Influence of oxygen on the track etch rate along light ion trajectories in CR-39

The role of oxygen for stabilising radicals to form permanent damages in irradiated samples which are basically important for the etching process of track detectors has been studied. Samples of CR-39 were irradiated with 4 MeV -particles with a fluence of 30 000 particles per cm². The irradiations were performed in three different regimes:

• variation of out-gassing time in vacuum before irradiation;

• no out-gassing time but varied post-irradiation storage times in vacuum;

• variation of both storage times in vacuum before and after irradiation.

Generally, the absence of dissolved oxygen in the irradiated sample results in a decreased detection sensitivity in comparison with the detection properties and track parameters of samples irradiated in air. To determine the sensitivity the depth dependence of the track etch rate v_T(x) along the particle trajectory has been measured. From the observation of the dependence of the decreasing sensitivity on the post-irradiation storage time in vacuum, a lifetime of free radicals of 30 min could be derived. The amount of permanent damages responsible for the etching mechanism is related to the concentration profile of back-diffused oxygen and the REL-dependent radical concentration. This confirms previous results that the detection sensitivity is not a simple function of REL but depends on both REL and x the stronger the deeper within the irradiated sample.