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

Abstract The behavior of l -forbidden M1 transitions around Z = 50 and N = 82 is investigated. In the decay of 131 Ce, lifetimes of excited states in 131 La 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 133 La, the lifetime of the 12.3 keV state in 133 Ba 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( M 1, 5 2 → 3 2 ) exp , possible conclusions on the structure of the ground state in 131 La are drawn. All the available data on l -forbidden M1 transitions n (2 d 3 2 ↔ 3 s 1 2 ) and p (1 g 7 2 ↔ 2 d 5 2 ) in nuclei with 100 A 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