We have developed a small liquid argon time projection chamber (LArTPC) called NanoGRAMS, whose fiducial volume is 5 × 5 × 10 cm3. A LArTPC has advantages of dense and large volumes sensitive to MeV gamma rays compared with multi-layer semiconductor detectors or gaseous ones. The aim of our work is proof-ofconcept studies for one of the important goals of Gamma-Ray and AntiMatter Survey (GRAMS). The imaging performance of Compton cameras is mainly determined by energy and position resolutions of the detector. The NanoGRAMS measures scintillation light and ionized electrons generated by interactions of gamma rays and argon atoms, which are reconstructed to information on the initial energies and momenta of incoming photons. The NanoGRAMS is equipped with photon detection boards loading a large-area array consisting of 4×4 single SiPMs, which totally cover 2.56×2.56 cm2, and low-noise electron readout boards. The SiPM boards operate at a liquid argon (87 K) temperature and exhibit a fast response time below 100 ns allowing pulse shape discrimination to reject atmospheric neutron background. The electron readout system comprises an anode electrode segmented into 16×16 pixels with a 3.2mm pitch. Charge signals from all pixels are processed by four 64-channel low-noise readout ASICs originally developed for semiconductor detectors. We confirmed that this TPC has the capability of detecting scintillation light and electrons generated by α-rays in gaseous argon and gamma rays in liquid argon.
GRAMS (Gamma-Ray and AntiMatter Survey) is a next-generation proposed balloon-borne/satellite-based mission aimed at high sensitivity MeV gamma-ray astrophysical observations and background-free indirect dark matter search via hadronic antiparticles. The main detector of GRAMS is a meter-scale liquid argon time projection chamber (LArTPC). The adoption of liquid argon as detector material allows us to produce an unprecedentedly large effective area instrument both for cosmic MeV gamma rays of 0.5-20 MeV and antiparticles of dark matter origin. This large effective area, which will exceed 1000 cm2, is necessary for measuring faint gamma-ray signals of nuclear line emissions from energetic phenomena such as supernovae as well as for observing short-duration transient objects including gamma-ray bursts with high photon statistics. In this talk, we present the mission concept and design, the current proof-of-concept studies using prototype LArTPCs, and an engineering balloon flight conducted in 2023.
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