Fundamental Development of “Floating‐Mode” Avalanche Photodiode for Low‐Energy Electron Measurements in Space Plasma

Abstract

AbstractWe conducted developmental experiments using avalanche photodiode (APD) for electron measurements applicable in future space plasma explorations. Electrons with energies of 10 eV to a hundred keV could be detected by applying “floating” voltages up to +5 kV to APD to achieve wide‐energy electron measurements. We detected 10‐eV electrons from an electron source as energized electrons of an energy of approximately 5 (5.01) keV due to the 5‐keV electrostatic acceleration (=Source energy: 0.01 keV + Acceleration energy: 5 keV; hereafter, (0.01 + 5) keV) with an energy resolution of ∼4.6 keV by simplified assumption and calculation based on peak energy and full width at half maximum of the pulse height distribution. The energy resolution for (5 + 0)‐keV electrons, ∼5.3 keV in our experiments, was equivalent to the previous results. On the other hand, we concluded that for (5 + 5) keV, the energy resolution was significantly improved to 1.3 keV compared with that for (5 + 0) keV. For (1 + 5)‐keV electrons, the energy distribution showed a broadening toward lower energies than that for (6 + 0) keV because the backscattered electrons produced secondary electrons at the APD chassis surface, which were accelerated to 5 keV by the floating voltage to be detected by the APD. We verified that the floating‐mode APD can contribute to space plasma measurements as a detector with characteristics different from those of microchannel plates. This is the first achievement to establish advanced in‐situ observation techniques based on APD for wide‐energy (a few eV to a hundred keV) electron measurements in space.