Toward new frontiers : Encounter and synergy of state-of-the-art astronomical detectors and exotic quantum beams

Program C01-1	Development of multiple scattering Compton cameras for high sensitivity cosmic MeV observations and nuclear medicine
Principal Investigator	ODAKA, Hirokazu (The University of Tokyo)

In observational astronomy using the electromagnetic wave, medium energy gamma rays which range from 0.1 to 10 MeV remain as an unexplored window. This energy band contains gamma-ray lines emitted by atomic nuclei, providing us with unique direct probes of nuclear reactions associated with high energy objects such as supernova explosions, neutron star mergers, and black holes. The direct observations of these nuclear reactions should bring us information on nucleosynthesis in the universe and the origin of matter. In spite of their importance, those observations have been extremely limited due to their technical difficulties. We therefore introduce a new approach to the detector design and its data analysis methods in order to open the MeV gamma-ray window.

The GRAMS project, which we proposed together with collaborators in the United States, uses a liquid argon time projection chamber for a balloon-borne experiment to perform simultaneously the indirect search for dark matter via cosmic anti-deuterons and the astronomical observations of MeV gamma rays. The adoption of the liquid detector, which can have a high density and a large volume, realizes a cost-effective large collection area instrument. However, this requires a new development of gamma-ray analysis technique as well as hardware development. The purpose of this research plan is to develop an event reconstruction algorithm of multiple Compton scattering inside the detector and an image reconstruction algorithm by using statistical methods and techniques of machine learning. Based on these software techniques, we will make a concrete design of the hardware and observational plans of astronomy.

Our products should be applied to state-of-the-art semiconductor Compton cameras which have been developed in Japan for astrophysics, largely improving their sensitivities. This is also useful for Compton cameras of nuclear medicine, particularly for image diagnostics using radioactive isotope markers and beam position monitors of radiation therapy, from a viewpoint of reducing an exposure dose and a measurement time.
A schematic drawing of the liquid argon time projection chamber (TPC) for the GRAMS experiment. The combination of the TPC and scintillation light enables three-dimensional determination of an interaction position in a single volume of the detector. This research project concentrates on study of events with Compton scattering of an incident gamma ray shown at the center.

Members

Reference Materials

• T. Aramaki et al., “Dual MeV gamma-ray and dark matter observatory - GRAMS project,” Astropart. Phys., in press (2019).
• Hitomi Collaboration, “Detection of polarized gamma-ray emission from the Crab nebula with the Hitomi Soft Gamma-ray Detector,” Publ. Astron. Soc. Jpn. 70, 113 (2018).
• H. Odaka et al., “Modeling of proton-induced radioactivation background in hard X-ray telescopes: Geant4-based simulation and its demonstration by Hitomi's measurement in a low Earth orbit,” Nucl. Instrum. Methods Phys. Res. A 891, 92–105 (2018).
• S. Ikeda et al., “Bin mode estimation methods for Compton camera imaging,” Nucl. Instrum. Methods Phys. Res. A 750, 46–56 (2014).
• H. Odaka et al., “High-resolution Compton cameras based on Si/CdTe double-sided strip detectors,” Nucl. Instrum. Methods Phys. Res. A 695, 179–183 (2012).