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The EXO-200 experiment

The EXO-200 experiment searches for the neutrinoless double beta decay of 136Xe with a Q-value of 2458 keV using a single-phase liquid xenon time-projection-chamber. The discovery of this exotic, hypothetical decay would increase the knowledge of neutrinos in particular and the universe in general. The EXO-200 experiments is located underground in the Waste Isolation Pilot Plant (WIPP) in New Mexico with a natural shielding of 1585 m water-equivalent rock.

Figure 1: Scheme of the EXO-200 detector setup at WIPP.

Figure 1 shows a sketch of the experimental setup and the time-projection-chamber (see [1] for details). The time-projection-chamber is located in the center of a cryostat surrounded with a lead shielding as well as the cooling agent HFE. Muons are vetoed by large area plastic scintillators. The time-projection-chamber is a 44 cm long copper vessel with a diameter of 40 cm. It contains approximately 200 kg of liquid xenon enriched to about 80 % in 136Xe. The conducting parts of the detector are made from copper and bronze. The isolating parts from acrylic, PTFE or polyimide. At nominal conditions, the liquid xenon has a temperature of 167 K and a density of about 3.0 g/cm3.

In the middle plane of the vessel, the cathode grid is placed and biased with a negative high voltage with respect to the anode planes at both sides of the copper cylinder (see figure 2). Charged particles ionize xenon atoms along their trajectory. Depending on the ionization density, a certain portion of the released electrons does not recombine and is separated from the positively charged region of the trajectory by the applied electric field. On each planar side of the vessel, two crossed planes of wires form the anode planes. Electrons approaching the wires induce currents during their movement towards the wires and generate an electrical signal. The two wire planes are inclined by 60 degrees to reconstruct the two coordinates (x,y) of drifting electron distributions in the plane of the cap of the vessel.

Figure 2: Scheme of a double beta decay (red) emitting two electrons which in turn produce secondary electrons via ionisation (drifted to charge wires by electric fields) as well as scintillation light (detected by APDs).

Besides ionisation, UV-light is produced in liquid xenon after impact of an ionising particle. The UV-photons can be detected at both caps of the detector with arrays of avalanche photodiodes (APD) behind the anode grids. The field cage within the vessel is surrounded by PTFE plates for scintillation light reflection. The use of APDs is motivated by their small background contribution however they suffer from high dark-rates compared to photomultiplier tubes.

The time difference between charge and light signal corresponds to the drift time of the released electrons to the anode plane and can be converted to the depth of interaction. The pulse-shape of the wire planes is used to identify clusters of released charges. For a given total energy deposition, the strength of the scintillation signal (number of photons) is anti-correlated to the strength of the ionisation signal (number of secondary electrons). The reason is, that electron recombination leads to scintillation photon emission but also leads to a reduction of the drifting charge. Fiducial volume cuts are applied by excluding events close to the walls based on the three-dimensional event position reconstruction. The total 136Xe-mass after fiducializing used for the first physics data analysis was 79.4 kg [1]. A specific background rate of (1.5 ± 0.1) × 10-3 counts/(kg × keV × yr) was obtained.

First measurements have been accomplished in 2012 [2] and 2014 [3][4]. In 2017, a half-life limit of 1.8x 1025 yrs (at 90% C.L.) was deduced for the neutrinoless double beta decay of 136Xe corresponding to an upper limit on mββ between 147 meV and 398 meV [5]. The relative energy resolution had been improved to 1.23% at the Q-value. The background level was about 1.6 × 10-3 counts/(kg × keV × yr).

Research at Erlangen

Since August 2015, the Erlangen Centre for Astroparticle Physics is a member of the EXO-200 collaboration. ECAP works on improvements of the charge drift and event position reconstruction simulations. We also will investigate the light and charge anti-correlation, as well as the energy calibration procedure with a special view on the axis of reconstructed total deposited energy, named the beta scale. The beta scale is influenced by the anti-correlation between scintillation and charge signal measured in the TPC.