An instrument for the rapid measurement of X-ray field flatness

This instrument has been designed primarily for use with the Metropolitan-Vickers and Mullard 4 MeV linear accelerators, but it could be used equally well with other accelerators and also, if necessary, with X-ray units of lower energy.

It comprises two easily transportable units: (i) the detector unit which includes five Philips ORP61 cadmium sulphide (CdS) detectors, four of which are motor driven, and (ii) the control unit which includes the power supplies, motor control and the detector current measuring circuits. The two units are connected by a 40 ft. multicore cable.

The detector unit is shown in Fig. 1. It is constructed of Perspex and comprises four movable arms, each 7 in. long, linked in identical fashion to a common motor drive. Each arm carries a Philips ORP61 CdS detector at the distal end, whilst a fifth stationary ORP61 detector is mounted at the centre of the beam to monitor the central dose-rate. The central CdS detector is elevated 3 cm above the four outer ones so that the variation in the scattered radiation reaching it as the four outer detectors are moved shall be negligible. The effect of the variation in the scattered radiation reaching any one outer detector as the other detectors are moved through their full traverse has been measured and found to be less than 1 per cent of the detector response at all positions.