Isobars are nuclides with the same atomic mass number A (= Z+N) but with different values of N and Z e.g. 14B, 14C, 14N. An isobaric transition is a nuclear transformation between two isobars, i.e., a transformation that does not change the mass number A. Three types of isobaric transitions are of interest: (1) beta decay, (2) positron decay, and (3) electron capture.
In nuclear stability, the neutron-proton ratio (N/P) is crucial. If it is too low or too high, the nucleus will eventually decay into a more stable configuration. Beta radiation, which is the emission of energetic electrons, results when an N/P ratio is too high for stability; positron emission or electron capture occurs when it is too low for stability. These two conditions are represented by specific areas of the nuclide chart. Beta emitters are above the stable nuclides, and positron emitters and electron capture nuclides are below.
(text contributions by Helmut Paul, 2014)