Testing the standard model of nuclear physics Available to Purchase

Shell model calculations using optimized effective interactions are becoming the standard model of nuclear structure physics. While earlier effective interactions were optimized for nuclei near the valley of stability, radioactive beam facilities have allowed the study of nuclei much farther from stability. These results suggest that with increasing proton-neutron imbalance significant modifications are needed to the interactions. For example, evidence is mounting that the filling of proton shells modifies the neutron shell structure and vice-versa. The s-d shell provides an excellent testing ground for confronting theory with experiment where intruder structures from higher orbitals have already been recognized, especially in the region where N ~ 2*Z called the “Island of Inversion.” The approach to this region is critical to understanding how the shell structure changes towards inversion. The nucleus ³⁴P with $Z = 15$ and $N = 19$ is a key one along this approach. The known level scheme of ³⁴P was doubled in a recent experimental study using the GAMMASPHERE gamma detector array with the reaction ¹⁸O + ¹⁸O at 24 MeV using the ATLAS accelerator facility at Argonne National Laboratory. Comparison of the observed nuclear structure with shell model calculations using both the WBP-a and SDPF-NR interactions shows relatively good agreement for the pure 1s-0d shell states and for those involving the promotion of one particle to the 0f_7/2 and 1p_3/2 orbitals. However, both calculations badly overpredict the energies of states involving two or more particles in the f-p shell. It remains to be seen whether adjustments to these interactions can substantially improve the agreement for the latter states while not worsening the fit to the states with fewer intruders.