Accident Cripples Super-Kamiokande Neutrino Observatory

While the tank was being refilled with 50 000 tons of water, a photomultiplier tube imploded and the shock wave set off a chain reaction that left more than 7000 PMTs shattered. That, at least, is what scientists surmise happened on the morning of 12 November in Super-Kamiokande, the world's largest neutrino detector.

Super-K, which is buried 1000 meters deep in a defunct zinc mine in the mountains west of Tokyo, made headlines worldwide in 1998 with strong evidence that atmospheric neutrinos oscillate--or morph from one of three possible flavors into another--and therefore have mass.

The accident crippled Super-K and stunned particle physicists everywhere. "The accident was severe, but we will rebuild," says Super-K director Yoji Totsuka. The aim, he says, is to start up with about half the original density of PMTs within a year, and fully fix Super-K by 2007.

Shards

For the first resuscitation phase, the surviving PMTs, plus some spares, will be spread evenly around Super-K. At half the usual density, the lowest-energy signals will be lost, so some solar neutrinos won't be detectable, and proton decay, if it actually happens, will be harder to observe, but atmospheric and other studies could resume. "This is good enough for the time being," says Totsuka.

Scientists are especially eager to restart the K2K experiment. In K2K, which is intended to verify Super-K's spectacular 1998 atmospheric neutrino results, manmade muon neutrinos are shot to Super-K from the KEK proton accelerator in Tsukuba, 250 kilometers away. Early data support oscillation, in that only 44 muon neutrinos were seen in Super-K, down from 64 predicted from measurements near the beam source. "It's a teaser," says Fermilab theorist Boris Kayser. "It's consistent with the atmospheric results, but it's not definite. You need to run longer."

Another reason to get K2K back on track quickly is so a follow-on known as JHF-Kamioka, a high-intensity neutrino beam that will shoot neutrinos to Super-K from 295 kilometers away, can go ahead as planned in 2007. Says Totsuka, "I am afraid this [accident] might cause delays because of some kind of distrust of the technique."

And, as long as Super-K is down, there's one less detector keeping watch for supernova explosions. That's a serious concern, says Hank Sobel of the University of California, Irvine, one of Super-K's US spokesmen.

Two committees are looking into the accident. By the end of January, they are supposed to report on its cause, advise on how to avoid a repeat, and outline a plan and the costs for repairing Super-K, whose original price tag topped $100 million.

Replacements for the 7000 burst 50-cm PMTs will cost about $3000 apiece and will take a few years to make. The experiment also lost about half, or 1000, of its 20-cm veto PMTs, which face away from the tank's center and monitor a concentric water sheath. The smaller PMTs cost about $1000 each, bringing the tab for fixing Super-K to at least $22 million. Officials of Japan's Ministry of Education, Culture, Sports, Science and Technology say they will try to squeeze the repair costs out of the ministry's 2002 budget. "We are hoping that the DOE [US Department of Energy] will also help support the repair of the detector," says Sobel.

"The accident is a big setback. It causes everybody to be nervous about future experiments, perhaps more than is justified," says John Bahcall, a neutrino expert at the Institute for Advanced Study in Princeton, New Jersey. "We will have to examine the possibility of such accidents more closely. But in point of fact, Super-K will be repaired and back on the air, doing nearly everything it was doing, and almost as well." And, he adds, "repairing the lab is so important that, if I had the requisite skills, I would go and help."