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arXiv · 1605.05702

Experimental observation of $β$-delayed neutrons from $^{9}$Li as a way to study short-pulse laser-driven deuteron production

Andrea Favalli·Nevzat Guler·Daniela Henzlova·Katerina Falk·Stephen Croft·Donald C. Gautier·Kiril D. Ianakiev·Metodi Iliev·Sasikumar Palaniyappan·Markus Roth·Juan C. Fernandez·Martyn T. Swinhoe
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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 $^9$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 $β$-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.

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Andrea Favalli, Nevzat Guler, Daniela Henzlova, Katerina Falk, Stephen Croft, Donald C. Gautier, Kiril D. Ianakiev, Metodi Iliev, Sasikumar Palaniyappan, Markus Roth, Juan C. Fernandez, Martyn T. Swinhoe. 2016-05-18. Experimental observation of $β$-delayed neutrons from $^{9}$Li as a way to study short-pulse laser-driven deuteron production. https://arxiv.org/abs/1605.05702

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