Born again: Japan's Super-K neutrino detector is back online after having its shattered sensors replaced. Credit: SUPER-K

Yoji Totsuka had planned for a quiet 2002, as he neared the end of his stint as director of Japan's Kamioka Observatory. But those plans were shattered in November 2001, along with almost 7,000 photomultiplier sensors, when a shockwave triggered by one imploding sensor swept through the observatory's water-filled neutrino detector.

Instead of quietly drawing his tenure to a close, Totsuka has been trying to figure out what happened — and working overtime to repair the damage. “I'm still wondering why we could not anticipate that stupid accident,” he says. “I don't have an answer yet.”

The detector, known as Super-Kamiokande or Super-K, is some 240 kilometres northwest of Tokyo. Its headline achievement was providing evidence that neutrinos, a type of subatomic particle, possess mass. But the accident derailed other ongoing experiments, including a study of neutrinos beamed to Super-K from KEK, Japan's High Energy Accelerator Research Organization at Tsukuba, north of Tokyo.

A year of repairs has paid off, however. The detector has now been refitted with plastic-cased sensors, which should prevent a similar catastrophe, and the refilling of the 50-million-litre water tank will be finished before Christmas. The KEK experiment is expected to resume in mid-January. Super-K's sensitivity is half of what it was before the accident, but funding for a full recovery by 2006 is likely to be made available.

Totsuka is now director of KEK and has been succeeded at Kamioka by Yoichiro Suzuki. “The most important thing,” says Suzuki, “is to get some great results as a means of thanking the people who have supported us.”