A new system for the 222Rn and 226Ra assay of water and results in the Borexino project

We describe a very sensitive system for the 222Rn and 226Ra assay of water that we have constructed within the framework of the Borexino solar neutrino experiment. The technique that is applied for 222Rn measurements is based on concentration of radon from water samples followed by gas purification and subsequent alpha counting in low-background miniaturized proportional counters. The same technique is applied to measure the 226Ra concentration by determining the amount of 222Rn in equilibrium with the 226Ra in the water. Sensitivities of ∼1 mBq/m3 for 226Ra and ∼100 μBq/m3 for 222Rn have been achieved. We present results of measurements illustrating the performance of the system as well as investigations of samples from the Borexino water plant and the shielding water of the Counting Test Facility (CTF, a test facility for the Borexino detector).