An Analysis of the Target Theory of Lea with Modern Data

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Abstract
The target theory of Lea has been re-examined using modern data. Various workers have obtained radiation cross-sections from the data given in Lea's book. These cross-sections are different for different radiations and are in many cases larger than the physico-chemical cross-sections. Energy transfer between molecules has been postulated by some authors to explain this. However, before making this assumption, it is necessary to use the best available data to find the effect on the calculated cross-sections. Lea's data were calculated for water, whereas in this paper calculations have been made for protein. The differences are small (∼ 10 per cent) except in the case of the stopping power for low-energy electrons and the cross-section for primary ionization. The stopping power for low-energy electrons is not known, and modern experiments show that the primary ionization in protein may be much larger than in water. Various assumptions have been made about these in the calculations. The sensitivity of an ideal protein to lightly- and heavily-ionizing radiation has been calculated using various models for the stopping power for low-energy electrons and the primary ionization. The results indicate that, for reasonable assumptions about these parameters, the cross-sections obtained from the radiation sensitivity of a molecule will be smaller than those calculated from Lea's data and therefore closer to the physico-chemical cross-sections.