Abstract
Electric monopole transitions in
172Yb and
174Yb have been studied following thermal neutron capture on
171Yb and
173Yb. It is found that the second excited 0
+ state in
172Yb at 1405 keV has a very large
X =
B(
E0; 0
2
+−0
g
+)
B(
E2; 0
2
+−2
g
+
value of 2.81±0.31. The E0 decay branches for the other 0
+ states in
172Yb were also measured. In the case of
174Yb 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
174Yb exhibit such a large
X-value. Various interpretations are discussed and it is concluded that the 1405 keV level in
172Yb 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
172Yb as well as for the systematics of large
X-values in the rare-earth region.