A Preliminary Inter-centre Comparison Study for Photon, Thermal Neutron and Epithermal Neutron Responses of Two Pairs of Ionisation Chambers Used for BNCT

The dual ionisation chamber technique is the recommended method for mixed field dosimetry of epithermal neutron beams. Its importance has been long recognised and it has featured highly in the dosimetry exchange programme of the MIT BNCT group. This paper presents initial data from an ongoing inter-comparison study involving two identical pairs of ionisation chambers used at the BNCT facilities of Petten, NL and of the University of Birmingham, UK. The goal of this study is to evaluate the photon, thermal neutron and epithermal neutron responses of both pairs of TE(TE) (Exradin T2 type) and Mg(Ar) (Exradin M2 type) ionisation chambers in similar experimental conditions. At this stage, the work has been completed for the M2 type chambers and is intended to be completed for the T2 type chambers in the near future. Photon calibration: The photon responses of the ionisation chambers were obtained in 6 and 10 MV clinical photon beams at the University Hospital Birmingham. Photon calibration factor ratios, in terms of dose to water (Petten/Birmingham) of 1.077 ± 0.006 and 1.029 ± 0.005 were found for the M2 and T2 type chambers, respectively. Thermal neutron response: The thermal neutron sensitivities of the M2 type ionisation chambers were determined using the thermal neutron beam available at the Low Flux Reactor, Petten. Ratios of the integrated charge measured for each chamber indicate a ratio (Petten/Birmingham) of 0.980 ± 0.007 for the M2 chambers. BNCT epithermal neutron beam: Measurements in a reference PMMA cubic phantom were performed using the M2 type ionisation chambers in the epithermal neutron beam of the High Flux Reactor, Petten. At a depth of 2.5 cm, a ratio of the integrated charge for the chambers yields a sensitivity ratio (Petten/Birmingham) of 0.985 ± 0.008.JRC.F.4-Safety of future nuclear reactor