Short lifetimes in mirror nuclei 25Mg-25Al.

Abstract

Mean lifetimes of levels in the mirror nuclei $^{25}\mathrm{Mg}$ and $^{25}\mathrm{Al}$ have been measured using the Doppler-shift-attenuation (DSA) method and the reactions $^{12}\mathrm{C}$${(}^{15}$N,pn${)}^{25}$Mg and $^{24}\mathrm{Mg}$(p,\ensuremath{\gamma}${)}^{25}$Al, respectively. The lifetime values or limits were determined for 13 bound levels in $^{25}\mathrm{Mg}$ below the excitation energy of 7 MeV and for the 1.61- and 1.79-MeV bound levels in $^{25}\mathrm{Al}$. The lifetime values or limits of the 7.79, 8.02, and 9.00 MeV neutron unbound states in $^{25}\mathrm{Mg}$, and the 2.72-, 3.42-, and 4.03-MeV proton unbound states in $^{25}\mathrm{Al}$ were also determined. The lifetimes of four levels in $^{25}\mathrm{Mg}$ are reported for the first time. The targets were prepared by implanting $^{12}\mathrm{C}$ and $^{24}\mathrm{Mg}$ into Ta substrates, to ensure effective slowing down of the recoils. Computer simulations with the Monte Carlo method and experimental stopping power were used in the DSA analysis. Experimental transition matrix elements, based on the measured mean lifetime values, are compared with predictions of the universal sd-shell model.