Surface effects of high-energy X rays at oblique incidence

The concept of an electron-range-surface (e.r.s.) has been developed from consideration of the mechanism of dose build-up when a beam of high energy X rays meets an absorbing surface at right angles. Its application to a beam at oblique incidence has made it possible to predict how suface dose and the depth of peak build-up vary with the angle of incidence. The theory was tested experimentally with a conventional build-up ionization chamber, using thin sheets of Perspex as build-up material. Good agreement was found between the theoretical thickness of Perspex necessary for peak build-up at a given angle of incidence and that obtained experimentally, while the pattern of variation of surface dose with angle of incidence was also as expected. Confirmation of the findings was obtained using LiF as dose detector, though some differences in the magnitude of surface dose emerged. The results explain why skin reactions occur with near-glancing angles of incidence, but they also indicate that with incident angles as large as 45 deg. very little increase in skin dose may be expected.