High-pressure xenon gas time projection chamber with scalable design and its performance around the Q value of 136Xe double-beta decay

Abstract

We have been developing a high-pressure xenon gas time projection chamber (TPC) called AXEL (A Xenon ElectroLuminescence detector) to search for neutrinoless double-beta (0νββ) decay of ¹³⁶Xe. The unique feature of this TPC is the electroluminescence light collection cell (ELCC), the part designed to detect ionization electrons. The ELCC is composed of multiple units, and one unit covers 48.5 cm². A 180 L size prototype detector with 12 units, 672 channels, of the ELCC was constructed and operated with 7.6 bar natural xenon gas to evaluate the performance of the detector around a Q value of ¹³⁶Xe 0νββ. The obtained FWHM energy resolution is $0.73 \pm 0.11\%$ at 1836 keV. This corresponds to $0.60 \pm 0.03\%$ to $0.70 \pm 0.21\%$ of the energy resolution at a Q value of ¹³⁶Xe 0νββ. This result shows the scalability of the AXEL detector with the ELCC while maintaining a high energy resolution. Factors determining the energy resolution were quantitatively evaluated and the result indicates that further improvement is feasible. Reconstructed track images show distinctive structures at the endpoints of electron tracks, which will be an important feature in distinguishing 0νββ signals from gamma-ray backgrounds.