Lifetimes of Excited Nuclear States using Delayed-Reaction Gamma Rays

Using the pulsed charged-particle beam from the Livermore variable-energy cyclotron, the delayed reaction gamma rays have been employed for a direct measurement of lifetime of excited nuclear states in the nanosecond region. This technique is particularly useful in the investigation of states not populated (or only weakly populated) by radioactive decay. The mean lives of excited states of several nuclei in the $f_{\frac{7}{2}}$ shell have been measured. Using the ${\mathrm{Ti}}^{48}(p,n\gamma )$ and ${\mathrm{Sc}}^{45}(\alpha ,n\gamma )$ reactions the 305-keV state of ${\mathrm{V}}^{48}$ yields (10.9±0.4) nsec. Using the ${\mathrm{V}}^{51}(p,n\gamma )$ and ${\mathrm{Ti}}^{48}(\alpha ,n\gamma )$ reactions the 750-keV state of ${\mathrm{Cr}}^{51}$ yields (10.8±0.5) nsec. Considering previously reported level spin and gamma-ray multipolarity assignments, the above reported lifetimes appear to be anomalously long, indicating an extremely high degree of retardation of these electromagnetic transitions within the $f_{\frac{7}{2}}$ neutron and proton shells. These measurements have been confirmed using the delayed gamma-gamma coincidences following the radioactive decays of ${\mathrm{Cr}}^{48}$ and ${\mathrm{Mn}}^{51}$, respectively.