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)...
A Si(Li) detector fabrication procedure has been developed with the aim of satisfying the unique requirements of the GAPS (General Antiparticle Spectrometer) experiment. Si(Li) detectors are particularly well-suited to the GAPS detection scheme, in which several planes of detectors act as the target to slow and capture an incoming antiparticle into an exotic atom, as well as the spectrometer and tracker to measure the resulting decay X-rays and annihilation products. These detectors must provide the absorption depth, energy resolution, tracking efficiency, and active area necessary for this technique, all within the significant temperature, power, and cost constraints of an Antarctic long-duration balloon flight. We report here on the fabrication and performance of prototype 2 -diameter, 1-1.25 mm-thick, single-strip Si(Li) detectors that provide the necessary X-ray energy resolution of ∼ 4 keV for a cost per unit area that is far below that of previously-acquired commercial detectors. This fabrication procedure is currently being optimized for the 4 -diameter, 2.5 mm-thick, multi-strip geometry that will be used for the GAPS flight detectors.
Continuum x rays induced by bombardments of a Be target with 20.14-MeV/amu protons and He2+ ions have been measured with a Si(Li) detector in the direction of 90' with respect to the incident beam. Differences in the x-ray -production cross sections multiplied by the x-ray energy 8'ay Pro [o-I, (tao)/4 -oz(leo) jJ and ratio of the cross sections R (A'au) f= -o&(Aced) j4az(has)] for the proton and He +-ion impact, where (7&(fo)) and o. z(A'e) are the x-ray -production cross sections for proton and He2+-ion impact, respectively, were obtained as a function of the x-ray energy. Both the difference and the ratio show peaks in the region where the x-ray energy is equal to the relative kinetic energy between the projectile and the inner-shell electron to be scattered by the projectile. From the comparison with a theory of quasi-free-electron bremsstrahlung based on the impulse approximation, it is found that this peak corresponds to the maximum of the velocity-distribution function of the inner-shell electron. Furthermore, the contribution of the radiative electron-capture process in the case of proton and 3He2+ impact is clearly found.Recently, ' we have bombarded targets of Be, C, and Al with 6 -40-MeV protons and have found a new component of continuum x rays. These continuum x rays are predominant in the region where the x-ray energy tao is smaller than the energy T, = --,m, V»', where m, is the electron mass and V~is the projectile velocity, and the Doppler effect is definitely observed in the angular distributions of these x rays. From these experimental results, the x rays have been well understood in terms of the bremsstrahlung produced by the orbital electrons scattered by the projectile Coulomb field; we call this process the quasi-free-electron bremsstrahlung (QFEB). The lllIX r '1/2 projectile-energy dependence and the angular dependence of QFEB have been well explained by a simple theory assuming that the orbital electrons are free.Further, a calculation taking into account the velocity distribution of orbital electrons has been developed, ' and it was found that the spectrum of QFEB in the vicinity of the high-energy limit of Ace = T, is sensitively dependent on the velocity distribution of orbital electrons.On the basis of the impulse approximation which Jakubassa and Kleber have developed for the bremsstrahlung in heavy-ion reaction, 3 a more accurate formula for the cross section of QFEB can be given by with v, '"= -, '
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