The evolution of shell closures in the region of Z ≤ 20 and N ≥ 28 is a topic of particular interest in the field of nuclear structure. It has been observed that the standard magic number N = 28 weakens in this region, and shell gaps begin to emerge at N = 32, 34. As such, there is considerable impetus for experimental studies of the cross-shell 𝜋sd ⊗ 𝜈fp interactions, which are expected to influence this shell evolution in proton-deficient N = 28 isotones. The isotope ⁴⁸K is uniquely situated for this purpose, as it is both one proton below Z = 20, and one neutron above N = 28. Additionally, the neighbouring ⁴⁷K isotope is known to be primarily π(s_1/2¹ d_3/2⁴) structure in its ground state – as opposed to the naïvely expected π(s_1/2² d_3/2³) structure, which is dominant in potassium isotopes – allowing the exotic πs_1/2 ⊗ νfp interaction to be probed by the selective neutron transfer reaction, ⁴⁷K(d,p)⁴⁸K.

The first experimental study of states arising from the interaction between πs_1/2 and the orbitals νp_3/2, νp_1/2 and νf_5/2 has been conducted, by way of the ⁴⁷K(d,p) reaction in inverse kinematics. The radioactive beam of ⁴⁷K from the GANIL-SPIRAL1+ facility had a beam energy of 7.7 MeV/nucleon and was of exceptional quality, as it had a typical intensity of 5×10⁵ particles per second and was estimated to be > 99.99% pure. The beam impinged upon a 0.32(3) mg/cm² CD₂ target. The coupling of the MUGAST-AGATA-VAMOS++ detection systems allowed for triple coincidence gating, providing a great amount of selectivity in the detection of light ejectiles, heavy recoils and prompt γ-ray emissions in transfer and scattering reactions. An analysis based both on excitation and γ-ray energy measurements has revealed many previously unobserved states in ⁴⁸K, up to and above the neutron separation threshold. Spin-parities and direct transfer spectroscopic factors of these states have been determined.

The experimentally measured excited structure of ⁴⁸K is compared to calculations performed using two modern shell model calculations, revealing several key failures of these interactions, and indicating areas for improvement. The insights gained from this work have implications extending down the rapidly evolving proton-deficient N = 28 isotonic chain, where the singly-occupied πs_1/2 orbital is expected to couple with neutrons in the orbitals immediately above N = 28 in the short-lived ⁴⁴P nucleus.