AXEL: High-pressure xenon gas TPC for neutrinoless double beta decay search

A Xenon ElectroLuminescence (AXEL) is a high-pressure xenon gas time projection chamber with a unique ionization-electron counter called Electroluminescence Light Collection Cell (ELCC), which has been developed for neutrinoless double beta decay ( 0 ν β β ) search. We have developed a dedicated Cockcroft-Walton high-voltage generator installed inside the chamber of a 180L-size prototype and applied up to − 34.3 kV in xenon gas at 6.8 bar. A full-width at half-maximum energy resolution of ( 0 . 68 ± 0 . 01 ) % was obtained for 2615-keV electrons. Three-dimensional tracks of electrons were reconstructed, and the ability of the background rejection in 0 ν β β search is being evaluated. Toward the first 0 ν β β search with the AXEL detector, a 1000L-size detector is being constructed, and R&D of the components is in progress such as ELCC with large-area silicon photomultipliers, readout electronics, Cockcroft-Walton generator and scintillation detector.