Proof-of-concept study of MeV gamma-ray imaging using a liquid argon time projection chamber for GRAMS

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-of-concept 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 3.2 mm 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.