A. Roca, V. A. Nievaart, R. L. Moss, F. Stecher-Rasmussen, N. V. Zamfir; Validating a MCNPX model of Mg(Ar) and TE(TE) ionisation chambers exposed to 60CO gamma rays. Radiat Prot Dosimetry 2008; 129 (4): 365-371. doi: 10.1093/rpd/ncm484
For an accurate determination of the absorbed doses in complex radiation fields (e.g. mixed neutron-gamma fields), a better interpretation of the response of ionisation chambers is required. This study investigates a model of the ionisation chambers using a different approach, analysing the collected charge per minute as a response of the detector instead of the dose. The MCNPX Monte Carlo code is used. In this paper, the model is validated using a well-known irradiation field only: a 60Co source. The detailed MCNPX models of a Mg(Ar) and TE(TE) ionisation chamber is investigated comparing the measured charge per minute obtained free-in-air and in a water phantom with the simulated results. The difference between the calculations and the measurements for the TE(TE) chamber is within ±2% whereas for the Mg(Ar) chamber is around +7%. The systematic discrepancy in the case of Mg(Ar) chamber is expected to be caused by an overestimation of the sensitive volume.