HIGHLY SENSITIVE PASSIVE DETECTORS FOR SHORT-TERM PRE-AND POST- MITIGATION MEASUREMENTS

It is required to evaluate the achieved radon reduction, preferably shortly after the mitigation system is activated. As instantaneous measurements are affected by short term radon variations, few days pre- and post-mitigation integrated measurements of sufficient sensitivity is preferred. Within the European MetroRADON project novel detectors of sufficient sensitivity and with compensated temperature dependence of the response were developed. They are based on using DVDs of low intrinsic background as track detectors, covered with Makrofol N foils. The absorption of radon by Makrofol N is very high (concentration ratio foil/air is > 100). The Makrofol N foil serves as radon absorber/radiator that greatly amplifies the signal (net track-density at etched DVDs). The achievable sensitivity is sufficient to prove quantitatively (within one-week exposure) that the reduction to low radon levels (< 100 Bq m -3 ) is achieved after mitigation. The detectors are cheap and usable for measurements at many points in large buildings. A novel technical concept (patent pending) to reduce the temperature dependence of the detectors and to eliminate the influence of thoron and humidity is described. The results of pilot experiments shown demonstrate the feasibility of this concept.