6.3. SOLAR NEUTRINO PROBLEM 349
wheresnustands for solar neutrino unit:
1 snu= 10 −^36 reactions/(^37 Cl atom·s).In 1968, R. Davis et al (Davis, Harmer and Hoffman, 1968 ) reported the experimental
results, their measuring rate is
(6.3.10) NExp= ( 2. 0 ± 0. 3 )snu.
the experimental value (6.3.10) is only about one third of the theoretically expected value
(6.3.9). It gave rise to the famous solar neutrino problem.
Super-K experiment
In 2001, the Super-Kamiokande collaboration presented itsresults on solar neutrinos.
Unlike the Homestake experiment, Super-Kuses water as the detector. The process is elastic
neutrino-electron scattering:
νx+e−→νx+e,
whereνxis one of the three flavors of neutrinos. This reaction is sensitive toμandτneutrinos
as well ase-neutrinos, but the detection efficiency is 6.5 times greater fore-neutrinos than for
the other two kinds. The outgoing electron is detected by theCherenkov radiation it emits in
water. They observed the rate at
r=45% of the expected value.The Super-Kamiokande detector is located in the Mozumi Minenear Kamioka section of the
city of Hida, Japan.
Sudbury Neutrino Observatory (SNO)
Meanwhile, in the summer of 2001 the SNO collaboration reported their observation
results. They obtained
r=35% of the predicted value.
The SNO used heavy water(^2 H 2 O)instead of ordinary water(H 2 O), and the SNO detection
method is based on the following reactions:
(6.3.11) νe+^2 H−→p+p+e−,
(6.3.12) νx+^2 H−→p+n+νx,
(6.3.13) νx+e−−→νx+e−.
SNO detects electronse−, but notτ−andμ−, as there is not enough energy in the solar
electron-neutrino such that the transformed tau and mu neutrino can excite neutrons in^2 H to
produce eitherτ−orμ−.
KamLAND
The loss of reactor electron anti-neutrinoνeis verified by the KamLAND experiment.A potential alternative experiment