High-pressure xenon gas TPC for neutrino-less double-beta decay in 136Xe: Progress toward the goal of 1% FWHM energy resolution

A high-pressure xenon gas (HPXe) TPC offers attractive possibilities in the search for neutrino-less double-beta decay in ¹³⁶Xe. In the gas phase, near-intrinsic energy resolution is available from the ionization signal only, in contrast to the liquid phase which displays anomalously large fluctuations in the partition of energy between scintillation and ionization. In addition, events in the gas phase extend over several cm, permitting topological tests to discriminate efficiently against gamma-ray backgrounds. Our TPC exploits the nearly noiseless gain mechanism of electroluminescence to detect the signal. We report on progress with a small HPXe TPC that probes pressures up to 17 bars, also permitting variation of drift fields over an interesting range. The TPC provides excellent performance at the 662 keV gamma ray of ¹³⁷Cs. This TPC was developed as part of the prototyping stage of the experiment NEXT (Neutrino Experiment with a Xenon TPC), to take place at Canfranc underground laboratory.