Complete spectroscopy of 16c and 20o with solid and active targets using transfer reactions USC

Abstract

Over the past years, different experiments have shown the emergence or collapse of different shell gaps. For instance, the vanishing of the N =20 shell gap in the neutron-rich oxygen isotopes demonstrated how the study of this evolution can shed light into the fundamental proton-neutron interaction, and therefore the understanding of the nuclear force. Following this premise, the study of the evolution of other shell gaps in light nuclei can contribute to further constrain the mentioned interaction. This work presents the study of the N =8, 16 and Z=6 gaps in different neutron-rich carbon and oxygen isotopes, namely 16C and 20O. The shell gaps were investigated by two transfer reaction experiments performed in the LISE3 spectrometer at GANIL. The 16C beam was delivered to a 'traditional' transfer reaction setup, where a solid target (CD2 ) was surrounded by different Si stripped detectors, TIARA and MUST2, which were used to detect the protons and tritons coming from the (d,p) 17 C and (d,t) 15 C reactions. Additionally, a Si-Si-CsI telescope (CHARISSA) was placed at zero degrees and allowed atomic number identification of the beam-like particle. Four HPGe clovers (EXOGAM) were used to measure the gamma-rays in coincidence with the charged-particle detection. The study of the spectroscopic factors of the unbound states in 17C using the cross-section and the decay width allowed to locate the 0d3/2 strength in this nucleus, being able to determine for the first time, a lower limit for the N =16 gap. Moreover, the neutron removal reaction populated characteristic p-hole states in 15C and the relative spectroscopic factors, deduced from the normalization of the single-particle cross-section, were used to study the N =8 gap. The experimental results were compared to shell model calculations, using different state-of-the-art interactions such as YSOX and SFO-tls, showing their sensitivity to the N =8 gap. The comparison to previous results of 17 O allowed to constrain the cross-shell part of the interaction. In addition, the experimental results provided important input for ab initio calculations using the self-consistent Green's function (SCGF) method with the NNLO sat interaction. In the second experiment, the 20O beam was delivered to an active target setup formed by ACTAR TPC filled with a 90/10 mix of D2 and iC4H10 coupled to pad Si detectors. The light particles coming from different reactions were identified with the energy loss in the TPC vs the residual energy detected in the Si. The PID allowed unambiguous identification of 3/4He particles, which was used to select the (d,3He)19N channel. The use of active targets allowed to measure the vertex of the reaction together with an improved determination of the ejectile angle. Oxygen isotopes have their proton sp orbits completely filled, therefore the proton-removal reaction populated characteristic 0p 1/2 and 0p 3/2 states in 19 N. Except for the ground state, new excited states have been identified in this experiment. The angular distributions confirmed their l=1 character and allowed to determine the spectroscopic factors. With the previous information, the effective single-particle energies of the two orbitals involved in the spin-orbit splitting 0p 1/2 and 0p3/2 , were used to determine the Z=6 gap in 20O.Profiting from experimental data on less neutron-rich oxygen isotopes, the evolution of the Z=6 gap was studied. The results confirm a quenching of the spin-orbit splitting as neutrons are added to the 0d 5/2 orbital. The observed reduction is in agreement with shell model calculations using the YSOX interaction suggesting an effect of the proton-neutron tensor force. The study of transfer reactions with both 'classic' and 'atypical' setups was used to learn the limitations and potential of the active targets for such kind of experiments. The results highlighted that minor improvements together with additional auxiliary detection can enhance the intensity limits needed to perform such experiments with solid targets.