The (p,3p) two-proton removal from neutron-rich nuclei and the development of the STRASSE tracker

Abstract

The knockout of nucleons from nuclei is a powerful tool to investigate nuclear structure. In particular, the knockout of nucleons at energies above 200 MeV/nucleon from a hydrogen target, so called quasi free scattering, is believed to be a clean probe for nuclear structure and has led to several recent experimental programs and theoretical developments. In this work, we are interested in reactions that lead to the removal of two nucleons. Indeed, it was observed in several occurrences that different final states in a nucleus are populated when produced from one nucleon knockout (p,2p) or from two nucleon knockout (p,3p). The understanding of the latter could provide a new tool for nuclear spectroscopy. In this work the analysis of two experimental campaigns conducted at the RIBF in RIKEN, Japan, is presented. The scattered-proton angular distributions from several neutron-rich medium-mass nuclei were analysed. The radioactive nuclei were impinging onto a 100 mm long cryogenic liquid hydrogen target. The protons emitted from the reaction were measured with the MINOS time-projection chamber surrounding the target. For the first time, it gave access to angular correlations of the three protons in the final state with high angular coverage. The obtained proton distributions have been benchmarked against kinematical models assuming three different reaction mechanisms. The (p,3p) reaction happens sequentially. Twenty-one inclusive one-proton and two-proton knockout reaction cross sections of 68-70Co, 70Ni, 74Ni, 74-75Cu, 75-76Zn, 80-82Ga, 89-90Se, 97Rb, 100-102Sr, 102-103Y and 112Mo are presented as well. To improve the vertex resolution and enable missing-mass spectroscopy, a new setup called STRASSE is being developed. It is a silicon tracker array, which is located in a hexagonal shape in two layers around a liquid H2 target. The development started with a demonstrator setup PFAD, which consists of one third of STRASSE. An experiment with PFAD 10He(p,pα)6n at RIBF, RIKEN is already approved. A part of the DAQ is required to operate in vacuum, which has been validated in this work using a test-setup. A major part of the DAQ is adapted from the CBM tracker electronics, which were adapted to the PFAD/STRASSE requirements. The electronic noise was also optimised and a γ-spectrum of 241Am was taken. For PFAD, a cooling system has been designed, and the full electronic chain (DAQ) was set up and optimised.