A Device for the Measurement of Dose as a Function of Specific Ionization

Abstract

The physical parameters influencing the biological effect of ionizing radiation are the energy delivered to the tissues affected and the ion density along the paths of the charged particles producing the ions. Variations of the latter quantity are believed to be responsible for differences in relative biological effectiveness (R.B.E.). Zirkle has suggested the use of the concept of “linear energy transfer” (L.E.T.) in tissue. This is more fundamental than specific ionization in a gas, but the latter, which may be measured much more readily, may be presumed to be proportional to the former within about 10 per cent in most cases. At present there are several methods by which the total energy imparted to tissue by ionizing radiations can be determined. In this laboratory, tissue-equivalent ionization chambers (1) have been developed on the basis of the Bragg-Gray principle. Employing these chambers, one can obtain a measure of the total energy delivered per gram of tissue by particles of any specific ionization. Since these chambers, when exposed to the same radiation field, are traversed by ionizing particles of the same type, energy, and direction as is tissue, it appeared feasible to develop a method whereby the specific ionization of individual particles crossing the cavity might be determined. Since the ionization produced by individual particles will often be too small to be detected directly, recourse must be taken to gas multiplication. Hence the chamber must operate as a proportional counter. In this case the pulse obtained will be proportional to the ionization produced by the particle traversing the counter. The total ionization will be the product of specific ionization and path length in the gas volume. Discrimination on the basis of pulse height alone cannot lead to a clear distinction between particles of different specific ionization, since the pulse produced by a lightly ionizing particle having a long trajectory may be equally as large as that produced by a heavily ionizing particle having a short path. If the particles involved are of greatly different specific ionization, e.g., electrons and protons, a reasonably satisfactory separation of pulses produced by either type can be achieved, and it is upon this basis that Hurst and Ritchie (2) developed their neutron dosimeter. If, however, detailed information on specific ionization is required, allowance must be made for the distribution of geometric path lengths. This may be done most easily for the simplest kind of cavity, which is a sphere. This shape has the additional advantage of being isotropic and directional response is thus absent. Formula 1 in the Appendix gives a general expression for the distribution of path lengths intercepted by a spherical cavity at any distance from a point source. The most important practical case is the one where the source is effectively at infinity (i.e., many sphere radii away).