Barrier transmission of 6 MV x rays for oblique angles of incidence.

Abstract

Useful beam transmission data for both normal and oblique angles of incidence are required for optimum shielding design. This is particularly true for installations that utilize an isocentric source mounting. However, transmission data for 6 MV x rays are available only for normal incidence. To determine the effect of oblique angles of incidence on barrier transmission, measurements were made at a 6 MV linear accelerator installation. The useful beam was directed toward a 40-inch concrete barrier at various angles of incidence. The transmitted exposure rates were measured with a 590 cc ionization chamber rate meter. A 6.0 mm Lucite cover on the chamber was found to be sufficient to insure electronic equilibrium. All measurements were made with a 20 x 20 cm field at 1 m and with the detector 1 foot from the distal surface of the barrier. For normal incidence the detector was 3.5 m from the isocenter. The isocenter was 1 m from the source. The results for various angles of incidence are shown in Figure 1. The circle on each curve represents the point of interception of the central ray and the plane of measurement. For clarity, this point is defined as the “intercept.” As expected, for normal incidence (α = 0°) the variation of exposure rate with distance from the central ray is much greater than indicated by simple geometrical consideration. For oblique angles of incidence the experimentally determined exposure rate was compared with the rate computed from transmission data obtained with normal angle of incidence (1). The measured exposure rate at the “intercept” is in accord with the calculated value based on the barrier thickness along the central ray. However, as shown, the location of the maximum exposure rate is not coincident with the “intercept.” The ratio of the maximum exposure rate to the exposure rate at the “intercept” is very dependent upon the angle of incidence and ranges from 1.2 for 20° to 2.5 for 45° (Fig. 1). These preliminary results were obtained for the specified barrier thickness and geometrical conditions. It is intended to extend this investigation for a range of photon energies, barrier thicknesses, and geometrical conditions.