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Photon Detector Categories Detector Microphonics
After more than 30 years of germanium detector production the phenomenon
of microphonics is still not well understood. A back-of-the-envelope calculation leads to
the false conclusion that no germanium detector will ever operate. For example, consider a
metal part, such as the cup that holds the detector, which has a small, but non-zero
capacitance with respect to the FET gate. Assume that sound waves, such as from a voice,
induce a variation of merely 0.5 femtofarads in the value of this capacitance; the result
would be a signal equivalent to 10 keV!Although there is no IEEE standard on the measurement of the extent of
microphonics, considerable work has been done in this field:
1) Special design: ORTEC has always been at the forefront in this field,
for example production of a rugged detector designed for the U.S. Navy (Ref. 11). ORTEC has also provided arrays of germanium
detectors for helicopter aerial surveillance.2) Proper electronic setup: As the microphonics spectrum is primarily in
the few kcps range, a high pass filter (shorter amplifier time constants and baseline
restorer "on") will often improve detector performance.
3) Vibration decoupling: Users typically obtain improvement by using soft
foam rubber around and under the detector. |
