Projectile fragmentation as a tool to observe shell effects close to the neutron drip-line

In recent experiments, production cross sections for a large number of neutron-rich nuclei produced from the fragmentation of $^{76}$Ge (130 MeV/u) and $^{82}$Se (139 MeV/u) beams were measured, including 21 new isotopes of the elements 17 $\le \quad Z \quad \le $ 26. Enhanced cross sections of several new nuclei were compared to a thermal evaporation framework, and also have been well reproduced by the LISE$^{++}$ Abrasion-Ablation model with masses derived from the full pf shell-model space with the GXPF1B5 effective interaction. It seems that the systematic trends in the production cross sections demonstrate changes in the nuclear mass surface, that can be explained with a shell model that predicts a subshell closure at $N=$34 around $Z=$20. In this contribution it will be discussed why projectile fragmentation is so sensitive to changes in the nuclear mass surface close to the neutron drip-line, and a new dBE production cross section systematics will be presented.