Subatomic Physics

19.6. Neutrino Astronomy and Cosmology 601
extensively. (57) The investigations of supernovae show that they can provide the
energy required for cosmic rays, about 10 49 ergs/y. However, supernova shock wave
acceleration models have difficulty in accounting for particles above 10 15 eV. Recent
detection of cosmic rays from the binary system Cygnus X-3 and Hercules X-1
suggest that for energies beyond the knee most cosmic rays may come from pulsars
or binary systems consisting of a neutron star and a giant companion.
It is possible that the sources emit cosmic rays with the energy spectrum
Eq. (19.15). It is, however, also possible that nature uses the same technique as
present day high-energy accelerators, acceleration in stages (Section 2.6). A mechanism
for acceleration in interstellar space, collision of the particles with moving
magnetic fields, has for instance been suggested by Fermi. (58) However, it is now
more generally believed that the primary sources of cosmic rays are supernovae
explosions and their remnants. (55,56)
For cosmic rays of highest energies no unique source has yet been identified.
They could come from black holes or even from remnants of the early universe. (49)
19.6 Neutrino Astronomy and Cosmology
Classical astronomy is based on observations in the narrow band of visible light, from
400 to 800 nm. In the past few decades, this window has been enlarged enormously
through radio and infrared astronomy on one side and through X-ray and gammaray
astronomy on the other. The charged cosmic rays provide another extension.
However, all these observations have one limitation in common: They cannot look
at the inside of stars, because the radiations are absorbed in a relatively small
amount of matter (Chapter 3). As a ballpark figure, it takes a photon ∼ 10 4 − 10 5
years to come out from the center of the sun. Fortunately, there is one particle
that escapes even from the inside of a very dense star, the neutrino; and neutrino
astronomy, (22) even though extremely difficult, has become an irreplaceable tool
in astrophysics. The properties that make the neutrino unique have already been
treated in Sections 7.4 and 11.14:
1. The absorption of neutrinos and antineutrinos in matter is very small. For
the absorption cross section, Eq. (11.78) gives
σ(cm 2 −44 pe
)=2.3 × 10
mec
Ee
, (19.16)
mec2 where pe and Ee are momentum and energy of the final electron in the reaction
νN → eN ′ . With Eqs. (2.17) and (2.18) it is then found that the mean free
path of a 1 MeV electron neutrino in water is about 1021 cm. It far exceeds
the linear dimensions of stars, which range up to 1013 cm. (See also Fig. 1.1.)
57V. Trimble, Rev. Mod. Phys. 60, 859 (1988); S. Woosley and T. Weaver, Sci. Amer. 261,
32 (August 1989).
58E. Fermi, Phys. Rev. 75, 12 (1949). (Reprinted in Cosmic Rays, Selected Reprints, American
Institute of Physics, New York.)