Secondary γ Transitions in 159Gd After Neutron Capture at Isolated Resonances

The ¹⁵⁸Gd(n,γ)¹⁵⁹Gd reaction was studied at 12 isolated neutron resonances by the TOF method at the IBR-30 Fast Pulse Reactor at JINR Dubna. Totally 15 secondary γ transitions in ¹⁵⁹Gd were recorded in the range from 450 keV to 750 keV. Of these, six previously unseen transitions were placed on the established ¹⁵⁹Gd level scheme. The depopulation of strongly populated levels at 507.7 keV and 558.2 keV (the head and the first excited members of band 1/2⁻ [521]) was observed for the first time. It was shown that the observed 507.7 keV γ line, masked by the annihilation peak, originates from an unresolved doublet of transitions from the 507.7 keV level to the ground state and from the 558.2 keV level to the level at 50.7 keV. The 507.7 keV level decays exclusively to the ground state, while the 558.2 keV level decays via two transitions with a branching ratio that agrees well with the prediction according to Alaga’s rule.