Microdosimetry of light ions and neutrons

Dosimetry of energetic charged nuclei and neutrons during manned space missions and radiotherapy requires identification of mixed particle fields in order to determine LET which is necessary for estimating RBE or Quality Factors. A Tissue Equivalent Proportional Counter (TEPC) gives details of the absorbed dose and dose rate. It can also provide direct information on the quality or type of the radiation field. The interior cavity of the detector is filled with tissue equivalent gas such that the density thickness, cm2/g, of the gas is equivalent to the density thickness of tissue with dimensions approaching the nucleus of a mammalian cell (1-5 μm). The motivation for this was that the proportional counter serves as a microdosimeter that can detect energy deposition events similar to those encountered by biological systems having the similar dimensions. When high energy charged particles enter a person, energy deposition is dominated by near-minimum ionizing Coulomb interactions (Bragg plateau). Energy deposition increases as the particle slows down when it reaches the tumor. There can also be nuclear interactions that produce densely ionizing secondary particles both from elastic scattering and inelastic events that result in target fragmentation. These secondary particles have very short ranges and deposit most of their energy very near the point of interaction. Since the specific ionization is large, absorbed dose from these events could have a large relative biological effectiveness (RBE) compared with that of the primary particle. The types of nuclear interactions induced by high- energy protons are similar to those from high- energy neutrons where the RBE is known to be greater than one. Characterization of these effects is essential for both hadron therapy and radiation protection of astronauts. (author)