Stars as Laboratories for Fundamental Physics - MPP Theory Group

392 Chapter 10
to be attributed to a small astrophysical S 17 factor. In this case, the explanation
for the deficiency of beryllium neutrinos could not be resolved
by this detector.
10.9.2 Sudbury Neutrino Observatory (SNO)
The Sudbury Neutrino Observatory (SNO), also scheduled to take up
operation in 1996, is a heavy-water Cherenkov detector which is expected
to be able to measure the appearance of “wrong-flavored” neutrinos
if the MSW effect solves the solar neutrino problems (Sudbury
Neutrino Observatory Collaboration 1987; Lesko et al. 1993). The SNO
detector consists of 1000 tons of heavy water (D 2 O) in a spherical acrylic
vessel of 12 m diameter, immersed in an outer vessel of ultrapure light
water (H 2 O), surrounded by about 9600 photomultiplier tubes of 20 cm
diameter each. The detector is located 2000 m underground in the
Creighton mine, an operating Nickel mine, near Sudbury in Ontario
(Canada). Neutrinos can be detected by three different reactions in
this detector: by electron elastic scattering ν + e → e + ν and by the
deuterium dissociation reactions ν e +d → p+p+e and ν+d → p+n+ν.
The electron elastic scattering reaction is analogous to the Kamiokande
and Superkamiokande detectors: the recoiling electron is measured
by the detection of its Cherenkov light. The effective detection
threshold is expected to be at 5 MeV as in Superkamiokande. This
reaction is sensitive to both ν e and ν µ,τ , albeit with a reduced cross
section for the latter (Eq. 10.17).
The charged-current deuterium dissociation ν e d → ppe has a threshold
of 1.44 MeV, i.e. the final-state electron kinetic energy is essentially
T e = E ν − 1.44 MeV. The electron is detected by its Cherenkov light;
the effective energy resolution is about 20%. The angular distribution
relative to the incident neutrino is given by 1 − 1 cos Θ. The cross
3
section for this reaction is large. For an incident spectrum of boron
neutrinos one expects 9 times more electron counts above 5 MeV than
from electron elastic scattering; above 9 MeV even 13 times as many.
One can search for neutrino oscillations by a spectral distortion of
the electron spectrum, similar to Fig. 10.21 for Superkamiokande. In
fact, the spectral distortion is more pronounced as it is not washed out
by a broad final-state distribution of electron energies. Also, one can
search for a day/night effect.
The most important detection reaction, however, is the neutralcurrent
deuteron disintegration νd → pnν which has the same cross
section for all flavors and so it measures the total (left-handed) neutrino