Development of Large-area Lithium-drifted Silicon Detectors for the GAPS Experiment

TL;DR

This paper reports the development of large-area lithium-drifted silicon detectors designed specifically for the GAPS experiment, aiming to detect cosmic-ray antinuclei with high energy resolution at relatively high temperatures.

Contribution

The paper introduces a novel fabrication process for large-area Si(Li) detectors with improved leakage current suppression suitable for balloon-borne cosmic-ray detection.

Findings

Achieved <4 keV energy resolution at -35 to -45°C

Developed a high-yield production process for 10-cm diameter detectors

Successfully suppressed leakage current with guard ring and surface etching

Abstract

We have developed large-area lithium-drifted silicon (Si(Li)) detectors to meet the unique requirements of the General Antiparticle Spectrometer (GAPS) experiment. GAPS is an Antarctic balloon-borne mission scheduled for the first flight in late 2020. The GAPS experiment aims to survey low-energy cosmic-ray antinuclei, particularly antideuterons, which are recognized as essentially background-free signals from dark matter annihilation or decay. The GAPS Si(Li) detector design is a thickness of 2.5 mm, diameter of 10 cm and 8 readout strips. The energy resolution of <4 keV (FWHM) for 20 to 100 keV X-rays at temperature of -35 to -45 C, far above the liquid nitrogen temperatures frequently used to achieve fine energy resolution, is required. We developed a high-quality Si crystal and Li-evaporation, diffusion and drift methods to form a uniform Li-drifted layer. Guard ring structure and optimal etching of the surface are confirmed to suppress the leakage current, which is a main source of noise. We found a thin un-drifted layer retained on the p-side effectively suppresses the leakage current. By these developments, we succeeded in developing the GAPS Si(Li) detector. As the ultimate GAPS instrument will require >1000 10-cm diameter Si(Li) detectors to achieve high sensitivity to rare antideuteron events, high-yield production is also a key factor for the success of the GAPS mission.