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
- The full energy is detected since one is
performing a kind of calorimetry and
- 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. |
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Filling liquid nitrogen (with a temperature of -196oC) in the Gran Sasso underground laboratory
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