Transverse shielding for KENS, 2

This note describes on calculation of the transverse (overhead) shielding required for the KEK (Japan High Energy Research Laboratory) Intense Neutron Source (KENS). It is an unfortunate fact that the literatures on radiation shielding at this energy are confusing. In view of these uncertainties, two methods of calculation are presented here: calculation based on measurement of the neutron yield from thick uranium targets bombarded by 740 MeV protons, and scaling upwards calculation of the transverse shielding for proton linear accelerators with energy up to 200 MeV. Two radiation design goals are used at KEK: a maximum dose equivalent rate of 1 mrem/hr at shield surface and a total accelerator produced contribution of 5 mrem/year at the site boundary of the laboratory. The note describes about values of the parameter assumed, and values concerning shielding calculation models. Model 1 predicts total shield thickness of 1920 gm/cm2 to achieve the radiation intensity of 1 mrem/yr on the shield roof. However, Model 1 overstimates the shield thickness because the data of Madey et al. of 740 MeV protons were taken, and revised to 1860 gm/cm2. Similarly Model 2 underestimates the shield because the target material assumed in O'Brien's calculation is copper/iron. If high energy neutron yield is scaled by the mass numbers of targets, the thickness of 1880 gm/cm2 is obtained. Thus, Model 1 and Model 2 predict values of 1860 and 1880 gm/cm2, respectively. These values are in fortuitously good agreement. The shield should be constructed with heavy concrete and iron. (Wakatsuki, Y.)