Exploring the n-n and p-n interactions close to the dripline

Abstract:  The study of nuclei with large neutron to proton asymmetries is very useful to investigate the inter-action between neutrons and protons in extreme conditions which exist for instance in explosive stars. It could also improve our knowledge about new effects that occur when we move far from the stability. In a first part, I will present my recently published results [1] on the n-n correlations observed in the decay of unbound states in 18C and 20O (viewed as 14C+4n and 16O+4n, respectively) populated via the sudden knockout of a proton in 19N and a neutron in 21O. I will discuss a novel method that allows to reveal neutron correlations in the nucleus. This was achieved by studying the decay of high-energy states above S2n populated after the sudden knockout of a deeply bound nucleon. This experiment, performed at GSI, required the complex R3B-LAND setup to determine the full kinematics of the reaction. We have studied the evolution of the n-n correlations as a function of the increasing energy Ed of the system and compared the decay patterns of the two systems. We used a simulation that takes into account the different decay mechanisms (direct and sequential decay) and the final state interactions to interpret the experimental data. Using information on n-n and core-n momenta, we show that we can clearly distinguish direct from sequential decays. Remarkably, direct decay is strongly dominant in 18C up to Ed=8 MeV, beyond which sequential decay amounts to only 20%. This is in contrast to the case of 20O, in which sequential decay dominates already at low Ed, and in which much weaker n-n correlations are observed. In a second part, I will discuss the evolution of the p-n interaction towards the neutron dripline in the fluorine isotopic chain. Recent experiments have shown that the dripline occurs in the oxygen isotopes at the doubly magic 24O. However, with the exception of 28F and 30F, which are unbound, six more neutrons can be added in the fluorine isotopic chain before reaching the dripline at 31F. One can therefore speculate that the extension of the dripline between oxygen and fluorine, as well as the odd-even binding of the fluorine isotopes, arise from a delicate balance between two-body p-n and n-n interactions, the coupling to the continuum, and three-body forces. Within this context, we have investigated the evolution of the residual interaction between the 26F and 28F nuclei that were studied at RIKEN during the SAMURAI21 experiment. The setup benefited from some powerful and innovative devices, such as the NeuLAND demonstrator and the MINOS TPC. I will present the results on the structure of 26F unbound states populated via neutron knockout from 27F as well as the ground and first excited states of the unbound 28F, populated via knockout reactions from 29F and 29Ne at 250 MeV/nucleon. Our results complete previous studies on 26F, where bound excited states [2] and unbound states populated via proton knockout from 27Ne [3] have already been investigated. Moreover our results on 28F highly improve and complete a previous study [4] due to the high statistics and resolution achieved during this experiment. References [1] A. Revel et al., Phys. Rev. Lett. 120, 152504 (2018). [2] A. Lepailleur et al., Phys. Rev. Lett. 110, 082502 (2013). [3] M. Vandebrouck et al., Phys. Rev. C 96, 054305 (2017). [4] G. Christian et al., Phys. Rev. Lett. 108, 032501 (2012).