APD will measure kinetic energies and pitch angle distributions of electrons over an energy rage of 3 to 50 keV based on utilization of avalanche photodiodes. Although avalanche photodiodes are usually applied for photoelectronic devices, application of avalanche photodiodes in space for low energy electron detection is a novel technique. Comparing with conventional SSDs, which are widely used for high-energy electron measurements, utilization of avalanche photodiodes can drastically improve energy resolution (~1keV in FWHM) and the lowest limit of detectable energy of impinging electrons (~3keV). The avalanche photodiode is a kind of p-n junction semiconductor that has an internal gain due to the avalanche amplification of electrons and holes in the strong electric field within its depletion region.
The APD instrument consists of two parts: a sensor part, APD-S and an electronics sub-system, APD-E.
In the sensor part, photon rejection system and four avalanche photodiodes (Type Z7966-20, Hamamatsu Photonix K.K.) are installed. In order to reject incoming photons, trajectories of incident electrons are deflected with a homogenous magnetic field supplied by a permanent magnet (NEC Tokin). Since electron trajectories are different depending on their energies, four avalanche photodiodes are necessary to cover the entire energy range, i.e., 3-50 keV. The APD instrument itself is capable of obtaining energy spectrum with time resolution (delta T) of 10 msec, since no energy sweeping is needed in contrast to conventional electrostatic analyzers in which delta T of 10 msec is challenging. However, actual time resolution of the APD instrument is determined by spin rate of the rocket (~1 sec), since it provides pitch angle coverage. The APD instrument has two observation modes (normal mode and direct mode). In the normal mode, the fundamental data set is to be delivered every 10 msec. Using look-up tables, raw pulse height data from preamplifiers are utilized to increment the specific counter; each detector system has a set of 12 (8bit) and 1 (13bit) counters. In the direct mode, raw data are directly sent to the telemetry system without any processing, but still total count of each detector is sent every 10ms. During the flight, observation will be made with the normal mode (5s) and the direct mode (30ms) alternately.
Table 1. APD specifications.
|Field of View||8°x22°|
|Energy Resolution||~1keV (Direct mode)|
11 energy bins (Normal mode)
|g-factor||8.1E-3 (cm2 str keV)|
|Time Resolution||10ms /1 energy spectrum|
~1s (1 rocket spin) / 1 angular sweep
Figure 1. 3D-view of the APD instrument.
Figure 2. Sectional view of the APD instrument with sample trajectories of incoming electrons.
K. Ogasawara, K. Asamura, T. Takashima, T. Mukai and Y. Saito (ISAS/JAXA)
AD Co., Ltd., YS design, Clear Pulse Co., Ltd., NEC Tokin Co., Ltd., Hamamatsu Photonics K.K.