Radon problem in an underground low-level laboratory

The low-level gamma-ray spectroscopy as well as the investigation of rare nuclear processes require not only low, but also constant levels of relevant background radiations. In underground laboratories dedicated to this type of measurements, one of the main and hard to control sources of unwanted radiation is the radioactive gas radon. It is well-known that radon concentration varies daily and seasonally, primarily due to the variation of atmospheric parameters. This introduces unwanted and hard to evaluate systematic uncertainties in long-term low-level measurements. In this paper, the system for radon reduction in the underground Low-Background Laboratory for Nuclear Physics at the Institute of Physics in Belgrade is presented in some detail. The laboratory exists for ten years and different measurements of radon concentration were carried out during this period. The indoor radon measurements are performed using nuclear track detectors (type CR-39 and LR-115) for long-term measurements and the commercially available radon monitor for short-term measurements. In this work we present the results of these measurements for the period 2003-2008.