Cryogenic particle detectors operate at very low temperature, typically only a few degrees aboveabsolute zero. Thesesensors interact with an energeticelementary particle (such as aphoton) and deliver a signal that can be related to the type of particle and the nature of the interaction. While many types of particle detectors might be operated with improved performance atcryogenic temperatures, this term generally refers to types that take advantage of special effects or properties occurring only at low temperature.
The most commonly cited reason for operating any sensor at low temperature is the reduction inthermal noise, which is proportional to the square root of theabsolute 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 even more than that from reduction in thermal noise. Two such commonly used properties areheat capacity andelectrical resistivity, particularlysuperconductivity; other designs are based on superconductingtunnel junctions,quasiparticle trapping,rotons insuperfluids, magneticbolometers, 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 asneutrinos,axions, andweakly interacting massive particles (WIMPs).[2][3]
Acalorimeter is a device that measures the amount ofheat deposited in a sample of material. A calorimeter differs from abolometer in that a calorimeter measures energy, while a bolometer measurespower.
Below theDebye temperature of a crystallinedielectric material (such assilicon), 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 that 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 ofthermistor, as in a classical calorimeter. In general, small sample size and very sensitive thermistors are required to make a sensitive particle detector by this method.
In principle, several types ofresistance thermometers can be used. The limit of sensitivity to energy deposition is determined by the magnitude of resistance fluctuations, which are in turn determined bythermal fluctuations. Since allresistors exhibit voltage fluctuations that are proportional to their temperature, an effect known asJohnson noise, a reduction of temperature is often the only way to achieve the required sensitivity.
A very sensitive calorimetric sensor known as atransition-edge sensor (TES) takes advantage ofsuperconductivity. Most pure superconductors have a very sharp transition from normal resistivity to superconductivity at some low temperature. By operating on the superconducting phase transition, a very small change in temperature resulting from interaction with a particle results in a significant change in resistance.
Thesuperconducting tunnel junction (STJ) consists of two pieces ofsuperconducting material separated by a very thin (~nanometer)insulating layer. It is also known as asuperconductor-insulator-superconductor tunnel junction (SIS) and is a type of aJosephson junction.Cooper pairs cantunnel across the insulating barrier, a phenomenon known as theJosephson effect.Quasiparticles can also tunnel across the barrier, although the quasiparticle current is suppressed for voltages less than twice the superconducting energy gap. A photon absorbed on one side of a STJ breaks Cooper pairs and creates quasiparticles. In the presence of an applied voltage across the junction, the quasiparticles tunnel across the junction, and the resulting tunneling current is proportional to the photon energy. The STJ can also be used as aheterodyne detector by exploiting the change in the nonlinearcurrent–voltage characteristic that results from photon-assisted tunneling. STJs are the most sensitive heterodyne detectors available for the 100 GHz – 1 THz frequency range and are employed forastronomical observation at these frequencies.
Thekinetic inductance detector (KID) is based on measuring the change inkinetic inductance caused by the absorption of photons in a thin strip ofsuperconducting material. The change in inductance is typically measured as the change in the resonant frequency of amicrowaveresonator, and hence these detectors are also known as microwave kinetic inductance detectors (MKIDs).
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 perfectdiamagnetism and excludes the field completely 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.
Paramagneticrare-earth ions are being 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.
Calorimeters assume the sample is inthermal 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, orphonons, caused by the interacting particle. This can be done by several methods including superconductingtransition edge sensors.
Thesuperconducting nanowire single-photon detector (SNSPD) is based on a superconducting wire cooled well below the superconducting transition temperature and biased with a dccurrent that is close to but less than the superconducting critical current. The SNSPD is typically made from ≈ 5 nm thickniobium nitride films which are patterned as narrow nanowires (with a typical width of 100 nm). Absorption of a photon breaksCooper pairs and reduces the critical current below the bias current. A small non-superconducting section across the width of the nanowire is formed.[4][5] This resistive non-superconducting section then leads to a detectable voltage pulse of a duration of about 1 nanosecond. The main advantages of this type of photon detector are its high speed (a maximal count rate of 2 GHz makes them the fastest available) and its low dark count rate. The main disadvantage is the lack of intrinsic energy resolution.
In superfluid4He the elementary collective excitations arephonons androtons. 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.
In the B phase, below 0.001 K, superfluid3He acts similarly to a superconductor. Pairs of atoms are bound asquasiparticles similar to Cooper pairs with a very small energy gap of the order of 100 nanoelectronvolts. This allows building a detectoranalogous to a superconducting tunnel detector. The advantage is that many (~109) pairscould be produced by a single interaction, but the difficulties are that it is difficult to measure the excess of normal3He atoms produced and to prepare and maintain muchsuperfluid at such low temperature.