Decay spectroscopy of $^{257,258}$Db in the vicinity of the N=152 deformed shell gap*

Valuable information on the existence and the position of the island of superheavy stability can be obtained by studying the structure of elements in the transfermium region (Z>100). Of particular interest are the isotopes around the deformed shell gaps of N=152 and N=162, where some of the single particle states relevant for the opening of these gaps are also relevant for the spherical shell closure in the superheavy region at N= 172 or N=184. In our study we have investigated $^{257}$Db (N=152) and $^{258}$Db. We confirmed that $^{257}$Db has at least two states with different half-lives decaying by $\alpha$ particle emission into different states of $^{253}$Lr, which further decays by $\alpha$ emission with different half-lives. Indication of two states with different half-lives for $\alpha$ decay in $^{258}$Db was confirmed. Additional information on the $\gamma$-ray decay of excited levels in $^{250}$Md was gathered and a tentative level scheme was proposed. The electron capture branch of $^{258}$Db was measured directly, and the $\alpha$ decay of $^{258}$Rf was confirmed. Yet, the branching ratio is significantly lower compared to the previously published value.