5-Bromo-2''-deoxyuridine labelled bacteriophage T1 was irradiated with monoenergetic X-rays obtained from synchrotron radiation with the energies at 13.51 keV and 12.40 keV, just above and below the K-shell absorption edge (13.41 keV) of bromine, respectively. Phage samples were irradiated under three conditions, wet N₂ gas, dry N₂ gas and in vacuum, with water contents (g H₂O/g sample) of 60, 2.6 and 0%. At 12.40 keV the Do in kGy were 1.1 (“wet”), 1.9 (“dry”) and 2.5 (“vacuum”) for the Br-labelled phages and 1.6 (“wet”), 2.4 (“dry”) and 6.2 (“vacuum”) for unlabelled phages. The results clearly demonstrated that the X-ray sensitivities decreased with decrease in water contents. Br-enhancement ratios, ER=D₀ (unlabelled)/ D₀ (Br-labelled), varied from 1.4 (“wet”, 12.40 keV) to 2.5 (“vacuum”, 13.51 keV). Auger enhancements which were defined by energy-dependent enhancement factor, F_en=[ER(13.51 keV)-ER(12.40 keV)]/ ER(12.40 keV), were 0.09±0.09, 0.29±0.07 and 0.02±0.03 under “wet”, “dry” and “vacuum” conditions, respectively. The change in Auger enhancement under “dry” condition in comparison with “wet” condition could be explained due to less of water. However the Auger enhancement decreased sharply under “vacuum” condition as the water content was zero. The reason for the sharp decrease in Auger enhancement under “vacuum” condition is difficult to understand. A possible explanation is discussed in the text.

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Two groups of normal human blood cells, one stimulated with phytohaemaglutinin (PHA) for 24 hr (G₁-S phase of the cell cycle) and one unstimulated (G₀ phase), were irradiated with ⁶⁰Co gamma rays or ²⁵²Cf radiation. A comparison of radiation-induced micronucleus frequencies showed that the high-dose-rate gamma rays were more effective in inducing micronuclei than low-dose-rate gamma rays. In the cells exposed to low-dose-rate irradiation, there was little difference between the frequency of micronuclei in the G₀ phase and the G₁-S phase. However, cells in the G₁-S phase were more sensitive than G₀-phase cells to high-doserate gamma rays. The relative biological effectiveness of ²⁵²Cf neutron irradiation measured in micronucleus assays was consistent with the value obtained for the lethal effect of ²⁵²Cf on cultured cells.

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Pregnant mice were exposed to whole-body X-irradiation at a total dose of 0.25 Gy split into two 0.125 Gy exposures at 0.5-, 2 or 6-hour interval on day 13 of pregnancy. Fetuses were obtained from dams at various post-exposure periods and their brains were processed for microscopy. Undifferentiated neural cells in the ventricular zone of telencephalon (ventricular cells) were examined, and incidence of cells involved in pyknosis was evaluated. The curves of incidence of pyknotic cells plotted against time after the exposures to two split-doses at 0.5-hour and 2-hour intervals overlapped that of a single 0.25 Gy exposure; they had a common peak at 8-10 hours after the first exposure. Following two 0.125 Gy exposures at 6-hour interval, two peaks with similar elevations from background levels appeared at 6 and 12 hours after the first exposure. These results indicated that low-dose X-irradiation shows simply additive effects of split doses on cell death, without induction of adaptive response of ventricular cells of the telencephalon at day 13 of pregnancy in mice.

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