The NEXT-100 experiment aims at searching the neutrinoless double decay ($\beta\beta 0\nu$) of the Xe-136 isotope, using a TPC filled with 100 kg of enriched and highly pressurized gaseous xenon. The key technological feature of NEXT TPC is the electroluminescence (EL), which requires the optical readout of the signals using Photomultiplier Tubes (PMT) and Silicon Photomultipliers (SiPMs) for energy and tracking measurements respectively. This technology has been implemented and extensively tested during the last years in the demonstrator prototype NEXT-DEMO, achieving high energy resolution ($<$ 1% FWHM) and tracking capabilities required for a robust $\beta\beta$ decay experiment.

The interaction of the $\beta$ particles in the xenon gas produces excitation and ionization signals that are both used in the TPC for the identification of the $\beta\beta 0\nu$ events. The excitation signal is a prompt VUV scintillation, that is read out by an array of PMTs at the TPC cathode, providing a start-of-event signal. The ionization signal, which is proportional to the deposited energy in the gas, is amplified through the electroluminescence process that converts proportionally the charge signal to an optical signal, with very low statistical fluctuations. This optical signal known as the secondary scintillation, is of much larger amplitude and duration (few $\mu$s) than the primary scintillation signal. It builds up in the EL region at the TPC anode, several hundreds of $\mu$s after the starting of the event and is read out by the PMTs for the energy measurement. It is also read out by the array of SiPMs located right behind the EL region for the events topology measurement.

In the proposed talk, I will describe the NEXT concept and the signals induced by charged particles in the high-pressure xenon TPC read out by PMTs and SiPMs. I will especially address the SiPM readout technique implemented in the demonstrator prototype NEXT-DEMO, and its upgrade for the upcoming NEXT physics runs in the Canfranc Underground Laboratory (LSC) with the detectors NEXT-NEW and NEXT-100.

Instituto de Física Corpuscular (IFIC), CSIC & Universitat de València. Spain