Beam current monitoring and radiachromic film dosimetry at the NPS Linac.

Abstract

The beam current monitoring system at the NPS electron Linac was studied to determine its performance under varying conditions of machine operation. The efficiency of the secondary emission monitors (SEM efficiency), defined as the ratio of SEM current of Faraday cup current, was found to increase with increasing beam energy, increase with the introduction of targets into the beam, and vary erratically with SEM foil surface conditions. No dependence on beam current was observed. The spatial variation of electron fluence (time-integrated flux) was investigated using RACM 203 radiachromic film developed by the EG&G Corporation. The optical density of this film was found to vary linearly with absorbed dose over a range of 5 to 40 megarads . Beam profiles were obtained at A locations in the Linac end station for beam energies of 25 and 63 MeV. In addition, the film was used to observe and measure the effect of cascade showers in thick absorbers.