The progress of nuclear mass measurement experiments based on HIRFL-CSR

Abstract

As one of the most fundamental properties of nuclei, nuclear mass data are widely used in many research fields such as nuclear structure and nuclear astrophysics. At present, mass measurement of short-lived nuclei is an important research topic in nuclear physics. However, due to their small production rates and short lifetimes, it is technically challenging to measure their masses accurately. Isochronous mass spectrometry (IMS) based on the storage ring is an effective technique for measuring masses of short-lived nuclei. We established the IMS at the experimental cooler-storage ring (CSRe) in Lanzhou and developed the techniques continuously, achieving the highest precision of IMS measurement in the world. Since 2009, 10 isochronous mass measurement experiments have been conducted and high-precision mass data were obtained. In the earlier researches, we focused on mass measurements for medium-mass neutron-deficient nuclei. Recently, we have extended the research to heavier and neutron-rich regions. The masses of 82Zr and 84Nb were measured for the first time, and the masses of 79Y, 81Zr, and 83Nb were re-determined with a higher precision. The results do not support the existence of the hypothetical island of pronounced low α separation energies for neutron-deficient Mo and Tc isotopes, making the formation of Zr-Nb cycle in the rp-process unlikely. The results also partly remove the overproduction of the p-nucleus 84Sr in the νp-process simulations. The masses of the neutron-rich 52–54Sc and 54,56Ti nuclei were measured. The empirical shell gap extracted from the experimental results shows a significant subshell closure at N =32 in scandium. Combined with the theoretical calculations, it turns out that the Sc isotopes are located in the transition region of the shell evolution. Exploiting the high mass resolution of the IMS at CSRe, we also studied the nuclear isomeric states. Masses of 52g,52mCo were measured for the first time with an accuracy of ~10 keV. Combining the results with the previous β-γ measurements of 52Ni, the T =2, J π=0+ isobaric analog state (IAS) in 52Co was newly assigned, questioning the conventional identification of IASs from the β-delayed proton emissions. Using the new results, the isobaric multiplet mass equation is restored for the masses of the T =2 multiplet. Decay of the 8+ isomer in fully stripped ions 94Ru44+ was directly observed by monitoring the revolution times during its circulation in the CSRe. The isomeric half-life was found to be longer by about 44% than that in the neutral atoms. A new isomer in 101In was discovered and its excitation energy was measured. Using the experimental results, the configuration-dependent shell evolution, type-II shell evolution, in odd-A nuclei is discussed for the first time. High intensity heavy-ion accelerator facility (HIAF) is a large-scale research facility for nuclear science. It is under construction and is expected to be delivered in 2025. HIAF, as the world-leading next generation heavy ion accelerator device, has the ability to generate nuclides extremely far away from the stable line and provide superior research conditions for accurately measuring masses of short-lived nuclei far away from the stable line. Based on the accumulated experience of CSR mass measurement, we will develop advanced detection devices, fully exploit the performance of HIAF, and lead the development of storage ring mass spectrometry.