On the basis of simulations of water radiolysis, it has been postulated that the yield of OH radicals, which become homogeneously distributed, is dependent upon energy when irradiated with low-energy photons. The aim of this study is to determine experimentally the dependence of the yield of OH radicals, which escape intratrack recombination, on photon energy of incident radiation using plasmid DNA as a probe. The yields of single strand breaks (ssb) induced in plasmid DNA (pUC18) when irradiated with photons varying in energy from 0.28 keV to 1.25 MeV was determined and, when normalized relative to the yield for 60Co irradiation, may be used as a measure of the yield of OH radicals escaping radiation tracks. As the photon energy decreases from 1.25 MeV to 1.5 keV the OH radical yield decreases from 0.290 to 0.072 μmol J-1, respectively, in line with an increased ionization density of the radiation and hence an increasing probability of radical recombination. However, with a further decrease in photon energy from 1.5 to 0.28 keV there is an upturn in OH radical yields. Carbon-K X-rays are found to have a significantly higher yield of 0.257 μmol J-1 than that associated with the higher energies. The experimental dependence is compared with a number of theoretical calculations, which predict an upturn in OH radical yields between 0.1 and 1 keV. Such a dependence of OH radical yields on energy provides experimental data suitable for direct comparison with simulations, aiding their refinement and development.

The observed radiotoxicity of 65Zn is consistent with its behaviour as an Auger-emitting radionuclide that is localized to some extent in the nucleus.

Gamma-rays contribute 33% of the absorbed dose from an unfiltered 252Cf fission neutron source. To reduce this gamma-ray component and to enable radiobiological experiments at as high a dose rate as possible, Monte Carlo calculations for several filter materials (Al, Fe, Pb and concrete) have been made using MCNP neutron and photon transport code version 4a. A lead filter of thickness 4 cm was found to reduce the gamma-ray component to 6.7% of the total dose whilst reducing the neutron dose by only about 10%. Such a filter was installed at the MRC 252Cf neutron irradiation facility and dosimetric measurements were made using a TE-TE chamber and a 7LiF(Mg, Cu, P) TLD. Monte Carlo simulations agree with experimental measurements of neutron and gamma-ray doses within 6%. V79-4 Chinese hamster cells were irradiated with lead-filtered and unfiltered neutrons and also with 60Co gamma-rays at two dose rates. The survival fraction obtained for each radiation was consistent with the reduced gamma-ray dose. The relative biological effectiveness for neutrons alone, corrected for gamma-ray effects, was found to be 9.2 +/- 3.4 from the initial slopes and 3.1 +/- 0.5 at 10% survival, both relative to the acute gamma-rays.