A pioneering experiment using the CLF's Vulcan laser has successfully demonstrated efficient moderation of laser-driven fast neutrons (~MeV) to epithermal energies (eV-100keV) with intrinsically short burst durations, illustrating the feasibility of driving intense bursts of epithermal neutrons with short pulse lasers.
Over the past few decades there has been sustained interest in laser-driven neutron sources - owing to their ability to generate sub-nanosecond bursts of fast (MeV) neutrons with high brightness - that can be used in fields such as imaging and material testing for fusion reactor vessels.
The field of neutron science, however, predominantly requires slow neutrons (sub-meV to keV energies) which have typically been generated by moderating fast neutrons at reactor and accelerator based facilities. What this paper [1] - published in the journal Applied Physics Letters -successfully demonstrates, is an alternative method for generating epithermal neutrons, namely using laser-based sources which, as the article describes, are “fast-approaching a crucial stage in their development for neutron science."
In collaboration with CLF application development scientists and ISIS neutron beamline and detector scientists, a team led by Queen's University Belfast used the 100 TW arm of the Vulcan laser facility and a specifically designed downstream compact moderator (10 cm x 10cm x 8cm), to produce a significant epithermal neutron flux of the order 105 neutrons/sr/pulse in the energy range of 0.5-300eV. The moderator in question, placed around 11cm from the source, was specifically designed with the help of MCNPX radiation and particle transmission simulations, to provide efficient and directional moderation of the fast neutron spectrum produced by the laser.
Whilst there is still scope for improvement, for example with regards to the attainable epithermal neutron flux, the group hope that the results of their experiment will pave the way for scientists to acquire novel insights into the structure and properties of matter using laser-based systems.
“Moderated neutrons are much more applicable than fast (MeV) ones for imaging and inspecting hydrogen products like water, corrosion and oil within large dense objects, so this is a great step forward in the development of laser-driven accelerator sources for industrial applications," explains co-author Dr. Ceri Brenner, of the Central Laser Facility.
The research was supported by EPSRC and STFC and the full publication is available to view in Applied Physics Letters
For further information about the research, please contact Dr. Satyabrata Kar (s.kar@qub.ac.uk)
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