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Why we use Gallium doped Germanium (Ge:Ga)
Our main science goals require
that we optimize observing time and one of the ways we
will accomplish this, is to simultaneously observe
in two of the important far-infrared bands
(see our design criteria).
We find that best detector material to detect radiation at these wavelengths
is Gallium doped Germanium.
In the below figure the standard responsivity of the material
is shown from 45-110 microns and by stressing the material
a longer wavelength response of 110 - 210 microns is obtained.
The Challenge
The biggest challenge for us has been how to properly stress such
a large array such that all of the 16 pixels
are equally stressed, therefore having similar responsivity.
The below image shows the detectors from our old instrument (FIFI)
on the right in comparison to what is now needed for FIFI-LS on
the left.
Our Solution
We have solved the problem of uniform stressing by developing a stressing
mechanism which stresses each column independently and a detector housing
(in the figure below middle image)
which uniformly applies stress along the right side and
the ball-and-socket mechanism (upper left) for centering the stress on
each detector pixel.
In addition, the middle image on top shows the light cones that uniformly
fill the filed of view.
In the photographs below, we show the finished assembly of the first
complete detector stack.
Our Results
The effect of the total stress on the resistance of 5 detectors in the
first stack is shown in the below image. Since the stress is applied uniformly throughout the
stack, R1 (the resistance of detector 1) varies like R15 (the resistance
of detector 15). This shows that our implementation of stress is working very
uniformly.
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