Development of a Detection Technique for Medium-Energy Electrons, 15-R9755

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Principal Investigators
Keiichi Ogasawara
James Cravens
Stefano Livi
David J. McComas

Inclusive Dates:  10/01/07 – 04/01/09

Background - Because distribution functions of medium-energy electrons vary from thermal (several keV) to non-thermal (in the range of 100 keV), investigating the energy spectra of several keV to 100 keV electrons will provide an important clue to understanding the heating and acceleration mechanisms of magnetospheric plasmas. Therefore, precise measurements of medium-energy electrons will directly address the scientific paradigms in magnetospheric physics, such as the particle acceleration by magnetic reconnection and the physics of collisionless shocks. However, it is difficult to accurately and reliably detect electrons in the medium-energy range because this range borders the techniques for lower-energy and higher energy. This project offers a potential solution by applying avalanche photodiodes (APDs) to the detection of these electrons. The APD is a kind of p-n junction semiconductor with an internal gain caused by the avalanche amplification. Electron-hole (e-h) pairs are created as a result of dissociations by injected electrons. These e-h pairs will be accelerated in the strong electric field inside the APD. If getting sufficient kinetic energy, electrons or holes create other e-h pairs in an 'avalanche' way. This gain enables high-resolution detection of low-energy electrons of several keV. The objective of this project is to build a foundation for a brand-new detection technique of electrons with energies from 3 to >100 keV (medium-energy range). This project will ideally complement SwRI's expertise in low-energy and energetic plasma detectors and position the Institute as the pioneer of the medium-energy electron detector.

Approach -

Task 1: Test and evaluate two types of single pixel APDs to make selection for fabrication of custom eight pixilated APDs using radioactive source and electron beams

  • Set up and make test measurements of APDs
  • Evaluate the single pixel devices performance from the pulse height distributions (PHDs) to make selection for fabrication of custom eight-pixel APD

Task 2: Perform temperature dependence test of single pixel APDs and the ability of bias control to offset this dependence

  • Test thermal vacuum equipment and procedure
  • Thermal vacuum test of two types of single pixel APDs

Task 3: Test and evaluate performance of eight pixilated APDs

  • Test 8 pixilated APDs to evaluate the performance and gain uniformity
  • Evaluate performance of two adjacent single pixels in the eight pixilated APD array to investigate the cross talk and the additional noise due to multiple pixels.

Accomplishments - The project team investigated the temperature dependence of Hamamatsu spl 6815 Avalanche Photodiode (APD) as an electron detector under temperatures from –9 °C to 30 °C. The gain variation on the temperature was –1.2 percent/K at 10 °C and typical bias voltage. The linearity of the response was excellent under all measured temperatures, and the variation of the energy resolution was acceptably small to maintain good performance. The team also successfully demonstrated the performance of a large-area avalanche photodiode array as a medium-energy electron detector. This avalanche photodiode array comprises 8 pixels with 1.2 cm2 of total active area. The energy resolution (ΔE/E) is as low as 4.5 percent in FWHM for 25 keV electrons, and the linearity is excellent. 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.

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