Underground Chinese dark-matter detector is deepest yet

IEEE Spectrum : Weakly interacting massive particles (WIMPs) are a theoretical form of dark matter. By nature, they rarely interact with ordinary matter. To try to find evidence of them, researchers have built large detectors deep underground to shield the detectors from cosmic rays and other radiation sources that might mimic WIMPs. China’s Particle and Astrophysical Xenon (PandaX) project, which will begin operations early this year, is the deepest yet, at 2400 m underground. Like other similar detectors, it features a large tank filled with liquid xenon. It looks for the release of photons and electrons caused by collisions between outside particles and the xenon atoms. By comparing the signals of the photons and electrons, the researchers can determine where in the tank the collisions occurred. Only collisions in the central part of the tank would be considered valid, as the tank’s walls themselves can emit radiation. PandaX is uniquely designed so that it can easily be scaled up to increase the central target area to 1 metric ton, twice its current volume. That would be significantly larger than any other detector, although one in Europe is scheduled to be scaled to a similar size in the next several years.