Forbidden decay modes

Fig.7.9. Experimental arrangement of Marrus and Schmieder (1972) for the investigation of the forbidden decay modes of one- and two-electron ions of high Z.

Motivated by the observed decay rate discrepancy between QED theory and experiment for At, numerous searches have been performed for forbidden, small or exotic decay modes. An exotic decay branch, besides o-Ps —> 37, with roughly 10-3 branching ratio could be causing the higher decay rate and is given by A 0bs = + A exotic- Many candidate decay branches have been proposed in... 

Finally the high Ps rate could be used to improve on the search for rare or forbidden decays of ground state Ps. Table II summarizes the present values of such decay modes and theoretical predictions. 

A decay momentum p is given for each decay mode. For a 2-body decay, p is the momentum of each decay product in the rest frame of the decaying particle. For a 3-or-more-body decay, p is the largest momentum any of the products can have in this frame. For any resonance, the nominal mass is used in calculating p. A dagger ( t ) in this column indicates that the mode is forbidden when the nominal masses of resonances are used, but is in fact allowed due to the nonzero widths of the resonances. 

Positronium, being a readily available purely leptonic system and also a particle-antiparticle pair, has attracted considerable experimental interest over the years as a testing ground for the existence of exotic particles or couplings. The latter may perhaps manifest themselves in the decay properties of positronium, so that attempts have been made to observe forbidden modes. In particular, the longstanding discrepancy between the Michigan experimental value for oAo-ps and the results from QED calculations, described in subsection 7.1.1, has acted as a spur to such investigations. 

For certain nuclides, different physical properties (half-lives, mode of decay) are observed. They are due to different energetic states, the ground state and one or more metastable excited states of the same nuclide. These different states are called isomers or nuclear isomers. Because the transition from the metastable excited states to the ground states is forbidden , they have their own half-lives, which vary between some milliseconds and many years. The excited states (isomers) either change to the ground state by emission of a y-ray photon (isomeric transition IT) or transmutation to other nuclides by emission of cc or particles. Metastable excited states (isomers) are characterized by the suffix m behind the mass number A, for instance Co and Co. Sometimes the ground state is indicated by the suffix g. About 400 nuclides are known to exist in metastable states. 

In the case of the 6s ions the SO interaction is so strong that the emission can be interpreted in terms of the SO-split levels Pi and Po. The vibrational progression in the emission band, if present, is always due to coupling with the. symmetric r>i mode. At low temperatures the decay time becomes very long (ms range), since the Pq - So emission is strongly forbidden. At higher temperatures the P level becomes thermally occupied and the decay time becomes much faster. This runs parallel with the discussion above on the Eu ion (see Fig. 3,16). 

For neutron-deficient nuclides both modes of decay, i.e., emission and electron capture, are possible if the Qg value is higher than 1.02 MeV. Calculated ratios for the probabilities of the two modes of decay are published in [48, 49]. The dependence of these ratios on the maximum energy for allowed transitions (Al = 0 or 1, no parity change) and first forbidden nonunique transitions (Al = 0 or 1, parity change) is shown in Fig. 1-9, p. 17. 

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