MODES OF RADIOACTIVE DECAY
Atomic and Nuclear Physics
Alpha Decay ( )
Alpha decay is the emission of alpha particles (helium nuclei) which may be represented as either
He or . When an unstable nucleus ejects an alpha particle, the atomic number is reduced by 2
and the mass number decreased by 4. An example is uranium-234 which decays by the ejection
of an alpha particle accompanied by the emission of a 0.068 MeV gamma.
The combined kinetic energy of the daughter nucleus (Thorium-230) and the particle is designated
as KE. The sum of the KE and the gamma energy is equal to the difference in mass between the
original nucleus (Uranium-234) and the final particles (equivalent to the binding energy released,
since m = BE). The alpha particle will carry off as much as 98% of the kinetic energy and, in most
cases, can be considered to carry off all the kinetic energy.
Beta Decay ( )
Beta decay is the emission of electrons of nuclear rather than orbital origin. These particles are
electrons that have been expelled by excited nuclei and may have a charge of either sign.
If both energy and momentum are to be conserved, a third type of particle, the neutrino, , must be
involved. The neutrino is associated with positive electron emission, and its antiparticle, the
, is emitted with a negative electron. These uncharged particles have only the
weakest interaction with matter, no mass, and travel at the speed of light. For all practical purposes,
they pass through all materials with so few interactions that the energy they possess cannot be
recovered. The neutrinos and antineutrinos are included here only because they carry a portion of
the kinetic energy that would otherwise belong to the beta particle, and therefore, must be considered
for energy and momentum to be conserved. They are normally ignored since they are not significant
in the context of nuclear reactor applications.
Negative electron emission, represented as e, , or simply as e or , effectively converts a neutron
to a proton, thus increasing the atomic number by one and leaving the mass number unchanged.
This is a common mode of decay for nuclei with an excess of neutrons, such as fission fragments
below and to the right of the neutron-proton stability curve (refer to Figure 6). An example of a
typical beta minus-decay reaction is shown below.