Gamma and Cosmic Ray Astrophysics

 
 
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Detector Development

Germanium detectors

New technologies for fabricating germanium detectors are under development by NRL in collaboration with several other institutions.
 
 

Paul Luke of the Lawrence Berkeley National Laboratory has fabricated a double sided strip detector using an amorphous germanium contact technology that he developed.  The detector (shown above) has been tested at NRL, and demonstrated excellent imaging and energy measurement performance as expected.    The device has an active volume 1 cm thick and 5x5 cm in area.  There are 25 strip electrodes centered on a 2 mm pitch on each face. 

A third collaboration with an industry partner will transfer electronics technology developed by NRL and Oak Ridge National Laboratory into a demonstration germanium strip detector system.   This development is intended to lead to a marketable product of general interest to the radiation detector community. 

Silicon detectors

Imaging silicon detectors have been developed for a variety of applications at NRL.  One project called ATIC is a cosmic ray experiment designed to make direct measurements of the cosmic rays to ~100 TeV.  This system will use silicon pad detectors like the one pictured pictured above to measure cosmic ray charge.  The principle investigator for this project is Dr. Jim Adams (now at MSFC).  This technology is also being developed for NASA's ACCESS mission. 

Thick silicon detectors are an important goal for many of our objectives.  The maximum thickness of a practical semiconducting detector is proportional to the square-root of the impurity concentration.  The maximum thickness of a silicon detector is limited to about 2 mm using the purest silicon material that is now manufactured (so called high resistivity float-zone material).  Thicker silicon devices are still possible by effectively "masking" the remaining impurities.  Trace amounts of lithium are carefully drifted into the silicon.  The lithium effectively compensates the remaining impurities, allowing the bulk material to behave as if it had a higher purity.  These devices are called "Si(Li)," and have been made over 1 cm thick. Pictured above is a 5 mm thick Si(Li) device with a 6 cm diameter active area  produced at the Lawerence Berkeley National Laboratory by Dr. Craig Tindall.  The device has been segmented into two haves.  Both halves provide excellent spectra at temperatures below -100 C.  This work is supported by programs from both NASA, and DTRA, Dr. James Kurfess, PI.  We are now working on improving segmentation techniques, and to raise the effective operating temperature.

NRL is also developing double-sided silicon strip detectors for the Hard X-ray telescope (HXT) on Constellation-X.   Silicon detectors are a relatively mature and stable technology with well developed manufacturing techniques.  A total thickness of ~1 cm of silicon will easily satisfy the requirements of the HXT focal plane detector.  The proposal is to stack several thick silicon detectors like this one which is 1.5 mm thick.  The principle investigators are Drs. Neil Johnson and Bernard Phlips. 

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last updated: 9-Oct-2001