Page 304 - Radiochemistry and nuclear chemistry

288 Radiochemistry and Nuclear Chemistry

"something" which either can be described as a wave or as a particle (see w The
carrier of the gravitational force is the graviton. Experimenters have tried to detect
gravitational waves, but so far the results are inconclusive.
A second force of nature with which we are all relatively familiar is that of the
electromagnetic force. The electromagnetic force is expressed by Coulomb's law and is
responsible for the attraction and repulsion of charged bodies. Just as the gravitational force
holds the planets in their orbits about the sun and explains the stability of the planetary
systems, so the electromagnetic force explains the attraction between electrons in atoms,
atoms in molecules, and ions in crystals. It is the force that holds the atomic world
together. It is approximately 1036 times stronger than the gravitational force. If gravity is
the force underlying the laws of astronomy, electromagnetism is the force underlying the
laws of chemistry and biology. The carrier of the electromagnetic force is the photon.
The third major force in nature has been discussed briefly in chapter 3 where we called
it the nuclear force. This force is also known as the strong interaction force and is the one
responsible for holding nuclear particles together. Undoubtedly it is the strongest in nature
but operates only over the very short distance of approximately 10 -14 m. Whereas
electromagnetism binds electrons to nuclei in atoms with an energy corresponding to a few
electron volts, the strong interaction force holds nucleons together in nuclei with energies
corresponding to millions of electron volts. The carrier of the strong interaction force is
now recognized to be the gluon; we will return to this point in w
The fourth force is the one which is involved in the radioactive B-decay of atoms and is
known as the weak interaction force. Like the strong interaction, this weak interaction force
operates over extremely short distances and is the force that is involved in the interaction
of very light particles known as leptons (electrons, muons, and neutrinos) with each other
and as well as their interaction with mesons, baryons, and nuclei. One characteristic of
leptons is that they seem to be quite immune to the strong interaction force. The strong
nuclear force is approximately 102 times greater than the Coulombic force, while the weak
interaction force is smaller than the strong attraction by a factor of approximately 1013. The
carrier of the weak interaction force is still a matter of considerable research; we will return
to this point later.
The strong interaction manifests itself in its ability to react in very short times. For
example, for a particle which passes an atomic nucleus of about 10-15 m in diameter with
a velocity of approximately 108 m s -1 (i.e. with a kinetic energy of - 50 MeV for a
proton and 0.03 MeV for an electron), the time of strong interaction is about 10 -23 s. This
is about the time of rotation of the atomic nucleus. The weak interaction force requires a
much longer reaction time and explains why leptons such as electrons and photons do not
react with atomic nuclei but do react with the electron cloud of the atom which has a
diameter on the order of 10-10 m. There is sufficient time in passing this larger diameter
for the weak interaction force to be effective.
Scientists have long doubted that all the particles produced with masses between the
electron and the proton (loosely referred to as mesons, i.e. "intermediate'), and with masses
greater than the proton (referred to as baryons, "heavy') really are "elementary'. It was
proposed that they have a substructure or constitute excited states of each other. Are they
waves or particles since they serve as carriers of force. At this point it is important to
understand what is meant by "particle" in nuclear physics.