Highly sensitive measurements of radioactive noble gas nuclides in the Borexino solar neutrino experiment

Low background miniaturized proportional counters as developed for the Gallex solar neutrino experiment can be applied to the detection of radioactive noble gas nuclides at very low activities. We have developed an apparatus that allows the activity of trace amounts of isotopes of the four noble gases Ar, Kr, Xe and Rn to be measured. The technique includes contamination-free chromatographic purification of raw gas samples and subsequent low-level counting. Minimum detectable activities of 100 μBq and below have been attained. The developed techniques can be used to determine the ²²²Rn and ⁸⁵Kr concentration in nitrogen for the solar neutrino experiment Borexino. By applying efficient techniques to concentrate noble gases from nitrogen, minimum detectable activity concentrations below 1 μBq/m3 of nitrogen (STP) have been reached for both nuclides