Results from a solar cycle of neutrino data.

Since it began watching the sun in 1987, Japan's Kamiokande neutrino detector has provided data important for understanding how nuclear fusion reactions power the sun and for testing theories of stellar evolution. Fusion reactions in the sun's core produce huge quantities of neutrinos, elusive subatomic particles that interact weakly with ordinary matter. Earth-based detectors pick up only a tiny fraction of these particles.

By the time it was shut down last year to make way for a considerably more sensitive experiment, the original Kamiokande detector had accumulated data covering nearly a complete solar cycle, the roughly 11-year period during which sunspot activity goes from a minimum to a maximum and back to a minimum.

Earlier studies, based on data from a different solar neutrino detector, had hinted at a correlation between the number of neutrinos arriving at a collector and solar activity, with the detection rate decreasing as the number of sunspots increases (SN: 12/8/90, p. 358). If such a connection had been confirmed, it would have provided evidence that the neutrino has a mass and could change from one type of neutrino to another (SN: 5/18/96, p. 319).

In their final report, however, members of the Kamiokande collaboration conclude that they observed no month-to-month or year-to-year variations in the number of neutrinos detected that would match the pattern of solar activity throughout this period.

"No strong correlation of the solar neutrino flux with the sunspot numbers was found within experimental [uncertainties]," Yoshiyuki Fukuda of the Institute for Cosmic Ray Research at the University of Tokyo and the other members of the Kamiokande collaboration report in the Aug. 26 Physical Review Letters.

Located in a mine about 200 kilometers west of Tokyo and a kilometer underground, to shield it from less penetrating rays, the Kamiokande detector consisted of a cylindrical tank containing 3,000 tons of pure water. Sensitive photodetectors lining the tank's walls picked up light flashes generated by speeding electrons knocked aside on the rare occasions-averaging less than once a day-when neutrinos interacted with water.

The detector was the first to show directly that the observed neutrinos came from the direction of the sun rather than from other possible sources (SN: 10/28/89, p. 280). The energy of the neutrinos indicated that they originated in the decay of boron-8, an isotope created near the end of the chain of nuclear fusion reactions fueling the sun.

Kamiokande's detection rate of boron-8 neutrinos was about half that predicted by theorists calculating the sun's output using the standard solar model.

Other experiments have found a similar, persistent shortfall, suggesting either that models of the sun's interior are incomplete or that neutrino physics may be more complicated than expected.

The Super Kamiokande detector, which replaces the original Kamiokande, started up April 1. It is expected to detect about 100 times as many neutrinos per day as its predecessor.