Isospin sum rules for photonuclear reactions are derived; they relate the strengths of the two isospin components of the giant resonance to the isoscalar, isovector, and isotensor radii of the nucleus. The connection between these radii and both the number and correlations of excess neutrons is discussed. A semiempirical formula for the fraction of the dipole strength in the $T+1\mathrm{}$ giant resonance is derived and various experimental data are discussed in the light of the results obtained using this equation.

• Received 10 November 1971

DOI:https://doi.org/10.1103/PhysRevC.5.846