RBE of Neutrons at Energies of 36 keV-15 MeV and Photons at Energies of 1

We examined the induction of dicentrics in human lymphocytes irradiated in vitro with nearly monoenergetic neutrons at energies in the range of 36 keV-15.0 MeV and monochromatic X-rays in the range of 1.8 keV-40 keV. For the assessment of the relative biological effectiveness (RBE) both 220 kV X-rays and Co γ-rays were used as reference radiations. To avoid potential confounding factors that would influence the outcome of the experiments, only blood from one individual was used. The neutron and photon RBE culture conditions ensured that the chromosome analysis could be performed exclusively in metaphases of the first cell cycle in vitro. For the reference radiation of 220 kV X-rays, the values of the low-dose RBE of neutrons were found to increase from 16.6 (En = 36 keV) to the maximum value of 23.4 (En = 385 keV) with a further decrease to 4.1 (En = 14.6 MeV), as well as a strong indication for a systematic change of the low-dose RBE of photons with an increase from 0.59 (E = 40 keV) to 2.1 (E = 6.9 keV) and a slight tendency to drop with further decrease of energy. For Co γ-rays utilized as the reference radiation, the corresponding RBE values were found to be higher by a factor of 4. These results agree well – with some restrictions – with previously published large data sets of three laboratories.