水村 好貴

Although the MeV gamma-ray band is a promising energy-band window in astrophysics, the current situation of MeV gamma-ray astronomy significantly lags behind those of the other energy bands in angular resolution and sensitivity. An electron-tracking Compton camera (ETCC), a next-generation MeV detector, is expected to revolutionize the situation. However, the energy band observable with ETCC has been limited to < 2 MeV. Here, we study ETCC events in which the Compton-recoil electrons do not deposit all energies to the electron tracker but escape and hit the surrounding pixel scintillator array (PSA). We developed an analysis method for this untapped class of events and applied it to laboratory and simulation data. We also evaluated the detector performance using the simulation data and found that this new method has enabled us to extend the observable energy range in the previous studies with the ETCC to the higher energy.

MeV gamma rays from celestial objects provide unique information about nucleosynthesis in supernovae or neutron star mergers, the diffusion of matter in the galaxy, the existence of low-energy cosmic rays, and so on. However, the detection sensitivity in this band is not yet sufficient to discuss astrophysical phenomena because of the huge background. For future observations, we are developing an electron-tracking Compton camera (ETCC) with powerful background rejection tools based on the Compton recoil electron tracks. In 2018, our second balloon experiment was conducted to demonstrate the detection of bright sources, and it successfully detected the Crab Nebula and the Galactic Center region with the designed sensitivity. Therefore, we are planning some scientific observations using the ETCCs loaded on long-duration balloons to reveal the origin of galactic diffuse gamma rays and to discover new MeV gamma-ray sources. In this paper, we will present the scientific motivation of SMILE-3 and the preparations for the next flight.

Since the discovery of possible bacterial particles in the stratosphere in the 1930s, bioaerosol particles in the stratosphere have been studied to understand how far the biosphere extends, what types of bioaerosol particles and how they exist in the extreme stratospheric environment and how these bioaerosol particles can reach above the troposphere. Although little is known about the ecology in the stratosphere, not much work has been done in the stratosphere due to its limited accessibility. The physical and biological dynamics of bioaerosol particles in the stratosphere are closely related to aerobiological and astrobiological research topics such as global-scale long-range transport of bioaerosol particles, planetary protection, and the emerging and evolution of life. Thus, the development of the new experimental methodologies in the stratosphere will lead to the acquisition of the new high-altitude aerobiological research opportunities. In the presentation, we will present the details of our experimental platforms for aerobiological research projects in the stratosphere that we have established using scientific balloons, and the research objectives of each research project.

MeV gamma-ray observations provide unique information about nucleosynthesis, diffusion in our galaxy, low-energy cosmic rays, particle acceleration, and other phenomena. However, the detection sensitivity in this band is significantly lower than that in other bands due to a large background contamination. To address this issue, we are developing an electron-tracking Compton camera (ETCC) with powerful background rejection tools based on Compton recoil electron tracks. This will enable future observations to be conducted with greater sensitivity. We have successfully demonstrated the detection technology and performance of the ETCC with two balloon experiments. We are preparing for the next balloon flight, SMILE-3, to observe galactic diffusion gamma rays and some bright celestial objects.