Large Underground Xenon Detector
Encyclopedia
The Large Underground Xenon Detector (LUX) is a 350 kg two-phase liquid xenon
detector of dark matter
particles. Liquid xenon both scintillates and becomes ionized when hit by particles (e.g. photons, neutrons and potentially dark matter). The ratio of scintillation over ionization energy caused by the collision provides a way of identifying the interacting particle. The leading theoretical dark matter candidate, the weakly interacting massive particle (WIMP), could be identified in this way.
Dark matter comprises most of the matter in the universe but its nature has yet to be determined. One of the leading candidates for the non-baryonic dark matter is the WIMP. WIMPs are expected to interact only with nuclei. Most of the events observed in noble liquid detectors such as LUX will be photon
s which interact predominantly with the electrons, which result in a different ionization signature than that of the WIMP nuclear collisions. This difference allows such detectors to remove much of the background events.
Light emitted from the excitation of atoms caused by a particle interacting will be directly detected by the phototubes. This initial pulse of light is referred to as "S1". The charge released by a collision will then be indirectly measured by the phototubes. Once an atom is ionized, the electrons will drift towards the top of the chamber, where they will scintillate as they approach the upper grids. This secondary pulse is referred to as "S2". The ratio of S1 and S2 is what's used to determine the nature of the detected particle.
In order to discern WIMPs from neutrons, the number of single interactions must be compared to the number of double and triple events. Since WIMPs are so weakly interacting, most of these particles will pass through the detector unnoticed. Those that do interact will have an almost non-existent chance of interacting a second time in the volume. Neutrons, on the other hand, have a reasonably large chance of having multiple collisions within the target volume. In a given volume, it is known statistically what percentage of neutrons will scatter a certain number of times. Using this knowledge, once the ratio of single interactions to multiple interactions exceeds a certain value, the detection of dark matter can be confirmed.
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
detector of dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
particles. Liquid xenon both scintillates and becomes ionized when hit by particles (e.g. photons, neutrons and potentially dark matter). The ratio of scintillation over ionization energy caused by the collision provides a way of identifying the interacting particle. The leading theoretical dark matter candidate, the weakly interacting massive particle (WIMP), could be identified in this way.
Dark matter comprises most of the matter in the universe but its nature has yet to be determined. One of the leading candidates for the non-baryonic dark matter is the WIMP. WIMPs are expected to interact only with nuclei. Most of the events observed in noble liquid detectors such as LUX will be photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s which interact predominantly with the electrons, which result in a different ionization signature than that of the WIMP nuclear collisions. This difference allows such detectors to remove much of the background events.
General design
The apparatus is a cylindrical volume of liquid and gaseous xenon (hence "two-phase"). A uniform axial electric field is maintained using voltage rings, which are evenly spaced along the length of the detector. There are two arrays of photomultiplier tubes (phototubes), situated with one array at the top and the other at the bottom. The interior walls are designed to reflect as much light as possible, while trapping as few drifting electrons as possible, to maximize the detection of both.Detector function
In order to identify the nature of particles interacting with the detector, the charge and light released in the collision must be recorded as thoroughly as possible.Light emitted from the excitation of atoms caused by a particle interacting will be directly detected by the phototubes. This initial pulse of light is referred to as "S1". The charge released by a collision will then be indirectly measured by the phototubes. Once an atom is ionized, the electrons will drift towards the top of the chamber, where they will scintillate as they approach the upper grids. This secondary pulse is referred to as "S2". The ratio of S1 and S2 is what's used to determine the nature of the detected particle.
Finding dark matter
WIMPs interact exclusively with the nuclei of the liquid xenon atoms, resulting in nuclear recoils that appear very similar to neutron collisions. In order to single out WIMP interactions, the number of neutron measurements must be reduced as much as possible. This is done through the use of proper shielding and ultra-quiet building materials.In order to discern WIMPs from neutrons, the number of single interactions must be compared to the number of double and triple events. Since WIMPs are so weakly interacting, most of these particles will pass through the detector unnoticed. Those that do interact will have an almost non-existent chance of interacting a second time in the volume. Neutrons, on the other hand, have a reasonably large chance of having multiple collisions within the target volume. In a given volume, it is known statistically what percentage of neutrons will scatter a certain number of times. Using this knowledge, once the ratio of single interactions to multiple interactions exceeds a certain value, the detection of dark matter can be confirmed.