Irradiators for measuring the biological effects of low dose-rate ionizing radiation fields

Abstract

Biological response to ionizing radiation differs with radiation field. Particle type, energy spectrum, and dose-rate all affect biological response per unit dose. This thesis describes methods of spectral analysis, dosimetry, biological assays, and mathematical modeling for determining the relative biological response for low dose-rate fields. The spatial dimensions of optically stimulated luminescence dosimeters make them ideal for measuring dose at a specific location. However the response of these dosimeters varies with photon energy. A method is presented for measuring dose delivered by several fields with photon energies less than 60 keV using these optically-stimulated luminescence dosimeters. This method is confirmed using an ion chamber dosimeter and computer simulation. The construction of 24Am irradiators for tissue culture and animal experiments using this dosimetry method is also described. The results of tissue culture experiments performed using these irradiators are presented, and the relative biological effectiveness (RBE) is determined for two fields with approximately equal dose-rates produced by shielding 24Am foil sources with aluminum and polyethylene. Biological effects can result from single instances of energy deposition within a cell or from the combination of separate instances, but at low dose-rates biological repair mechanisms reduce the probability of effects resulting from the combination of separate instances. At a sufficiently low dose-rate the effects due to combination of separate instances are negligible. A model of low dose-rate energy deposition within a cell nucleus was developed to determine this doserate. In this model the proportion of biological effects due to single instances of energy deposition within a cell nucleus is described in terms of the DNA repair rate of the biological 'system and the dose-rate and lineal energy transfer of the radiation field. This model also describes the projection of RBE values for fields with dose-rates below this threshold.