Experimental study on the aging process of the LR 115 cellulose nitrate radon detector

An experimental determination of the aging process of cellulose nitrate det ector material was based on the examination of special properties of the LR 115 solid state nuclear track detectors (SSNTDs) of various ages up to 18 years. The examined relevant parameters are the bulk etching rate upsilon ( b) and the track etching rate upsilon (t). These parameters are responsible for the appearance, the size and the registration efficiency of tracks of alpha -particles from radon gas in the detector. To find a correlation betw een these material parameters and the detector sensitivity an experimental calibration of indoor room and outdoor soil detector devices based on LR 11 5 took place at the Umweltforschungszentrum Leipzig-Halle (Germany). To avo id routine calibration work in external radon exposure facilities a correct ion of the age dependent calibration factors with the material parameters m easured in one's own laboratory was targeted. In this study a general age d ependence, however, was not found. The following statements for practical a pplications can be made. (i) the bulk etching rate upsilon (b) for detector s of the same batch has a depth dependence and this dependence is constant over 2 years (LR 115 September 1994). (ii) detectors of different batches o lder than 5 years and stored at room temperature show an odd upsilon (b) be haviour when upsilon (b) is used for describing track shapes. (iii) the cal ibration factor of detectors of different batches that were stored at about +4 degreesC is constant over 5 years (LR 115 September 1994 and February 1 999, Table 2). The conclusion is that LR 115 detectors not older than 5 years and stored i n a refrigerator at about +4 degreesC should be preferred for radon measure ments. Furthermore these detectors should be recalibrated every year and th e microscope work of this calibrations should be performed by the same pers on who performs the measurements. In addition, a phenomenon related to fundamental track formation mechanisms was found, that the time straggling of the time t(through) when vertical t racks penetrate the 12 mum thick detector layer is independent of the age o f the detectors and the energy of the alpha -particle at the detector surfa ce. (C) 2001 Elsevier Science Ltd. All rights reserved.