Identification of the Lowest T=2 , Jπ=0+ Isobaric Analog State in

Abstract

Masses of 52g,52m Co were measured for the first time with an accuracy of ∼ 10 keV, an unprecedented precision reached for short-lived nuclei in the isochronous mass spectrometry. Combining our results with the previous β -γ measurements of 52 Ni, the T = 2, J π = 0 + isobaric analog state (IAS) in 52 Co was newly assigned, questioning the conventional identification of IASs from the β -delayed proton emissions. Using our energy of the IAS in 52 Co, the masses of the T = 2 multiplet fit well into the Isobaric Multiplet Mass Equation.We find that the IAS in 52 Co decays predominantly via γ transitions while the proton emission is negligibly small. According to our large-scale shell model calculations, this phenomenon has been interpreted to be due to very low isospin mixing in the IAS.PACS numbers: 21.10. Dr, 27.40.+z, 29.20.db The concept of isospin was introduced by Heisenberg [1] and developed by Wigner [2] to describe the charge independence of nuclear forces. This concept is being widely used in particle and nuclear physics [3, 4]. Within the isospin formalism, a nucleus composed of Z protons and N neutrons has a fixed isospin projection of T z = (N − Z)/2, while all states in the nucleus can have different total isospins T ≥ |T z |. In other words, states of a given T can occur in a set of isobaric nuclei with T z = T, T − 1, ..., −T . These states with the same T and J π are called the isobaric analog states (IAS). The states with T = |T z | are the ground states of the corresponding nuclei and the ones with T > |T z | are excited states, except for some oddodd N = Z nuclei [5,6]. A set of IASs with fixed A and T are believed to have very similar structure and properties and to be energetically degenerated in the framework of isospin symmetry. This energy degeneracy is mainly altered due to the Coulomb interaction, the proton-neutron mass difference, and the charge-dependent forces of nuclear origin [7]. In an isobaric multiplet, the masses of the IASs of a given T can be described in first order approximation by the famous quadratic * Corresponding author.

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