Precision mass measurement of neutron-rich nuclei around A=90 (Z=32-34) and towards ⁷⁸Ni via multi-reflection time-of-flight mass spectrograph at ZeroDegree spectrometer

Abstract

Nuclear masses are one of the most fundamental properties of atomic nuclei, which serve as an important probe to investigate the shell evolution and as crucial input for astrophysics nucleosynthesis calculations. To perform the high-precision mass measurements of exotic nuclei, a new multi-reflection time-of-flight mass spectrograph (MRTOF-MS) was built at RIBF, coupled with the BigRIPS-ZeroDegree beam line. The new MRTOF-MS achieved excellent mass resolving power ($m/\Delta m \sim 500\,k$) and good transport efficiency in the offline test. In addition, to identify exotic nuclei in the MRTOF-MS without ambiguity, a new type of ion detector, namely ``$\beta$-TOF'', is developed in the work of this thesis for the coincidence measurement of $\beta$-decay and time-of-flight.

In the first commissioning experiment of the new MRTOF-MS, mass measurements of neutron-rich nuclei $A\sim90$ ($Z=32\sim34$) were performed. Since the neutron-rich $A\sim90$ region locates on the path of r-process nucleosynthesis, the mass data of this region are essential inputs to the nucleosynthesis network calculations and impact the solar r-process abundance. Besides, sub-shell closure and sudden deformation onset are observed in the nuclei close to this region. The mass measurements of this experiment will also probe the nuclear structure evolution in this region. In the experiment, masses of 35 atoms and 20 stable molecules in the region of $A=82\sim92$ were successfully measured with high precision. The masses of \ts{88,89}As (-50677.8(31), -46686.5(48)\,keV) were determined for the first time, while the mass uncertainties of \ts{86}Ge (-49596.7(17)\,keV) and \ts{90,91}Se (-55881.3(43), -50267.4(26)\,keV) were reduced by around a factor of 100 compared with that in AME2020. The systematic deviation of 1.2(0.8)\,keV, which was evaluated by stable molecular masses, demonstrates the excellent precision of this MRTOF system. The extracted $S_{2n}$ curve of the Se isotopic chain shows a smooth decreasing trend, indicating no existence of $N = 56$ sub-shell in \ts{90}Se. Compared to the AME2020 values, our new mass values as the input to the Hauser-Feshbach (HF) statistical model significantly reduce the uncertainties of the calculated single neutron-capture reaction rates by up to two orders of magnitude. A slight impact on the solar r-process abundance distribution has already been subsequently seen, which indicates nuclear masses of more neutron-rich nuclei are crucial.

The second experiment targeted neutron-rich Ni isotopes towards \ts{78}Ni, which is a doubly-magic nucleus while being far from the valley of stability, leading to the masses of Ni isotopes in the vicinity are highly desirable to clarify the shell evolution towards the $N = 50, Z = 28$ core. In this experiment, masses of \ts{73-75}Ni were measured. The steadily declining $S_{2n}$ tread suggests no dramatic change in shell evolution. Measurements of \ts{76-79}Ni will be carried out to study the magicity of \ts{78}Ni. This first $\beta$-decay-correlated time-of-flight measurement with the ``$\beta$-TOF'' detector demonstrates the validity of the $\beta$-TOF technique for clear identification of ions and their isomeric states.