Isotope Shift Anomalies and Nuclear Structure

Abstract

When observed isotope shifts in the electronic spectra of the heavy elements have been corrected for electronic structure, the resulting shifts give new insight into nuclear structure. In comparison with the shifts predicted for constant density spherical nuclei, the reduced observed shifts are anomalous (1) in magnitude, being on the average too small by a factor 0.5, (2) in trend with neutron number, showing an oscillation in magnitude, and (3) in the staggering of shifts between even and odd isotopes. The present work shows that the anomaly in magnitude largely disappears when account is taken of the compressibility and polarizability of nuclear matter, as a result of which the nuclear radius is found to vary less rapidly with change in neutron number than is predicted by the average ${A}^{\frac{1}{3}}$ law. The analysis of experimental data also suggests that nuclear particle density variations are slightly greater than indicated by earlier theoretical estimates. Likewise, the oscillatory trend in isotope shifts ceases to appear anomalous when associated with the regular progression in nuclear shapes as derived from configuration theory, from the analysis of quadrupole moments and the first excited states of even-even nuclei, and from other sources. The general size of the amplitude and the nodal positions of the shift variation are consistent with expectations. Finally, the even-odd staggering in isotope shifts is reasonably to be connected with the staggering to be expected in the progression of nuclear deformations through odd and even isotopes. Thus the over-all pattern of isotope shifts appears to fit together with present ideas and to provide new specific information on nuclear structure.