Effective proton-neutron interaction near the drip line from unbound states in F25,26

Abstract

Background: Odd-odd nuclei, around doubly closed shells, have been extensively used to study proton-neutron interactions. However, the evolution of these interactions as a function of the binding energy, ultimately when nuclei become unbound, is poorly known. The 26 F nucleus, composed of a deeply bound π 0d 5/2 proton and an unbound ν0d 3/2 neutron on top of an 24 O core, is particularly adapted for this purpose. The coupling of this proton and neutron results in a J π = 1 1 + − 4 1 + multiplet, whose energies must be determined to study the influence of the proximity of the continuum on the corresponding proton-neutron interaction. The J π = 1 1 + , 2 1 + , 4 1 + bound states have been determined, and only a clear identification of the J π = 3 1 + is missing. Purpose: We wish to complete the study of the J π = 1 1 + − 4 1 + multiplet in 26 F, by studying the energy and width of the J π = 3 1 + unbound state. The method was first validated by the study of unbound states in 25 F, for which resonances were already observed in a previous experiment. Method: Radioactive beams of 26 Ne and 27 Ne, produced at about 440A MeV by the fragment separator at the GSI facility were used to populate unbound states in 25 F and 26 F via one-proton knockout reactions on a CH 2 target, located at the object focal point of the R 3 B/LAND setup. The detection of emitted γ rays and neutrons, added to the reconstruction of the momentum vector of the A − 1 nuclei, allowed the determination of the energy of three unbound states in 25 F and two in 26 F. Results: Based on its width and decay properties, the first unbound state in 25 F, at the relative energy of 49(9) keV, is proposed to be a J π = 1/2 − arising from a p 1/2 proton-hole state. In 26 F, the first resonance at 323(33) keV is proposed to be the J π = 3 1 + member of the J π = 1 1 + − 4 1 + multiplet. Energies of observed states in 25,26 F have been compared to calculations using the independent-particle shell model, a phenomenological shell model, and the ab initio valence-space in-medium similarity renormalization group method. Conclusions: The deduced effective proton-neutron interaction is weakened by about 30-40% in comparison to the models, pointing to the need for implementing the role of the continuum in theoretical descriptions or to a wrong determination of the atomic mass of 26 F.

An ab-initio Gamow shell model approach with a core

“…The spectrum of 26 F is presented in Figure 4 and compared to experimental data [19,20]. Considering the new shell closure discovered with 24 O [3,4], 26 F can simply be treated as one Excitation Energy (MeV) 26 F Figure 4: Energy spectrum for 26 F calculated with MPBT+GSM and compared with experimental data [19,20] and other calculations where continuum coupling is discarded: SM with USDB interaction and VS-IMSRG with NN+3NF.…”

Effective interactions in the sd shell

“…Recently, significant attention has been given to a dripline nucleus 26 F [59,62]. The low-energy theoretical spectra of 26 F are shown in Fig.…”

“…If one assumes that, as in the independent particle picture, the low-energy multiplet of states (1 + −4 + ) is mainly provided by (πd 5/2 )(νd 3/2 ) configuration, then one can estimate empirical protonneutron matrix elements as proposed in Refs. [59,62],…”

Extending the Southern Shore of the Island of Inversion to F28

“…Christian et al [10] found that 28 F is unbound by 220(50) keV, and based on the agreement with the predictions of USDA/USDB shell-model calculations 28 F was placed outside the "Island of Inversion" (IoI) [11]. This means that the ground state of 28 F could be described by a particle-hole configuration (π0d 5/2 × ν0d −1 3/2 ) with respect to an unbound core of 28 O, forming a multiplet of J π = 1 + -4 + states as in 26 F [12,13]. On the other hand, the relatively low energies of the first excited states in 27,29 F suggest the presence of intruder neutron pf -shell contributions [14].…”

“…The first positive-parity states are predicted too low in energy, which could be explained by effects of the continuum (not taken into account explicitly in the present calculations) that change the effective two-body matrix elements [13,56] and induce lingering of the ℓ = 1 states compared to ℓ = 2 [8]. Another feature that could be related to the effects of the continuum, discussed in Ref.…”