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Cryogenic instruments in science
Cryogenic instruments are used in many areas of science. Operating an
instrument at cryogenic temperatures increases the cost and complexity
over room temperature operation, and a compelling reason is therefore
required to do so. There are two main such reasons. The first is to
increase the signal-to-noise of the detectors that are at the heart of
many instruments. Many noise sources decrease as temperature is
decreased, thermal noise being an obvious
example. In addition to this, for some types of detector, the sensitivity
increases as temperature is decreased, further increasing
signal-to-noise. The second reason to employ cryogenics is so that
superconductors can be used. Some detectors rely on superconductivity
for operation, such as transition edge sensors and kinetic inductance
detectors used in astronomy. Such detectors are often read out with
superconducting electronics such as SQUIDS (superconducting quantum
interference devices). Superconducting magnets are also used to generate the
high magnetic fields required in applications such as particle physics
and magnetic resonance imaging.
More information on some of the scientific fields using cryogenics is
given below.
Astronomy
Cryogenic instrumentation has a long and continuing history in astronomy.
The main drive to low temperatures is the need to increase
signal-to-noise, both by reducing background noise
from black-body radiation, and increasing detector signal-to-noise.
As temperature is reduced,
many sources of detector noise reduce, and for some
detectors the sensitivity is also greatly increased.
Needs range from temperatures of 150 and higher for
the reduction of
dark noise in CCD detectors, to temperatures below 100 mK for X-ray
microcalorimeters.
As instruments increase in complexity and size, new materials and techniques
will be required to construct instruments to a reasonable cost and
timescale.
A further complication is that at some wavelengths it is desirable or even
necessary to carry out observations in space.
More information on the use of cryogenics in astronomy is given here.
Dark matter and fundamental physics
Some experiments to directly detect dark
matter or rare events such as double beta
decay require the measurement of extremely small temperature
changes in large crystals. To achieve a useful sensitivity, the crystals
have to be cooled to extremely low temperatures (as low as 10 mK). Cooling
such large masses to such low temperatures is extremely challenging,
particularly when coupled with the need to use radiopure materials and to
use large volumes of lead shielding to reduce the background on the
detectors from radiation.
Plans to use helium-3 as a direct detection medium for dark matter
would involve operation at temperatures as low as 130 μK.
While this is less than 10 mK below the operating temperature of the
systems described above,
achieving temperatures below 1 mK is extremely difficult.
The difficulty of cooling a material depends on the ratio of
initial to final temperatures;
cooling from 100 mK to 100 μK is therefore comparable to
cooling from room temperature to 100 mK.
Gravitational Wave Detection
The majority of work on gravitational wave detection is carried out
using large laser interferometers (e.g. LIGO).
A major noise source in such systems is thermal noise (Brownian
motion) of the suspended mirrors in the interferometer.
In current systems, the
mirrors operate
at room temperature, but moving to cryogenic temperatures has the
potential to reduce this noise and thus increase sensitivity by orders
of magnitude.
Research towards the operation of such systems is underway,
and an intereferometer in Japan, CLIO, has already
started operation with cryogenic mirrors.
An alternative method of detecting gravitational waves employs large
resonant masses. In order to operate with sufficiently low noise,
these have to be operated at cryogenic temperatures, ideally
temperatures well below 100 mK (for example Auriga,
and Nautilus).
Magnet technology
Superconducting magnets are necessary in various research fields,
including particle physics and nuclear magnetic resonance, and in
medicine (magnetic resonance imaging). In the field
of particle physics, upgrades to the LHC instrument at CERN will require
a new generation of superconducting magnets to give higher fields and
tolerate higher radiation environments and heat loads than has been
the case until now.
Other areas
The advent of reasonably priced and reliable closed-cycle mechanical
coolers is opening up many commercial applications for cryogenic
instrumentation beyond traditional "niche" markets. New applications
include spinoffs from astronomy in which detectors originally
developed for astronomy are used in areas as diverse as X-ray
detection for materials analysis in the semiconductor industry and THz
(sub-mm) detection for airport security. The commercial development of
cryogenic instruments will require reliable engineering data, and few
companies have the facilities or the desire to carry out material
property measurements in-house.
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Page created: Adam Woodcraft
Last edited 2008-4-25
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