CHAPTER 17 17.4.2 Factors Affecting the Interaction of F.P.C. Radiation With a Target Ship (1) Factors Affecting Neutron Radiation. The amount of neutron radiation received at a target some distance from a nuclear detonation is dependent on several factors: the characteristics of the nuclear device; the distance of the target from the detonation (the neutron source); and the shielding around the target point. The device characteristics markedly affect both the number of neutrons emitted and the energy spectrum at the source.! ’ The bomb materials, particularly the hydrogenous high explosives used, capture neutrons efficiently and hence affect the number and energy of the prompt neutrons that escape into the air. Furthermore, several times as many neutrons are released per kiloton of fusion yield as per kiloton of fission yield. 18 Te neutron--energy spectrum at the source affects the distribution of energies (the spectrum) at the target, and the neutron energy spectrum at the target, in turn, affects the neutron radiation dose at the target. Prompt neutrons released by the detonation of a fission weapon have a continuous energy spectrum that peaks at about 1 Mev at the source, while almost all the neutrons mesultingfrom detonation of a fusion device are 14 Mev at the source .t 8 According to Ref.19, field-test data indicate that the slow neutrons with energies of less than about 1 ev contribute no more than 2% of the total neutron dose received at distances of biological interest, whereas the faster neutrons with energies greater than 0.75 Mev contribute about 75% of the dose. The distance from the detonation to the target affects both the number of neutrons reaching the target and the energy spectrum at the target. As the prompt neutrons leave the environment of the bomb they undergo collisions with nuclei of elements present in the atmosphere and either are captured or scattered (lose energy) with each collision. The mean free path between the collisions is dependent on neutron energy, and can vary from about 100 meters (thermal neutrors) to greater than 300 meters (14 Mev neutrons). Each collision will result in either a decrease in neutron energy or in neutron capture and hence removal. The longer the path to the target, the more collisions are possible; therefore fewer neutrons will reach more distant targets since more capture reactions are possible. The neutron energy spectral characteristics at the target depend on the relative importance of the scatter and capture processes during these collisions. Capture is usually much more probable for very low energy neutrons. Hence, after neutrons traverse a few mean free paths in air, just as many low-energy neutrons are lost by capture as are produced when higher energy neutrons lose energy through the scattering process. The result is an equilibrium neutron energy spectrum after the radiation has traversed a few mndred meters of air or a few centimeters of iron or other solid material. 17-21