Photodisintegration of the Lightest Nuclei

It is shown that for nuclei in which the ground state wave function is symmetric in the space coordinates of all the nucleons, such as ${\mathrm{H}}^2$, ${\mathrm{H}}^3$, ${\mathrm{He}}^3$, ${\mathrm{He}}^4$, the integrated bremsstrahlung-weighted cross section for electric dipole absorption is simply related to the mean-square radius of the nucleus, independent of the existence of correlations between the motions of the nucleons. This relationship was first derived by Levinger and Bethe on the assumption of absence of correlation, and was found to be at variance with known experimental results in heavy nuclei. Experimentally, the relationship is well verified by data on ${\mathrm{H}}^2$ and ${\mathrm{He}}^4$. The relationship of the integrated bremsstrahlung-weighted cross section to certain nuclear parameters in the case of ${\mathrm{Li}}^6$ is briefly discussed.