TOKYO--A single cracked photomultiplier tube apparently triggered the accident on 12 November that has closed Japan's Super-Kamiokande neutrino detector for at least a year. An investigation into the accident has confirmed early suspicions about the sequence of events that destroyed about 65% of the observatory's 11,000 light-detecing sensors (ScienceNOW, 13 November 2001).
The $100 million detector has produced convincing evidence that neutrinos have mass, contrary to decades of theoretical predictions (ScienceNOW, 5 June 1998). The wispy particles cannot be observed directly, however, so the 39-meter-diameter, 41-meter-high observation tank is filled with water and lined with photomultiplier tubes that can catch a distinctive glow, known as Cerenkov radiation, produced when neutrinos smash into atomic particles in the water. Last summer, for the first time since the facility was completed in 1996, the water was drained so some 100 burned-out tubes could be replaced. The tank was being refilled when one of the tubes, damaged during the repairs, imploded and started a chain reaction that destroyed almost all of the submerged tubes.
By analyzing the sequence in which the sensors stopped sending signals, the investigators narrowed down the initial break to one of two tubes--one original, one a replacement--on the floor of the tank. Technicians had stood on thick Styrofoam pads placed directly atop the tubes, after deciding that the tubes could withstand the stress. But a computer simulation after the accident showed that a concentrated load on the edge of the 52-centimeter-diameter tubes might create microcracks on the neck that could give way when the tank was filled.
To test the theory that a single imploding tube could destroy thousands of others, the investigating team three times submerged an array of nine tubes and deliberately punctured the central tube. Each time the shock wave resulting from the implosion broke all the surrounding tubes. "This makes it obvious that the implosion of just one tube can cause a shock wave that results in a large amount of destruction," investigators tersely note in a report made public today.
The Japan-U.S. collaboration running the experiment hopes to resume some observation within a year using a limited number of photomultiplier tubes. But bringing the facility back to full strength could take 5 years and cost between $15 million and $25 million.