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What is
microcaloirmetry?
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. A microcalorimeter is used to measure the energy
of a single photon. 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.
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Microcalorimeters 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. Much effort now goes
into creating second generation detectors capable of improving this
performance.
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The picture here depicts the basic components of a microcalorimeter. |
What
they look like:
Microcalorimeters are made up of three basic components: an absorber, a
thermometer and a weak thermal link. The thermometer is extremely sensitive that works by sitting
at the sharp transition temperature of a superconductor. To know more
about the future of these thermometers read about Transition Edge Sensor
(TES). The weak thermal link is
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An X-ray calorimeter basically consists of a thermal mass to absorb
incident X-ray photons, a thermometer to measure the resulting
temperature rise, and a weak link to a low-temperature heat sink that
provides the thermal isolation needed to sense a temperature change.
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Why
do microcalorimeters need to be so cold?
The calorimeter should operate at a low temperature so that the energy
deposited is large relative to the overall temperature of
the . In
order to effectively measure the temperature change caused by one
photon, the thermometer must be sensitive and thus cold. The
absorber needs to have a low heat capacity so that the change in energy
can be measured and must also reproducibly and efficiently distribute
the energy of the initial photon across a thermal distribution of
photons. |
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Requirements for a good microcalorimeter are an absorber with a small
heat capacity able to convert the radiation from one photon into
thermal energy quickly and with high efficiency, and a sensor with low
heat
capacity and high sensitivity to temperature variations.
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