Xie, Fang; (2021) Radon Background Studies for the SuperNEMO Experiment. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

SuperNEMO is a neutrinoless double beta decay (0ν2β) experiment, aiming to reach a half-life sensitivity of 1026 years corresponding to an effective Majorana neutrino mass of ­ mββ® < 40 - 100 meV. The sensitivity target has posed a significant challenge of radiopurity for both the detector design and the construction process. One of the major backgrounds of the SuperNEMO detector is due to Radon, of which the decay daughter 214Bi (from 222Rn) presents a major challenge due its high Qβ values and ability to mimic a double beta decay process. Two radon emanation chambers have been built to be used in conjunction with an electrostatic detector to measure the detector components and construction materials. The chambers have been used to carry out 222Rn emanation assays for multiple samples of detector components and materials for the SuperNEMO and LZ experiments with a sensitivity of 0.09 - 0.19 mBq at 90% Confidence Level (C.L.). The 222Rn contamination of the tracker gas is required to be less than 0.15 mBq/m3 . This activity is beyond the sensitivity of current best radon detectors. Therefore a “Radon Concentration Line” was designed and built at UCL. This facility can deliver a concentrated gas sample to the detector, and for the SuperNEMO tracker gas it can reach a sensitivity of < 5 µBq/m3 (at 90% C.L.). Radon emanation measurements of fully instrumented SuperNEMO tracker submodules have been performed showing that the challenging 222Rn background requirements have been met.

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