Definition of source for head shielding requirements in linear accelerators may affect room shielding design.

Siemens Corporation defines the source of the Mevatron 20 linear accelerator as a line source approximately 2 m in length. Calculations for secondary barriers for leakage radiation are generally based on 0.1% of the useful beam 1 m from the target. The difference in source terms can lead to calculating thinner barriers than are required. The placement of a 30 X 10 X 10-cm block of lead behind the head was required to achieve acceptable radiation levels.