The gamma dose component In A p(66)/Be neutron therapy beam

Although previous measurements [1-3] have been made of the gamma dose contamination in the neutron therapy beam, the experimental conditions have since been improved and more time was available for more comprehensive investigations. All the measurements have been redone and have been made, as before, using the twin-detector method [4]. The detectors used were A-150 tissue-equivalent (TE) ionization chambers flushed with methane-based TE gas and energy-compensated Geiger-Muller (GM) counters. The latter were fitted with LiF caps to reduce their sensitivities to thermal neutrons. The experimental improvements were: (1) A small steel collimator (2 cm diameter) was fitted inside the clinical collimator to better approximate narrow-beam conditions for the attenuation measurements required to determine the neutron sensitivities of the GM counters. Previously a field size (at the iso-centre) of 5.5 x 5.5 cm2 was used. (2) A better and more accurate beam monitoring system was used in which dose-rate linearity was preserved even at the very low currents used for GM counter measurements. An output was taken directly from one of the beam monitor ionization chambers into an electrometer, bypassing the gantry electronic system, which had been used for the previous measurements