Is selective absorption of ultrasoft X-rays biologically important in mammalian cells?

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Abstract
Tests whether photon absorption processes in particular atomic element(s) may be responsible for the observed high relative biological effectiveness (RBE) of ultrasoft X-rays. The effectiveness of titanium K characteristic X-rays (4.55 keV) is compared with previous observations for aluminium (1.5 keV) and carbon (0.28 keV) K ultrasoft X-rays. For a given absorbed dose, five times more TiK than AlK photons are absorbed in phosphorus; since AlK X-rays are observed to be more effective in killing human and hamster cells it is concluded that absorption in phosphorus does not play a dominant lethal role. This is supported by the observation that the absolute number of AlK photons absorbed in phosphorus of DNA of human fibroblasts is <1 per lethal event. For no element is the relative number of absorbed photons of the three X-ray energies even approximately proportional to their observed RBEs. The effectiveness of ultrasoft X-rays is apparently not due to selective absorption but rather to the secondary electrons; consequently the mechanism of action should be common to the large numbers of low energy secondary electrons produced by most other ionising radiations, including gamma -rays.

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