Lifetime, gamma states

Eig. 22 a and b. Determination of lifetime of states of O by recoil-distance method (Devons et al. [2d]), (a) Apparatus for pair emitting and gamma emitting states, (b) Results for 6.13 MeV level. 

Fig. 23 a and b. Determination of lifetime of gamma emitting state of 0 by recoil-DoppLER shift method (Devons... 

The PAC experiment makes use of a nucleus which decays through successive gamma radiations yj and yj- The observation of yj in a fixed direction determines the spin orientation of the nucleus and the subsequent radiation j2 then has a definite angular correlation with respect to the direction of yj. If in the intermediate state of the gamma-ray cascade the nucleus is under the influence of a magnetic hyperfine interaction, the second radiation y2 carries with it information on this interaction. Hence the relaxation of the environment within the lifetime of the intermediate state of the nucleus manifests itself in a time-dependent magnetic hyperfine interaction. 

Gamma (y) rays are photons deriving from isomeric transitions. Isomeric transitions occur when a nucleus remains in an excited state after a particle emission or a decay by electron capture. These intermediate levels are referred to as isomeric states (or metastable states), and each decays to a lower state (either the ground state or another intermediate state) with lifetimes from picoseconds to years. Gamma ray emissions are characteristic of the radionuclide, and the energies of the emitted photons depend on the energy differences between the initial excited state and the next one. 

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