Twin ionisation chambers for dose determinations in phantom in an epithermal neutron beam

The validity of the twin ionisation chamber technique for determination of gamma and neutron doses in an epithermal neutron beam has been studied by comparison of measured and calculated dose distributions in a water phantom. The measurements were performed in two epithermal neutron beams, designed for boron neutron capture therapy, at the FiR 1 TRIGA nuclear reactor using two pairs of A150 tissue-equivalent and non-hydrogenous Mg(Ar) ionisation chambers. Doses were calculated by the two-dimensional discrete ordinates radiation transport code applying the fluence-to-dose conversion factors by ICRU. At the dose maximum in the phantom 5% consistency between measured and calculated results was achieved for both neutron and gamma doses. Improvements in chamber materials and precise knowledge of the fluence perturbation effects of the chambers are needed to decrease the measurement uncertainty to an acceptable level.