Low-²²²Rn nitrogen gas generator for ultra-low background counting systems

A new stand-alone system for purification of boil-off nitrogen from ²²²Rn is described. The purification principle is based on the radon adsorption on charcoal at low temperatures (cryo-adsorption). The device can be attached to ordinary dewars, which are usually used for liquid nitrogen storage or transportation. The measured ²²²Rn concentration in unpurified boil-off nitrogen is in the range of 0.5 mBq/m³, while the unit described can supply ultra-low background detectors (e.g. gamma spectrometers or dark matter detectors) with gaseous nitrogen of ²²²Rn specific activity lower by a factor 40. The lowest concentration achieved was 12±3 μBq/m³. Using a 100-l dewar it is possible to produce effectively at least 40 m³ of highly pure gas with the available rate between 0.3 and 2.5 m3/h. The nitrogen flux can be set by adjusting the power to an electrical heater placed inside the dewar