Serious shortfall of solar neutrinos.

Serious shortfall of solar neutrinos

The mystery of the "missing" solar neutrinos deepens. Thermonuclear reactions at the sun's core should produce copious quantities of neutrinos, but preliminary data from a neutrino detector operating since January in the Soviet Union indicate that the new detector has apparently failed so far to capture any of these elusive particles. This follows the pattern of long-running solar neutrino experiments elsewhere, which have all revealed that the number of neutrinos detected on Earth falls significatly short of the apparent rate of neutrino production in the sun (SN: 10/28/89, p. 280).

The new data come from the Soviet-American gallium experiment (SAGE) at the Baksan underground laboratory in the mountainous North Caucasus region. The apparatus consists of a tank containing 30 tons of liquid gallium metal. When an atom of the isotope gallium-71 absorbs an electron-neutrino (one of the three types of neutrinos known to exist), it turns into radioactive germanium-71. By extracting germanium-71 from the gallium and measuring its radioactivity, researchers can count the number of neutrino interactions. What makes gallium special is that relatively low-energy eleectron-neutrinos -- including those produced in the main nuclear-fusion reactions occurring at the sun's core -- can trigger the reaction. In contrast, the detector in the Homestake gold mine near Lead, S.D., and Japan's Kamiokande II detector are senstive only to certain high-energy electron-neutrinos created in a rare type of fusion reaction. Thus, the number of neutrinos in a gallium-based detector would presumably reflect more closely the sun's rate of energy production, which is directly related to the sun's brightness.

Theoretical calculations indicate that the Baksan gallium detector should pick up roughly one neutrino per day. Data from the first few months of operation suggest that the actual rate of neutrino capture is virtually zero. Because gallium interacts with low-energy neutrinos created in the prime energy-producing reactions in the sun, such a low rate provides strong evidence that electron-neutrinos somehow disappear between their creation and reaching Earth.

However, the number of neutrino events counted in the experimental is only what one would expect as a background count, and the results may represent some kind of statistical fluke or an unforeseen problem in the apparatus or analysis. To see if the pattern holds, researchers are now awaiting further results from SAGE and the first data from a gallium detector that has just begun operating at the Gran Sasso underground laboratory in the Apennines east of Rome. If further work confirms the initial results, then theorists will have to look more closely at the properties of neutrinos to account for their mysterious behavior.