The angular distribution of multiply-scattered γ-radiation

Abstract

The intensity of ionization up to a distance of 400 yards (2 1/2 mean free paths) in air from a forward-emitting 500 curie source of 60 Co has been measured using air-tilled condenser integrating ion chambers with air equivalent walls. These chambers have been used unshielded, and also shielded by lead cones cutting off all radiation within + 5° of the main beam and within increasing angles up to ± 90°. It has been found that of the total, unshielded dose, 25 % is due to scattered radiation arriving over angles outside +5° of the main beam at 100 yards, with a corresponding figure of 52 % at 400 yards. Back-scattered radiation accounts for 5 % of the dose at 100 yards and 10% at 400 yards. Figures for intermediate angles and distances are also given. It appears possible that equilibrium is reached at about 600 yards (4 mean free paths) with values only slightly above those quoted for 400 yards.