Surface effects of high-energy X rays at oblique incidence

In a recent article, Jackson (1971) discussed the partial loss of skin sparing when high-energy X or γ radiation is incident obliquely on the surface. He quite correctly stresses the importance of tangentially incident beams by stating that: “… Serious effects of beam angulation are confined to angles nearing glancing incidence…” Unfortunately, he was unable to make experimental measurements of the depth at which peak build-up occurs at glancing incidence but concludes, on the basis of a theoretical analysis, that this depth “… is not affected by air ionization or size of field”. Results of our experiments, shown in Table I, do not agree with this conclusion.