The Search for Solar Neutrinos 1968-1976
The Chemistry Department is justifiably proud of The Solar Neutrino Experiment, since it resulted in the 2002 Nobel Prize for Physics to our longtime (1948-84) colleague, Ray Davis. With the clarity of hindsight, it would be easy to regard the path from the experiment's conception to the Nobel Prize as straight and smooth. Well, it wasn't. When the early measurements were published, and they didn't fit the standard solar model, the response was massive skepticism...
Discussions of the experiment are manifold. A few links are here. Briefly, the basic idea is as follows. Hydrogen fusion and b decay reactions of 7Be, 8B, 13N and 15O in the sun generated neutrinos which were then emitted and detected on earth. Neutrinos were be detected via their capture by 37Cl: n + 37Cl → 37Ar + e-. Radioactive 37Ar then decayed back to 37Cl with a 35-day half life, during which time its concentration could be determined using sensitive counting techniques developed at Brookhaven and at the Weizmann Institute of Science. The concentration of 37Ar directly reflected the concentration of neutrinos intercepted by the 37Cl atoms. The probability of the reaction cited above is very low: most neutrino-chlorine encounters produce nothing. Therefore, in order to produce sufficient 37Ar to be measured, a large amount of 37Cl had to be exposed to the neutrino flux: 379,000 liters (680 tons) of C2Cl4, (cleaning fluid, perchlorethylene). In addition, concentrations of 37Ar can be produced spuriously by cosmic rays. Therefore, the tank of C2Cl4 had to be shielded from cosmic rays by placing it in a gold mine about a mile underground. Initial measurements were made starting in 1967, and results indicated a neutrino flux considerably below the prediction of the standard solar model. The combination of the difficulty of determining the very low 37Ar concentrations, plus the possibility of cosmic ray interference, plus the poor agreement between experiment and theory made the result seem spurious.
During this whole period, experimental runs were conducted, methods of 37Ar detection were refined and proposals for explaining why the experiment yielded the "wrong" results were dismissed one-by-one by careful experimental demonstration. The experimental result converged on a neutrino flux about one-third of the predicted value and stayed there, standing up to each criticism.
While 37Cl experiments were continuing, discussions began about other methods that could buttress the result. The 37Cl experiment is insensitive to neutrinos produced by the reaction H + H → D + e+ + n, because the neutrinos produced are not sufficiently energetic for the 37Cl reaction to take place efficiently. Neutrino capture by 71Ga to produce 71Ge can occur with neutrino energies from the hydrogen fusion reaction. Initial calculations showed that at least eighteen metric tons of gallium would be required in order to produce quantities of 71Ge that could be measured reliably. Ultimately, this experiment was performed, as the Gallex Experiment, at the Gran Sasso National Laboratory in Italy, with the participation of Ray Davis' successors at Brookhaven. The results from GALLEX supported the Brookhaven result.
Another reaction considered that was sensitive to low energy neutrinos was based on neutrino capture by 7Li, producing 7Be. However, in this case, there were (at least) two difficulties to be overcome: one involved an investigation of how to achieve chemical separation of a few atoms of Be from a large quantity of concentrated LiCl solution. Another big problem was the necessity to develop a low-background counting technique for measuring 7Be activity. In any case, this experiment would still have been large scale, requiring 109 metric tons of LiCl.
Of the many possible reasons for the "neutrino deficit," one appealing explanation discussed during this period involved so-called "neutrino oscillation", where during the 500 s trip from the sun to the earth, neutrinos transformed from one type (to which the Davis experiment was sensitive) to another form, to which the experiment was insensitive. The problem with this explanation is that the principle of lepton conservation is violated, which made acceptance of this theory difficult. However, Frederick Reines, who shared the 1995 Nobel Prize for Physics, showed that neutrino oscillation could occur, which is the likely explanation for Ray Davis' result.
"The Search for Solar Neutrinos" R. Davis, Jr. Umschau 5 153 (1969).
"A Progress Report on the Brookhaven Solar Neutrino Experiment" R. Davis, Jr. Acta Phys. Acad. Sci. Hung. 29 371 (1970).
"The Search for Neutrinos from the Sun" R. Davis, Jr., Probe '70 13 (1970)
"Progress Report on the Brookhaven Solar Neutrino Experiment" R. Davis, Jr., Academia Nazionale dei Lincei 157 59 (1971).
"The Brookhaven Solar Neutrino Detector as a Detector of Neutrino Pulses from Collapsing Stars" J. C. Evans, R. Davis, Jr. and J. N. Bahcall, Nature 251 486 (1974).
"Solar Neutrinos: A Scientific Puzzle" J. N. Bahcall and R. Davis, Jr., Science 191 264 (1976).
Last Modified: June 28, 2012