Speaker
Description
Single-particle structure has been observed to evolve away from stability. The ordering and separation between levels varies to the extent that the established magic numbers change. For example, in the neutron-rich region where $Z = 8$-20, the $N = 20$ shell closure weakens, with a new closure emerging at $N=16$ in $^{24}$O [1]. In order to understand the evolution of these shell closures, and provide robust data for the development of shell-model interactions, it is crucial to measure the single-particle properties of nuclei in this region. Of particular interest is the behaviour of the negative-parity intruder orbitals from above $N=20$, the $f_{7/2}$ and $p$ orbitals. The difference in energy between these and the positive-parity $d_{3/2}$ defines the $N=20$ shell closure. Historically, shell-model calculations have failed to reproduce the energies of the observed negative-parity levels moving towards a region of the chart known as the ``island of inversion" (IOI). Here cross-shell excitations become prevalent and these intruder configurations make up the ground-states. Data on negative-parity levels outside the IOI provide data for validating new interactions
Single-particle transfer reactions are an ideal probe of the single-particle properties of nuclei. Away from stability, it is necessary to perform them in inverse kinematics using radioactive ion beams. Here we present a measurement of the $d(^{28}$Mg,$p)^{29}$Mg reaction using the ISOLDE Solenoidal Spectrometer, probing the single-particle properties one neutron outside $N=16$, in $^{29}$Mg. Cross sections, excitation energies, and angular momenta for the observed nuclear states were extracted, and spectroscopic factors were deduced. These data can then be used to test new shell-model interactions developed to better describe cross-shell interactions in this region of the nuclear chart, including the EEdf1 [2] and FSU [3] interactions, and to explore the evolution of single-particle centroids along the $N=17$ isotones.
This work is supported by the UK Science and Technology Facilities Council, and the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under Contract Number DE‑AC02‑06CH11357 (ANL), and the European Union's Horizon 2020 Framework research and innovation program under grant agreement no. 654002 (ENSAR2), and the Marie Skłodowska-Curie grant agreement No. 665779, and the Research Foundation Flanders (FWO, Belgium), and the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement number 617156.
[1] R. V. F. Janssens, Nature 459, 1069 (2009).
[2] N. Tsunoda, et al., Phys. Rev. C 95, 021304 (2017).
[3] R. S. Lubna, et al., Phys. Rev. C 100, 034308 (2019).
