Stars as Laboratories for Fundamental Physics - MPP Theory Group

Solar Neutrinos 349
Fig. 10.3. Normalized spectra of neutrino source reactions in the Sun.
It is much more difficult to obtain the spectrum from 8 B decay because
the final-state 8 Be ∗ nucleus is unstable against spontaneous fission
into two α particles. Even though several states of 8 Be contribute to
the transition, it is dominated by the 2.9 MeV excitation and so the
neutrino spectrum can be determined with relatively little ambiguity
by folding Eq. (10.2) with the experimental α spectrum (Kopysov and
Kuzmin 1968). A more recent and more detailed analysis was performed
by Bahcall and Holstein (1986). An analytic approximation to
their tabulated spectrum is
dN/dE ν = 8.52×10 −6 (15.1 − E ν ) 2.75 E 2 ν , (10.3)
where the neutrino energies are in MeV. This normalized spectrum
is also shown in Fig. 10.3 where, again, the difference between the
tabulated values and the analytic approximation would be hidden by
the line width (maximum deviation less than 0.02 in units of the vertical
axis in Fig. 10.3).
Neutrinos from the hep reaction extend to the highest energies of
all solar sources, but their overall flux is very small and very uncertain
because of large uncertainties in the low-energy 3 He p cross section—
see Bahcall and Pinsonneault (1992) for a detailed discussion. The
tabulated spectrum (Bahcall and Ulrich 1988) can be represented by
dN/dE ν = 2.33×10 −5 (18.8 − E ν ) 1.80 E 1.92
ν , (10.4)
where the quality of the fit is equally good as that for the 8 B neutrinos.