Experimental observation of -delayed neutrons from Li as a way to study short-pulse laser-driven deuteron production
arXiv:1605.05702
Abstract
A short-pulse laser-driven deuteron beam is generated in the relativistic transparency regime and aimed at a beryllium converter to generate neutrons at the TRIDENT laser facility. These prompt neutrons have been used for active interrogation to detect nuclear materials, the first such demonstration of a laser-driven neutron source. During the experiments, delayed neutrons from Li decay was observed. It was identified by its characteristic half-life of 178.3 ms. Production is attributed to the nuclear reactions Be(d,2p)Li and Be(n,p)Li inside the beryllium converter itself. These reactions have energy thresholds of 18.42 and 14.26 MeV respectively, and we estimate the (d,2p) reaction to be the dominant source of Li production. Therefore, only the higher-energy portion of the deuteron spectrum contributes to the production of the delayed neutrons. It was observed that the delayed-neutron yield decreases with increasing distance between the converter and the deuteron source. This behavior is consistent with deuteron production with energy greater than 20 MeV within a cone with a half-angle greater than 40. Prompt-neutron time-of-flight measurements at varying separation between the converter and the laser target indicate that the fast deuteron population above threshold is severely depleted on axis out to 20. These measurements are consistent with emission of the fast deuterons (i.e., above 10 MeV/nucleon) in a ring-like fashion around the central axis. Such an inferred ring-like structure is qualitatively consistent with a documented signature of the breakout afterburner (BOA) laser-plasma ion acceleration mechanism. The measurement of -delayed neutrons from Li decay could provide an important new diagnostic tool for the study of the features of the deuteron production mechanism in a non-intrusive way.