XXXVII. Isotopic spin selection rules. VII: Breakdown of the rules and the situation in^l6O

Abstract

Abstract Factors governing the breakdown of the isotopic spin selection rules are discussed. Two energy ranges exist within which we expect the rules to be obeyed; at low excitation individual excited states are well separated in energy and have good isotopic spin purity so that the rules are obeyed in reactions passing through such states only; at high excitation individual states no longer have a well-defined isotopic spin but the rules are obeyed because many overlapping levels are simultaneously excited in a total state of initially well-defined isotopic spin which breaks up before the Coulomb forces have had time to cause much isotopic spin mixing. In between these regions where the rules are obeyed (although for very different reasons) severe breakdown is to be expected. The approximate extent of these reasons is discussed. The experimental situation as regards the operation of the rules and other consequences of charge independence is examined in l6O where we have knowledge relating to the effective isotopic spin purity of ten energy levels or energy ranges between 6 and 30 MeV excitation. Good correspondence with theoretical expectation is found in the above-described general trend of effective impurity versus excitation. It is concluded that the situation is adequately described by the Coulomb perturbation of an otherwise charge independent (symmetric) Hamiltonian.