Shape coexistence in light krypton isotopes

Matière Nucléaire - Commune avec NEX et ENLInternational audienceShape coexistence in the light krypton isotopes was studied in a series of experiments at GANIL using various experimental techniques. A new low-lying $0^+$ state, a so-called shape isomer, was found in delayed conversion-electron spectroscopy after fragmentation reactions. The systematics of such low-lying $0+^$ states suggests that the ground states of the isotopes $^{78}$Kr and $^{76}$Kr have prolate deformation, while states with prolate and oblate shape are practically degenerate and strongly mixed in $^{74}$Kr, and that the oblate configuration becomes the ground state in $^{72}$Kr. This scenario was tested in experiments performing low-energy Coulomb excitation of radioactive $^{76}$Kr and $^{74}$Kr beams from the SPIRAL facility. Both transitional and diagonal electromagnetic matrix elements were extracted from the observed $\gamma$-ray yields. The results find the prolate shape for the ground-state bands in $^{76}$Kr and $^{74}$Kr and an oblate deformation for the excited $2^{+}_{2}$ state in $^{74}$Kr, confirming the proposed scenario of shape coexistenc