Stars as Laboratories for Fundamental Physics - MPP Theory Group

Solar Neutrinos 393
flux above its threshold of 2.2 MeV, independently of the occurrence of
oscillations. The main problem here is the measurement of the finalstate
neutron by the detection of γ rays from the subsequent neutron
capture, or by a neutron detector array in the heavy water. Ultrapure
water, acrylic, and other materials are needed to prevent an excessive
radioactive background that would spoil this measurement. One anticipates
to obtain the full unsuppressed 8 B flux with a precision of
about 1% after 5 years of operation. The measured ratio between the
charged-current and neutral-current deuterium disintegration will give
an immediate measure of the electron survival probability and thus of
the occurrence of neutrino oscillations.
If spin or spin-flavor oscillations occur such that a sizeable ν e flux
is produced, it can be detected by the reaction ν e + d → n + n + e +
which produces three detectable particles (Balantekin and Loreti 1992).
Of course, a possible ν e flux is already constrained by Kamiokande
(Fig. 10.14), and can be detected or constrained by Superkamiokande.
Suggestions for solar model independent methods of analyzing future
solar neutrino data were made, for example, by Spiro and Vignaud
(1990), Bilenky and Giunti (1993, 1994), and Castellani et al. (1994).
10.9.3 BOREXINO
Superkamiokande and SNO are both limited to a measurement of the
boron neutrino flux because of their relatively high detection thresholds.
If the boron flux is partly or mostly suppressed by a low S 17 factor or
a low central solar temperature instead of neutrino oscillations these
experiments may have difficulties at identifying oscillations which would
still be indicated by the missing beryllium neutrino flux. Therefore, it is
interesting that another experiment (BOREXINO 60 ) is being prepared
which would be sensitive dominantly to the beryllium neutrinos.
The main detection reaction is elastic ν-e scattering as in the lightwater
Cherenkov detectors. However, the kicked electron is detected by
virtue of scintillation light rather than Cherenkov radiation, the former
60 The name of this experiment is derived from BOREX (boron solar neutrino
experiment), a proposed detector that was to use 11 B as a target (Raghavan, Pakvasa,
and Brown 1986; see also Bahcall 1989). For a practical implementation it
was envisaged to use a boron loaded liquid scintillator (e.g. Raghavan 1990); because
of the relatively small size of this detector the Italian diminutive BOREXINO
(baby BOREX) emerged. Ultimately, the idea of using a borated scintillator was
dropped entirely, leaving boron only in the name of the experiment. Confusingly,
then, BOREXINO is unrelated to a boron target, and also unrelated to the solar
boron neutrinos because the experiment is designed to hunt the beryllium ones.