Chinese hamster cells cultured in vitro were used as a test system to determine the radiobiological properties of 3.9-GeV nitrogen ions. Samples were irradiated at the Bragg peak of the depth-dose distribution. The oxygen enhancement ratio (OER) was found to be 1.25 for nitrogen ions, compared with 2.2 for x-rays. The relative biological effectiveness (RBE) of the heavy ions is a function of dose and, therefore, of the level of biological damage. For a cell-surviving fraction of 10-2 the RBE is approximately 2. At lower doses of 50-100 rads, corresponding to a cell-surviving fraction of 0.5, the RBE is close to 6.

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