Cryogenic Calorimeter
A cryogenic calorimeter is basically a device that measures small changes in temperature. Energy changes as small as those produced by a single photon can be measured with great accuracy. This has applications in X-ray astrophysics, the search for the dark matter in the universe, and the missing neutrinos from the Sun. Other areas of research have also benefited from these developments, such as double Beta-decay experiments, neutrino mass experiments, single photon counting spectrometry, mass spectrometry of large molecules (DNA), and X-ray microanalysis for industry.
Each
area of research has made modifications to this basic design to best fulfill
its unique requirement, but the underlying physical principles of the calorimeter
remain the same. The calorimeter shown in the accompanying figure
consists of a volume of material (absorber) connected to a thermometer,
which is connected to a heat sink. An incoming photon knocks an electron
loose in the absorber material. This electron will then bounce around
in the absorber, raising the material's temperature by a few thousandths
of a degree. To ensure that the absorber has time to come to equilibrium
before the thermometer begins to see the temperature change, the thermometer
is partially isolated from the absorber. Usually the choice of a thermometer
is a thermistor which produces dramatic changes in temperature for a small
change in resistance. If a current is run through the thermistor,
the change in electrical resistance, and thus the change in temperature,
can be measured by observing the drop in voltage across the thermistor.
Finally, the thermistor reaches equilibrium with the absorber by giving
its heat off to the heat sink, and the process can begin anew. Depending
on the materials used for the thermistor, semiconductor or superconductor,
this can take anywhere from milli to micro seconds.
X-ray astrophysics
The calorimeters used in X-ray astrophysics are often referred to as
microcalorimeters, because it is desirable to have very compact and light
weight instruments for Space based missions. A recent collaboration
between NASA
and the University of Wisconsin/Madison launched a microcalorimeter
onboard a sounding rocket, and recorded 240 seconds of data observing the
interstellar diffuse X-ray background. The calorimeter consisted
of 36 (0.5mm by 2mm) detectors with silicon thermistors and Hg/Te absorbers,
operating at a temperature of 60mK. The entire device, including
liquid helium cryogen, fit into a cylinder 35cm in diameter and 40cm high.
Microcalorimeters are valuable to x-ray astrophysics, because they offer
the best of both worlds: the ability to detect very small amounts of energy
with high precision resolution, and high quantum efficiency. Dispersive
techniques that spread out the light, such as crystals or gratings offer
high energy resolution, especially at low energies, but are inefficient,
have limited energy coverage (crystals), and may not be suited for extended
sources
(grating). Other non-dispersive techniques, such as charge coupled
devices (CCD), have higher quantum efficiency, but the energy resolution
is rather poor, because of charge production and collection in silicon.
The search
for dark matter in the universe
There is evidence that a significant amount of the dark matter in the
universe is exotic in nature, possibly massive neutrinos, or other weakly
interacting massive particles (WIMPs). Scientists can detect WIMPs
by measuring the nuclear recoil energy from WIMP-nucleus scattering.
Depending on the size of the absorber nucleus, recoil energies range from
eVs to keVs, and detectors must be located in deep underground laboratories
to be shielded from cosmic ray background. In addition detectors
need to be shielded locally against radioactivity from surrounding rocks
and materials.
There are currently several collaborations under way, one of the largest
is the Cryogenic Dark Matter Search
(CDMS) collaboration which employs two different techniques to measure
the Energy loss of a particle in the absorber. The first, called the
Berkeley Large Ionization and Phonon based detector (BLIP), uses two NTD
Germanium thermistors bonded to 165g Ge crystals, operating at a temperature
of 20mK. The second type, called Z-sensitive Ionization and Phonon
based detector (ZIP), uses tungsten aluminum sensors covering a larger
proportion of area of a 100g silicon absorber. Utilizing superconductor
technology ZIP is quite a bit faster at returning to equilibrium than BLIP,
but this won't make a significant difference until larger detectors are
built, because the expected event rate is only on the order of one per
kg of detector per day.
Double beta-decay
experiments
The largest
calorimeter used in double beta-decay experiments is in Milan's Gran
Sasso Laboratory, and consists of an array of 20 TeO2 crystals totaling
almost 7kg. Each crystal has a Ge NTD thermistor
attached to it. This device is mainly used to search for the neutrinoless
double beta-decay of Te-130, taking advantage of the large isotopic abundance
(34%), and the large transition energy (2528keV), but is also used to search
for WIMPs. For the future the Milan Group plans to greatly increase
the number of detectors, in the process increasing the weight of the entire
device to about a ton.
Neutrino
mass experiments
Most experiments to find a non-zero mass for the neutrino in beta decay
are using tritium decay into He-3, because the transition has a rather
low endpoint energy of 18.6keV. However, to date only upper limits
have been found, because the fitting of the electron energy spectrum consistently
results in a negative value for the square of the neutrino mass.
This could be the result of final state interactions, such as tritium decaying
into excited states of He-3. Cryogenic calorimeters should be able
to achieve high enough energy resolution to avoid this problem.
Single
photon counting
The importance of measuring the energy of a single photon has already
been discussed in the section on x-ray astrophysics, but until recently
it has been limited to the upper end of the electromagnetic spectrum.
Now, a group at the European Space Agency
is working to extend single photon counting capabilities into the visible
and near infrared range. Using superconductors as thermometers, they
have been able to observe energy changes as small as 1eV. This technology
was recently tested when a cryogenic camera was installed in the William
Herschel Telescope in La Palma, and accurately measured the period
of the Crab Pulsar (33ms) to within 5 microseconds.
X-ray
microanalysis in industry
A group at the National Institute of
Standards and Technology (NIST) has recently developed a high powered
x-ray microcalorimeter, which is cooled by a compact adiabatic demagnetization
refrigerator (ADR) to 70mK, and mounted to an electron scanning microscope.
This device's excellent energy resolution allows it to measure changes
in X-ray Spectra caused by changes in electron binding due to chemical
bonding. The microcalorimeter also looks promising in the detection
of large accelerated masses, such as proteins or DNA. Should they
succeed, the sequencing of DNA would be made several orders of magnitude
faster compared to traditional gelelectrophoresis methods.
Neutron transmutation doping (NTD). Thermal neutrons from a reactor are captured by nuclei which transform into isotopes. These can then be the donors or acceptors for the semiconductor.
Reference:
Pretzl K., Cryogenic
calorimeters in astro and particle physics; University of Bern;
Contribution to the VIII International Conference on Calorimetry in
High Energy Physics, 13-19 June 1999, Lisbon, Portugal
Project Formulation Manager:
Jean F. Grady Mission Development Office Code 740.2 NASA/GSFC Greenbelt, MD 20771 |
Project Scientist:
Nicholas White Laboratory for High Energy Astrophysics Code 662 NASA/GSFC Greenbelt, MD 20771 |
Facility
Science Team Chair:
Harvey Tananbaum Smithsonian Astrophysical Observatory 60 Garden Street Cambridge, MA 02138 |
High Energy
Astrophysics Division at the Smithsonian
Astrophysical Observatory