Solar neutrino experiments : past, present and future

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Session 40 -- Particle Astrophysics
Oral presentation, Tuesday, 2:00-6:30, Dwinelle 155 Room

[40.03] Solar neutrino experiments : past, present and future


Solar neutrino detection started 25 years ago by the famous radiochemical chlorine experiment installed by Davis and his collaborators (600\, tons of $\rm C_2Cl_4$ in the Homestake mine, South Dakota). Neutrinos produce radioactive $\rm^{37}Ar$ atoms ($\rm T_{1/2} \,=\,35$ days). Standard solar models (SSM) predict a value between 6.4 and 8 SNU (1 SNU or solar neutrino unit is 10$\rm^{-36}$ capture/atom/second) and Davis observes $\rm 2.3\,\pm\,0.25$ SNU. This reduction by a factor of about 3 is known as the "solar neutrino problem".

Since 1988 the real time Kamiokande experiment (Japan) measures a solar neutrino flux also reduced by a factor of about 2 ($\rm 0.49\,\pm\,0.04\,\pm\,0.06$ of SSM predictions). However, the reaction threshold either in the chlorine ($>$\,0.814\,MeV) or in the Kamiokande ($>$\,7.5\,MeV) experiments is too high to be sensitive to neutrinos produced in the primary pp fusion reaction ($\rm E_{max}=0.42\,MeV$).

From 1990, a new generation of radiochemical experiments, using gallium as a target (0.233\,MeV reaction threshold) started to take data. The first result of SAGE (Soviet-American Gallium Experiment in Baksan, Caucasus) ($\rm 20\,^{+15}_{-20} \,(stat.)\,\pm\,32\,(syst.)$ SNU) has not been confirmed by GALLEX (Heidelberg-Karlsruhe-Munich-Gran Sasso-Milan-Rome-Saclay-Nice-Rehovot-Brookhaven in the Gran Sasso Underground Laboratory) which published, in June 1992, $\rm 83\,\pm\,19\,(stat.)\,\pm\,8\,(syst.)$ SNU (SSM predictions between 124 and 132 SNU). During the summer 1992, SAGE did not confirm its low value using its 1991 data ($\rm 58\,^{+17}_{-24} \,(stat.)\,\pm\,14\,(syst.)$ SNU, preliminary) and its final result should not be too far from the GALLEX one. GALLEX observes about 2/3 of the expected signal, almost $\rm 2\sigma$ below SSM predictions. This constitutes the first evidence for the pp fusion reaction in the core of the Sun.

Interpretations of the flux reductions observed are discussed in terms of astrophysics (reduction of the central temperature of the Sun) or of neutrino oscillations. The future of solar neutrino experiments (Sudbury, SuperKamiokande and Borexino) is briefly presented.

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