Detector upgrade for 222Rn concentration in high purity nitrogen measurement

To ensure the low background of the Jiangmen UndergroundNeutrino Observatory (JUNO) detector, high purity nitrogen (HPN) isused as a cover gas in direct contact with the liquid scintillator,which serves as the target material for neutrinos. The radonconcentration in this nitrogen should be less than10 μBq/m3. Given the background of the old detector, whichis 2.20 ± 0.64 mBq/m3, combined with its enrichmentefficiency of 51.0 % at a flow rate of 25 standard liters perminute (SLM), the effective measurement of radon concentrationsbelow 10 μBq/m3 in HPN is a major challenge. A highlysensitive radon detector was upgraded to measure the radonconcentration in HPN. A low background radon detection chamber wasconstructed based on the method of electrostatic collection andelectrolytic polishing of low background steel with an internalsurface roughness of ≤ 0.2 μm. The background of the radondetection chamber was reduced to 1.47 ± 0.19 mBq/m3. Anautomatic enrichment system based on 472 g low background activatedcarbon was developed. The introduction of a large amount ofactivated carbon has led to a significant increase in the enrichmentflow rate to 200 SLM. In addition, the enrichment efficiency of thesystem simultaneously improved to 71 %. The relationship betweenthe collection efficiency of the detection chamber and the appliedvoltage was calibrated. The result measured with this detector,shows that the radon concentration in HPN is2.38 ± 0.30 μBq/m3.