Laser Breaks Neutron Beam Record

The largest neutron beam ever achieved using a short-pulse laser could lead to more advanced materials measurement.

Neutron beams typically are made with particle accelerators or nuclear reactors and are commonly used in a variety of scientific research, particularly in advanced materials science.

“Neutrons are a unique probe with many scientific applications,” said Frank Merrill of Los Alamos National Laboratory’s (LANL) neutron science and technology group. “Neutrons are used to study fundamental properties of the universe, advanced materials and have potential applications such as active interrogation of cargo containers, monitoring for clandestine nuclear explosives at border crossings, and as a test bed for fusion-relevant neutron diagnostics — the initial impetus for this study.”

Now, Merrill and colleagues from LANL, the Technical University of Darmstadt, Germany, and Sandia National Laboratories have used LANL’s Trident — a 200 trillion W short-pulse laser — to generate the largest neutron beam ever created.

Tom Hurry of Plasma Physics adjusts the target positioner and particle beam diagnostics prior to an experiment at Trident. (Image: LANL)

Five times larger than the previous record, it requires less than a quarter of the laser energy, said Markus Roth of Technical University.

The scientists used the short-pulse laser to focus high-intensity light on an ultrathin plastic sheet infused with a deuterium isotope of hydrogen. The 200 quintillion W/cm² laser light interacts with the plastic sheet, creating a plasma that accelerates large numbers of deuterons — the nucleus of the deuterium atom — into a sealed beryllium target, which converts the deuterons into a neutron beam. Using the relativistic transparency property of plasmas, the deuterons are accelerated in a fraction of a millimeter, rather than the many meters required by standard accelerator technologies.

“So far, only at Trident has this new plasma acceleration mechanism been successfully implemented,” said Roth, who serves as the 2012 Rosen Scholar at Los Alamos. “This result is the world’s record for short-pulse laser-generated neutron flux, four quintillion neutrons per square centimeter for an object one centimeter from the source. In this generation scheme, the neutrons are emitted along the direction of the initial laser beam and can reach very high energies, in excess of 50 million electron volts.”

This record neutron beam has the speed and energy range that makes it an ideal candidate for radiography and a variety of high-energy-density physics studies.

“An object placed one centimeter behind the source would be exposed to more than 40 neutrons per square micrometer (one-millionth of a meter) in less than a nanosecond (one-billionth of a second) making it an impressive probe for radiography applications,” Merrill said.

“Also, for the first time, in these experiments a neutron image driven by a short-pulse laser was realized and showed excellent agreement with numerical calculations,” Roth said. Using short-pulse lasers for the production of neutrons can open the field of neutron research to universities and to a broader research community in general.

For more information, visit: www.lanl.gov