Physics for Geologists, Second edition

Atomic structure and age-dating 61
There are several different forms of radioactive decay. The summary of
the important ones that follows is extracted largely from Faure (1986) to
which reference should be made for more detailed discussion.
B decay. The prime event is the emission of a B-particle from the nucleus,
along with some neutrinos and often y-rays. The atomic number, Z, is
increased by one, and the neutron number, N, is reduced by one. This
changes the element. For example, potassium becomes calcium with the
emission of a B-particle,
:;K + ;:ca + B-.
This can be interpreted as the transformation of a neutron into a proton
and an electron.
Positron decay. A positron is a positively-charged electron from the
nucleus and when one is emitted the atomic number, 2, is decreased by
one, and the neutron number, N, is increased by one. This changes the
element. For example,
18 + ':o + @+.
This can be interpreted as the transformation of a proton in the nucleus
into a neutron, a positron and a neutrino.
Electron capture. When an electron from the 'cloud' is captured by the
nucleus (usually an electron nearer the nucleus), the nucleus emits a neutrino.
The atomic number, Z, is decreased by one, and the neutron
number, N, is increased by one. This changes the element. For example,
I~K + e- -+ :t~r.
This can be interpreted as a reaction between an electron and a proton
in the nucleus to form a neutron and a neutrino.
Branched decay. Some nuclides decay by more than one process, such
as ;K to both ; i~r and ::ca, which will be discussed later.
a decay. The em~ssion of a-particles from atoms having atomic numbers
(2) greater than 58, and from a few with small atomic numbers. This
reduces both the atomic number and the neutron number by two and so
changes the element. For example,
238 S2U -+ 2i;f~h + ; ~e.
Copyright 2002 by Richard E. Chapman
Ionization is a process that converts neutral atoms or molecules to
charged atoms or molecules. It has slightly different meanings in physics
and chemistry. Ionization can occur by collision, ejecting an electron
from a neutral atom if the energy is sufficient. It may occur at very high
temperatures, if the temperature is high enough to excite the atoms suf-
ficiently to part with an electron. In aqueous solutions, the molecules
dissociate into cations (+) and anions (-), which can be manipulated
by electric currents.