Lifetime measurements of excited states in 219Rn by observation of conversion-electron doppler shifts following α-decay

Abstract

A method is described in which the observation of conversion- electron Doppler shifts is used to measure nuclear lifetimes in the picosecond range. The isometric nuclei recoil into the vacuum chamber of an orange-type β-spectrometer following α-emission. The recoil velocity is measured directly by a time-of-flight technique. For the lifetime measurements the recoil distance is limited by a stopper foil, the position of which is varied in a reproducible way using the thermal expansion of a metal holder rod. Since the distances are in the micrometer region, an optical interference method can be applied to measure directly the variation in distance. The fraction of conversion electrons in the unshifted conversion line represents the fraction of isomeric nuclei which do not decay in flight. The nuclear lifetime is derived directly from the variation of this quantity with the recoil distance. This method should be applicable to many excited states of nuclei in the Z > 82 region, where common techniques fail. Lifetimes are accessible in the range 1 ps<T12<100 ps. An application to three excited states in 219Rn gave the following results: T12(159 keV) = (17.7±2.6) × 10−12s, T12(269 keV) = ( 4.4±1.0) × 10−12s, and T12(339 keV) = ( 8.1±2.8) × 10−12s.