Cryodetection

At very low temperatures the components of ordinary matter become so still that very small amounts of energy can have surprisingly large effects. For example, in a massive CRESST detector of about 300 grams the deposit of just a few hundred electronvolts (eV) from a single particle interaction leads to a measureable temperature jump in a superconducting film thermometer. Or, at 10 Millikelvin the deposit of just 14 eV - the binding energy of a single hydrogen atom - can be enough to flip a micron-sized superconducting sphere, a macroscopic object with 1013 atoms, into the normal state. Indeed if one naively extrapolates standard formulas, one obtains astonishing sounding results. Extrapolating the ideal heat capacity formula for silicon to the microkelvin (µK) range, one finds that a few electronvolts would double the temperature of a ton of silicon!

While the ideal formulas should not be naively extrapolated and the practical realization of such systems involves many challenges, they do indicate there is a great potential for detecting small energies with cryogenic devices. To reach these very low temperatures one uses dilution refrigerators.

In the seach for the interaction of dark matter particles in the laboratory one must combine two almost mutually exclusive requirements. On the one hand one needs massive detectors in order to obtain measurable interaction rate given the very low interaction probability which is anticipated for dark matter particles. On the other hand one needs the ability to observe small energy deposits because it is anticipated that the recoil energy of the struck nucleus is small, with most of the events in the keV region and below. Cryodetectors, with their ability to detect small energies, even when they have been diluted in a substantial volume, are ideally suited to this task.

Cryodetectors may be contrasted with familiar radiation detectors such as Geiger counters and their derivatives, photomultipliers, scintillation counters, etc. in that
  1. The full energy is detected since one is performing a kind of calorimetry and
  2. The energy of the basic excitation involved is much lower than that of the traditional devices.
The second point implies that for a given energy many more basic excitations are produced in the detector. Therefore statistical fluctuations are reduced and energy resolution can be improved. In the classical devices the initial interaction is inevitably with an electron in the detector. Therefore, the characteristic energy involved is that of electron binding, namely eV. But for a cryodetector the basic energy unit is much lower, typically 10-3 eV for a phononic excitation. Hence the to-be measured energy is distributed over 103 more excitations with the associated reduction in fluctuations.

The combination of total energy measurement, high sensitivity and high resolution has meant that cryodetectors, in addition to being the natural instrument for the direct detection of dark matter, can open other new fields, and improve capabilties in existing ones. Some examples are low energy neutrino scattering, (neutrinoless) double beta decay, detection of large biomolecules, and the detection of microfractures.
  Filling Nitrogen

Filling liquid nitrogen (with a temperature of -196oC) in the Gran Sasso underground laboratory
 


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