Some effects of low-energy accelerated helium ion irradiation of essentially the whole skin surface of the mouse (penetration depth <tex-math>$51\ {\rm mg}/{\rm cm}^{2}$</tex-math>) have been investigated, using doses ranging from 75 to 2800 rads delivered in a single exposure. At this penetration depth, dose-response relationships were obtained for moist desquamation and mortality. For the moist desquamation reaction, 50% of the mouse surface area was involved at a dose of 2918 rads and extrapolation of the moist reaction curve to zero percent skin involvement yields a threshold dose of 1132 rads. Mortality of mice was found with helium-ion doses of 1600 rads or more with an <tex-math>${\rm LD}_{50(60)}$</tex-math> value of 2660 rads. This <tex-math>${\rm LD}_{50(60)}$</tex-math> value corresponds to involvement of about 43% of the mouse surface in a moist reaction. Mortality occurred between 22 and 42 days postirradiation. The mean survival time of the mice varied between 28.5 and 32.0 days. No dependence of survival time on dose was found. Skin reactions were first seen at about 15-20 days postirradiation, and maximal skin involvement occurred at about 28 days postirradiation for the moist reaction. Changes in epidermal cell nuclear volumes and mitotic indices were also measured as a function of dose and time postirradiation.

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