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【Prof. Hsiang-Kuang Chang’s team】Compton Spectrometer and Imager (COSI) was selected by NASA’s SMEX program

Compton Spectrometer and Imager (COSI) was selected by NASA’s SMEX program

The Compton Spectrometer and Imager (COSI) project, which is led by Space Sciences Lab of UC Berkeley and in which Prof. Hsiang-Kuang Chang’s team at NTHU/IoA has been participating since 2006, was selected by NASA’s SMEX program to launch in 2025 in a low Earth orbit to observe cosmic gamma-rays. This is an important milestone for the development of the next generation Compton telescope in astronomy.

The core of COSI is an array of 16 pieces high-purity germanium detectors. Compton scatterings between photons and electrons in the detector volume are exploited to detect the incoming gamma rays of MeV energies. These cosmic soft gamma rays carry information of many phenomena and unsolved mysteries. There is strong emission at 511 keV, for example, from the Galactic center. That is electron-positron annihilation line emission. The origin of the ample supply of positrons has remained unknown for more than half a century. It could be related to the supermassive black hole at the Galactic center, some systems containing black holes or neutron stars, or even some kind of hypothetical light dark matter. Besides, supernova events and many systems of black holes and neutron stars all emit gamma rays at this MeV energy range. Furthermore, Gamma Ray Burst (GRB) is one of the major subjects in multi-messenger astronomy.     

COSI is a next generation Compton telescope. Its main thrust is to achieve high sensitivity with a smaller volume and mass, compared to earlier technologies. It is very important to achieve high sensitivity. For example, limited by sensitivity, the detection of gamma-ray lines associated with nucleosynthesis of many heavy elements in supernova events has been elusive. COSI can record accurately the location of Compton scattering in its detector volume and can also have 3-D tracking of multiple scattering events. With all these, background level can be significantly reduced and therefore a high sensitivity can be achieved. In addition, high-sensitivity gamma-ray detectors, driven by the high demand of astronomical research, can also be applied to various kinds of radiation detection, such as medical imaging. Much lower dose is needed because of higher sensitivity of the imager.

Several groups in the world are developing different technologies to build the next-generation, high-sensitivity Compton telescope. Among them, the COSI collaboration is the forerunner. Just like X rays, gamma rays cannot penetrate the Earth’s atmosphere. To observe cosmic gamma rays, instruments must be flying on board satellites or space crafts. Usually before having such space programs, which are expensive, instrument performance is often verified and demonstrated with high altitude balloon flights. COSI had a 40-hour stratospheric balloon flight launched from New Mexico, USA, in 2009 and another very successful 47-day flight from Wanaka, New Zealand, in 2016. COSI has demonstrated its nice performance in these flights and therefore stands out from all the strong competitors.      

Prof. Chang’s COSI project was supported by National Space Organization (NSPO) and the Ministry of Science and Technology (MOST). Dr. Chih-Hsun Lin of AS/IoP is another key member of this project.

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