The measurement of cosmic rays and Solar energetic particles in space is basic to our understanding of the Galaxy, the Sun, phenomena in the Heliosphere and the emerging field of space weather . For these reasons, cosmic ray instruments are common on both scientific spacecraft and operational spacecraft such as weather satellites. Cosmic rays (CRs) and Solar energetic particles (SEPs) include ions over the full range of elements found in the Solar System. High-resolution measurements of the energy spectra of space radiation are key to understanding both acceleration and propagation processes. An inherent challenge is the large range of energies of such spectra. Cosmic ray energies range up to over 1 0 21 eV , while SEPs can reach a few GeV. Multi-instrument measurements are currently required to cover the full range of particle energies. Indeed, the highest energy particles, due to the rarity, can only be measured with ground-based instruments using the atmosphere as a calorimeter . Over limited energy ranges, SEP spectra are often approximated by a power law; however, over the full energy range, SEP events exhibit changing spectral shapes (e.g. “knees”, “roll-overs” and “cut-offs”). These features give information about the acceleration processes, such as size of the acceleration region, time of acceleration, morphology of the magnetic field during acceleration, and others, all of which can vary from event to event. Measurements of such features are often compromised by the need to combine measurements from more than one instrument, each with its own limited energy range. Even if there are no gaps between the energy intervals of the instruments, differing systematics can severely impact data analysis. A single instrument capable of measurements over a continuous and extended energy range would offer vastly more reliable measurement of the energy spectra of SEP events as well as replacing multiple instruments on resource-limited spacecraft. The most common method to measure GCRs and SEPs from a few to ∼ 100 MeV for protons, is Si Solid-State Detector (SSD) stacks. Above these energies, Cherenkov detectors are typically used together with SSDs. Ideally, to provide full energy coverage with no gaps, this requires a Cherenkov radiator with a threshold of ∼ 100 MeV. No suitable Cherenkov detector with such a low threshold has been developed. We are in the process of developing a synthetic diamond Cherenkov detector for this purpose. Diamond’s high index of refraction (2.42) results in a theoretical threshold of 92 MeV for protons. Even with a practical threshold of ∼ 110 MeV, this is ideal for extending the energy range from that of SSDs alone to that of sapphire Cherenkov detectors (202 MeV threshold) with higher energies attainable using plastic Cherenkov detectors. Both Sapphire and plastic Cherenkov radiators have spaceflight heritage.
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Mar 15, 2020
Susumu Minato + 1
Open Access
