EXCESS SPIN NUCLEI


Nuclei in excited states with higher spin than their ground state spin are called nuclear isomers. These isomers are radioactive emitting gamma radiation as they decay back into their ground state spin. They are metastable. There are two very different ways nuclei can possess spin. Only non-spherical nuclei (usually prolate spheroids) can have collective rotation, where the nucleus rotates as one. These nuclei have fast transitions (short halflives) back to the ground state. Alternatively, several nucleons of spherical or prolate nuclei can orbit the nucleus independently in a non-collective rotation. In these nuclei, it is less likely that all the independent nucleons will lose spin simultaneously; they are trapped into high spin states by each other, so they have much longer halflives. In any nucleus, a mixture of the two can exist.

If, in decaying, the emitted gamma ray has to carry away several units of angular momentum simultaneously, the probability of decay is low. An excess energy is associated with the excess spin. The lower the energy difference and the higher the spin difference between the excited state and the ground state, the longer will be its halflife. Nuclei with an odd number of both protons and neutrons(odd/odd nuclei) can possess high excess spin states near the ground state, satisfying the criteria for long halflives. It is possible that the high-spin isomer will have a longer halflife than the ground state, if that is radioactive: a halflife difference of 108 fold has been noted in polonium-212 and its isomer (300ns for the ground state; 45s for the metastable state).

Nuclei with even-Z and even-N (those with a whole number of He-4 nuclei) can have such high excess rotational spin that the nucleus deforms into a long whirling string of alpha particles. This happens in very high-spin states of carbon-12, oxygen-16, neon-20 and magnesium-24. Others may be found. Here the nuclei are on the verge of flying apart (fission).

See Isomeric Transition.

OBSERVATIONS On Integral/Half-Integral Spins
The nature of the spin, integral or half-integral, remains un-changed throughout the decay chain. i.e. The decay chains of integral spin nuclides are all integral spin, and those of half-integral spin are all of half-integral spin.