Production cross sections from $^{82}$Se fragmentation as indications of shell effects close to the neutron drip-line

Production cross sections for neutron-rich nuclei from the fragmentation of a $^{82}$Se beam at 139 MeV/u with beryllium and tungsten targets have been measured for a large number of nuclei. The nuclides $^{64}$Ti,$^{67}$V,$^{69}$Cr,$^{72}$Mn, the most neutron-rich isotopes of the elements $22\le Z\le 25$, have been observed for the first time. The measured cross sections were used to search for trends in the structure of nuclei around $^{54}$Ca and were compared with Abrasion-Ablation calculations under the assumption of various mass models. The results confirm our previous investigations from a similar measurement using a $^{76}$Ge beam and can be explained with a modified GXPF1B Hamiltonian where the energy of the $f_{5/2}$ orbit is lowered by 0.5 MeV for neutron-rich isotopes around $Z$=20. The subshell gap at $N$=34 is reduced compared to the unmodified Hamiltonian.