February 15, 2014
The tank for liquid xenon used in PandaX experiment. Image credit: Scott Stephenson
In China's Sichuan province, an underground dark matter detector is about to be powered up. The PandaX project has a very large tank, filled with liquid xenon and is set under 2,400 meters of marble rock.
Dark matter is hypothesized matter which we use to explain the gravitational pull of galaxies, which is stronger than it should be if the visible matter is all there is. By current calculations, dark matter makes more than 80% of the matter in the universe.
It is made of very elusive, so-called WIMPs, short for weakly interacting massive particles. They are also hypothetical and the PandaX is going to attempt their detection. WIMPs only interact with ordinary matter through gravity and weak interacting force when they bump into nucleus of an atom of ordinary matter.
These interactions are rare and make WIMPs very hard to detect, if they are there at all. Like other similar experiments, PandaX (Particle and Astrophysical Xenon) uses xenon which WIMPs should bump into. Because other particles that interact with xenon could give false signals, most notably cosmic radiation and radiation from the elements in rocks, these detectors are built underground. PandaX is surrounded by marble, which gives it additional protection since there are no radioactive materials in such rocks.
The tank contains 2.4 metric tons of liquid xenon, but due to radioactivity in the walls of the tank itself, only about 1 metric ton in the middle of the tank make the target area. There, WIMPs are expected to collide with xenon nuclei and cause them to emit photons and electrons which are then detected at the top of the tank.
The PandaX project is relatively cheap, at US $8 million. The reason is already mentioned natural shielding that makes additional shielding material unnecessary. If the initial results lead to increase in funding, the xenon tank is easily expandable, making the detection even more likely.