Nuclear Isomer - Nucleus

Nucleus

The nucleus of a nuclear isomer occupies a higher energy state than the corresponding non-excited nucleus, which exists in the lowest energy state, called the ground state. In an excited state, one or more of the protons or neutrons in a nucleus occupy a nuclear orbital of higher energy than an available nuclear orbital of lower energy. These states are analogous to excited states of electrons in atoms.

Excited atomic states decay by fluorescence which usually involves emission of light near the visible range. However, because of the much higher binding energies involved in nuclear processes, most nuclear excited states decay instead by gamma ray emission. For example, a well-known nuclear isomer used in medical procedure is technetium-99m, which decays with a half-life of about 6 hours, by emitting a gamma ray of 140 kiloelectron-volts of energy (this is close to the energy of medical diagnostic X-rays).

Nuclear isomers owe their long half lives to the fact that their gamma decay is (relatively, not absolutely) forbidden due to a large change in nuclear spin needed to emit a gamma. For example, Ta-180m has a spin of -9 and must gamma decay to Ta-180 with a spin of +1. Similarly, Tc-99m has a spin of +1/2 and must gamma decay to Tc-99 with a spin of +9/2.