In order for NEXT to be competitive with the new generation of 0nuBB experiments already in operation or in construction, we need a detector with very good energy resolution (< 1%), very low background contamination (~ 10^-4 counts/(keV kg y)) and large target mass. In addition, it needs to be operational as soon as possible.
The NEXT design optimizes energy resolution thanks to the use of proportional electroluminescent amplification (EL), which provides a large yield of photons as a signal; it is compact, as the Xe gas is under high pressure; and it allows the measurement of the topological signature of the event to further reduce the background contamination.
The SOFT (Separated Optimized FuncTion) design takes advantage of different sensors for tracking and calorimetry. On the tracking side, we'll make use of SiPMs (MPPCs) coated with a suitable wavelength shifter, while radiopure photomultipliers will be installed for the measurement of the energy and the primary scintillation needed to estimate the t0.
In the figure an asymmetric SOFT TPC is illustrated. An event, shown as a wiggly track, generates primary scintillation recorded at both planes (this is called the S1 signal, following the slang used by the experiments searching for direct detection of Dark Matter). EL light generated at the anode (S2) is recorded in the SiPMs plane right behind it and used for tracking. It is also recorded in the photosensor plane behind the transparent cathode and used for a precise energy measurement.
This design requires very little R&D and most of the proposed solutions have already been tested in the NEXT-1 prototypes. The detector may be upgraded to a fiducial mass of 1 ton after the initial physics runs, following the successful approach of GERDA and XENON experiments.