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

The behaviour 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 Tsub(1/2) (26.2 keV) = 0.85 +- 0.10 ns, Tsub(1/2) (145.5 keV) <= 0.3 ns. and Tsub(1/2) (195.5 keV) = 0.20 +- 0.08 ns. In the decay of 133La, the lifetime of the 12.3 keV state in 133Ba has been measured as Tsub(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)sub(exp), possible conclusions on the structure of the ground state in 131La are drawn. All the available data on l-forbidden M1 transitions n(2dsub(3/2) reversible 3ssub(1/2)) and p(1gsub(7/2) reversible 2dsub(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. (orig.)