Unstable to beta decay
Unstable to electron capture decay

ODD MASS BETA STABILITY


ODD ATOMIC WEIGHT ISOBARS and BETA / INVERSE BETA STABILITY
The atomic masses of isobars having an odd atomic weight lie on a parabola, with just one isobar at the mass minimum. Isobars on the right of this minimum can decay by beta decay, those on the left by inverse beta decay, always to an isobar with lower atomic mass. The decays occur singly, one isobar decaying to its next-nearest isobar with lower atomic mass.

The graph at the bottom of the Segre chart displays the masses of seven isobars (nuclides with equal mass, but of different elements) relative to that of the selected nuclide (shown in white in the middle). This is part of a diagonal section of the Segre chart, and shows the stability against beta- or inverse beta-decay. The difference in mass of the surrounding nuclides is readily seen. Nuclides with lower relative mass are depicted in green, while those with higher are shown in blue, together with the mass difference (measured in MeV). Nuclides with higher mass can decay into those with lower mass by emitting an electron (Beta decay) or a positron or electron capture (inverse beta decay/electron capture), releasing most of the difference in mass as kinetic energy. Generally, the greater the mass decrease, the shorter the half-life.


RELATIVE MASS of A=141 ISOBARS
Fig 1 shows a cross section of isobars of atomic weight 141, with cerium-141 decaying into the stable (lowest mass isobar) praseodymium-141 by beta decay, releasing a total energy of 580keV.


RELATIVE MASS of A=83 ISOBARS
Fig 2 shows a cross section of isobars of atomic weight 83, with rubidium-83 decaying into the stable (lowest mass isobar) krypton-83 by inverse beta decay, releasing 900keV of energy.