WIMP Dark Matter Detection
All direct WIMP dark matter detectors aim to observe nuclear recoils produced by WIMP-nucleon scattering. WIMPs interact with all target materials in exactly the same way as neutrons and generally scatter off nuclei rather than electrons due to their relatively large mass. There a few different methods that can be used to detect nuclear recoils, including collecting ionisation, scintillation or thermal energy deposition data. The energy spectra of nuclear recoils for different target materials and WIMP masses can be simulated and calculated then used to compare with real data from detectors to aid in the identification, or in placing an upper limit on characteristics (usually mass and WIMP-nucleon scattering cross-section), of WIMPs.
Another means to identify a WIMP signal is through annual modulation in the total signal count and by observing the daily fluctuations in the incident direction of recoils. All dark matter detectors with a sufficiently large target mass can be made sensitive to the annual modulation, provided the energy resolution is good enough. Only detectors with directional capabilities can observe the daily fluctuations expected of signals from a galactic source.
Some dark matter projects detect the ionisation produced when a particle scatters off the nucleus of the experiment's target material. The first ionisation detectors used germanium as a semiconductor, since it has a band gap of 0.69 eV at liquid nitrogen temperatures and so can detect low energy nuclear recoils. Many ionisation detectors still use Ge crystal targets, however there are novel gaseous ionisation detectors now operating, including the DRIFT detector, part of the compliment of UKDMC experiments. There are several different charge readout devices available to be used with such detectors. These include Multi-Wire Proportional Chambers (MWPCs), Gas Electron Multipliers (GEMs), microstrips and MicroMEsh GASeous detectors (MicroMEGAS). These devices are used to avalanche electrons from the ionisation, produced in the gas of the detector by particles interacting, in order to increase the signal received by the data acquisition system of the detector.
Scintillating crystals and liquids are used in conjunction with light collectors such as photomultiplier tubes (PMTs) to observe the scintillation light emitted when a particle interacts with a nucleus within the target material, which absorbs energy from the interaction and then releases this energy in the form of photon emission. Scintillator detectors are able to discriminate between WIMP and background events in the low energy region using pulse shape discrimination, which compares signals from gamma and neutron calibration runs to data taken when running normally (without calibration sources present). The ZEPLIN experiment uses this technique with a liquid scintillator of xenon.
Bolometers, or phonon detectors, are used to collect data on thermal energy deposition within different absorbers in several dark matter experiments. This technique detects and measures tiny temperature changes in the target material caused by energy deposited in the target nuclei from particle interactions, which eventually converts to heat. A bolometer is basically a resistor that is thermally isolated and exposed to the incident radiation (phonons). As the bolometer absorbs the radiation energy its temperature changes and so the resistance of the device also changes. This resistance change is measured by passing a small current through the bolometer and measuring the voltage across it, which can subsequently be converted back into the equivalent amount of energy deposited in the detector. This measurement can hence enable calculation of relevant parameters of interest for dark matter research should a WIMP event be observed, as mentioned above, and is particularly sensitive in the low WIMP mass region.
Some dark matter detectors combine two or more of the techniques described above to improve their resolution and overall sensitivity to WIMP-nucleon interactions. Detectors that combine technologies have, to date, provided the best limits on WIMP characteristics.