Subatomic Physics

598 Nuclear and Particle Astrophysics
and 6.11, the interaction will produce a large number of hadrons; pions predominate,
but antinucleons, kaons, and hyperons also occur. These hadrons can again interact
with oxygen or nitrogen nuclei; the unstable ones can also decay weakly. The decays
result in electrons, muons, neutrinos, and photons (Chapter 11). The photons can
produce pairs; the muons decay, but because of the time dilation (Eq. (1.9)), many
penetrate into the Earth’s solid mantle before doing so. Overall, a very-high energy
proton can give rise to a large number of photons and leptons (Fig. 3.10); such a
cosmic-ray shower can cover an area of many km 2 on the surface of the Earth. (46)
In contrast, a photon produces a shower with very few muons. We shall not discuss
the phenomena in the atmosphere further, but shall turn to the primary radiation.
Abundance relative to Carbon = 100
10 6
10 4
10 2
10 0
10 -2
10 -4
Nuclear abundance: cosmic rays compared to solar system
H
Li
He
Cosmic ray
Solar system
C
O
N
Ne Mg Si
S
A Ca Ti Cr
Fe
Cu
Na Al
F
P Cl K
Mn
Ni
B
V
Sc
10 -6 Be
0 5 10 15
Nuclear charge
20 25 30
Figure 19.10: Composition of the nuclear component of
the primary cosmic rays. Shown for comparison are the
solar abundances. [From T.K. Gaisser and T. Stanev, Nucl.
Phys. A777, 98 (2006).]
The composition of the nuclear
component of the primary
cosmic rays is shown
in Fig. 19.10. (47) Also shown
for comparison is the universal
distribution of the elements
observed in the solar
atmosphere and in meteorites.
A few remarkable facts
emerge from a comparison of
the cosmic-ray and the universal
data: (1) The elements
Li, Be, and B are about 10 5
times more abundant in cosmic
rays than universally. (2)
The ratio 3 He/ 4 He is about
300 times larger in cosmic
rays. (3) Heavy nuclei are
much more prevalent in cosmic
rays.
The first two facts can be explained by assuming that the cosmic rays have traversed
about several g/cm 2 of matter between their source and the top of the Earth’s
atmosphere. In such an amount of matter, nuclear reactions produce the observed
distribution. Since the interstellar density is about 10 −25 g/cm 3 ,thecosmicrays
must have wandered around for 10 6 –10 7 y. Two more facts have been established
that may prove important for theories of the origin of cosmic rays: (4) So far, no
antihadrons have been found in the primary cosmic rays. (48) (5) Electrons are about
46 D.E. Nagle, T.K. Gaisser, and R.J. Protheroe, Annu. Rev. Nuc. Part. Sci. 38, 609 (1988);
M.V.S. Rao and B.V. Sreekantan, Extensive Air Showers, World Scientific, 1998.
47 Composition and Origin of Cosmic Rays,( M. M. Shapiro, ed.) Reidel , Boston, 1982; J.
A. Simpson, Annu. Rev. Nuc. Part. Sci. 33, 323 (1983); N. Lund in Cosmic Radiation in
Contemporary Astrophysics, (M. M. Shapiro, ed.) Reidel, Boston, 1986, p. 1.
48 See e.g., M. Bongi et al, IEEE Transc Nucl. Sci. 51, 854 (2004).