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Ultrafast science

Ultrafast science

Neutrons on a lab bench

30 Jan 2013
Shining bright

A new compact high-flux source of energetic neutrons has been built by physicists in Germany and the US. The new laser-based device has the potential to be cheaper and more convenient than the large neutron facilities currently used by physicists and other scientists. The inventors say the source could be housed in university laboratories and might also be used to identify illicit nuclear material.

Neutrons are a valuable tool for scientists in many fields, allowing them to probe the structure and dynamics of a range of materials. Today, the main drawback of neutron science is that intense beams of neutrons must be produced in either nuclear reactors or dedicated accelerator facilities – making a laser-based table-top source very attractive.

Low fluxes

Laser-based sources involve creating very brief pulses of high-energy electromagnetic radiation, which ionize a small solid target and then propel the liberated electrons to the back of the target, so creating a very strong electric field that in turn accelerates the ions. The ions – typically deuterons, which comprise one proton and one neutron – then stimulate nuclear reactions in a second target, producing neutrons. Despite a decade of research, however, the resulting neutron fluxes have remained low. This is largely because charged molecules such as water vapour contaminate the target surface and are accelerated at the expense of the ions.

In 2006 Lin Yin and Brian Albright at Los Alamos National Laboratory in the US showed how this problem might be overcome. They used computer simulations to show that an intense laser beam can penetrate a thin solid target. Usually a solid object is opaque because the frequency with which its constituent electrons vibrate exceeds that of the incoming light. But Yin and Albright calculated that a very intense laser beam should be able to boost the speed of electrons in a plasma to such an extent that their relativistic mass significantly reduces the electrons’ frequency to below that of an infrared laser.

Breakout afterburner

Yin and Albright named this effect the “laser breakout afterburner” because in “breaking out” to the far side of the target the laser beam would re-energize electrons that have lost energy in accelerating ions, so allowing those ions to reach higher energies. The beam would also interact with the entire target, rather than just the atoms on the surface, meaning that many more deuterons would be accelerated, so increasing the neutron flux.

This scheme has now been put into practice by Markus Roth of the Technische Universität Darmstadt and colleagues at Los Alamos and Sandia National Laboratories. Roth’s team directed extremely powerful and well defined pulses from the Los Alamos TRIDENT laser onto a 400-nm-thick plastic target doped with deuterium atoms. This was positioned just 5 mm in front of a secondary target made from beryllium.

Even though the pulses delivered less than a quarter of the energy employed in previous experiments, they produced neutrons that were nearly 10 times as energetic – up to 150 MeV – and also nearly 10 times as numerous. In addition, many of these neutrons were emitted in the forward direction, which the researchers attribute to one specific kind of nuclear reaction, the break-up of deuterons.

First radiographs

Roth’s group also took the first radiographs using a laser-driven neutron beam, by placing a series of tungsten, steel and plastic objects between the neutron source and a scintillating fibre array that was linked to a CCD camera.

Hopefully this will make neutron science available to many university students
Markus Roth, Technische Universität Darmstadt

Roth says that although his group’s device produces fewer neutrons than reactors or accelerators do, it packs the neutrons into extremely short pulses – each lasting just a few 10-billionths of a second. This, he explains, makes it suitable for applications that need high temporal resolution, such as pump-probe investigations of neutron damage inside nuclear reactors or monitoring simulations of conditions inside planetary cores. And he claims that, once commercialized, the entire device will fit on a lab bench and that only the target will need shielding. “The really cool thing for me as a university professor is that we replaced an accelerator hundreds of metres long with a laser,” he says. “Hopefully this will make neutron science available to many university students.”

The group will now work on tailoring the device’s energy spectrum – low-energy neutrons being useful for studying matter under extreme conditions, for example, whereas high energies are needed for the inspection of sensitive material inside containers. It is for this counter-terrorism application that the device could find its first customers. “We have started a network with US laboratories and universities to develop a system that can be sold commercially within the next five to six years,” he explains.

Boosting repetition rate

Laser-driven neutron expert Scott Wilks of Lawrence Livermore National Laboratory in the US points out that non-laser based neutron sources small enough to fit in a suitcase can generate comparable numbers of neutrons, but, he says, over a time interval measured in seconds and at much lower energies. This makes them less good at imaging very short-lived phenomena. The next step, Wilks adds, will be to increase the laser’s repetition rate, which, he predicts, “will be no small feat, but, given laser technology’s rapid evolution, inevitable”.

The device is likely to have its limits, cautions Bob Cywinski of the University of Huddersfield in the UK. He agrees it could be useful for applications requiring single shots of neutrons, such as nuclear-materials monitoring or radiation-damage studies, and might, if its time-averaged flux can be made high enough, be suited to nuclear-waste transmutation. However, Cywinski thinks the average flux will be too low to replace reactors and accelerators for conventional neutron-scattering applications.

The research is published in Physical Review Letters.

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