Contact Information:
Prof. Massimiliano Galeazzi
P.O. Box 248043
Coral Gables, FL 33124
Tel: (305) 284-2326 x2
Fax: (305) 284-4222
galeazzi@physics.miami.edu
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Cryogenic Microcalorimeters
For Starters
Your opponent's serve was almost
perfect, but you vigorously returned it beyond his outstretched raquet
to win the point. Now the tennis ball sits wedged in the chain-link
fence around the court. What happened to the ball's kinetic energy? It
has gone to heat the fence, and you realize that if the fence were
quite a bit colder, you might be able to measure that heat and
determine just how energetic your swing really was.
Calorimetry has been a standard measurement technique since James Joule
and Julius von Mayer independently concluded, about 150 years ago, that
heat is a form of energy. But only in the past 15 years or so has
calorimetry been applied, at millikelvin temperatures, to the
measurement of the energy of the individual photons and particles with
exquisite sensitivity.
The picture here depicts the basic
components of a microcalorimeter.
An X-ray microcalorimeter is
composed of three parts, an absorber that converts the energy of the
incident x-rays into heat, a thermometer that detects the temperature
variations of the absorber and a weak thermal link between the detector
and a heat sink. The operating principle is simple. When an x-ray hits
the absorber its energy is thermalized, that is, is distributed among
thermal phonons, and the temperature of the detector first rises and
then returns to its original value due to the weak thermal link to the
heat sink. The temperature change is proportional to the energy of the
incident x-ray and is detected by the thermometer. The thermometer is
generally a resistor whose resistance has a strong dependence on the
temperature at the working point.
The Old Way of Doing Things
In the precalorimeter era, the
choice between wavelength dispersive devices (such as Bragg crystals or
grazing incidence diffraction gratings) and nondispersive spectrometers
(solid state detectors or proportional counters) presented a dilemma.
Dispersive spectrometers offer a very good energy resolution, but at
low throughput. Nondispersive spectrometers, on the other hand, have
very high efficiency, but relatively poor resolution.
The Advantages of the New
Microcalorimeters provide a number
of advantages over these methods. Detectors can be relatively large and
still be sensitive to small amounts of deposited energy. This energy is
sensed after it has been converted to heat, so that even interactions
that produce little or no ionization can be detected. Another useful
property is that thermal detectors do not depend on the charge
transport properties of the absorber. Only a very few materials can be
used to make ionization detectors, while a calorimeter can incorporate
a wide variety of materials.
The Basics of X-ray Calorimetry:
- The calorimeter should operate at a low temperature so that
the energy deposited is large relative to the thermodynamically
unavoidable random transfer of heat across the weak link.
- The absorber should be opaque to X-rays, and yet have a low
heat capacity so that a small deposition of energy is translated into a
measurable temperature change.
- The absorber must thermalize well -that is, it must
reproducibly and efficiently distribute the energy of the initial
photon across a thermal distribution of phonons (or electrons depending
on the thermometer). <>The thermometer must be highly
sensitive to temperature changes.
And the thermal link should be weak enough such that the time for the
base temperature to be restored is the slowest time constant in the
system, yet not too slow otherwise the device cannot handle the
incident x-ray flux.
Microcalorimeters Put to Use
With the use of arrays of
microcalorimeters it is possible to extend the limits of a singe
microcalorimeter, and create a better detector. A typical array for the
XQC experiment is composed of 36 elements, each with a collecting area
of 1 mm 2, for a total collecting area of 0.36 cm 2.
Each pixel is composed of a silicon thermistor implanted in a
micro-machined silicon chip, thermally connected to a HgTe absorber.
The absorber is 0.75 micrometers and has more than 99% quantum
efficiency of the detector below 1 keV. The detector is installed in an
adiabatic demagnitization refrigerator at a base temperature of 60
mK.
  Where is this Used and Where is
it Going?
X-ray astrophysics is one of the
fields where most of the efforts in the development of cryogenic
microcalorimeters are spent. The first microcalorimeters for X-ray
astronomy were developed by the University of Wisconsin / NASA Goddard
Space Flight Center collaboration. They were first employed on a
sounding-rocket experiment (the X-ray quantum calorimeter - XQC) that
had an array of microcalorimeters for the study of X-rays from the
interstellar medium in the energy range 30-1000 eV.
Much effort now goes into creating second generation detectors capable
of improving this performance. A major project is aimed at the
development of large detectors arrays (more than 1000 elements) with
Transition Edge Sensor (TES) thermometers.
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