Application of the Inverse-Square Law to Oil-Immersed Tubes

Abstract

During the past several years, doubt has been expressed regarding the validity of applying the inverse-square law to the small oil-immersed x-ray tubes (140 kv. p. and less) which are used for both diagnostic and therapeutic purposes. This doubt has been particularly emphasized in respect to the short target-skin distances which are possible with the modern shock-proof oil-immersed tube. According to the inverse-square law, the radiation intensity at a given site varies inversely as the square of the distance from the source. It implicitly includes two factors which are often overlooked. One is that the radiation be emitted from a point source; the other that there be no absorbing or scattering media between the source and the site in question. While radiation from an x-ray tube is, of course, never actually emitted from a point source, the focal spots of these small tubes are generally so small that they can be considered practically point sources, even at short distances. The insertion of a layer of oil...