Energy deposition and ionization fluctuations induced by ions in small sites : an analytical approach

A concise, analytical approach is developed for calculating energy deposition and ionization fluctuations in volumes within ion-irradiated media which have dimensions as small as 1 nm. The method accounts for both direct ion interactions with the site and interactions of secondary electrons which are produced by ions in the surrounding medium. Particular attention is given to the way the contributions of the two types of events are combined. Since energy deposition fluctuations are simply related to the fundamental quantities $\overline{z_{{\rm D}}}$ and $\overline{y_{{\rm D}}}$ employed in microdosimetry theory, this new approach provides a convenient means to obtain these parameters. Results obtained with the analytical method show good agreement with Monte Carlo charged-particle track-structure calculations of $\overline{y_{{\rm D}}}$ for 0.5 to 20 MeV protons incident on spherical sites of water vapor with diameters ranging from 1 nm to 10 μm. In contrast to Monte Carlo techniques, the analytical meth...