In nuclear physics and radioactivity, isodiaphers refers to nuclides which have different atomic numbers and mass numbers but the same neutron excess, which is the difference between numbers of neutrons and protons in the nucleus. For example, for both 234
90Th and 238
92U the difference between the neutron number (N) and proton number (Z) is N − Z = 54.
One large family of isodiaphers has zero neutron excess, N = Z. It contains many primordial isotopes of elements up to calcium. It includes ubiquitous 12
8O, and 14
The daughter nuclide of an alpha decay is an isodiapher of the original nucleus. Similarly, beta decays (and other weak-force-involving decays) produce isobars.