Radiation Protection Dosimetry Advance Access published online on November 3, 2009
Radiation Protection Dosimetry, doi:10.1093/rpd/ncp250
PERSONAL DOSE EQUIVALENT CONVERSION COEFFICIENTS FOR NEUTRON FLUENCE OVER THE ENERGY RANGE OF 20–250 MEV
Health Physics Measurements Group RP-2, MS J573, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
Corresponding author: tmclean{at}lanl.gov
Received April 2, 2009, amended August 27, 2009, accepted October 5, 2009
Monte Carlo simulations were performed to extend existing neutron personal dose equivalent fluence-to-dose conversion coefficients to an energy of 250 MeV. Presently, conversion coefficients, Hp,slab(10,
)/
, are given by ICRP-74 and ICRU-57 for a range of angles of radiation incidence ( = 0, 15, 30, 45, 60 and 75°) in the energy range from thermal to 20 MeV. Standard practice has been to base operational dose quantity calculations <20 MeV on the kerma approximation, which assumes that charged particle secondaries are locally deposited, or at least that charged particle equilibrium exists within the tally cell volume. However, with increasing neutron energy the kerma approximation may no longer be valid for some energetic secondaries such as protons. The Los Alamos Monte Carlo radiation transport code MCNPX was used for all absorbed dose calculations. Transport models and collision-based energy deposition tallies were used for neutron energies >20 MeV. Both light and heavy ions (HIs) (carbon, nitrogen and oxygen recoil nuclei) were transported down to a lower energy limit (1 keV for light ions and 5 MeV for HIs). Track energy below the limit was assumed to be locally deposited. For neutron tracks <20 MeV, kerma factors were used to obtain absorbed dose. Results are presented for a discrete set of angles of incidence on an ICRU tissue slab phantom.