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Cosmic Radiation and Elementary Particles 287

natural background that is measured by nuclear detection devices in laboratories. 1 Some
of the cosmic radiation interacts to make atmospheric radioactivity which is principally the
nuclides 3H and 14C. This important radioactivity is treated in chapter 5.

10.3. Elementary particles and forces of nature

Cosmic radiation contains a large number of the kind of particles which used to be called
elementary particles. We have so far mentioned protons, electrons, neutrons, positrons,
pions, muons, photons, and neutrinos. These particles can be sorted into different classes
according to their quantum properties, Table 10.1. We have here added one more particle,
the K-meson (or kaon), because of its similarity to the w-meson (pion). Kaons and pions
appear in cosmic rays and high-energy nuclear reactions, cf. Fig. 10.4. As we shall see
later, the baryons and mesons of Table 10.1 are not elementary in a strict sense. However,
to avoid confusion we will refer to the particles in Table 10.1 as elementary in the
appropriate nuclear reactions (cf. later Table 10.2).
This group of elementary particles began to be considerably expanded about 1947 when
physicists discovered the first of the so-called "strange" particles in cloud chamber pictures
of cosmic rays. These new elementary particles were called strange because they lived
almost a million million times longer than scientists had any reason to expect at that time.
The population of elementary particles has literally exploded since then, as physicists have
built larger and larger particle accelerators, by which it is possible to impart sufficient
kinetic energy to protons so that interaction with nuclides transform a large fraction of the
kinetic energy into matter. With the present limit in the 2000 GeV range (Ch. 13) it is
possible to produce particles with a mass of up to -2000 proton masses, and hundreds of
new "strange" particles have been observed. Figure 10.4 is a typical picture of a reaction
observed in a liquid hydrogen bubble chamber at an accelerator center.
This has created a scientific area called elementary particle physics. It is quite different
from nuclear physics, which is concerned with composite nuclei only. A principal objective
of elementary particle physics has been to group the particles together according to their
properties to obtain a meaningful pattern which would describe all particles as parts of some
few fundamental building blocks of nature. One step in this direction is to study how the
elementary particles interact with each other, i.e. what kind of forces are involved.
Considering what an immense and incredible diverse assembly the universe is - from the
cosmos to man and microbes - it is remarkable that scientists have been able to discover
only four basic forces which govern the attraction and repulsion of all physical objects of
nature. Let us consider these forces of nature in a qualitative way.
The first and weakest force of nature is that of gravity. This is the force that causes all
objects to attract one another and is responsible for the attraction of the planets to the sun
in the solar system and of the solar system to the rest of the galaxy. It is also the force that
holds us to the earth. It seems paradoxical that the weakest attraction of the four basic
forces of nature is the force that is responsible for the assembly of the largest objects on
the greatest scale. In modem physics it is believed that all forces arc carried by

1
The remainder of the natural background comes from naturally occurring radioactive elemems in the laboratory
materials and surrounding building.