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Nuclear magic numbers off stability

Final Report Summary - MINOS (Nuclear magic numbers off stability)

Rare isotopes, both by their significant neutron-over-proton unbalance or their weak binding, offer unique features to understand the atomic nucleus. The completion of facilities dedicated to produce radioactive ion beams worldwide has allowed to span new regions of the nuclear landscape where new properties of nuclei, in particular the evolution of shell structure, can be investigated. It is now established that the nuclear shell structure, as unraveled from stable nuclei, is not universal across the entire nuclear chart but evolves depending on which neutron and proton orbitals are occupied.

MINOS is a new device composed of a very thick liquid hydrogen target and a time projection chamber surrounding the target. The target thickness is 100-150 mm and can be adapted according to the requirement of an experiment. Since a typical reaction cross section of a nucleus is of the order of 1 barn (10-24 cm2), the target thickness (6.1023 /cm2) correspond to a mean-free path, and thus it is expected an incident nucleus reacts with a high probability in the target! It has the advantageous geometry of being free of any absorbing material around the target and therefore efficiently be surrounded by any type of radiation detector. Note that such a target could generally not be used in standard in-beam gamma or invariant mass measurement because of its thickness. For that purpose, a vertex tracker, technically chosen to be a time projection chamber (TPC) based on a Micromegas amplification stage, was developed to be positioned around the target. The achieved goal of the TPC is to determine the vertex of the reaction with a precision better than 5 mm FWHM and to achieve a total detection efficiency of better than 80% for either proton from a (p,2p) reaction. MINOS is readout by a dedicated electronics based on new generation chips. The system was entirely developed at the Commissariat à l'Energie Atomique et aux Energies Alternatives (CEA) in France. MINOS has been successfully used at the RIBF since 2014.

This compact combination is the first of its kind in nuclear physics. The first physics questions to be investigated are the mechanisms of shell evolution in neutron rich nuclei.
During the first two years of operation of MINOS at the RIBF, the first spectroscopy of very neutron rich nuclei such as 78Ni, 110Zr, important in our understanding of nuclear structure, have been performed. First preliminary results question the so-far established magic character of N=50 for Cr and Fe isotopes. The detailed spectroscopy of 78Ni should lead very soon to the first insight into this question. 110Zr, central in the modeling of the astrophysical r process, is show to be a well-deformed nucleus with no stabilization due to shell effects as it has been claimed in the past.
A dedicated program was established to understand the origin of di-neutron correlations in halo nuclei, with the combination of MINOS with other detectors from the RIBF, including the large acceptance SAMURAI spectrometer.

MINOS plays a central role in the study of neutron-rich nuclei at and beyond the neutron drip line. As a highlight, the role of three-body forces and continuum states along the oxygen isotopic chain has been investigated through the first spectroscopy of 27O and 28O. A complete study of two-neutron halo composition in 11Li and 14Be will allow understanding di-neutron correlation at an unprecedented level of accuracy. These recent experiments are currently under analysis.