Neutron emission and dose distribution from natural carbon irradiated with a 12 MeV amu−1 12C5+ ion beam

Abstract

Measured neutron energy distribution emitted from a thick stopping target of natural carbon at 0°, 30°, 60° and 90° from nuclear reactions caused by 12 MeV amu−1 incident 12C5+ ions were converted to energy differential and total neutron absorbed dose as well as ambient dose equivalent H*(10) using the fluence-to-dose conversion coefficients provided by the ICRP. Theoretical estimates were obtained using the Monte Carlo nuclear reaction model code PACE and a few existing empirical formulations for comparison. Results from the PACE code showed an underestimation of the high-energy part of energy differential dose distributions at forward angles whereas the empirical formulation by Clapier and Zaidins (1983 Nucl. Instrum. Methods 217 489–94) approximated the energy integrated angular distribution of H*(10) satisfactorily. Using the measured data, the neutron doses received by some vital human organs were estimated for anterior–posterior exposure. The estimated energy-averaged quality factors were found to vary for different organs from about 7 to about 13. Emitted neutrons having energies above 20 MeV were found to contribute about 20% of the total dose at 0° while at 90° the contribution was reduced to about 2%.