing 60Co calibration source and a 90Sr check source which has been designed and constructed to perform ionisation chamber stability measurements. Both irradiation sources as well as the two ionisation chambers are modelled with MCNPX. This code has been used because it is capable of simulating neutrons among many other types of particles and rays. The model of the two ionisation chambers is investigated comparing the measured charge collection rate when the detectors are exposed in the 60Co gamma-ray field and in the 90Sr beta field with the calculated results. For the 60Co experiments the calculations agree within 3% with the measured values. For the 90Sr source the simulated Mg-Ar charge is 8% higher than the measurement and the simulated TE-TE charge is 6% lower than the measured charge from the ionisation chamber.JRC.F.4-Safety of future nuclear reactor