Lifetimes of the First Excited 0+ States ofCa40andZr90

The first excited states of ${\mathrm{Ca}}^{40}$ and ${\mathrm{Zr}}^{90}$ are 0+ states which decay to the 0+ ground state primarily by the emission of internal pairs or conversion electrons. These states have been excited by neutron inelastic scattering and their decay times have been observed by detecting the 0.51-Mev annihilation gamma rays as a function of time. The experimental method utilized a pulsed neutron source and the standard ring geometry often used for inelastic scattering measurements. Proton excitation of ${\mathrm{Ca}}^{40}$ in a different geometry was also used. To measure the lifetimes, the pulse-height spectra from a NaI(Tl) scintillation counter were recorded as a function of the delay time after the exciting pulse, and the intensity of the annihilation radiation was obtained from the area under the photopeak at 0.51 Mev. Selection of the time delays was made with the aid of either a time-to-pulse-height converter or a fast coincidence circuit. Mean lives for the first excited states of ${\mathrm{Ca}}^{40}$ and ${\mathrm{Zr}}^{90}$ were found to be (3.4±0.2)×${10}^{-9\mathrm{}}$ sec and (90±6)×${10}^{-9\mathrm{}}$ sec, respectively. Corresponding values of the reduced matrix elements $\rho$ are 0.15 and 0.056.