Some Physical Aspects of Relative Biological Efficiency

After surveying the broad nature of the problem of relative biological efficiency, an account is given of methods which have been used to characterise the quality of ionizing radiations. These former methods have ignored the phenomenon of secondary or δ-track formation and reference is made to treatments which take this factor into account. The concept of “local” energy losses is defined, and tables are given for various radiations, showing the amounts of “local' energy dissipated in water with respect to particle kinetic energy range. The radiations considered are 1 MeV and 210Po α particles, tritium β particles, 1 and 2 MeV monochromatic electrons, 25 MVp, 220 and 100 kVp X rays and 60Co γ rays. The special problem of the L.E.T. of electrons of less than 2 keV is considered and new values are derived from recently published experimental results. A mean L.E.T. is defined and calculated; its limitations are emphasised.