Personal dose equivalent conversion coefficients for neutron fluence over the energy range of 20 to 250 MeV Page: 3 of 13
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INTRODUCTION
Presently, neutron conversion coefficients, Hp,1a(10,a)/lD, are given by ICRP-74{' and ICRU-57 2a for a
range of angles of radiation incidence (a =0, 15, 30, 45, 60, and 75 degrees) in the energy range from thermal to 20
MeV. Since neutron personal dose equivalent around high-energy accelerators is dominated by neutron leakage in
the energy range of 10 to 200 MeV, neutron dosimetry at such facilities requires accurate conversion coefficients
over this energy range. Previous calculations35 were based on the kerma approximation, which assumes that
charged particle secondaries are locally deposited, or at least that charged particle equilibrium (CPE) exists within
the tally cell volume. The kerma approximation was taken as a safe assumption for the energy range below 20 MeV,
but it was recognized that it is no longer valid in the energy range above 20 MeV. In particular, the range of elastic
recoil protons and protons produced during nonelastic nuclear interactions is significantly large in tissue (e.g., 4.5
cm at 75 MeV and 25 cm at 200 MeV). Thus, rigorous transport of secondary protons and other charged particle
secondaries is a requirement for accurate absorbed dose calculations above 20 MeV.
The Los Alamos Monte Carlo radiation transport code MCNPXf6) was used to generate all of the
conversion coefficients. Recent versions of this code incorporate the necessary physics models to transport all
secondary charged particles produced in neutron elastic and inelastic interactions. Starting with version 26Et7 , the
Heavy Ion (HI) physics model has enabled the transport of recoil nuclei. Hence, heavy recoil nuclei (C, N and O) in
tissue can now be transported in addition to the light ion types: proton, deuteron, triton, He-3, and alpha. The HI
model automatically transports all residuals that are produced from any model physics (such as CEMNs , Bertini 9 ,
etc.) interaction - even if the source particle is not a heavy ion. Neutron-produced carbon, nitrogen, and oxygen
elastic recoils are transported (to a lower energy limit of 5 MeV) in the model regime if heavy ions are designated as
live particles. Previously, it was only possible to transport light elastic recoils (proton, deuteron, He-3, and alpha)
using the Light Ion Recoil model.
Transport within the cross section table regime does not normally create secondary particles, so any
interaction using proton and neutron libraries typically will not produce transportable ions. The one significant
exception is the LAI50 cross section library 10 which provides neutron, proton, and photonuclear cross-sections up
to 150 MeV (to 250 MeV for protons) for several ions (including H, C, N and O) based on experimental data and
nuclear model calculations using the GNASH nuclear reaction model code. Above 20 MeV, secondary charge
particle transport may be invoked for neutron interactions. Sampling is performed for proton, deuteron, triton, He-3
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Mclean, Thomas D; Justus, Alan L; Gadd, S Milan; Olsher, Richard H & Devine, Robert T. Personal dose equivalent conversion coefficients for neutron fluence over the energy range of 20 to 250 MeV, article, January 1, 2009; [New Mexico]. (https://digital.library.unt.edu/ark:/67531/metadc927729/m1/3/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.