Nuclear Resonance Fluorescence inCu63: Lifetimes of Excited States and the Slowing Down of Recoils (3-100 ev) in Condensed Media

Measurements of the lifetimes of the 669-kev and 963-kev states of ${\mathrm{Cu}}^{63}$ have been performed using the nuclear resonance fluorescence technique. Self-absorption experiments yield mean lives of (2.94±0.24)×${10}^{-13\mathrm{}}$ sec for the spin ½ 669-kev level and (7.2±1.8)×${10}^{-13\mathrm{}}$ sec for the spin $\frac{5}{2}$ 963-kev level. The resonance yields from sources inside Cu metal and in water solution were measured. The reduction of the resonance yields due to slowing down of the recoils from the ${\beta }^{+}$ decay was calculated with the use of the model of Vineyard et al. for the solid and with an elastic collision model for the liquid environments. These calculations are in good agreement with the measured yields and result in a unique spin assignment to the 669-kev state. The effect of the scatterer temperature on the resonance yield was shown to be in agreement with the "effective temperature" predicted by the theory of Lamb.