Dosimetry of fast neutron produced by Cockcroft-Walton neutron generator

Neutron produced by a Toshiba Cockcroft-Walton neutron generator at Nagasaki University School of Medicine was measured in air and in a tissue-equivalent phantom. Two dosimetric methods were applied; the paird ionization chamber method and the activation method. The paired ionization chamber method was composed of a tissue-equivalent ionization chamber flowed with tissue-equivalent gas and a graphite one with carbon dioxide gas. The activation method was applied with 56Fe (n, p)56Mn, 48Ti (n, p)48Sc and 58Ni (n, 2n)57Ni reactions for fast neutron dose, and a 115In (n, r)116mIn reaction for thermal neutron dose. Fast neutron dose measured by the paired ionization chamber method was 7.07 ± 0.56 (x 10-3 Gy) at 10 cm in air per 1 x 104 counts of the plastic scintillation counter used as a monitor. Fast neutron dose obtained by activations of 56Fe, 48Ti and 58Ni was 5.65 ± 0.79 (x 10-3 Gy), 4.69 ± 0.68 (x 10-3 Gy) and 5.18 ± 0.63 (x 10-3 Gy), respectively. The fraction of the thermal neutron dose to the fast neutron dose was 3.29 x 10-5 in air and 3.42 x 10-3 at 10 cm depth in the phantom. (author)