Isodiaphor

Isodiaphers are nuclides with the same excess of neutrons . A distinction is made in the nuclide map

• Isotopes (same number of protons with variable number of neutrons), ${\displaystyle Z}$
• Isotons (constant number of neutrons with variable number of protons), ${\displaystyle N}$
• Isobars (constant mass number ) ${\displaystyle A}$
• Isodiaphere (nuclides with the same excess of neutrons ) ${\displaystyle N_{1}-Z_{1}=N_{2}-Z_{2}}$

In the nuclide map in the most used representation according to Segrè , i.e. H. Increasing upwards and increasing to the right, these four related nuclide relationships are on top ${\displaystyle Z}$ ${\displaystyle N}$

• horizontal lines (isotopes)
• vertical lines (isotones)
• Bisector with negative slope (isobar)
• Bisector with positive slope (isodiaphers)

The starting and ending nuclei in alpha decay are isodiaphic nuclei, for example:

${\displaystyle {}_{\ 62}^{146}\mathrm {Sm} \to {}_{\ 60}^{142}\mathrm {Nd} +{}_{2}^{4}\mathrm {He} }$.

While lighter, stable nuclides (few nucleons in the nucleus) each have a ratio of around 1: 1 (protons: neutrons), the electrostatic repulsion of the protons in the nucleus requires an excess of neutrons that increases with the number of protons (kink of the valley of stability in the nuclide map below or right for higher nuclide masses). The neutron surplus is thus an important stability criterion for nuclides (in addition to other parameters taken into account in the semiempirical Weizsäcker formula; see " droplet model of the atomic nucleus ").