  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.
