Detector packaging
Large arrays of germanium detectors are needed for many exciting new instrument concepts in astrophysics.  These arrays present unique problems in handling since the detectors are very sensitive to surface contamination and handling.  We are developing sophisticated encapsulation techniques to to address this issue, but also to provide desirable electrical and gamma ray properties.

Our first ceramic encapsulated detector is shown above mounted on a test fixture for preliminary characterization. It is protected from abrasion and surface contamination by sealing inside a ceramic canister.   The encapsulated detectors may be handled in a shirt-sleeve environment without damage.  The canister is designed to withstand vibration levels and loads of a Delta or Pegasus launch vehicle.  An engineering model of this design was successfully tested at a vibration level of 11.4 Grms (random vibration on all axes). 

Ceramic was chosen for strength, light materials (low-Z), thermal conductivity, and high electrical resistance.  The advantage of light materials is that they have lower activation background and higher gamma ray transmittance.  The use of insulating materials reduces stray capacitance between the electronics and the detector.  Electronics noise increases with higher capacitance.  Thus we expect superior energy resolution than what could be achieved in a metal canister. 

The detector is spring loaded inside the canister between the base unit and the ceramic cover.  Contacts are made by metal pins braised into the ceramic.  The can is sealed with an indium joint and and four corner bolts. 

We took delivery of four encapsulated strip detectors in March 1999, one of which is shown above.  The detectors will be tested in the test cryostat pictured below.

This cryostat test facility features a large cold plate (5.875 inch diameter), two 50 pin vacuum connectors, connectors for high voltage and test signals.  The cold plate is cooled to ~90K by liquid nitrogen in the dewar beneath the cryostat.  An infrared shield covers the detectors, shown here on the table  to the left of the cryostat (small can).

We now have six germanium strip detectors in our laboratory with an additional four on order.  Our next project is to configure these devices into two small arrays to demonstrate the principles and performance of an Advanced Compton Telescope (ACT).  The ACT has applications in Astrophysics (NASA), environmental remediation (DOE), nuclear power, and elsewhere.

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