Speaker
Description
To achieve sustainable human planetary exploration, world-wide space agencies are collaborating to advance crewed mission programs. The Moon and Mars, key targets of these missions, lack the thick atmosphere and strong geomagnetic shield such like the Earth. As a result, the intensity of Galactic Cosmic Rays (GCRs) in these environments can reach an order of magnitude higher than in the low Earth orbit. In the lunar environment, it is estimated that particles composed of GCR above 500 MeV account for approximately 70% of astronauts’ GCR dose during an extravehicular activity and over 80% during an intra-vehicular activity. On deep-space missions, astronauts are continuously exposed to GCR, significantly increasing their risks of central nervous system (CNS) damage, cataracts, and cancer. Among the various constituents of GCRs, protons are particularly important as they account for more than half of the GCR dose to astronauts, making proton measurement essential for radiation risk assessment.
The energy spectrum of GCR protons typically peaks between 200 and 1000 MeV/n, depending on solar activity. Since its intensity decreases with a simple power-law beyond the peak, measuring at least five points in the 200–2000 MeV/n range would provide a reasonable approximation of the GCR spectral shape. Up to now, most of radiation protection detectors portable for astronauts have been measured energy loss of particles to determine their kinetic energies, such as semiconductor or gas detectors. However, measuring high-energy particles up to 2000 MeV with these detectors requires large and complex systems, which are not suitable for portable use. For compact devices capable of GeV-scale energy measurements, a Cherenkov detector is one of optimum choices.
Our team—comprising Japan Aerospace Exploration Agency (JAXA), Tokyo University of Science (TUS), and RIKEN—is developing the Lunar-RICheS (Ring Imaging Cherenkov Spectrometer). Lunar-RICheS integrates two independent detector systems: a stacked semiconductor detector for low-energy measurements (–250 MeV), developed by JAXA, and a Ring Imaging Cherenkov detector for high-energy measurements (250–2000 MeV), developed by RIKEN and TUS. The high-energy measurement unit, called the Compact Cherenkov Counter (CCC), consists of a ring imaging Cherenkov detector combined with a position-sensitive strip detector (Double Sided Si-strip Detector, DSSD).
The ring imaging Cherenkov spectrometer is a detector that combines a crystal radiator with a 64-channel Multi-Anode Photomultiplier Tube (MAPMT), and determines particle energy based on the number of Cherenkov photons detected. A key advantage of the CCC is its ring imaging–based background suppression capability, which reduces the impact of secondary particles such as a delta ray and a spallation reaction fragments —one of the main challenges in high-energy particle measurements with a Cherenkov detector.
To date, we have conducted proof-of-principle experiments using high-energy proton and heavy-ion beams at accelerator facilities, as well as integration tests with the position-sensitive strip detector. In this time, we will report on the principles and current development status of the CCC.
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