Sensitivity of a low background proportional counter used in the GALLEX experiment to environmental 222Rn

The response to environmental 222Rn of a miniaturized proportional counter as used in the ultra low background solar neutrino experiment GALLEX was investigated. The increase of the number of counts per unit time of the counter placed in gas containing 222Rn with concentrations of the order of 300–1500 kBq/m3 was registered using electronics similar to the original GALLEX data acquisition system. The measured sensitivity of the counter to the environmental 222Rn for the passive array (passive shield only) of the GALLEX spectrometer is (0.15 ± 0.01) cpd/(kBq/m3) and (0.041 ± 0.005) cpd/(kBq/m3) for the GALLEX relevant L and K energy — rise-time acceptance windows, respectively. The sensitivity for the active array (passive shield and anticoincidence shield) is (2.4 ± 0.2) cpd/(kBq/m3) and (0.67 ± 0.07) cpd/(kBq/m3) for the L and K windows, respectively. The integral response of the counter to the 222Rn for all energies > 0.5 keV is 36.8 cpd/(kBq/m3) and 2.03 cpd/(kBq/m3) for the active and passive array, respectively. We derive the required correction of (0.3 ± 0.3) SNU, due to time variations of the ambient 222Rn concentration on the counting site, for the first 15 GALLEX solar neutrino runs.