In theory, millions of WIMPs pass through the detector stacks every second. By their very nature, these dark-matter particles barely interact with ordinary matter, but in some rare instances one should collide in just the right way to make its presence known. Germanium, which is chemically similar to silicon, makes a particularly good target because its nucleus—a clump of 32 protons and 38 to 44 neutrons—is close to what theorists say is the lower size limit for a WIMP. As in billiards, the object being hit responds especially effectively if the cue ball (the WIMP) and the target ball (the germanium nucleus) are roughly the same size.
Work and play inside the physics laboratory
If a passing WIMP bumps directly into a germanium atom, the nucleus should vibrate, producing a tiny amount of heat. Each CDMS detector is outfitted with sensitive layers of aluminum and tungsten designed to record that minuscule heat signal. At the same time, the dark particle should also jar loose some electrons from the germanium atoms, triggering an electric charge that will be recorded by an electrode. The ratio of charge to heat tells researchers whether the particle struck the nucleus, and therefore might be a WIMP, or if it is just a rogue electron or some other familiar particle that is simply stirring up the atomic neighborhood.
This whole process is unimaginably sensitive. That is why the detectors are kept in an icebox chilled to just 70 thousandths of a degree Fahrenheit above absolute zero. To understand how cold this is, remember that even in the depths of space, residual radiation from the Big Bang keeps things at a relatively toasty 4.86 degrees Fahrenheit above absolute zero, making the inside of the CDMS detector one of the coldest places in the entire universe. At that temperature, vibrations in the germanium crystals virtually cease, eliminating jiggles that could interfere with a WIMP signal. “The WIMP is like a pebble dropping into a pond,” Bauer says. “It’s hard to see the ripples on a windy day, so you want the water to be as quiet as possible.”
The need for quiet also explains all the shielding around the detectors: successive blankets of polyethylene and lead. The lead—15 tons of it—blocks 99.995 percent of gamma rays that would normally overwhelm the detectors. In fact, even ordinary lead generates its own radiation. Instead, much of the detector’s flak vest is made of ancient lead obtained from a 2,000-year-old Roman shipwreck. It takes a few hundred years for radioactive leftovers in the metal to fully decay, so there is a brisk market in old lead for experiments like CDMS. Ancient lead sells for $40 per pound, about 40 times the going rate for ordinary lead.
As the data accumulate at roughly 10,000 particle detections per day, the team has a physicist on-site 24-7 whose job it is to watch the refrigerator, make sure nothing breaks down, and monitor the experiment for the one telltale event that could finally prove WIMPs really exist. The crew also swaps out empty liquid nitrogen and liquid helium bottles for new ones every other day, a nerve-racking job that, if botched, can end up bleeding the cryogenics more quickly than the new bottles can replace them. The icebox takes weeks to rechill. “I wouldn’t call it tedious; we’re certainly excited about the big picture,” Bauer says. He acknowledges that “the hardware stuff” also appeals to the inner Erector set in all physicists. Nevertheless, he concedes the job can be wearing. It carries the full burden of command without the promise of any interim eureka moments. To blow off steam and pass the time, staff members play rousing bouts of Ping-Pong on the lab floor in the shadow of inflatable palm trees.
