Search for double beta decay of 82Se with the NEMO-3 detector and development of apparatus for low-level radon measurements for the SuperNEMO experiment

• Main Author: Mott, J. E
• Format: Dissertation
• Language: English
• Published: University College London (University of London) 2014

The 2νββ half-life of 82Se has been measured as (9.93 ± 0.14 (stat) ± 0.72 (syst)) × 10^19 yr using a 932 g sample measured for a total of 5.25 years in the NEMO-3 detector. The corresponding nuclear matrix element is found to be 0.0484 ± 0.0018. In addition, a search for 0νββ in the same isotope has been conducted and no evidence for a signal has been observed. The resulting half-life limit of > 2.18 × 10^23 yr (90% CL) for the neutrino mass mechanism corresponds to an effective Majorana neutrino mass of <mββ> < 1.0 - 2.8 eV (90% CL). Furthermore, constraints on lepton number violating parameters for other 0νββ mechanisms, such as right-handed current and Majoron emission modes, have been set. SuperNEMO is the successor to NEMO-3 and will be one of the next generation of 0νββ experiments. It aims to measure 82Se with an half-life sensitivity of 10^26 yr corresponding to <mββ> < 50 - 100 meV. Radon can be one of the most problematic backgrounds to any 0vBB search due to the high Q value of its daughter isotope, 214Bi. In order to achieve the target sensitivity, the radon concentration inside the tracking volume of SuperNEMO must be less than 150 μBq/m3. This low level of radon is not measurable with standard radon detectors, so a “radon concentration line” has been designed and developed. This apparatus has a sensitivity to radon concentration in the SuperNEMO tracker at the level of 40 μBq/m3, and has performed the first measurements of the radon level inside a sub-section of SuperNEMO, which is under construction. It has also been used to measure the radon content of nitrogen and helium gas cylinders, which are found to be in the ranges 70 - 120 μBq/m3 and 370 - 960 μBq/m3, respectively.