Velocity distribution in Recoil-Distance Doppler-Shift experiments

The Recoil-Distance Doppler-Shift (RDDS) or Plunger technique is a well established method to measure lifetimes of excited nuclear states in the pico-second range. In standard RDDS experiments, the velocities of the nuclei of interest emerging from a usually thin target foil are distributed around a mean velocity v¯=⟨v⟩v with a relatively narrow width and it is sufficient to assume that all nuclei move with the average velocity. In this paper we investigate the influence of a broader velocity distribution especially for lifetimes τ determined using the DDCM and its basic relation τ=−(R(x)−Rfeed(x))/(dR(x)/dx)v and simulated experimental data (R(x) decay curve of the level of interest, Rfeed(x) feeding decay curves). It turned out that it is favorable to use ⟨1/v⟩v instead of 1/⟨v⟩v. Further, deviations from the correct lifetimes practically vanish at target to stopper separations close to the maximum amplitude of the function dR(x)/dx. As a consequence in a plunger experiment target-to-stopper separations should be selected symmetrically around the maximum amplitude of the function dR(x)/dx in order to minimize the effect of a broad velocity distribution