Stars as Laboratories for Fundamental Physics - MPP Theory Group

Supernova Neutrinos 417
All of the relevant cross sections per water molecule are shown in
Fig. 11.10 as a function of E ν . The curves incorporate a factor of 2 for
proton targets (two per H 2 O), and a factor of 10 for electrons (ten per
H 2 O). Above its threshold, the ν e 16 O cross section rises very fast; it is
then the dominant detection process for ν e ’s. Still, the ν e p reaction is
the absolutely dominant mode of observing SN neutrinos.
The BST detector is filled with an organic scintillator based on
“white spirit” C n H 2n+2 with n ≈ 9. The dominant detection reaction is
also ν e p → ne + . In addition, elastic scattering on electrons is possible,
and the process ν e 12 C → 12 N e − occurs for E νe ∼ > 30 MeV.
The trigger efficiencies relevant for the three detectors are shown
as a function of the e ± energy in Fig. 11.11. Analytic fit formulae to
these curves were given by Burrows (1988) for IMB and Kamiokande.
IMB reports a dead time of 13% during the SN burst (Bratton et al.
1988); the IMB curve includes a factor 0.87 to account for this effect.
The fiducial volume of Kamiokande II relevant for the SN 1987A observations
was 2,140 tons, for IMB 6,800 tons, and for BST 200 tons. It
corresponds to a target of 1.43×10 32 protons at Kamiokande, 4.6×10 32
at IMB, and 1.88×10 31 at BST.
With the ν e p cross section of Eq. (11.6) and the efficiency curves
of Fig. 11.11 one may compute a prediction for the number of events
Fig. 11.11. Trigger efficiency for electron (positron) detection at the
Kamiokande (Hirata et al. 1988) and IMB (Bratton et al. 1988) water Cherenkov
detectors, and the Baksan scintillator telescope (Alexeyev et al. 1988),
relevant for the SN 1987A neutrino observations. In the IMB curve a factor
0.87 is included to account for their reported dead time of 13%.