Abstract
Background count of CR-39 detector (allyl diglycol carbonate) was measured as a function of storage time to evaluate the background levels at delivery and during storage, and so as to find the way of storage to suppress the background increase. An image processing TV device was used for measuring them. The etch pit density at delivery was 3.9 to 8.4mm-2; on the other hand, it was 0.13 to 0.38mm-2 for the pits larger than 12μm, where the surface removed by the etching was 19μm. Under this background the radon of 60Bq/m3 could be detected at 50% relative standard deviation for one month exposure on the area above 10mm2 by optimizing a threshold for etch pit diameter to be counted. The increase rate of background was 5.5mm-2/month when left exposed to the atmosphere. On the other hand, the CR-39 sealed in polyethylene bag showed the increase rate less than one forth of that of the exposed detector.
