Can heavy charged particles really be regarded as high-LET radiations with respect to their radiobiological actions? I: LETs and fluences of heavy charged particles and associated δ-rays

The structures of the energy transfer of heavy charged particles (HCP) to nm-size targets have been investigated, taking account of δ-ray generation by HCP as well as associated δ-rays. From the microdosimetric viewpoint, the radiobiological effects of HCP have to be analysed in terms of the microdosimetric quantities based on the local energy transfer (LET) to the target, that is the restricted linear energy transfer (LΔ, and the fluences or the degradation spectra of charged particles including δ-rays. LΔ of HCP and δ-rays, and the fluences of δ-rays have been calculated for various HCP (H-Ar) with energies 1, 10 and 100 MeV amu-1. From the results obtained, it is concluded that heavy ions (HI) should be expressed as two-component radiations which are composed of high-LΔ heavy ion tracks of low fluence and low-LΔ δ-rays of high fluence; in other words, the currently used expression 'HI are high-LET radiations' is not valid. (author)