Seedlings of Vicia faba were used as a test system to investigate the radiobiological properties of a beam of 3.9 GeV nitrogen ions. Samples were irradiated on the entrance plateau and in various positions close to the Bragg peak of the depth-dose distribution. The oxygen enhancement ratio (OER) was found to be 1.55-1.65 at the Bragg peak, but had a value indistinguishable from that of x-rays at all other positions in the beam. The relative biological effectiveness (RBE) is a function of dose, and therefore of the level of biological damage. For 50% growth inhibition the RBE is between 1 and 1.9 on the entrance plateau, and is about 6 at the Bragg peak. In the region a few millimeters upstream of the Bragg peak the RBE remains nearly constant in spite of a rapid variation of average LET. Over the same region the OER rises rapidly from the low value determined at the peak (1.55-1.65) to the value characteristic of conventional x-rays (2.5-3).

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