Page 302 - Radiochemistry and nuclear chemistry

286 Radiochemistry am1 Nuclear Chemistry

FIG. 10.3. Holes in helmets of Apollo 12 astronauts caused by high energy cosmic rays.
The holes have been made visible in a microscope by etching. (From New Scientist, April
22, 1973.)

,r o
pon +-x+; nop + lr-; pop; non;

Pions are the particles of strong interaction. Since the pion rest mass is 0.147 u, the energy
to produce a pion is at least 137 MeV (in practice, in order to conserve momentum it
exceeds 400 MeV, cf. (12.4)). The pions are unstable in free form.

10.2. Secondary reactions in the earth's atmosphere

Few of the pions formed in the annihilation process reach the earth's surface. They
undergo radioactive decay (life-time about 10 -6 s) to muons and neutrinos, or they collide
with other particles in the atmosphere and are annihilated. The muons have properties
similar to the electron, but are unstable, decaying with a life-time of about 2 x 10 -6 s to
electrons and neutrinos. The collision reactions of the pions result in the formation of a
large number of other particles such as electrons, neutrons, protons, and photons. Some of
the electrons so formed are captured in a thick zone around the earth known as the inner
van Allen belt.
The main part (50 - 80%) of the cosmic radiation which reaches the earth's surface
consists of high energy muons. Muons have much less tendency to react with atomic nuclei
than pions and, therefore, can penetrate the atmosphere and the solid material of the earth
relatively easily. The remaining part, which is the lower energy component of the cosmic
radiation that strikes the earth, consists of photons, electrons, and positrons. At sea level
this part of the cosmic radiation gives rise to approximately 2 - 3 ion pairs s- 1 cm-3 of air.
It is this component of the cosmic radiation that gives rise to the cosmic ray portion of the