Fundamental Development of "Floating-Mode" Avalanche Photodiode for Low-Energy Electron Measurements in Space Plasma

We 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 & SIM;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, & SIM;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. We conducted fundamental developments of floating-mode avalanche photodiode (APD) for reducing lowermost measurable energy of electrons in spaceExperimental results indicate that the floating-mode APD applied by +5 kV as floating voltage can detect 10-eV electronsWe identified contaminations by secondary electrons that were produced by backscattered electrons and accelerated by the floating voltage