A theoretical model for event statistics in microdosimetry. II: Nonuniformdistribution of heavy ion tracks

A microdosimetry model, described in Part I, applies to the case of a conve x site immersed in a uniform distribution of heavy particle tracks, and ass umes no restrictions in site geometry or the kind of randomness. In Part II , this model is extended to include nonuniform distributions of particle tr acks. This situation is relevant to the study of microdosimetry, for exampl e, in boron neutron capture, in irradiation experiments using heavy ion par ticle beams, where the sources of particle tracks are external to the cell, or in irradiation from internally incorporated particle-emitting radionucl ides, such as environmental radon or occupational exposure to radioactive m aterials. The formalism developed permits the calculation of statistical pr operties, track length distributions, and microdosimetric spectra for conve x sites where the ''inner'' and ''outer'' concentrations of sources may be different, or for tracks originating on the surface of a convex site. Expre ssions applicable to the case of surface-distributed sources of tracks are presented that may represent situations such as boron compounds bound to th e membrane of a cellular nucleus in boron neutron capture. A series of Mont e Carlo calculations and analytical solutions, illustrating the case of sph erical site geometry, are presented and compared. Finally, microdosimetric spectra and specific energy averages are calculated for alpha and lithium p articles originating from thermal neutron capture in B-10, showing their de pendence on B-10 localization (extra-site, uniform, intra-site, or surface- distributed). (C) 2001 American Association of physicists in Medicine.