The potential capability of solid state devices to detect suprathermal solar-wind plasmas

The medium-energy range (from 1 keV to 100 keV in the total energy of the particle) plays of vital importance in order to identify and study the physical mechanisms associated with heating and acceleration, source and loss processes, boundary and discontinuity phenomena in the solar-wind plasmas, because the energy spectra often follows characteristic non-thermal spectra (power-laws, exponential distributions, and kappa distributions) depending on the related physical mechanisms in this energy range. Space scientists have been trying hard to achieve as low-energy threshold as possible for solid state devices which covers higher part of this energy range, in order to obtain the full and accurate picture of these non-thermal energy spectra by using one instrument. However, the noise issues limit the low-energy threshold and the energy resolution no less than 20 keV with the conventional techniques. Considering the source particle in the solar wind with the energy of ~1 keV, this limitation makes it difficult to accurately reveal the mechanisms hidden in the turbulent solar-wind plasmas. Potentially, there are three ways to break this limit by reducing the noise: keeping the device in extremely low temperature; employing the extremely small pixel devices; and using devices with an intrinsic gain by Avalanche Photodiode, APD. We have been focusing on the APD because of its light and power saving feature, as well as the reliable efficiency, and the fast and linear response. In this study, we will present our new results of conventional Solid State Detectors (SSDs) and APDs to discuss the potential capability of the low-energy threshold and the energy resolution by using H, He, N, Ne and Ar ions ranging up to 300 keV/e. Our APD worked with an extremely low threshold (2 keV/e for H+, and 3.4 keV/e for He+) and high energy resolution (~15%) with the system noise level of 1.1 keV in silicon. At the same time, there were intrinsic noises caused by the physical detection processes happening in the device, such as the fluctuations in the pulse height defect, statistical dispersion of the number of electron-hole pairs. These effects stood out especially in the medium-energy range. Based on our results, we shall discuss the potential capabilities and limitations of the solid state devices for the future instrumentation for the energetic particles.