Optical isotope shifts and nuclear deformation in dysprosium

Abstract

The isotope shift in six Dy I lines has been investigated by means of the technique of exchanging lightsources containing enriched isotope samples and a recording Fabry-Perot spectrometer with a digital output. The analysis of output data is made by means of an electronic computer. The results prove that the isotope shift is a mixture of mass and field effect, and that the minimum value of the specific mass effect is about 6 times the general mass effect. Assuming this minimum value as the most probable one, the relative field shift is obtained. This is interpreted as a combined volume and quadrupole shift and compared with nuclear data. The results are discussed in connection with the collective model of the nucleus taking account of nuclear deformation. A fairly consistent picture is obtained, both with respect to the irregularities in the relative field shift and the spins and parities of the odd A nuclei. The values of the nuclear deformation parameter are lower than those known from rotational level structures. Evidence is presented that the transition from the single particle model to the collective model, usually considered to take place when the 45th pair of neutrons enters, may be slightly Z-dependent so as to occur between N = 90 and N = 92 in dysprosium.