Developing a mass-production model of large-area Si(Li) detectors with high operating temperatures

Abstract

This study presents a fabrication process for lithium-drifted silicon (Si(Li)) detectors that, compared to previous methods, allows for mass production at a higher yield, while providing a large sensitive area and low leakage currents at relatively high temperatures. This design, developed for the unique requirements of the General Antiparticle Spectrometer (GAPS) experiment, has an overall diameter of 10 cm, with ∼9 cm of active area segmented into 8 readout strips, and an overall thickness of 2.5 mm, with 2.2 mm ( 90%) sensitive thickness. An energy resolution 4 keV full-width at half-maximum (FWHM) for 20−100 keV X-rays is required at the operating temperature ∼ − 40 • C, which is far above the liquid nitrogen temperatures conventionally used to achieve fine energy resolution. High-yield production is also required for GAPS, which consists of 1000 detectors. Our specially-developed Si crystal and custom methods of Li evaporation, diffusion and drifting allow for a thick, large-area and uniform sensitive layer. We find that retaining a thin undrifted layer on the p-side of the detector drastically reduces the leakage current, which is a dominant component of the energy resolution at these temperatures. A guard-ring structure and optimal etching of the detec- tor surface are also confirmed to suppress the leakage current. We report on the mass production of these detectors that is ongoing now, and demonstrate it is capable of delivering a high yield of ∼90%.We present here a high-yield mass production process for lithium-drifted silicon (Si(Li)) detectors that meet the unique requirements of the General Antiparticle Spectrometer (GAPS) experiment. GAPS is a balloon-borne experiment that aims to survey low-energy (<0.25 GeV/n) cosmic-ray antinuclei for the first time, by adopting a novel detection concept based on the physics of exotic atoms [1][2][3][4]. Low-energy cosmic-ray antinuclei, especially antideuterons, are predicted to be distinctive probes for the dark matter annihilation or decay occurring in the Galactic halo [1,[5][6][7][8][9]. Precise measurement of the low-energy antiproton spectra will also provide crucial information on the source and propagation mechanisms of cosmic rays [10][11][12][13]. GAPS sensitivities to antideuterons and antiprotons are discussed in [14] and [13], and capabilities for antihelium detection are being evaluated. The first flight of GAPS via a NASA Antarctic long duration balloon is planned for late 2021.GAPS is comprised of a 1.6 m W × 1.6 m D × 1.0 m H tracker made of Si(Li) detectors surrounded by a time-of-flight (TOF) system made of plastic scintillator paddles. A low-energy antinucleus triggered by the TOF is slowed and captured by the Si(Li) detector array, forming an excited exotic atom with a silicon nucleus. It immediately decays, radiating de-excitation X-rays of characteristic energies. The antinucleus then annihilates with the silicon nucleus, producing pions and protons with a multiplicity that scales with the incident antinucleus mass. The surrounding Si(Li)...

Large-area Si(Li) detectors for X-ray spectrometry and particle tracking in the GAPS experiment

“…The low leakage current is achieved at relative high temperatures (∼ −40 • C) by implementing a thin undrifted layer on the detector's p-side and a guard ring structure ( Fig. 1) that limits surface leakage current in the detector's active area [8]. The capacitance and leakage current are further controlled by segmenting the detectors into strips, which also improves spatial resolution for particle tracking.…”

Reconstruction of antinucleus-annihilation events in the GAPS experiment

“…All scintillators are 6.35 mm thick and 16 cm wide, with a variable lenght between 1.51 and 1.8 m. Each paddle system provides the measurement of energy deposit, time and longitudinal position along its largest dimension [10]. The tracker systems is made of 1440 Si(Li) detectors arranged in 10 evenly spaced planes [11][12][13][14]. Each detector has a cylindrical shape of ∼10 cm diameter and ∼2.5 mm thickness and it is segmented into eight strips of equal area.…”

Searching for cosmic antihelium nuclei with the GAPS experiment

“…Searching for cosmic antihelium nuclei with the GAPS experiment A. Stoessl 2.5 mm thick lithium-drifted silicon (Si(Li)) detectors, arranged into ten tracking planes. Each Si(Li) detector has a cylindrical geometry with an active area of about 70 cm 2 that is segmented into eight single-sided strips of equal area [6][7][8][9]. The energy resolution of these detectors is close to 4 keV in the 20−100 keV range, and the electronics provide a high dynamic range for measuring particle tracks with individual energy depositions up to 100 MeV before saturation.…”