The behavior of l-forbidden M1 transitions near Z = 50, N = 82

The behavior of l-forbidden M1 transitions around Z = 50 and N = 82 is investigated. In the decay of ¹³¹Ce, lifetimes of excited states in ¹³¹La have been measured for the first time as T _{1 2}(26.2 keV) = 0.85 ± 0.10 ns, T _{1 2} (145.5 keV) ≦ 0.3 ns, and T _{1 2} (195.5 keV = 0.08 ns. In the decay of ¹³³La, the lifetime of the 12.3 keV state in ¹³³Ba has been measured as T _{1 2} (12.3 keV) = 7.0 ± 0.3 ns in contradiction to a recently published result. From the value B(M1, 5 2 → 3 2)_exp, possible conclusions on the structure of the ground state in ¹³¹La are drawn. All the available data on l-forbidden M1 transitions n(2d _{3 2} ↔ 3s _{1 2}) and p(1g _{7 2} ↔ 2d _{5 2}) in nuclei with 100 < A < 150 are systematically considered. Their remarkable sensitivity towards filling of the jj closed shell N = 82 is discussed in terms of quadrupole deformations. This approach is supported by the systematics of rotational B(M1) values in deformed nuclei (150 < A < 190) found to increase appreciably with increasing quadrupole deformation.