The determination of basic properties of the so-called isobaric analog states (IASs) which are states with the same isospin T and spin-parity Jπ in a fixed A isobar has long been an important research subject in nuclear physics. A famous and simple formula, isobaric multiplet mass equation (IMME), which was derived 60 years ago on the basis of the perturbation theory in Quantum Mechanics and the isosopin symmetry, can successfully describe the masses of IASs. However, a breakdown of the IMME was found in the four members of the A=52, T=2 isospin quintuplet system. This phenomenon cannot be explained by present nuclear theory.
To clarify this disagreement, isochronous mass spectrometry was applied to neutron-deficient 58Ni projectile fragments at the HIRFL-CSR facility in Lanzhou. In this experiment, a slit was introduced in the dispersive straight section of the CSRe for the first time to reduce the momentum spread of the secondary beams in the CSRe. As a result, the mass resolving power was improved by a factor of 2, and thus the ground states and its 2+ low-lying isomeric states were unambiguously identified. An unprecedented relative precision 1.6*10-7 was reached, which is the best isochronous mass spectrometry worldwide at present.
Combining the researchers’ new mass values with the β-delayed γ decay measurements of 52Ni in the literature, the energy of the T = 2 IAS in 52Co is determined to be 135 keV higher than previously assumed on the basis of the β-delayed proton decay data from the 52Ni decay studies. With this new IAS assignment, the mass excesses of the four members of the A = 52, T = 2 isobaric multiplet are found to be consistent with the IMME. Furthermore, a remarkably different decay scheme of 52Ni could be constructed, in which the proton group with the highest relative intensity corresponds to the decay from the 1+ excited state in 52Co and not from the 0+, T = 2 IAS. This finding questions the conventional identification of IASs from the β-delayed proton emissions. The newly determined level scheme of 52Co can well be reproduced by large-scale shell model calculations using an isospin non-conserving Hamiltonian. These theoretical calculations indicate that the isospin mixing in the 0+, T = 2 state in 52Co is extremely low, thus leading to a negligibly small proton emission from this state.
This work was jointly supported by funding agencies such as Major State Basic Research Development Program of China, National Natural Science Foundation of China and “Light of West China” Program of Chinese Academy of Sciences. This work was published in Physical Review Letter (Phys. Rev. Lett. 117, 182503 (2016)).
The article can be linked as follows: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.117.182503
FIG.1. Part of the revolution time spectrum zoomed in at a time window of 608 ns ≤ t ≤ 619 ns. The red and black peaks represent the Tz = −1 and −1/2 nuclei, respectively. The insert shows well-resolved peaks of the ground and 2+ isomeric states of 52Co.(Image by IMP)