Determination of radon and thoron exhalation rate from the walls surface by CR-39 track detectors

Solid-state nuclear track detectors are finding expanded uses in various environment research investigations. Inhalation dose due to indoor radon/thoron and their short lived progeny concentrations constitute a major part of the total natural background radiation received by the population. Many measurements of radon concentration in the environment also involve using a dosimeter containing solid state nuclear track detector. The walls of houses included building materials such as granite, bricks, sand, cement etc., contain uranium and thorium in various amounts. Therefore the knowledge of true value exhalation rate of Rn and Tn from building materials represents scientific and practical interest in environmental radiation protection and ecology. In the present work, we have used calibrated plastic cups with two CR-39 detectors. The first detector fixed on bottom of cup in distance of cup height from walls surface and records alpha particles from radon only. The second detector fixed on the top of cup and records alpha particles from thoron and radon. The detected surface of the cup is situated in perpendicular position to surface of the wall. The detectors were exposed for 20-30 days. After exposition, the detectors were chemically etched and analyzed. The exhalation rate per unit area of the concrete, fired and unfired brick walls varied 0.091, 0.078 and 0.1 Bq m-2 h-1 for the radon, 3470, 200 and 5800 Bq m-2 h-1 for the thoron, accordingly. The number and location of radon and thoron alpha tracks were determined using an optical microscope and an image analysis system based on special developed software. The software for track images treatment by application of digital image processing has been developed. All program modules are made in DELPHI execution environment by using Pascal object-oriented language.