Orateur
Description
Current experimental and theoretical efforts focus on shell-structure evolution to explain the variation of magic numbers with isospin asymmetry and to constrain the underlying nucleon-nucleon interactions. Advanced shell-model interactions attribute this to the monopole part of the nuclear Hamiltonian, for which additional experimental information on the two-body matrix elements is essential to improve predictive power [1]. Moreover, nucleon-nucleon correlations limit the fraction of measurable spectroscopic strength to \qty{60(10)}{\percent} at low excitation energy [2]. Constraining this quenching of spectroscopic strength is essential for understanding shell-structure changes towards exotic nuclei.
To shed more light on these two topics, two experiments were performed. The first, conducted at LISE, measured one-nucleon transfer reactions $^{10,12}$Be(d,t) and $^{10,12}$Be(d,$^3$He) to study the quenching of spectroscopic factors in the final nuclei. Using MUST2 telescopes [3] to detect and identify the light ejectiles, low-lying states in $^{9,11}$Be and $^{9,11}$Li were populated. Their spectroscopic factors, compared with shell-model calculations, yielded a quenching factor consistent with the literature and showed no significant dependence on the asymmetry energy. In $^{11}$Li, an anomalously large quenching indicated geometrical-mismatch effects [4] that theoretical models fail to completely reproduce.
The second experiment, also at LISE, measured the one-neutron removal reaction $^{20}$O(d,t)$^{19}$O to investigate shell migration in $^{20}$O. Using ACTAR TPC [5-7] to perform 3D particle tracking coupled to silicon pad detectors to measure the light-particle residual energy, eleven excited states were populated in $^{19}$O, demonstrating the suitability of active targets for transfer-reaction experiments [8]. Effective single-particle energies for the $1s_{1/2}$, $0d_{5/2}$, $0p_{1/2}$ and $0p_{3/2}$ neutron orbitals were extracted with full strength except for the $\nu0p_{3/2}$ (only \qty{40}{\percent}). Comparison with the SFO-tls shell-model interaction [9] suggests that more attractive neutron-neutron monopole matrix elements of the $p-sd$ shells are needed to better reproduce experimental ESPEs. Further theoretical developments are required towards a universal interaction describing spectroscopy in this region of the nuclear chart.
References
[1] T. Otsuka et al., Rev. Mod. Phys. 92, 15002 (2020).
[2] T. Aumann et al., Prog. Part. Nucl. Phys. 118, 103847 (2021).
[3] E. Pollacco et al., in 4th conference on exotic nuclei and atomic masses (ENAM’04), Vol. 25 (2004).
[4] A. Matta et al., Phys. Rev. C 92, 41302 (2015).
[5] T. Roger et al., Nucl. Instrum. Methods Phys. Res., Sect. A 895, 126–134 (2018).
[6] B. Mauss et al., Nucl. Instrum. Methods Phys. Res., Sect. A 940, 498–
504 (2019).
[7] J. Giovinazzo et al., Nucl. Instrum. Methods Phys. Res., Sect. A 953, 163184 (2020).
[8] J. Lois-Fuentes et al., Accepted to Phys. Rev. Lett. (2026).
[9] T. Suzuki et al., Phys. Rev. C 78, 061301 (2008).