Mutant fractions have been measured in the HPRT gene of the Chinese hamster cells line V79 after the exposure to accelerated heavy ions. In the course of experiments, the cells were irradiated under 4 different irradiation conditions. The accelerated ions were: B-11, O-18, and Ne-20 (two different energies). The linear energy transfer (LET) values were in the interval 50 ­ 153 keV/µm. The dependence of the mutant fraction on expression time (the incubation period of the cells from the end of irradiation until reseeding on a selective medium containing 6­thioguanine for the isolation of HPRT mutants) was measured for the doses: 0.5, 1, and 2 Gy.
The dependence expressed non-linear progression under all conditions. It was observed that mutant fraction was rising with the expression time; reaching a maximum; and decreasing again. For the high-LET radiation, the mutant fraction reached its maximum in various expression times depending on the LET. If the culture of cells was exposed to higher LET radiation the position of the maximum was shifted towards longer expression times. After this peak, the value of mutant fraction was falling down with increasing expression time, until reaching the levels typical for spontaneous (non-irradiated) mutants – in the region of 35 - 50 days (75 – 95 cell generations) after irradiation. No clear dependence on the dose was found.