K. Ogasawara

A Linear Mode Avalanche Photodiode for Ion Detection in the Energy Range 5–250 keV

We investigated the response linearity and the energy resolution of a silicon based reach-through avalanche photodiode (APD) for low energy ions ranging from 5 keV to 250 keV. We found that space charge affects the APD gain, and that this could be compensated by applying a simple correction formula to the measured pulse height distributions. The comparison between former theoretical or experimental studies and our results showed that there was no evidence of additional performance degradation within the tested energy range in terms of the linearity and the energy resolution compared to conventional non-avalanching silicon detectors. Thanks to the low noise level (equivalent to 0.9 keV in silicon) at the room temperature even for a large detection area (15 mm2), APDs would be potentially used for detection of low-energy ions in various environments such as space plasma physics, particle physics, and many industrial applications.

Avalanche Photodiode Arrays Enable Large-Area Measurements of Medium-Energy Electrons

We demonstrate the performance of a large-area avalanche photodiode array as a medium-energy electron detector. This avalanche photodiode array is composed of eight pixels with 1.2 cm2 of total active area. The energy resolution (DeltaE/E) is as low as 4.5% in full width at half maximum for 25-keV electrons, and the linearity is excellent. Monte Carlo modeling by Geant4 shows that the detection efficiency of this device is appropriate for the detection of electrons ranging from < 5 to > 200 keV. This new device can be coupled with magnetic deflectors, electrostatic analyzers, and other physics instruments requiring position-sensitive or large-area solutions for the detection of electrons in this energy range.

Properties of suprathermal electrons associated with discrete auroral arcs

We report on the properties of suprathermal electrons observed over three discrete auroral arcs from a sounding rocket by applying shifted kappa distributions. By analyzing kappa parameters (density, temperature, and kappa), we found three novel characteristics to provide clues to understanding the auroral acceleration mechanisms and magnetosphere-ionosphere coupling processes. First, the auroral potential drop was proportional to the inverse-square of kappa, consistent with previous theoretical investigations by [Dors and Kletzing, 1999]. The observed dependency was slightly stronger than their calculations, suggesting additional contribution from non-linear plasma processes. Second, the polytropic relation showed non-adiabatic (near isothermal) state of the source electrons. This can provide a restriction on the pressure balance issues in the plasma sheet convection. Third, there was a clear difference in the polytropic and the kappa index for the first arc as opposed to the second and the third arcs, suggesting different source locations in the plasma sheet for precipitating electrons to cause these near-by arcs.

Development and performance of a suprathermal electron spectrometer to study auroral precipitations

The design, development, and performance of Medium-energy Electron SPectrometer (MESP), dedicated to the in situ observation of suprathermal electrons in the auroral ionosphere, are summarized in this paper. MESP employs a permanent magnet filter with a light tight structure to select electrons with proper energies guided to the detectors. A combination of two avalanche photodiodes and a large area solid-state detector (SSD) provided 46 total energy bins (1 keV resolution for 3-20 keV range for APDs, and 7 keV resolution for >20 keV range for SSDs). Multi-channel ultra-low power application-specific integrated circuits are also verified for the flight operation to read-out and analyze the detector signals. MESP was launched from Poker Flat Research Range on 3 March 2014 as a part of ground-to-rocket electrodynamics-electrons correlative experiment (GREECE) mission. MESP successfully measured the precipitating electrons from 3 to 120 keV in 120-ms time resolution and characterized the features of suprathermal distributions associated with auroral arcs throughout the flight. The measured electrons were showing the inverted-V type spectra, consistent with the past measurements. In addition, investigations of the suprathermal electron population indicated the existence of the energetic non-thermal distribution corresponding to the brightest aurora.

Next‐generation solid‐state detectors for charged particle spectroscopy

The performance of silicon avalanche photodiodes (APDs) and single crystal chemical vapor deposit diamond detectors (DDs) are reviewed in comparison with conventional silicon based solid-state detectors (SSDs) from the perspective of space plasma applications. Although the low-energy threshold and the energy resolution are equivalent to SSDs, DDs offer a high radiation tolerance and very low leakage currents due to a wider band gap than silicon. In addition, DDs can operate at higher temperatures, are insensitive to light (>226 nm), and are capable of timing analysis due to the higher intrinsic carrier mobility. APDs also offer several advantageous features. Specificially, APDs have a lower energy threshold (<0.9 keV) and a higher energy resolution (<0.7 keV FWHM at room temperature), along with a linear response due to a strong electric field causing signal amplifications within the detector. Therefore APDs can be used to detect lower-energy particles, covering a larger portion of the energy spectrum than conventional SSDs. Further, the strong internal electric field gives them a sub-nanosecond response time by the charge mobility saturation, allowing them to make precise timing measurements of ions. These novel detector techniques can be potentially applied to improve the measurements of suprathermal particles, whose energies lie between typical ranges of conventional sensors for low-energy plasmas and energetic particles. Although the origin and evolution of the suprathermal particles are the key to understanding the acceleration and heating processes in space plasma, they are not well understood due to the technical difficulties of making the measurement.

Avalanche photodiode based time-of-flight mass spectrometry

This study reports on the performance of Avalanche Photodiodes (APDs) as a timing detector for ion Time-of-Flight (TOF) mass spectroscopy. We found that the fast signal carrier speed in a reach-through type APD enables an extremely short timescale response with a mass or energy independent <2 ns rise time for <200 keV ions (1-40 AMU) under proper bias voltage operations. When combined with a microchannel plate to detect start electron signals from an ultra-thin carbon foil, the APD comprises a novel TOF system that successfully operates with a <0.8 ns intrinsic timing resolution even using commercial off-the-shelf constant-fraction discriminators. By replacing conventional total-energy detectors in the TOF-Energy system, APDs offer significant power and mass savings or an anti-coincidence background rejection capability in future space instrumentation.

Interplanetary magnetic field dependence of the suprathermal energetic neutral atoms originated in subsolar magnetopause

Using energetic neutral atom (ENA) emission observations of the subsolar magnetopause measured by the Interstellar Boundary Explorer (IBEX), we study the correlation between the upstream interplanetary magnetic field (IMF) conditions and the spectral index of the source ion population. Our ENA data set includes hour-averaged ENA measurements at energies between ∼0.5 and ∼6 keV obtained by the IBEX High Energy ENA imager from January 2009 to May 2011. Under the condition of quiet geomagnetic activity (SYM-H index >−20 nT), we find that the shallower spectra in the suprathermal tail of the ion population of the subsolar magnetopause is weakly correlated (correlation coefficient of −0.30) with the shock angle of the Earths bow shock, but not correlated with parameters related to magnetic reconnection (i.e., elevation and clock angle of the interplanetary magnetic field orientation). The observed correlation suggests suprathermal ion energization from diffusive shock acceleration and thus that the suprathermal ions in the subsolar magnetopause are of shocked solar wind origin. We also argue that the roles of magnetospheric ion leakage or ion acceleration by magnetic reconnection are reduced in the magnetopause emissions compared to shock acceleration processes.

UV-Grade Silicon Photomultipliers for Direct Counting of Low-Energy Electrons and Protons

This paper reports a novel application of silicon photomultipliers (SiPMs) for directly counting kiloelectron-volt-range electrons and protons. Current instruments for measuring low-energy ions and neutral atoms (<30 keV) in space regularly use secondary electron multipliers, which pose practical limitations in small volume and low budget applications. A thin dead-layer SiPM, originally developed for vacuum-ultraviolet (VUV) photons, is tested for direct measurement of >keV particles replacing conventional detectors for certain applications in this paper. Our results demonstrated that the VUV-SiPM was sensitive above 1 keV both for electron pulses and protons. Using absolute beam monitor system, the proton detection efficiency of these SiPMs was evaluated. The efficiency was ~1% for 1-keV protons and plateaued above 5–6 keV at the expected 75% efficiency that agreed reasonably with the Monte Carlo numerical simulation results. The 75% plateau level was matching the open area ratio of the SiPM structure. The electron-pulse efficiency was also estimated by applying the same model, which shows ~1-keV detection limit and plateau above 3–4 keV. The agreement between the simulation and measurement shows that the dominating mechanism to trigger SiPM signals is the ionization loss of primary electrons and protons in the active volume of the pixel. An analytic investigation shows that the dark count rate can be significantly reduced with a double (or quadruple) coincidence measurement if multiple electrons can be produced from a single event.