Counting neutrinos from an 'artificial sun.'

The nuclear fusion reactions that power the sun produce huge quantities of neutrinos. Experiments designed to detect these elusive, weakly interacting particles provide a glimpse deep into the sun's core, where these reactions take place.

Over the last few years, however, several Earth-based neutrino detectors have found fewer solar neutrinos than scientists had expected, based on theoretical models of how the sun generates energy (SN: 6/13/92, p.388). That deficit has proved an enduring puzzle.

Now, researchers participating in the GALLEX solar neutrino project have completed an important check on the efficiency of their neutrino detector and have raised some intriguing questions.

"This is the first time that neutrinos, produced in known amounts by an immensely radioactive source, or 'artificial sun,' have been used to test the overall operation of a solar neutrino experiment," says Richard L. Hahn of the Brookhaven National Laboratory in Upton, N.Y. Hahn is a member of the GALLEX team.

Preliminary results indicate that the GALLEX detector picks up essentially all of the available neutrinos. Submitted for publication to PHYSICS LETTERS B, this finding helps rule out the possibility that detector inefficiency accounts for the solar neutrino deficit. It confirms that about 40 percent of the expected neutrinos are missing and focuses renewed attention on possible explanations for the conflict between theory and experiment.

At the Gran Sasso underground laboratory in Italy, GALLEX researchers looked for low-energy neutrinos produced by the decay of chromium-51, a specially prepared, highly radioactive isotope of chromium. This source generated neutrinos at a rate 16 times greater than that of the sun.

In the experiment, neutrinos from the decay of chromium-51 interacted with gallium atoms to convert them into germanium-71 atoms. Detection of these germanium atoms provided a count of neutrino interactions.

The researchers found that the measured rate of germanium production matched the rate expected from the known activity of the source to within 10 percent. Incorporating additional data, they expect to have a more precise result in the spring.

The GALLEX solar neutrino measurements, along with those from the Soviet-American Gallium Experiment (SAGE) in Russia and the Kamiokande detector in Japan, spotlight one particular step in the chain of nuclear fusion reactions occurring in the sun's core.

The sun's energy comes primarily from the fusion of two protons to form a deuteron, accompanied by the emission of a positron and a neutrino. A secondary reaction involves the production of boron-8 from the fusion of a proton with beryllium-7, a process that also creates a neutrino.

"It turns out that the energies of the neutrinos from the chromium-51 source are very close to those from beryllium-7," Hahn says. "Since we have just shown that we can detect essentially all the neutrinos from chromium-51 in the GALLEX detector, we should also be able to detect the solar beryllium-7 neutrinos. But, apparently, we don't see them."

In other words, although researchers detect neutrinos from boron-8 at the end of the nuclear fusion chain, they see very few of the neutrinos from beryllium-7, without which the boron-8 cannot be made.

"Solar neutrino science is entering a new phase," comments R.S. Raghavan of AT&T Bell Laboratories in Murray Hill. N.J., in the Jan. 6 SCIENCE. "The solar neutrino puzzle is deepening into a paradox that refutes the basic logic of the reaction chain that powers the sun by the fusion of protons into heavy elements."

Raghavan suggests that one way to avoid this "missing link" problem is to postulate that the neutrino, generally thought to be without mass, actually has a small mass.