Neutron spectra and neutron kerma derived from activation and fission detector measurements in a d+T neutron therapy beam

Abstract

Neutron spectra at different positions in a phantom have been derived from activation foil and fission counter measurements for the collimated beam of the Amsterdam d+T fast-neutron therapy facility. The spectra show that the fraction of lower-energy neutrons increases with increasing depth in the phantom as well as with increasing distance from the central axis of the beam. Calculation of the ratios of kerma values in tissue and tissue-equivalent (TE) plastic and of W_N values for the spectra at five positions in the phantom, showed that the variations in these quantities are less than 0.5%. Kerma values have been derived from the neutron spectra and from the fission counter measurements separately. These neutron kerma values were compared with the neutron absorbed dose values derived from combined TE ionisation chamber and GM counter measurements. The dose values have been obtained applying recommendations given in the European protocol for neutron dosimetry for external beam therapy. At 50 mm and 100 mm depth along the central axis of the beam in the phantom, the agreement between the three methods was within 2%. In the penumbra region a maximum difference of about 7% has been observed between the three methods. The contribution from thermal neutrons to the total kerma is less than 1% in the beam as well as in the penumbra.