Symmetry effect on the sum rule for bremsstrahlung cross-section

The effect of symmetry of the space co-ordinates of the nucleons of a nucleus on the integrated bremsstrahlung-weighted cross-section for electric dipole absorption σ_b has been investigated. It is found that the relationship of σ_b to the mean square radius of the nucleus is really model dependent. For a system heavier than Is nuclei, for example,⁶Li it is shown, in principle, that its σ_b is directly proportional to the mean square radius of⁶Li in a simple shell model picture which was believed to be contrary thus far. For a modified shell model wave function σ_b for⁶Li has been calculated and found to be close to the experimental value. The correction due to the centre-of-mass motion brings the r.m.s. radius of⁶Li in exact agreement to the value observed by electron scattering.