Single Photon Calorimeters
ALT="Diagram of an X-ray single photon calorimeter"
SRC="calorimeter_diagram.gif">
Perhaps the most intriguing advance in X-ray astronomy
instrumentation in the 1990s has been the development of single photon
calorimeters, spearheaded by work at NASA's Goddard Space Flight Center.
These devices detect X-rays by the temperature pulses they generate in a small
absorber which is cooled to a fraction of a degree kelvin.
Energy Resolution
Single photon calorimeters work by the low-noise conversion
of absorbed energy to heat. Such devices consist essentially of an absorber
and one or more thermistors, each linked through a load resistor to a
low-noise amplifier. The temperature rise induced by the absorption of an
X-ray produces a voltage waveform from which energy information can be
extracted. The limiting energy
resolution is given by the equation delta E = 2.36 x Eta x
sqrt(k To2C) where C is the heat capacity (in joules per
Kelvin) of the detector at a heat sink temperature To, k is
Boltzmann's
constant, and eta is a detector constant dependent primarily on the
properties of the thermistor. Typically, eta has a value in the range 1-3.
Spatial Resolution
The focal plane coverage of a calorimeter is severely limited by the need
to minimize heat capacity. So close-packing, or mosaicing, an array of
individual detectors won't work. It may be possible to create an imaging
device by using the thermal non-uniformity of a single absorber. However,
tests have shown that to produce good
spatial
resolution requires a high degree of thermal non-uniformity, which then
has to be traded against energy resolution.
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