Abstract
A method is developed for calculating the change in the mean square radius of the nuclear charge distribution due to the addition and rearrangement of neutrons. A single particle model is used and the effect of long range correlations is included through BCS theory, particular account being taken of the blocking effect of the odd particle on the pairing wavefunctions in the case of an odd nucleus. Most reasonable functional forms (both analytical and numerical) that have been suggested for the neutron-proton interaction can be used with the model by expressing them as a sum of Yukawa functions. In this work isotope shifts for Sn, Ba, Hg and Pb isotopes are calculated with several different forces. Density independent forces give shifts of the wrong sign, but it is found that even with a very simple density dependent force shifts of the correct sign and magnitude can be predicted. With this force, too, odd-even staggering of the shifts is predicted and some measure of agreement with experiment achieved. Isomer shifts, calculated for Hg, show good agreement with experiment.