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The role of the 14N(n,p)14C reaction in neutron irradiation of soft tissues
Authors:M.W. Swanepoel
Affiliation:Medical Radiation Department, iThemba Laboratory for Accelerator Based Sciences, P.O. Box 722, Somerset West 7129, South Africa
Abstract:The cell-killing potential of the 14N(n,p)14C reaction was considered with regard to neutron absorption in human nuclear DNA and respiratory phosphates for: (A) 1012 thermal neutrons in 1 kg of soft tissue, (B) a mono-energetic beam of 2 MeV neutrons incident in 1 kg of soft tissue such that the total collision kerma was 10 J/kg, and (C) an evenly distributed 0–66 MeV neutron beam, also incident in 1 kg such that the total collision kerma was 20 J/kg. For case (A) 0.0017 14N(n,p)14C reactions could be expected per DNA double strand, case (B) 0.053, and case (C) 0.0039. The probabilities that a proton emitted outside the nucleus would cross nuclear DNA were estimated from 14N tissue content for adult skeletal muscle, liver, and kidney tissues, for (1) nuclear DNA being concentrated in a sphere of 1.8 μm diameter, and (2) nuclear DNA being evenly distributed in a spherical nucleus 5 μm in diameter. It was concluded that even in a nitrogen-rich tissue exposed to a collision kerma of 20 J/kg by a 0–66 MeV fast neutron beam, the 14N(n,p)14C reaction directly kills at most 10 cells in every 1000, 4 of these by DNA nitrogen absorption and 6 by the 14N(n,p)14C protons emitted elsewhere in the cell. However, the dose due to the 14N(n,p)14C reaction should be measured where exposure to thermal neutron fluxes is significant. For therapeutic neutron doses the number of respiratory phosphate molecules in which the 14N(n,p)14C reaction occurs is insignificant, and doses from 14C-decay after neutron therapy are also negligible.
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