Commissioning of the SuperNEMO Demonstrator: A Neutrinoless Double Beta Decay Experiment

SuperNEMO is a neutrinoless double beta decay (0νββ) experiment, whose demonstrator module is located under the Frejus mountain in Modane, France, below 4800 m.w.e. It uses a unique technology combining a tracker and a calorimeter that allows it to unambiguously identify the two individual double-beta electrons, measure each particle's energy and their angular correlation. It aims to achieve an ultra-low background level of <10−4 events/(keV.kg.yr), with an exposure of 17.5 kg.y of 82Se. We also expect to be sensitive to 0νββ half-lifes of < 4×1024 years, corresponding to an effective Majorana mass of |mββ| < (260-500) meV (90% CL). We will as well make detailed measurements of the 2νββ, using all observables provided by this tracko-calo experiment, to constrain processes beyond Standard Model : Majoron, bosonic neutrino, Lorentz violation. We will also infer constrain of the axial-vector quenching constant, whose insufficient knowledge is a source of uncertainty for the 2νββ search.

SuperNEMO's calorimeter was commissioned in 2018 and since then, analyses were performed to calibrate the detector module in energy and time. This ensured that 98% of its 712 optical modules met our performance goals for gain and background level. Additionally, synchronisation of the optical modules in time achieved a precision better than 0.2 ns on the measured time difference, enough to make background rejections using time of flight calculations. The time resolution was calculated to be ~ 600 ps for gammas at 1 MeV.

Tracker commissioning began in September 2021. Initial data has enabled the tuning of gains and thresholds for performance optimisation and also validation that the detector is functioning with a high efficiency. Several studies are underway; these include the mapping and equalisation of the electric field by standardising plasma propagation time, HV stabilisation investigation, and evaluation of the spatial resolution with tracks. Additional commissioning is planned for calibration with 207Bi sources.

In September 2021, the calorimeter and the tracker were operated for the first time together. We have successfully demonstrated that we can clearly identify the individual electron tracks in double-beta-like events.