LIFETIMES OF THE FIRST EXCITED 0+ STATES OF Ca$sup 40$ AND Zr$sup 9$$sup 0$

The first excited states of Ca/sup 40/ and Zr/sup 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 palsed neutron source and the standard ring geometry often used for inelastic scattering measurements. Proton excitation of Ca/sup 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 palse 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 Ca/sup 40/ and Zr/sup 90/ were found to be (3.4 plus or minus 0.2) x 10/sup -9/ sec and (90 plus or minus 6) x 10/sup 9/ sec, respectively. Corresponding values of the reduced matrix elements (subscript p) are 0.15more » and 0.056. (auth)« less