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

High Energy Astrophysics Division at the Smithsonian Astrophysical Observatory