Purity of analog spin in isobaric analog resonances

It is shown that the position of the pole of the inverse averageS-matrix of an isolated isobaric analog resonance (IAR) is a very useful parameter, which allows one to distinguish between the weak and the strong coupling case of the finestructure distribution of the resonance. It gives also a direct measure of the purity of analog spin of the resonance. As an example the lowest IAR in²⁰⁸Bi is discussed.