Microdosimetric investigations on collimated fast-neutron beams for radiation therapy: I. Measurements of microdosimetric spectra and particle dose fractions in a water phantom for fast neutrons from 14 MeV deuterons on beryllium.

Lineal energy spectra of fast neutrons, produced by bombarding a thick beryllium target with 14 MeV deuterons, were measured with a proportional counter simulating a sensitive diamter of 2 micrometers. The mean neutron energy of the collimated beam was 6 MeV. The total lineal energy spectra, measured at different positions within a water phantom were separated into three different components: the gamma component, the recoil-proton component, and the component of heavier recoil ions. It is shown mathematically that the components of a linear energy spectrum thus derived allow the calculation of their fractional dose contributions. These dose fractions i.e. the gamma dose, the recoil-proton dose, and the dose fraction of heavier recoil ions, are shown as a function of the spatial position in the phantom.