Change in the sensitivity of CR-39 for alpha-tracks after the storage at different temperatures

The effect of storage on track registration property of CR-39 has been investigated. Pieces of CR-39 plate were irradiated with normally incident α-particles and fission fragments using a 252Cf source prior and posterior to the storage of them in air for one year at different temperatures of -80degC, -23degC, 4degC, 23degC and 35degC. Periodical etching was performed for the pieces stored at different temperatures using the solution of NaOH with 7 mol+l-1 at 70degC for 4 hours. Bulk etch rate (Vb) was obtained from the etch pit diameter (Df) of fission tracks using the equation: Vb=Df/(2t), where t is etching time (h). The sensitivity for α-tracks (S) was obtained from the ratio of the etch pit diameters between α-tracks (Da) and fission-tracks using the equation: S={1+(Da/Df)2}/{1-(Da/Df)2}-1. The present results show that both the bulk etch rate and α-track sensitivity are not constant, which tend to decrease with storage times and storage temperatures. At -80degC, -23degC and 4degC the change in the sensitivity was negligible. On the other hand, the storage at 23degC for 1 year decreased the sensitivity down to 74%. The most significant effect was observed on the α-track sensitivity at 35degC, which was reduced to 80% for one month, 61% for three months, 42% for six months and 32% for one year. By the comparison of experiments between fading and ageing it is shown that such storage effect is attributed not to fading of latent tracks but mainly to some changes in the detector itself. When CR-39 is used of radon monitoring or neutron monitoring, the following points should be paid attention to: (1) the detector should be stored in a refrigerator before exposure and until etching after the exposure, (2) the change in the sensitivity between the time of calibration and the time of use should be evaluated and the counting efficiency at the measurement should be corrected and (3) for comparison or for interpretation of experimental results in different experiments the effect of storage should be carefully considered. (author)