Stars as Laboratories for Fundamental Physics - MPP Theory Group

Solar Neutrinos 357
Xu et al. (1994) have discussed a unique relationship between S 17 (0)
and the nuclear vertex constant of the overlap wave function for the
virtual decay 8 B → 7 Be+p; the nuclear vertex constant can be predicted
from other nuclear data. These authors’ calculation of S 17 (E) agrees
remarkably well with the data points of Filippone et al. (1983) at low
energies, and with Vaughn et al. (1970) at higher energies; these are
the experiments which gave a low S 17 (0). Xu et al. (1994) find an even
lower value of S 17 (0) ≈ 17.6 eV b.
On the other hand, Brown, Csótó, and Sherr (1995) studied the
relationship between the Coulomb displacement energy for the A = 8,
J = 2 + , T = 1 state and S 17 . They found a high value S 17 (20 keV) =
(26.5 ± 2.0) eV b.
The S 17 (0) factor was recently determined from the Coulomb dissociation
of 8 B, i.e. by the Primakoff-type process 8 B → 7 Be + p in
the electric field of a 208 Pb nucleus (Motobayashi et al. 1994). These
authors find a very low preliminary value of S 17 (0) = (16.7 ± 3.2) eV b.
Langanke and Shoppa (1994) think that the true value may be significantly
lower still if allowance is made for a possible E2 amplitude in
the Primakoff reaction. However, this possibility is heavily disputed by
Gai and Bertulani (1995); see also the reply by Langanke and Shoppa
(1995).
All of these new results are somewhat preliminary at the present
time. Independently of their ultimate status it is evident that the S 17
factor must be considered the weakest link in the prediction of the
boron neutrino flux.
10.3 Observations
10.3.1 Absorption Reactions for Radiochemical Experiments
The longest-running solar neutrino experiment is the Homestake chlorine
detector which is based on a reaction proposed by Pontecorvo
(1948) and Alvarez (1949),
ν e + 37 Cl → 37 Ar + e (threshold 0.814 MeV). (10.9)
The two other data-producing radiochemical experiments use gallium
as a target according to the reaction
ν e + 71 Ga → 71 Ge + e (threshold 0.233 MeV). (10.10)
Because of its low threshold, the gallium experiments can pick up the
solar pp neutrino flux. The absorption cross sections as a function of