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Description
The Gamma-Ray and AntiMatter Survey (GRAMS) is a proposed mission to explore the MeV gamma-ray sky using a large effective-area Compton camera employing a liquid argon time projection chamber (LArTPC). As part of the concept study, we have developed a compact $5\times 5 \times 10\,\mathrm{cm^3}$ LArTPC prototype, named NanoGRAMS. When a MeV photon enters the LArTPC, it undergoes Compton scattering or photo-absorption in argon, producing scintillation light and ionized electrons. With information on the positions and energy depositions of these electron signals, we can estimate the spatial and energy distributions of incoming gamma rays.
NanoGRAMS is equipped with an arrayed silicon photomultiplier (SiPM) system, optimized for operation at liquid argon temperature ($87~\,\mathrm{K}$), for scintillation light detection, and a multi-pixel electron readout system. The SiPM system consists of $4\times 4$ array of $6\times 6\,\mathrm{mm^2}$ Hamamatsu SiPMs and a transimpedance amplifier. In liquid nitrogen tests ($77~\mathrm{K}$), the circuit demonstrated sensitivity from single photons up to approximately 100 photons with a timing response of nearly $80\,\mathrm{ns}$. This fast response is sufficient for not only event triggering but also rejection of background albedo neutrons through pulse shape discrimination. The electron readout system employs a 16x16 pixelated anode ($5.12\times 5.12\,\mathrm{cm^2}$) and ASICs originally developed for semiconductor detectors. In liquid argon, the electronics noise is 140 $\mathrm{e^-}$ rms per channel, which is sufficient for gamma-ray imaging. Combined with the SiPM light trigger, the system successfully detected energy depositions corresponding to multiple Compton scatterings from $^{60}\mathrm{Co}$ gamma rays ($1.17/1.33\,\mathrm{MeV}$). We present the design of the NanoGRAMS detector and the evaluation of gamma-ray measurement performance.
