The surprising absence of the $N=34$ subshell closure in the exotic nucleus ${}^{56}\mathrm{Ti}$ is highlighted by examining the experimental single neutron energies along the $N=28$ isotopes, which are extracted from previously existing data on single-neutron transfer reactions on stable isotones and nuclear masses. The results point out the role of the monopole effect of the tensor force on the $f_{7/2}\text{−}{f}_{5/2}$ splitting and confirm the prediction of the reduction of this splitting in the $Z>20$ isotones. In addition, the importance of making similar measurements of the distribution of single neutron and single neutron hole strength in the exotic $Z<20\hspace{0.5em}N=28$ isotones is discussed. Measurements of the ($d,p$) reaction and the neutron knockout reaction on the exotic nucleus ${}^{46}\mathrm{Ar}$ have provided proof of principle for the necessary reactions, and advances presently taking place in detector technology will soon provide the tools necessary for reliably mapping the distribution of neutron strength in detail.

• Received 14 July 2008

DOI:https://doi.org/10.1103/PhysRevC.78.037304