Back from the dead

 作者:俞匮侯     |      日期:2019-03-08 03:18:12
By Andrew Watson COOLING damaged particle detectors to ultralow temperatures could give them a new lease of life, say researchers at CERN, the European Laboratory for Particle Physics near Geneva. This trick could save scientists millions on the cost of new detectors and help to speed up the repair and maintenance of particle accelerators. Physicists studying the strange events that occur when one particle beam smashes into another watch the action with sensors called vertex detectors. Made from concentric cylinders of silicon, they record the energy and direction of the charged particles formed during the collision. Although vertex detectors are very good at spotting weak signals, they have an Achilles heel—they are damaged by the very radiation they measure. When a charged particle whizzes through silicon, it leaves a trail of free electrons and positively charged holes that tell physicists about the particle that created them. However, the particles sometimes bash into atoms of silicon, dislodging them and creating a “trap” that pulls in both the electrons and the holes. As more and more traps form, the detector slowly loses its sensitivity and eventually becomes useless. What particle physicists needed was a way to fill in the traps. One possible answer emerged a year ago when Vittorio Palmieri, a physicist at CERN, suggested that cooling the detectors might do the trick. Now an international research collaboration has shown that Palmieri’s idea works. In silicon cooled to 77 kelvin, electrons that fall into a trap lack the energy to escape and the trap quickly fills. With the traps out of action, the detector regains its sensitivity. “We got some dead detectors, dipped them in liquid nitrogen and brought them back to life,” says Palmieri. He and his colleagues have named this process the Lazarus effect. With detectors costing around £20 million,