Page 301 - Radiochemistry and nuclear chemistry

Cosmic Radiation and Elementary Particles 285

Incident cosmic ray
4He-particle of /
ca. 10 4 GeV ! w ~ . ,,o" ,,o o
..1 ~ . 9 ~ , ." t ~ if "o 9 " ' ,o ~176 "~ Narrow ower
,
9
meso

.

9 o
FIG. 10.2. Secondary particles produced by a 10 4 GeV helium atom in a photographic emulsion.

the particles at the lower end of the energy spectrum are assumed to originate from sources
within our galaxy (the Milky Way), while the particles of the higher energy end are
assumed to come from sources outside of our galaxy. Different hypotheses, which for the
most part are untested, suggest that the particles come from astronomical radio sources,
exploding super novae, or colliding galaxies, etc. At least a portion of this radiation is a
residue of the processes involved in the original formation of the universe. It is assumed
that the high energy particles obtain their tremendous kinetic energies through acceleration
in the magnetic field of galactic objects (synchrotron acceleration, Ch. 13).
When the primary cosmic particles enter the earth's atmosphere, they collide with the
matter of the atmosphere and are annihilated. In this annihilation process a large number
of new particles are formed whose total kinetic energy is less than that of the original
primary radiation but whose total rest mass is larger than that of the primary particle. A 104
GeV cosmic helium ion may produce a shower of 50 - 100 new highly ionizing particles,
cf. Fig. 10.2. The main reaction products are particles which are known as pions,
designated 7r.
Figure 10.3 shows the effect of high energy cosmic rays hitting the helmets of Apollo 12
astronauts. It is probable that the cosmic ray intensity will put a limit to how long man can
endure in outer space: it has been calculated that in a journey to the planet Mars about
0.1% of the cerebral cortex will be destroyed. The annihilation process occurs to such an
extent that below an altitude of approximately 25 km above the earth the number of primary
cosmic particles has been reduced to quite a small fraction of the original intensity.
The discovery of r-mesons (or pions) was reported by PoweU and Occhialini in 1948 after
they had analyzed tracks in photographic emulsions placed for some months on a mountain
top to get a high yield of cosmic ray interactions. Pions are produced in large amounts in
all high energy (> 400 MeV) nuclear reactions. In 1935 Yukawa suggested that the
nucleons in a nuclide were held together through the exchange of a hypothetical particle,
which we now recognize as the pion, just as hydrogen atoms in H 2 are held together
through the exchange of an electron: