Speaker
Description
A Xenon Electroluminescence (AXEL) detector is a high-pressure xenon gas time projection chamber for searching for neutrinoless double beta decay (0$\nu\beta\beta$), which is a key to solve neutrino mass mechanism and matter-antimatter asymmetry in the universe. For a high-sensitivity 0$\nu\beta\beta$ search, achieving both of backgrounds discrimination and large mass of decay nuclei is essential. To meet these requirements, AXEL detector has a unique ionization-electron detection plane called Electroluminescence Light Collection Cell (ELCC). Since ionization-electrons are detected thorough Electroluminescence process, it can measure the energy with high resolution. Also, due to the rigid cell structure, it enables track pattern recognition and has a scalability to increase the nuclei.
Detector’s performance has been evaluated using 180L-size prototype at 6.8 bar, demonstrating the energy resolution of (0.67$\pm$0.08) % FWHM at 2615 keV. This was achieved with the drift electric field supplied by –34.3 kV generated by a dedicated Cockcroft-Walton (CW) multiplier installed inside the gas vessel. Furthermore, tracks possessing characteristics that enable the discrimination of 0$\nu\beta\beta$ from environmental radiation have been successfully imaged. A machine learning model to do this discrimination has been trained using simulated tracks and achieved 95 % background rejection efficiency with 60 % signal efficiency. Clearer track images have been obtained by the Richardson-Lucy deconvolution method, which would enhance the discrimination power.
Many R&D for the next 1000L-size detector construction is on progress. An ELCC structure with more resistance to creeping discharge is being developed. We use Multi Pixel Photon Counter (MPPC) for the photon detection on ELCC. To enhance the detection efficiency, large sensitive region MPPC was developed. It is confirmed that new MPPC has enough performance, and readout electronics for this MPPC is also under development. To reduce backgrounds contamination and also to lower the manufacturing cost, a field shaper cage made with Flexible Printed circuit (FPC) was assembled. Its voltage endurance and the potential charging-up on its surface are being investigated. CW multiplier update is underway, and generated voltage is up to –76.4 kV in the air. Also, to reduce contamination in capacitors in CW circuits, development of FPC-made capacitors is ongoing. To enhance the position-reconstruction accuracy, a novel scintillation light timing counter is under development to replace the conventional photomultiplier tubes. To improve the understanding of energy resolution, the fluctuation in MPPC non-linearity correction is under study using simulation data. With 1000L-size detector, we will demonstrate AXEL detector’s performance and its excellent sensitivity for 0$\nu\beta\beta$ search.