Stars as Laboratories for Fundamental Physics - MPP Theory Group

446 Chapter 11
ber of events. Its fiducial mass for the detection of SN neutrinos is
about 32,000 t, to be compared with 2,140 t for Kamiokande, i.e. it has
a target mass about 15 times larger. Thus one may be able to recognize
a neutrino signal from a SN perhaps as much as 4 times farther
away than the Large Magellanic Cloud, which is out to about 200 kpc.
However, the closest large galaxy is M31 (Andromeda) at a distance
of about 700 kpc, allowing Superkamiokande (and all other near-future
detectors) to observe SNe only in our own galaxy and in the Large and
Small Magellanic Clouds.
The rate at which SNe occur in our Galaxy as well as in the LMC
is rather uncertain. From observations in other galaxies the rate of
core-collapse SNe in the Milky Way is estimated to be about 7.3 h 2
per century with h the Hubble parameter in units of 100 km s −1 Mpc −1
(van den Bergh and Tammann 1991). Thus, for a low h of order 0.5
one may expect only about 2 such events per century. Roughly the
same number was found in a more recent study by Tammann, Löffler,
and Schröder (1994). The record of historical SNe, on the other hand,
suggests a significantly larger number. Thus it is optimistic, but not
entirely implausible, to hope for an observation within a decade of
Superkamiokande running time. The rate for the LMC is thought to
be about 0.5 per century (Tammann, Löffler, and Schröder 1994)—one
cannot reasonably expect another SN there within our lifetime.
To reach beyond the limits of our own galaxy and the LMC one
would need much more sensitive (much bigger) detectors. It would not
be enough to go as far as Andromeda because this galaxy appears to
have an anomalously low SN rate (van den Bergh and Tammann 1991).
In order to achieve a SN rate of at least 1 per year one may need to use
the Virgo cluster of galaxies at about 15 Mpc (300 times the distance to
the LMC) although it may be enough to reach to the nearby starburst
galaxies M82 and NGC 253 within about 4 Mpc which have a very high
SN rate because of their high rate of star formation (Becklin 1990).
However, a recent estimate of the SN rate for each of these galaxies is
only about 1 per 10 years (van Buren and Greenhouse 1994).
A novel detection scheme (Cline et al. 1990) that may allow one
to build big enough detectors is based on the neutral-current reaction
ν + (Z, N) → (Z, N − 1) + n + ν which can have a much enhanced
cross section in some nuclei due to collective effects; one would detect
the final-state neutron. The necessary detector volume can be achieved
by using natural deposits of minerals which contain the relevant target
nuclei. Naturally, the main concern would be to reduce sources of
background in order to isolate the feeble signal from a distant SN.