title: Extended p₃/₂ neutron orbital and the N=32 shell closure in ⁵²Ca
Creators: name: Enciu, Madalina
Publisher: name: UNSPECIFIED
Year: 2023

description: The nuclear shell structure is not universal across the nuclear landscape. The standard shell closures, 2, 8, 20, 28, 50, 82, and 126 can weaken and fade away, while new ones can appear. The numbers of nucleons that correspond to a shell closure are commonly named as magic numbers. An example is the appearance of the N=32 and 34 new neutron magic numbers in the neutron-rich calcium isotopes region. Moreover, experimental measurements show an increase of charge radii and matter radii as one starts filling the 2p3/2 neutron orbital in potassium and calcium isotopes. One hypothesis is that the p neutron orbitals have a large size relative to the f neutron orbitals, and this would explain the measured large charge and matter radii values. The first part of the thesis presents the results of 52Ca(p,pn)51 Ca cross section measurements, supporting the N=32 shell closure in 52 Ca. It focuses also on the determination of the size of the single-particle neutron orbitals in the pf -shell via (p,pn) reactions and momentum distribution analysis. The quasi-free neutron scattering reaction was measured during an experimental campaign at the Radioactive Isotope Beam Factory (RIBF), at the SAMURAI fragment spectrometer, in inverse kinematics at an energy of ∼230 MeV/nucleon in a 15-cm long liquid hydrogen target and using the MINOS TPC. Inclusive and exclusive cross sections to bound states of 51Ca were evaluated using γ-ray spectroscopy measurements, as well as the momentum distributions corresponding to the removal of 1f7/2 and 2p3/2 neutrons were measured. The cross sections, interpreted within the distorted-wave impulse approximation (DWIA) reaction framework, are consistent with a shell closure at the neutron number N=32, found as strong as at N=28 and N=34 in Ca isotopes. The analysis of the momentum distributions leads to a difference of the root-mean-square radii of the 1f7/2 and 2p3/2 neutron orbitals of 0.61(23) fm, in agreement with the modified-shell-model prediction of 0.7 fm suggesting that the large root-mean-square (rms) radius of the 2p 3/2 orbital in neutron-rich Ca isotopes is responsible for the unexpected linear increase of the charge radius with the neutron number. The method for determining the rms radii of the single-particle neutron orbitals via momentum distribution measurements in (p,pn) reactions was further applied for the neighbouring isotopes, 53Ca and 54Ca. The size of the 1f7/2 , 2p3/2 , 2p1/2 , and 1f5/2 neutron orbitals was obtained and showed consistency with the results from 52Ca. The p orbitals show indeed a larger spatial extension compared to the f neutron orbitals (&gt;0.5 fm difference) as a result of this study. The second part of this thesis presents the research and development of a new particle tracker, STRASSE combined with a long liquid hydrogen target, aimed for the study of proton induced quasi-free scattering reactions such as (p,2p) and (p,3p). Despite the improved luminosity, the MINOS system was used only for tracking the reaction vertex inside the liquid hydrogen target. A similar setup allowing particle spectroscopy in addition to γ-ray spectroscopy could provide missing mass information. STRASSE is designed to be used together with CATANA, which is an array of CsI(Na) crystals aimed for measuring the total energy of the protons from the (p,2p) and (p,3p) reactions, as well as to perform γ-ray spectroscopy. This new experimental setup will be able to reconstruct the reaction vertex with sub-mm resolution and to perform missing mass measurements with a resolution of below 2 MeV. The production and testing of the liquid hydrogen target cell for STRASSE will be presented in this thesis, as well as offline and in-beam testing of the readout electronics aimed for STRASSE, using the prototype silicon tracker PFAD.