Shape Changes in the Mirror Nuclei Krypton-70 and Selenium-70

Abstract:  Unlike any other physical system the nucleus represents a unique dual quantum many-body system. Its constituents, protons and neutrons, are assumed to be identical, except for their electric charge. They can be seen as two representations of the nucleon, with isospin components t_z = ±1/2 for neutrons and protons, respectively. Under the assumption of charge independence of the strong interaction, hence invariance under rotation in the isospin space, the excitation energy spectra of mirror nuclei should be identical. Isospin breaking effects, besides the dominating electromagnetic force, are usually studied through mirror energy differences, testing the charge symmetry and triplet energy differences, testing the charge independence of the nuclear force. However, a more rigorous way to test isospin symmetry are electromagnetic matrix elements. In this talk, I will present the results of our study of the A = 70, T = 1 triplet performed at the Radioactive Isotope Beam Factory at the RIKEN Nishina Center in Japan. Proton-rich beams were produced by projectile fragmentation and identified using the BigRIPS separator. Coulomb excitation was induced by a secondary Au target, located in the center of the DALI2 gamma-ray detector array to measure the excitation probabilities. The measurement on krypton-70 revealed a much larger cross section for the excitation of the first 2+ state than would be expected from its mirror selenium-70. This can be interpreted as an unexpected shape change in the mirror nuclei and the new results present a challenge to the theoretical modeling of atomic nuclei.