Aerated and hypoxic primary roots of Vicia faba were irradiated in the entrance plateau and at the peak of the depth dose profile of 429 MeV/amu argon ions. Roots were sampled sequentially after irradiation and the spectrum and nature of chromosomal changes recorded on a per-cell basis. Plateau irradiation resulted in more cells damaged per rad than Bragg peak irradiation, i.e., the RBE was lower at the Bragg peak. There was a substantial cell age sensitivity for aberration frequency for plateau exposure but much less so at the Bragg peak. For exposure at both depth dose positions the oxygen enhancement ratio was similar and high. Irradiation at the Bragg peak resulted in higher frequencies of incomplete and complex aberrations than plateau irradiation. These manifestations of the passage of highly energetic argon ions doubtless contribute to the "overkill" capacity of Bragg-peak irradiation. While RBE and OER at the peak were comparatively low and high, respectively, if total cells were considered; "hit" cells were often grossly damaged, possibly indicating a higher RBE and a lower OER for those particular cells. Microdosimetric analysis is necessary before the distribution of energy deposition events at the nucleus level can be related to the distribution of observable cytological events.

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