Experimental observation of \(\beta\)-delayed neutrons from \(^{9}\)Li as a way to study short-pulse laser-driven deuteron production

• Published in: arXiv.org
• Main Authors: Favalli, Andrea, Guler, Nevzat, Henzlova, Daniela, Falk, Katerina, Croft, Stephen, Gautier, Donald C, Ianakiev, Kiril D, Iliev, Metodi, Palaniyappan, Sasikumar, Roth, Markus, Fernandez, Juan C, Swinhoe, Martyn T
• Format: Paper
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 18.05.2016
• Subjects: Acceleration; Afterburners; Beryllium; Converters; Decay; Deuterons; Diagnostic software; Diagnostic systems; Interrogation; Ion beams; Laser beams; Lasers; Lithium; Neutrons; Nuclear engineering; Nuclear reactions
• ISSN: 2331-8422

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 \(^9\)Li decay was observed. It was identified by its characteristic half-life of 178.3 ms. Production is attributed to the nuclear reactions \(^9\)Be(d,2p)\(^9\)Li and \(^9\)Be(n,p)\(^9\)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 \(^9\)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 \(\sim\)20 MeV within a cone with a half-angle greater than 40\(^{\circ}\). 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 \(\sim\)20\(^{\circ}\). 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 \(\beta\)-delayed neutrons from \(^9\)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.