A TWIN-TARGET TECHNIQUE FOR MEASUREMENT OF NUCLEAR LIFETIMES BY DOPPLER SHIFT ATTENUATION METHOD

Doppler shift attenuation measurements of nuclear lifetimes are described for a technique which minimizes the effects of instabilities in the γ-ray detector. The excited nuclei are produced by passing a single beam through two targets, only one of which has a backing to attenuate the Doppler shift. Particles emitted during the production of the excited nuclei are detected by surface barrier counters associated with each target and the de-excitation γ-ray spectra from both targets are viewed by a single NaI(Tl) crystal. The use of particle-gamma coincidences serves to identify not only the nuclear excited state and the direction of nuclear recoil but also the target in which the event occurred. Thus simultaneous measurements are made on the γ-ray with and without attenuation of the Doppler shift. The lifetime is derived form the relative positions of the two gamma-ray peaks. Corrections are discussed for the effects of the finite detector geometries and for the details of the slowing down of the recoil nuclei. As a test case, the lifetimes of the 1.37-MeV and 4.23-MeV states in 24Mg were measured to be (1.51±0.17)×10-12 sec and (8.3±1.6)×10-13 sec, respectively, in good agreement with earlier work. © 1968.