Dechanneling in single-crystals due to in-flight secondary decay: a method for measuring short nuclear lifetimes

Abstract

The propagation of excited heavy ions in single crystals is complicated by their possible in-flight decay, and the subsequent random perturbation on their path (dechanneling). We develop here a theory of the dechanneling valid when the decay occurs at the very beginning of the ion path. This permits us to calculate analytically the distribution of initial conditions for ion motion, that is expressed in terms of elliptic integrals. A large sample of ion trajectories is then constructed by a vectorized Monte Carlo code not containing in-flight decay. These trajectories can be appropriately weighted, and used many times, to simulate dechanneling due to different decay functions. All this produces a variant of the blocking method of measuring short interaction times between nuclei. This new method can be applied when the composite system, formed by the interaction of an ion beam with a crystal target, emits primary fragments in a too short time to be measured by the standard blocking technique. In this case, the blocking dip is sensitive to the secondary time delay, i.e. the lifetime of the primary fragment that decays in flight. Times measurable in this way range from some 10 −18 s to 10 −16 s. As an example, we re-analyse the data of a previous blocking experiment on the 16O + 28Si system, that revealed unexpected long reaction times. We show that, if interpreted as delays associated to in-flight decay, such results do not contradict standard statistical model calculations.