Proton-neutron structure of first and second quadrupole excitations of 90Sr A. P. Severyukhin, N. N. Arsenyev, N. Pietralla and V. Werner Eur. Phys. J. A, 54 1 (2018) 4 Published online: 22 January 2018 DOI: 10.1140/epja/i2018-12451-4