Lasers made muon beams, no massive accelerator needed

Producing the subatomic particles called muons is now a lighter lift than ever before.

Several teams of researchers have generated muons using small particle accelerators driven by lasers. Typically, enormous facilities are needed to accelerate particles and make muon beams. Like X-rays on steroids, muons can pass through solid materials and reveal what’s inside. So the feat opens future possibilities for portable scanners that could use muons to reveal contraband such as plutonium and uranium inside shipping containers.

“If you really want to penetrate through meters of concrete or stone or even metals, muons are the best particles to do that,” says Rajeev Pattathil of the Rutherford Appleton Laboratory in Didcot, England, who was not involved with the research.

Scientists have used naturally existing muons, created in reactions in Earth’s atmosphere, to glimpse the interiors of volcanoes, pyramids and other large, stationary structures. By detecting muons before and after they’ve passed through an object, scientists can determine how much the particles have been absorbed or scattered, giving a sense of what materials lie within.

But those naturally occurring muons are scarce. Just one muon per minute falls on each square centimeter of Earth’s surface, making imaging a slow process. In a busy shipping port, “you can’t really keep a container there for hours on end until you are able to get an image,” Pattathil says. Artificial beams of muons could make that process fast enough to be practical.

The new technique is based on scaled-down accelerators that use a laser to blast plasma, a soup of charged particles. That blast sets up a wave of electric charge in the plasma that accelerates electrons to high energies. When those electrons smash into a dense material like lead, they produce a beam of muons.

That’s what researchers did at Lawrence Berkeley National Laboratory in California. The electrons were accelerated over just 30 centimeters, physicist Davide Terzani and colleagues report October 8 in Physical Review Accelerators and Beams. That’s a bit shorter than the length of a bowling pin. The electrons had energies up to 10 billion electron volts, resulting in muons with energies of several billion electron volts. A traditional accelerator would need to be a thousand times that length to produce similarly energetic electrons.

“You can turn a kilometer-scale machine into something which fits inside of a laboratory,” says coauthor Jaron Shrock, a physicist at the University of Maryland in College Park. Terzani and other researchers from Berkeley Lab were unable to comment given the ongoing U.S. government shutdown. The researchers identified the muons based on how long they took to decay — an average lifetime of 2.2 microseconds.

Muons are similar to electrons, but heavier. That means “they can really penetrate deep into targets without being scattered too much,” says physicist Gianluca Sarri of Queen’s University Belfast in Ireland.

Sarri and colleagues conjured up muons at the Extreme Light Infrastructure–Nuclear Physics facility, or ELI-NP, in Măgurele, Romania. The team’s evidence for muons was based on measuring how the particles deposited energy in their detector, they reported in a paper posted at arXiv.org in March, which is not yet peer-reviewed. The muons had energies up to roughly one billion electron volts, Sarri says.

And an experiment at the Shanghai Superintense Ultrafast Laser Facility created muons of similar energies, researchers reported May 6 in Nature Physics. That study focused on characterizing the mechanisms by which muons are produced at those energies, rather than creating a tight beam honed for making images, says Wentao Wang, a physicist at the Shanghai Institute of Optics and Fine Mechanics.

The work is a result of the recent rapid development of laser-driven accelerators, Pattathil says. “You need to speed up electrons to very close to the speed of light for you to be able to produce an appreciable amount of muons,” says Pattathil. “It’s only now, in the last few years, it’s been possible to create high quality electron beams that are extremely energetic.” Although laser accelerators are not yet compact enough to be carted around, future advances in laser technology could make portable muon sources a reality.

Researchers are beginning to test the possibilities. At Colorado State University, researchers placed a muon detector and a lead object inside a truck, just outside a facility with another muon beam born from a laser-powered accelerator. The object cast a shadow in muons that was visible on the detector, researchers reported in March at an online workshop, Laser-Driven GeV Muon Sources at ELI. “This is kind of the proof-of-principle demonstration of an application,” says Shrock, a collaborator on the project, “the beginnings of an application at least — for these laser-produced muon beams.”