Electric monopole transitions from excited Kπ = 0+ levels in 172Yb and 174Yb

Abstract

Electric monopole transitions in 172 Yb and 174 Yb have been studied following thermal neutron capture on 171 Yb and 173 Yb. It is found that the second excited 0 + state in 172 Yb at 1405 keV has a very large X = B( E 0; 0 2 + −0 g + ) B( E 2; 0 2 + −2 g + value of 2.81±0.31. The E0 decay branches for the other 0 + states in 172 Yb were also measured. In the case of 174 Yb no E0 conversion lines were observed from the corresponding 0 + states but upper limits were placed on their intensities. These results indicate that none of the known 0 + states in 174 Yb exhibit such a large X -value. Various interpretations are discussed and it is concluded that the 1405 keV level in 172 Yb may contain large amplitudes of proton pair excitations. A model based on such excitations is presented which accounts qualitatively for the large X (1405) value in 172 Yb as well as for the systematics of large X -values in the rare-earth region.