Note on the Bragg-Gray Cavity Principle for Measuring Energy Dissipation

Abstract

Most measurements of the energy dissipated ionizing radiations in a material rest on the Bragg-Gray principle that the amount of ionization produced in a gas cavity serves as a measure of the energy dissipated in the surrounding material.' This procedure relies on the assumption that the cavity gas is traversed by the same flow of corpuscular radiation as exists in the material under consideration.2 Initially the Bragg-Gray principle rested on the fact that the flow of corpuscular radiation will remain undisturbed the presence of the cavity, provided only that the cavity size is sufficiently small compared with the range in the gas of the corpuscular radiations traversing it. However, this size limitation often becomes unacceptably restrictive.1 The application of the Bragg-Gray principle actually rests, most frequently, on another basis: The flow of corpuscular radiation will remain undisturbed the presence of the cavity, provided only that the crude chemical compositions of the gas and the surrounding material are equal, irrespective of the cavity size. The adequacy of this alternate proviso has been clearly recognized and accepted for a long time. It appears plausible at first sight and it has been justified, at least in part, qualitative argumentation and experimental tests. Nevertheless, it does not appear from the literature to have been treated exhaustively. An analytical proof that the gas-wall equivalence condition is indeed adequate has become more accessible in recent years, through increased familiarity with the problems of penetration and diffusion of secondary corpuscular radiation. A good qualitative discussion of this problem is given in the textbook Weyl and Warren.3 These authors consider first the ionization at a point in free air and then argue that this ionization remains unchanged if most of the surrounding air is compressed into a dense shell surrounding the point. As a justification they recall