engleski

Saturation of spark counter due to high track densities on strippable LR 115 II nuclear track detectors

Nuclear track etched detectors are widely used for radon measurements. At Department of Physics University of Osijek, in recent twenty years a systematical research on applying nuclear track etched detectors LR-115 type II in measuring radon concentration in air, soil and water has been performed. The strippable nuclear track detectors LR-115 type II are used for indoor radon measurements and their track densities are counted by a spark counter. It works by applying an electrical field between two faces of the detector film which acts as the dielectric part of the capacitor. When sparks dash through the tracks, given electrical pulses are counted by an electronic module. The local sublimation of aluminium (which is one of the plates of the condenser) is important in avoiding the multiple counting of the same track but it may cause the electric isolation of an adjoining track. Two tracks at mutual distance lower than one spot diameter of sublimated area could be resolved as a single one and this effect is called saturation. The probability of track counting loss by saturation rises with the track density. The aim of this work is to determine the saturation threshold of the spark counter we use. A hundreds of strippable nuclear track detectors LR-115 type II (manufacturer Kodak-Pathe, France) are placed 2 cm from Am-241 radioactive source and exposed to radiation in different times (5, 10, 15, …, 100 s) using KVOD (a device for controlling irradiation time of detectors). It has a constant flux of a- particles so the linear relationship between the track densities and time of irradiation is expected. After being exposed, strippable LR 115 type II detectors were chemically etched in 10% aqueous solution of NaOH at 50 °C for 150 min. The tracks were then automatically counted by the spark counter AIST-2 V (V.G. Khlopkin Radium Institute, St. Petersburg, Russia). The obtained results show there is a statistically significant linear relationship between number of tracks on detector and time of irradiations. The average track density on LR-115 type II detectors is 157.4 ± 9.5 tr cm-2 s-1. The observed discrepancy between the expected and really counted track densities pointed out that the saturation threshold was about 6000 tr cm-2. The obtained value enables a better estimation of reliability of radon detectors used at our Department of Physics because it corresponds to radon concentration of 1000 Bq m-3 (with the coefficient of sensitivity of the used detectors of 65 Bq m-3 / tr cm-2 d-1 and assuming detectors exposure of one year). This radon concentration is the highest value our detectors could measure without counting a correction of the spark counter (at one year exposure regime) and it is about 14 times higher than the average radon concentration in homes in Croatia. The saturation threshold was determined which enables a better estimation of reliability of radon detectors used in research. The flux of alphaparticles from the radioactive source is well defined, as well as the detector response on time of irradiation is linear. This will be helpful for further investigation of some particular characteristics of track etched detectors.

saturation ; nuclear track detector ; radon

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