The influence of pre-exposure thermal annealing on CR-39 etch rates has been studied here thoroughly. Annealing experiments were conducted on CR-39 detectors prior to their exposure to 252Cf fission fragment source. The bulk etch rates were measured by using the weight loss method (WLM) and the fission track diameter method (FTDM). We report here a new quantitative result that the bulk etch rate decreases from the original surface of each CR-39 detector initially rapidly and then more slowly along the depth mimicking the exponential trend. These quantitative results are more pronounced for cases of higher annealing temperatures and larger annealing time intervals. This exponential drop of the bulk etch rate suggests that the interior of the CR-39 detector material is almost out of pre-exposure annealing. Our results reveal that the etch rates and the registration properties of the heavy ions are considerably altered in the near-surface layer of the annealed PADC.
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