Cryogenic particle detectors
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Cryogenic particle detectors are radiation sensors that operate at very low temperature, typically only a few degrees above absolute zero. These sensors interact with an energetic elementary particle and deliver a signal which can be related to the type of particle and the nature of the interaction. While many types of particle detector might be operated with improved performance at cryogenic temperatures, this term generally refers to several types which require low temperature for their operation. These take advantage of special effects or properties occurring only at low temperature.
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[edit] Introduction
The most commonly cited reason for operating any sensor at low temperature is the reduction in thermal noise, which is proportional to the square root of the absolute temperature. However, at very low temperature, certain material properties become very sensitive to energy deposited by particles in their passage through the sensor, and the gain from these changes may be very much more than that from reduction in noise. Two such commonly used properties are heat capacity and electrical resistivity, particularly superconductivity; other designs are based on superconducting tunnel junctions, quasiparticle trapping, rotons in superfluids, magnetic bolometers, and other principles.
Originally, astronomy pushed the development of cryogenic detectors for optical and infrared radiation [1]. Later, particle physics and cosmology motivated cryogenic detector development for sensing known and predicted particles such as neutrinos, axions, WIMPs and the like. [2][3].
[edit] Types of cryogenic particle detectors
[edit] Calorimetric particle detection
A calorimeter is a device which measures the amount of heat deposited in a sample of material. In common usage a calorimeter differs from a bolometer principally in the ability to sense rates of energy deposition; bolometer was originally applied to devices for measuring thermal radiation by heating a sensor.
Below the Debye temperature of a crystalline dielectric material (such as silicon) the heat capacity decreases inversely as the cube of the absolute temperature. It becomes very small so that the sample's increase in temperature for a given heat input may be relatively large. This makes it practical to make a calorimeter which has a very large temperature excursion for a small amount of heat input, such as that deposited by a passing particle. The temperature rise can be measured with a standard type of thermistor, as in a classical calorimeter. Interaction with the particle heats the sensor and the thermistor measures the increase. In general, small sample size and very sensitive thermistors are required to make a sensitive particle detector by this method.
In principle, several types of resistance thermometers can be used. The limit of sensitivity to energy deposition is given by the smallest meaningful resistance fluctuation, which in turn is determined by thermal fluctuations. Since all resistors exhibit natural (Johnson noise) voltage fluctuations, a reduction of temperature is often the only way to achieve the required sensitivity.
[edit] Superconducting transition edge detectors
A much better calorimetric sensor takes advantage of superconductivity. Most pure superconductors have a very sharp transition from normal resistivity to superconductivity at some low temperature. By operating just below this transition temperature the superconductor switches from zero to a measurable resistance for a very small change in temperature, that is, it can be used as an extremely sensitive thermistor, and can measure much smaller changes in temperature near the transition.
[edit] Superconducting granules
The superconducting transition alone can be used to directly measure the heating caused by a passing particle. A type I superconducting grain in a magnetic field exhibits perfect diamagnetism and excludes the field completely excluded from its interior. If it is held slightly below the transition temperature, the superconductivity vanishes on heating by particle radiation, and the field suddenly penetrates the interior. This field change can be detected by a surrounding coil. The change is reversible when the grain cools again. In practice the grains must be very small and carefully made, and carefully coupled to the coil.
[edit] Magnetic calorimeters
Paramagnetic rare earth ions have been used as particle sensors by sensing the spin flips of the paramagnetic atoms induced by heat absorbed in a low heat capacity material. The ions are used as a magnetic thermometer.
[edit] Other methods
[edit] Phonon particle detection
Calorimeters assume the sample is in thermal equilibrium or nearly so. In crystalline materials at very low temperature this is not necessarily the case. A good deal more information can be found by measuring the elementary excitations of the crystal lattice, or phonons, caused by the interacting particle. This can be done by several methods including superconducting transition edge detectors.
[edit] Superconducting tunnel junctions
Supercurrents are carried by pairs of electrons called Cooper pairs which are weakly bound to each other but do not interact strongly with defects (such a tunnel gap) in the superconducting material. The energy required to break a Cooper pair into its constituents is much less than is required to excite an electron directly across the tunnel gap, so that much more current will result from a particle interacting with Cooper pairs within the gap, than with the electrons. For the same reason, fewer electrons are excited into crossing the gap by thermal noise, giving intrinsically better signal to noise.
[edit] Roton detectors
In superfluid 4He the elementary collective excitations are phonons and rotons. A particle striking an electron or nucleus in this superfluid can produce rotons, which may be detected bolometrically or by the evaporation of helium atoms when they reach a free surface. 4He is intrinsically very pure so the rotons travel ballistically and are stable, so that large volumes of fluid can be used.
[edit] Quasiparticles in superfluid 3He
In the B phase, below 0.001 K, superfluid 3He is acts similarly to a superconductor. Pairs of atoms are bound as quasiparticles similar to Cooper pairs with a very small energy gap of the order of 100 nanoelectronvolts. This allows building a detector analogous to a superconducting tunnel detector. The advantage is that many (~109) pairs could be produced by a single interaction, but the difficulties are that it is difficult to measure the excess of normal 3He atoms produced and to prepare and maintain much superfluid at such low temperature.
[edit] References
- Twerenbold, Damian (December 1996). "Cryogenic Particle Detectors". Rep. Prog.Phys 59: 349–426. doi:.
- Enss, Christian (Editor) (2005). Cryogenic Particle Detection. Springer, Topics in applied physics 99. ISBN 3-540-20113-0.
- ^ Glass, I.S. (1999). Handbook of Infrared Astronomy. New York: Cambridge University Press. ISBN 0-521-63311-7.
- ^ Primack, J. R.; D. Seckel, B. Sadoulet (December 1988). "Detection of Cosmic Dark Matter". Annual Review of Nuclear and Particle Science 38: 751–807. doi:.
- ^ Pretzl, K. (1988). "". Space Science Reviews 130 (1-4): 63–72. doi:.
