Atomic and Nuclear Physics
MODES OF RADIOACTIVE DECAY
When an unstable nucleus decays, the resulting daughter nucleus is not necessarily stable. The
nucleus resulting from the decay of a parent is often itself unstable, and will undergo an additional
decay. This is especially common among the larger nuclides.
It is possible to trace the steps of an unstable atom as it goes through multiple decays trying to
achieve stability. The list of the original unstable nuclide, the nuclides that are involved as
intermediate steps in the decay, and the final stable nuclide is known as the decay chain. One
common method for stating the decay chain is to state each of the nuclides involved in the standard
X format. Arrows are used between nuclides to indicate where decays occur, with the type of decay
indicated above the arrow and the half-life below the arrow. The half-life for decay will be
discussed in the next chapter.
Write the decay chains for rubidium-91 and actinium-215. Continue the chains until a stable
nuclide or a nuclide with a half-life greater than 1 x 10 years is reached.
Predicting Type of Decay
Radioactive nuclides tend to decay in a way that results in a daughter nuclide that lies closer to the
line of stability. Due to this, it is possible to predict the type of decay that a nuclide will undergo
based on its location relative to the line of stability on the Chart of the Nuclides.
Figure 9 illustrates the type of decay nuclides in different regions of the chart will typically undergo.
Nuclides that are below and to the right of the line of stability will usually undergo decay.
Nuclides that are above and to the left of the line of stability will usually undergo either decay or
electron capture. Most nuclides that will undergo decay are found in the upper right hand region
of the chart. These are general rules that have many exceptions, especially in the region of the heavy