Chapter 5Solar (and other) Neutrinos5.1 Solar neutrino detectorsCareful analyses of the experiments that will be described below indicate that the observedsolar neutrino fluxes differ substantially from standard solar model (SSM) expectations.φ(pp) ∼ 0.9 φSSM(pp)φ(7Be) ∼ 0φ(8B) ∼ 0.43 φSSM(8B)This pattern is difficult to reproduce in a solar model because of the temperature dependencesof the neutrino fluxesφ(pp) ∝ T−1.2cφ(7Be) ∝ T8cφ(8B) ∝ T18c(These results come from our standard formula, but with the constraint imposed that thesolar luminosity be correctly reproduced. This means that the ppI cycle production mustgo up as the temperature goes down in order to produce the desired luminosity.) A reduced8B neutrino flux can be produced by lowering the central temperature of the sun somewhat.However, such adjustments, either by varying the parameters of the SSM or by adoptingsome nonstandard physics, tend to push the φ(7Be)/φ(8B) ratio to higher values rather thanthe low one above,φ(7Be)φ(8B)∼ T−10cThus the observations seem difficult to reconcile with plausible solar model variations.As of 1998 five solar neutrino experiments had provided data, the Homestake37Cl experi-ment, the gallium experiments SAGE and GALLEX, Kamiokande, and SuperKamiokandeThe first three detectors are radiochemical, while Kamiokande and SuperKamiokande recordneutrino-electron elastic scattering event-by-event.The Homestake ExperimentDetection of neutrinos by the reaction37Cl(νe,e)37Ar was suggested independently by Pon-tecorvo (1946) and by Alvarez (1949). Davis’s efforts to mount a 0.61 kiloton experimentusing perchloroethylene (C2Cl4) were greatly helped by Bahcall’s demonstration that tran-sitions to excited states in37Ar, particularly the Fermi transition to the analog state at 4.99MeV, increased the8B cross section by a factor of 40. This suggested that Davis’s detectorwould have the requisite sensitivity to detect8B neutrinos, thereby accurately determiningthe central temperature of the sun. The experiment was mounted in the Homestake GoldMine, Lead, South Dakota, in a cavity constructed approximately 4850 feet underground[4280 meters water equivalent (m.w.e.)]. It operated almost continuously since 1967, finally1Figure 1: Here a variety of nonstandard solar models have been evaluated, where quantitieslike nuclear S-factors, the opacity, the solar age, and the metal content have varied outsidetheir accepted uncertainties. The resulting neutrino fluxes and the core temperature areevaluated, and the former are plotted versus the latter. One sees that core temperaturegives a very good one-parameter description of the results: this quantity seems to governthe neutrino fluxes, regardless of the source of the nonstandard model variation. One alsosees that a reduced8B neutrino flux requires a cooler sun, while a reduced7Be/8B neutrinoflux ratio would require a hotter one. This graph is from Castellani et al.20.0 0.2 0.4 0.6 0.8 1.0φ(7Be) / φ(7Be)SSM0.00.20.40.60.81.0φ(8B) / φ(8B)SSMMonte Carlo SSMsTC SSMLow ZLow OpacityWIMPsLarge S11Dar-Shaviv ModelSSM90% C.L.90% C.L.95% C.L.99% C.L.TC Power LawCombined FitFigure 2: Nonstandard solar models are again evaluated, and when their relative7Be and8Bfluxes are plotted, they form a path in the lower right quadrant. However the experimentaldata is in the upper left quadrant (and in fact prefers a7Be neutrino flux that is negative!)This shows that it is hard to fit the data assuming standard neutrino physics (especially anundistorted8B neutrino profile). From Hata et al.3terminating in 2002, when the Homestake Mine closed. The result of 25 years of measurementishσφi37Cl= 2.55 ± 0.17 ± 0.18 SNU (1σ)which can be prepared to two recent standard solar model predictions of 8.0 ± 1.0 SNU and6.4 ± 1.4 SNU, all with 1σ errors. The8B and7Be contributions account for about 75% and16% of the total.The experiment depends on the special properties of37Ar: as a noble gas, it can be removedreadily from perchloroethylene, while its half life (τ1/2= 35 days) allows both a reasonableexposure time and counting of the gas as it decays back to37Cl. Argon is removed from thetank by a helium purge, and the gas then circulated through a condensor, a molecular sieve,and a charcoal trap cooled to the temperature of liquid nitrogen. Typically ∼ 95% of theargon in the tank is captured in the trap. (The efficiency is determined each run from therecovery results for a known amount of carrier gas,36Ar or38Ar, introduced into the tank atthe start of the run.) When the extraction is completed, the trap is heated and swept by He.The extracted gas is passed through a hot titanium filter to remove reactive gases, and thenother noble gases are separated by gas chromatography. The purified argon is loaded into asmall proportional counter along with tritium-free methane, which serves as a counting gas.Since the electron capture decay of37Ar leads to the ground state of37Cl, the only signal forthe decay is the 2.82 keV Auger electron produced as the atomic electrons in37Cl adjust tofill the K-shell vacancy. The counting of the gas typically continues for about one year (∼10 half lives).The measured cosmic ray-induced background in the Homestake detector is 0.0637Ar atoms/daywhile neutron-induced backgrounds are estimated to be below 0.03 atoms/day. A signal of0.48 ± 0.04 atoms/day is attributed to solar neutrinos. When detector efficiencies,37Ardecays occurring in the tank, etc., are taken into account, the number of37Ar atoms countedis about 25/year.The Kamiokande and SuperKamiokande ExperimentsThe Kamiokande experiment used a 4.5 kiloton cylindrical imaging water Cerenkov detectororiginally designed for proton decay searches, but later reinstrumented to detect low energyneutrinos. It detected neutrinos by the Cerenkov light produced by recoiling electrons in thereactionνx+ e → ν0x+ e0Both νeand heavy flavor neutrinos contribute, with σ(νe)/σ(νµ) ∼ 7. The light was detectedby photomultiplier tubes that viewed the inner volume of the detector. Kamiokande hadan inner fiducial volume of 0.68 kilotons. Its successor, SuperKamiokande, then ran fullyinstrumented for a number of years, collecting 1496 days of data. (SuperKamiokande hada phototube accident and is now running with about half of the original number of tubes.)SuperKamiokande has a much larger fiducial volume of 22.5 kilotons.4tank_diagram.gif (GIF Image, 2990x2108 pixels) - Scaled