Stars as Laboratories for Fundamental Physics - MPP Theory Group

358 Chapter 10
neutrino energy are shown in Fig. 10.5 according to the tabulation of
Bahcall and Ulrich (1988).
Fig. 10.5. Neutrino absorption cross section on 37 Cl and 71 Ga according to
Bahcall and Ulrich (1988).
Because the absorption cross sections are steeply increasing functions
of energy while the predicted solar neutrino spectrum (Fig. 10.1)
steeply decreases, the low flux of boron neutrinos yields the dominant
contribution to the expected counting rate for chlorine, and a sizeable
contribution to gallium. This is illustrated in Fig. 10.6 where the predicted
counting rates from solar neutrinos, integrated between energy
E ν and infinity, are shown as a function of E ν . These plots correspond
to the lower panel of Fig. 10.1 if the differential flux is weighted with
the relevant absorption cross section. It is customary to express the
absorption rate per nucleus in “solar neutrino units”
1 SNU = 10 −36 s −1 , (10.11)
not to be confused with 1 SNu, the supernova unit, which quantifies
the rate of supernova occurrences in a galaxy. Because one measures
a fully integrated flux one only needs the absorption cross sections
folded with the spectra of the individual source reactions. For 37 Cl and
71 Ga they are given in Tab. 10.4. Multiplying the predicted fluxes with
these cross sections and applying a factor 10 36 gives the absorption rate
in SNUs.