How does the XRS work?
The XRS works by using a new kind of detector called a
microcalorimeter.
The figure on the left illustrates its components. The microcalorimeter
consists of three main components: an X-ray absorber (shown in red),
a thermometer (shown in maroon) and a heat sink (shown in blue).
When an X-ray hits the absorber, it raises the its temperature.
A type of thermometer called a thermistor is used to measure
this change. This temperature rise is approximately proportional
to the energy of the X-ray photon. This can be represented
by the equation
T ~ E/C
where, E is the energy of the X-ray and C is the heat capacity of
the absorber.
How are heat and energy and temperature all related? Heat is a
manifestation of energy. Everyday objects are made of a large
number of atmos or molecules that vibrate or move around randomly.
Energy in these vibrations and random mitions is heat. Temperature
is a the way we measure the average energy of atoms and molecules of
an object. The "heat capacity" tells us how much the
temperature rises in a material if we put in a certain amount of energy.
So, when an X-ray photon is absorbed by the absorber, each atom starts
to vibrate a little bit more than before the X-ray hit. That is,
the temperature of the absorber has gone up.
The heat then flows out through the legs of the detector to the heatsink,
and the temperature returns to normal. Thus there is a short-lived
temperature change in the thermistor whenever an X-ray photon is absorbed
(see animation on the right).
A thermistor is a device that changes its electrical resistance
with a small change in temperature. We simply read out the resistance
of the thermistor and measure how much it changes when an X-ray photon
is absorbed. The amount of this change in resistance is almost
proportional to the energy of the X-ray photon.
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Watch Drs. Caroline Kilbourne and Kevin Boyce explain why the XRS
has to be kept so cold. Click on the image above to view the
QuickTime video. (1.8 MB) (Description)
Watch Dr. Kevin Boyce explain the three stages of how the XRS is
kept cold. Click on the image above to view the QuickTime video. (1.6 MB) (Description)
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The microcalorimeter in the XRS needs to be kept extremely cold, almost
at absolute zero (60 milliKelvin or 0.06 Kelvin, -273 C, or -460 F).
This is so that it can detect the tiny changes in temperature, which
is only a few milliKelvin). The XRS achieves this
temperature by using a four stage cooling system: a mechanical cooler
(very similar to a household refrigerator), a shell of solid neon, an
insert filled with liquid helium, and a device called an Adiabatic
Demagnetization Refrigerator (ADR). The solid neon is the main source
of cooling power of the system. It has a temperature of 17 Kelvin.
Because 17 Kelvin is too high for proper functioning of the ADR,
whose function is to keep the microcalorimeter cold, the inner section
of the XRS is filled with liquid helium that maintains a temperature
of 1.3 Kelvin. This is low enough to allow the ADR to do its job.
The XRS has a limited life of 2.5 to 3 years before the neon
and/or helium runs out.
The figure above shows additional components of the XRS system.
"FEA" in the figure stands for Front End Assembly, and
that's where the microcalorimeter is. The filters
help to keep out stray light, radio waves, and other electromagnetic
radiation. Temperature pulses from X-rays are measured
by the microcalorimeter which is helped by two other devices, the
Calorimeter Analog Processor (CAP) and Calorimeter Digital Processor (CDP).
The CAP measures the detector resistance and amplifies the signals by
a factor of 20000. It then sends these amplified signals to the CDP which
analyzes each signal to determine its energy, and sends the result to
spacecraft. The spacecraft stores the results in its memory, and sends
them to ground control on earth.
