Research on sensitivity fading of CR-39 detectors during long time exposure

CR-39 PNTDs (Plastic Nuclear Track Detectors) have been widely used in radiation research for many years. Experiments with long-term exposure during space mission LDEF (Long Duration Exposure Facility, 5.7 years), EUROMIR95 (∼ 6 months), Matroshka-1 (616 days), Matroshka-2 (367 days) and ISS-Expeditions (>∼ 6 months) indicate that fading of CR-39 sensitivity exists and the longer the exposure time, the more the fading. Due to fading the values of etch rate ratio and LET (Linear Energy Transfer) as well as the radiation quantities were smaller than expected. In the past, there was no appropriate method and formula which could be used to deal with the sensitivity fading of CR-39 detectors, so all the results obtained from CR-39 exposed for long time are questionable.To solve CR-39 fading problem for ISS-Expedition and Matroshka work, a study on the sensitivity fading of CR-39 was conducted and a practical method was developed. The method is based on the internal LET calibration using GCR events identified as iron peak nuclei at ∼ 1 GeV/n (∼100 keV/μm in CR-39), combined with identification of the particles' charge using the effective etch rate ratio and fractional etch rate gradient. A correction formula for etch rate ratio as a function of exposure time was found using data of ISS-Expedition and Matroshka exposures.This paper introduces the correction method for the fading of CR-39 sensitivity and the method to determine particles' charge using CR-39 detectors, presents the correction formula for the sensitivity fading of CR-39 and compares the radiation quantities obtained without and with fading correction for CR-39 sensitivity for recent ISS-Expedition and Matroshka space missions.