The germanium strip detector (GSD) is a gamma ray detector that combines
excellent energy resolution with imaging. The next generation of
instruments in high energy astrophysics will need to exploit both of these
properties in order to make significant advances over the scientific achievements
of operational and planned future missions. New technologies
and approaches are essential for progress since simply building bigger
versions of present day instruments will only yield incremental improvements.
We are developing instrument concepts using GSDs which will achieve this
goal.
Germanium strip detector
Cartoon of a germanium strip detector. The crossed electrodes provide two-dimensional
position localization of interactions. The electrodes are read out individually
for best performance. A guard ring provides increased immunity to
leakage currents (more robust design though not essential). Two detectors
of this type have been fabricated and are operational in our laboratory.
The first has 5 strips on each surface with 9 mm strip pitch and 45x45
mm active area. Our second detector (shown below) has 25 strips on each
face with a 2 mm strip pitch and 50x50 mm active area. Both detectors
are 10 mm thick providing high total energy detection efficiency well over
100 keV, and 2% efficiency at 662 keV.
Photograph of a 25 x 25 strip detector. The detector operates at
a temperature of 80K. It is housed in a vacuum chamber with a copper
rod (top) to keep the detector cold. Signals from the individual
strips fan out on a ribbon cable that connect to preamplifiers just
outside the vacuum housing.
Energy resolution
Energy spectrum obtained using a 25x25 strip detector. The detector
was uniformly illuminated with 662 keV gamma rays (137Cs) to
produce this Figure. Data was selected to include only those events
where the interaction is contained in a single strip to measure the single
channel performance (i.e. the other 24 strips have no signal).
All of the strips were independently calibrated to within 0.1 keV at 662
keV and combined to form this spectrum. The prominent features in
the
figure are the 662 keV gamma ray line, the barium K-alpha and K-beta X-ray
lines (also from the 137Cs source) and the Compton shelf.
The 662 keV gamma ray line is resolved to 1.9 keV FWHM and was detected
with an efficiency of 2%.
Multiple strip interactions are also interesting but combine the noise
of several preamplifiers, thus provide slightly degraded energy resolution.
Conversely, combining the energy measurements made independently on both
sides of the detector actually improves the energy resolution. The efficiency
at 662 keV roughly doubles when multiple strip interactions are included.
Demonstration of the Detector Performance
This is a shadowgram of a brass key using 60 keV gamma rays from 241Am
projected onto the 25x25 strip detector. The shaft of the key is thinner
than the handle and therefore absorbs fewer gamma rays, appearing slightly
lighter in this image.
A 60 keV image of an Army medal awarded after parachute training is
shown below, followed by its photograph for comparison.
The medal has a parachute structure in the center tapering to a point beneath
the canopy which is surrounded by wings to either side. The thickest
parts of the medal are the darker blue colors in the gamma ray image.
The final image is of a 10 turn potentiometer. The central metal
shaft is clearly visible through the plastic case.
Please select one of the following links for more data on the characterization
of the strip detector or a discussion of the detector packaging
under development. We are also developing new germanium
and silicon detectors. Please visit our detector development
laboratory
for a description of some of our equipment.
For a more detailed discussion, please download one of our preprints.
Back to Position Sensitive Detectors Project
last updated: 30-Oct-1998
