Biological data obtained on Vicia faba, Tradescantia, and mammalian cells with a 3.9-GeV nitrogen ion beam are related to microdosimetric measurements. The relative biological effectiveness near the Bragg peak of the depth-dose curve can be explained on the basis of the microdosimetric data and the theory of dual radiation action. A comparison is performed with the results obtained on the same biological systems with monoenergetic neutrons, and essential agreement is found with earlier conclusions derived from the comparison of track-segment experiments and neutron irradiations. The oxygen-enhancement ratio for the nitrogen ions is higher than expected from the microdosimetric data; this is taken as an indication that the oxygen effect may at least partly depend on the energy concentration over distances of the order of nanometers. The implications of the results to the use of heavy ions in radiotherapy are considered.

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