This shielding is a necessity when one is looking for very rare events such as dark matter particle interactions with ordinary matter. Unfortunately some backgrounds are much harder to shield against. At the surface of the Earth, cosmic rays originating from outer space are constantly bombarding us with a flux of about 100 per square meter per second. Those very high energy, charged subatomic particles are extremely penetrating and the only way to protect against them to the degree that is required for a rare event search such as a dark matter experiment is to put kilometers of material between them and the detector. That is why a location such as the Sanford Underground Research Facility (SURF) is particularly attractive. One mile underground, the flux of cosmic rays is reduced by a factor of a million compared to the surface, which makes them just manageable.
There are many additional design features one can employ to improve a dark matter detector’s sensitivity, and several technologies have been explored over the past 20 years. Currently the LUX dark matter detector is being operated at the Sanford Laboratory, in the new Davis laboratory which was completed in the spring of 2012. LUX is a time-projection chamber, a traditional detector design dating back to the 70s which allows 3D positioning of interactions occurring within its active volume. LUX uses as its target 368 kilograms of liquefied ultra-pure xenon, which is a scintillator: interactions inside the xenon will create an amount of light proportional to the amount of energy deposited. That light can be collected on arrays of light detectors sensitive to a single photon, lending the LUX detector a low enough energy threshold to stand a good chance of detecting the tiny bump of a dark matter particle with an atom of xenon.
Because the xenon is very pure, the amount of intrinsic background radiation originated within the target itself remains limited. Because xenon is three times as dense as water, it can stop a lot of the radiation originating from outside the detector before it can reach the very center; combined with the 3D positioning capabilities of a time-projection chamber, this allows the definition of a very quiet region in the middle of the target where to look undisturbed for those rare dark matter interactions. Because the LUX detector is larger than any other similar detector currently in operation, it can make maximal use of this"self-shielding"feature, while retaining sufficient active detector mass to accumulate statistics rapidly.
This is a key feature for current and future dark matter detectors. Since the early 90s, detectors have been getting bigger and more sensitive, as dark matter keeps eluding us and physicists are forced to look for ever more tenuous interactions. In order to reach the degree of sensitivity required for positive dark matter detection, an experiment must be able to pick out a few events per year in hundreds of kilograms of material. Without targets built at least on that scale, the amount of time required to stand a chance of even seeing one is simply prohibitive.
The first dark matter search results from LUX will be released before the end of 2013. The experiment will continue running and release further results into 2015.