Electron contamination of large cobalt 60 photon beams

Experimental results are presented of the electron contamination of cobalt 60 gamma photon beams from the Mobaltron MS80, over a range of SSD's from 60 cm to 3.5 m. In the absence of an electron filter, the skin dose can exceed the dose at 4 mm depth by up to 1.6 times due to the high level of electron emission from the depleted uranium collimators. 4 mm of perspex effectively removes these electrons, and in the filtered beam, the skin dose cannot exceed the 4 mm dose. The electron contamination of a 6 MV X-ray beam is compared.