Determination of the relative neutron sensitivity of a C-CO2 ionization chamber

The neutron sensitivity of a C-CO2 ionization chamber has been experimentally determined in the neutron energy range of 0.1-1.2 MeV. Eleven kU data were obtained which ranged from 0.04 to 0.1 as a function of neutron energy. There were two peaks in the spectrum of the kU factor, and their positions corresponded to the resonance energies of the kerma factors of carbon dioxide, of which the energies are about 0.4 and 1 MeV. The present data agree with the data obtained at the National Institute for Radiological Sciences in Chiba, Japan (NIRS). The energy dependences of the sensitivity were calculated by using the kerma and the W-value ratios of gas to soft tissue for comparison. The experimental data were reproduced well by the calculations.