Superallowed transitions

The decay of the neutron into the proton is an important example of decay between mirror nuclei. In the (3 decay of mirror nuclei, the transformed nucleons (neutron —> proton or proton neutron) must be in the same shell and have very similar wave functions. This gives rise to a large matrix element Mif 2 and a very small log ft value. For the (3 decay of mirror nuclei to their partners, log ft values are about 3, which is unusually small. Such transitions are called superallowed transitions. 

Any charge independent theory would predict superallowed transitions between the states of a charge multiplet, but the fact that they are also found between the states of a supermultiplet (see Shap. Eli and EIII) is strong evidence for the Wigner classification. The observation of such superallowed transitions, which are discussed by Bolsterli and Feenberg [46], sometimes permits identification of analogous states in isobaric nuclei. 

In CF the superallowed transition of 1.6 sec half-life is the ground state decay. There is an isomeric state at 142 keV which was for some time thought to be the ground state, but which actually feeds it by an M3 transition whose internal conversion coefficient has been measured. This means that the ground state of Cl has spin 0 and isotopic spin 1 while the 142 keV level has spin y and isotopic spin zero. This level also decays by allowed positron emission to... 

A strong overlap of initial and final wave functions is expected for mirror nuclei, in which the mass numbers are identical, but the atomic number in one nucleus is equal to the neutron number of the other. The large overlap integral results in a very low fi value (superallowed transitions). Most of the known P emitters decay by allowed or superallowed transitions. Among the light nuclides there are many mirror pairs. 

Undoubtedly the Chalk River ISOL will be used, as others are, in the identification and spectroscopy of exotic nuclei. The nuclear chart in figure 5 illustrates the scope for such studies. However, the extreme purity of isotopes separated by our ISOL has been essential in the past to precision studies of the weak interaction, in one case the lifetimes of superallowed 0+ 0+ transitions [K.OS83], in another 8-v-a triple correlation coefficients in the decay of 2( Na [CLI83] both yielded measurements of the weak vector coupling constant. These types of measurements will be extended to other nuclei, since they exploit the best qualities of the accelerator and separator. 

The separation of states of different T implies that one of a set of isobaric nuclei is stable and the others unstable against beta decay. The Wigner theory predicts superallowed decay between the states of a given supermultiplet because no change of spatial wave function is needed. This is found (a) for the positron decay of odd mirror nuclei (b) for transitions between the low states of nuclei with mass number 4 + 2 in which both T= and P = 0 states are found in the (1,0,0) supermultiplet. 

The beta transition is superallowed and the positron energy agrees... 

The beta transition between the isobars has the superallowed character. The lower levels of the two nuclei show similar structure and spins, although the first... 

The beta transitions are both simple that of is of very low energy, so that the excitation of the first T = state in CP is at an energy approximately equal to the isobaric correction. The spin of the radioactive nucleus has been measured and is found to be f, in agreement with the // prediction for d l. The magnetic moment of CP agrees better with the prediction from this same configuration than with the Schmidt value for d /. The S ->CP decay is allowed, but not superallowed since these nuclei are not a true mirror pair. 

The decay of He to the ground state of Li is superallowed and is the most important evidence for the Gamow-Teller type of coupling in beta-decay. The Fermi allowed decay which would be expected from He to its isobar state at 3.57 MeV in Li is just energetically impossible. No trace of gamma radiation in the He decay has been found, nor has any transition to the 2.189 MeV level (/ = 3 ) been seen. 

The beta decay of is a simple allowed (unfavoured) transition to the 2 excited state of Ne o at I.63 MeV again it is not clear why the equivalent ground state transition does not take place, but the reason probably lies in the particular configurations. Thus if the F o and Ne states were described by a particularly complex set of configurations while the Ne ground state was simply s, the results might be explained. The Na o decay may contain a weak superallowed component to the first T — i level of Ne . 

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