Deposition of Energy by Neutrons in Spherical Cavities

The cavity principle, first applied to the measurement of fast neutron dose by Gray (1), continues to play a leading role in precision neutron dosimetry and in attempts to understand in detail the interactions of neutrons with biological materials. In mixed radiation dosimetry the cavity often takes the form of a proportional counter in which large pulses due to recoil nuclei ejected by neutrons may be distinguished from small pulses due to secondary electrons from y-rays. In the polyethylene-ethylene fast neutron dosimeter of Hurst (2, 3) and the neutron-insensitive 7-ray dosimeter (4), the dose determination is made by pulseheight integration of the desired pulses. A spherical-cavity proportional counter of tissue-equivalent material has been used by Rossi and Rosenzweig (5-7) for measurements of LET spectra from neutron radiation. Effective use of these devices requires a detailed understanding of the differential energy loss spectrum (corresponding to the pulse-height distribution) produced in the cavity by recoils generated by the neutrons. This paper is an attempt to provide this detailed understanding for the case of a spherical cavity by using a method that gives some insight into the physical processes taking place. The spherical cavity is the most important case because (1) its response is independent of the direction and angular distribution of the incident neutron radiation, (2) its chord-length distribution is simple and known, and (3) the distribution of energy losses in spherical cavities (from which a distribution of dose in Y, the energy loss divided by the sphere diameter in an event, may be obtained by a simple transformation) is important for an understanding of the biological effects of neutron radiation, as has been pointed out by Rossi (8). The type of analysis in this paper takes advantage of rotational symmetry which holds for (1) a spherical cavity with arbitrary direction of incidence of the radiation, (2) a cavity of arbitrary shape in an isotropic radiation field, and (3) a cavity of arbitrary shape rotated in angle to average out the effect of arbitrary direction of incidence of the radiation. Extension of these methods to nonspherical cavities (whose chord-length distributions depend on the size and shape of the cavity),