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 photons 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.
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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.
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.
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.