At the bottom of the valley
In the nuclei family with 127 nucleons, only iodine-127 nucleus is stable. If we represent the nuclei mass energy (mc2) of this family, the curve of the masses has the appearance of a valley. The number of protons increases from left to right, 49 (indium) to 57 (lanthanum), while the number of neutrons decreases from 78 to 70. The energy of iodine-127 being taken as 0, the energy differences are expressed in million of electron volts. Nuclei on the left flank of the valley, in excess of neutrons regain stability through a series of beta-minus decays, those from the right side (in excess protons) by a series of beta-plus decays.
IN2P3