Atomic and Nuclear Physics
MODES OF RADIOACTIVE DECAY
The important information in this chapter is summarized below.
Modes of Radioactive Decay Summary
Alpha decay is the emission of an alpha particle (2 protons and 2 neutrons) from
an unstable nucleus. The daughter nuclide has an atomic number 2 less than the
parent nuclide and a mass number 4 less than the parent nuclide. The daughter
nucleus commonly releases its excitation energy by gamma emission.
Beta-minus decay effectively converts a neutron to a proton and an electron,
which is immediately ejected from the nucleus. The daughter nuclide has its
atomic number increased by 1 and the same mass number compared to the
Beta-plus decay effectively converts a proton to a neutron and a positron, which
is immediately ejected from the nucleus. The daughter nuclide has its atomic
number decreased by 1 and the same mass number compared to the parent.
In electron capture, the nucleus absorbs an electron from the innermost orbit.
This electron combines with a proton to form a neutron.
Internal conversion occurs when a gamma ray, emitted by the nucleus as it goes
from the excited state to the ground state, interacts with one of the innermost
electrons of the same atom. The electron is ejected from the atom.
An isomeric transition is the decay of an excited nucleus to a lower-energy level
by the emission of a gamma ray.
Decay chains can be found by tracing the steps an unstable atom goes through
as it tries to achieve stability.
Many modes of radioactive decay result in a daughter nuclide that has an energy
level above the ground state.
This excitation energy is usually released
immediately in the form of a gamma ray.
The type of decay that a nuclide will typically undergo can be determined by its
relationship to the line of stability on the Chart of the Nuclides. Nuclides that
lie below and to the right of the line of stability will typically beta minus decay.
Nuclides above and to the left of the line will typically either beta plus decay or
electron capture. Most alpha emitters are found in the upper, right-hand corner
of the chart.