Cross-shell interactions at the N = 28 shell closure via 47K(d,p)48K and 47K(d,t)46K
Creators: name: Paxman, Charlie James
Publisher: name: University of Surrey
Year: 2024
Subjects: Nuclear Structure
Nuclear transfer reactions
Experimental Physics
Nuclear Physics

description: 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 ??sd ⊗ ??fp interactions, which are expected to influence this shell evolution in proton-deficient N = 28 isotones. The isotope &lt;sup&gt;48&lt;/sup&gt;K is uniquely situated for this purpose, as it is both one proton below Z = 20, and one neutron above N = 28. Additionally, the neighbouring &lt;sup&gt;47&lt;/sup&gt;K isotope is known to be primarily &lt;em&gt;π&lt;/em&gt;(s&lt;sub&gt;1/2&lt;/sub&gt;&lt;sup&gt;1&lt;/sup&gt; d&lt;sub&gt;3/2&lt;/sub&gt;&lt;sup&gt;4&lt;/sup&gt;) structure in its ground state – as opposed to the naïvely expected &lt;em&gt;π&lt;/em&gt;(s&lt;sub&gt;1/2&lt;/sub&gt;&lt;sup&gt;2&lt;/sup&gt; d&lt;sub&gt;3/2&lt;/sub&gt;&lt;sup&gt;3&lt;/sup&gt;) structure, which is dominant in potassium isotopes – allowing the exotic &lt;em&gt;π&lt;/em&gt;s&lt;sub&gt;1/2&lt;/sub&gt; ⊗ νfp interaction to be probed by the selective neutron transfer reaction, &lt;sup&gt;47&lt;/sup&gt;K(d,p)&lt;sup&gt;48&lt;/sup&gt;K. &lt;br&gt;The first experimental study of states arising from the interaction between &lt;em&gt;π&lt;/em&gt;s&lt;sub&gt;1/2&lt;/sub&gt; and the orbitals νp&lt;sub&gt;3/2&lt;/sub&gt;, νp&lt;sub&gt;1/2&lt;/sub&gt; and νf&lt;sub&gt;5/2&lt;/sub&gt; has been conducted, by way of the &lt;sup&gt;47&lt;/sup&gt;K(d,p) reaction in inverse kinematics. The radioactive beam of &lt;sup&gt;47&lt;/sup&gt;K 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 5×10&lt;sup&gt;5&lt;/sup&gt; particles per second and was estimated to be &gt; 99.99% pure. The beam impinged upon a 0.32(3) mg/cm&lt;sup&gt;2&lt;/sup&gt; CD&lt;sub&gt;2&lt;/sub&gt; 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 &lt;em&gt;γ&lt;/em&gt;-ray emissions in transfer and scattering reactions. An analysis based both on excitation and &lt;em&gt;γ&lt;/em&gt;-ray energy measurements has revealed many previously unobserved states in &lt;sup&gt;48&lt;/sup&gt;K, up to and above the neutron separation threshold. Spin-parities and direct transfer spectroscopic factors of these states have been determined. &lt;br&gt;The experimentally measured excited structure of &lt;sup&gt;48&lt;/sup&gt;K 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 &lt;em&gt;π&lt;/em&gt;s&lt;sub&gt;1/2&lt;/sub&gt; orbital is expected to couple with neutrons in the orbitals immediately above N = 28 in the short-lived &lt;sup&gt;44&lt;/sup&gt;P nucleus.