jn 1240-7 Shielding around the target point attenuates neutrons at a higher rate than does air, and thus reduces the neutron dose. The most effective neutron shielding involves a combination of scatter and capture materials. Some elements (such as barium or iron) are effective in slowing down fast neutrons ( 2 3 Mev)through inelastic scattering. Hydrogenous materials, euch as water or paraffin, are very effective in slowing dow fission neutrons (most of which have energies of less than 3 Mev) to thermal energies, and boron is effective in capturing thermal neutrons. (2) Factors Affecting Garma Radiation. Gamma radiations that contribute ea significant portion of the totel F.P.C.-radiation dose are (a) the gamma rays (of about 4 Mev average energy) produced when the neu- trons of greater than l-Mev energy undergo inelastic scattering, (b) the high-energy (up to about 11 Mev) gamma rays emitted when slow neu- trone undergo radiative capture by atmospheric nitrogen muclei, and (c) the early-time fission-product gamma raye that have an energy spectrum of about 3 Mev average energy, with energies up to 7 - 8 Mev. The amount of this F.P.C. gamma radiation that interacts with a target is dependent on several factors: the weapon type, the distance of the target from the source, the air density, the angle of incidence of the radiation, and the shielding around the target point. All these factors affect the gamma energy distribution at the target. The effects of these factors are briefly discussed in the following paragraphs. The weapon type (fission or fission-fusion) determines tne mumber and energy of the prompt neutrons emitted, and thus controls whether the gamma radiations resulting frominelastic scattering of neutrons and those from nitrogen capture Of neutrons contribute significantly to the total F.P.C. gamma radiation. Furthermore, the weapon type and yleld_ eleo affect the significance of the fission-product gamma radiation. !7 A few gamma-ray spectral measurements have been recorded st targets during weapon tests, but more detailed measurements have been made in laboratories, ©2! , The dose rate of the F.P.C. gamma radiation at a target decreases rapidly effect with distance from the source due to both the inverse-square and air attenuation. The ganma ree are both scattered am absorbed, to some extent, by passage through any material. Scattering through the *mis inverse-square relationship is valid only for a point source of radiation, but may be used to approximate the amount of direct radiation incident on a target at a distance equal to at least several times the diameter of a source of finite size. Y P O C E L B A L I A V A T S BE 17-22