Determination of the neutron capture sensitivity of ionisation chambers based on neutron capture reaction rates

An epithermal neutron beam from the High Flux Reactor is used for clinical BNCT at Petten. For the validation of the treatment planning simulations an accurate experimental determination of the different dose components in the mixed beam of neutrons and photons is important. Dosimetry is performed with paired ionisation chambers, among other techniques. Experiments have been made using Mg/Ar and TE/TE ionisation chambers. Calibration factors for both chambers have been obtained from measurements in a 60Co gamma-ray beam. Ionisation chambers are sensitive to neutron capture reactions in the structure of the chambers. This paper describes a novel approach to the correction for the parasitic sensitivity to neutron capture. The k' values for ionisation chambers in epithermal neutron beams have been determined previously using thermal neutron flux values. This procedure neglects the contribution of the epithermal neutrons which are present in the low energy part of the intermediate-energy neutron spectrum. In this work k' factors have been determined using reaction rate relations of neutron activation reactions. (author)