Low-energy Coulomb excitation of radioactive $^{70}$Se

An isobarically pure beam of $^{70}$Se ions was post accelerated to an energy of 206~MeV using {\small REX-ISOLDE}. Coulomb-excitation yields for states in the beam and target nuclei were deduced by recording de-excitation $\gamma$ rays in the highly segmented {\small MINIBALL} $\gamma$-ray spectrometer in coincidence with scattered particles in a silicon detector. At these energies, the Coulomb-excitation yield for the $2^{+}_{1}$ state in $^{70}$Se is expected to be strongly sensitive to the sign of the spectroscopic quadrupole moment through the nuclear reorientation effect. Experimental evidence is presented here for a prolate shape for this state, using an earlier published lifetime measurement, reopening the question over whether there are deformed oblate shapes close to the ground state in the neutron-deficient selenium isotopes.