Lasers made muon beams, no massive accelerator needed

Creating muons, the subatomic particles, has become significantly easier.

Multiple research teams have successfully produced muons using compact particle accelerators powered by lasers. Traditionally, generating muon beams required massive facilities. Muons, which can penetrate solid materials much like enhanced X-rays, hold the potential for portable scanning devices that could detect illicit substances like plutonium and uranium within shipping containers.

Rajeev Pattathil from the Rutherford Appleton Laboratory in Didcot, England, emphasizes, “For penetrating meters of concrete, stone, or metals, muons are unparalleled.”

Scientists have long utilized naturally occurring muons, formed from interactions in Earth’s atmosphere, to peer inside volcanoes, pyramids, and other substantial structures. By measuring muons’ behavior as they traverse an object, they can infer the materials present by noting how much has been scattered or absorbed.

However, natural muons are rare, with only one landing on each square centimeter of Earth’s surface every minute. This scarcity makes imaging a sluggish endeavor, especially in busy shipping ports, where holding a container for hours is impractical. The advent of artificial muon beams may streamline this process.

The cutting-edge method is grounded in miniaturized accelerators that utilize lasers to ionize plasma, creating charged particles. This process engenders a wave of electric charge within the plasma, propelling electrons to high energies. When these speedy electrons collide with dense materials, such as lead, they generate a muon beam.

Researchers at Lawrence Berkeley National Laboratory in California achieved this by accelerating electrons traversing merely 30 centimeters, as reported by physicist Davide Terzani and colleagues on October 8 in Physical Review Accelerators and Beams. This distance is slightly less than that of a bowling pin, resulting in electrons with energies reaching 10 billion electron volts, which led to the creation of muons with energies in the billions of electron volts. Conventional accelerators would need to span a thousand times that length to yield electrons of similar energy levels.

“We can convert a kilometer-sized setup into something that fits within a laboratory,” states co-author Jaron Shrock, physicist at the University of Maryland in College Park. Given the ongoing U.S. government shutdown, researchers at Berkeley Lab could not be reached for comments. Muon identification was based on measuring their decay time, which averages 2.2 microseconds.

Muons, although akin to electrons, are heavier. This property allows them to penetrate deep into materials without significant scattering, explains Gianluca Sarri of Queen’s University Belfast in Ireland.

Sarri and his collaborators successfully generated muons at the Extreme Light Infrastructure–Nuclear Physics facility in Măgurele, Romania. Their findings, based on energy deposition measurements in their detector, were documented in a March paper on arXiv.org, still pending peer review. These muons reached energies of around one billion electron volts, according to Sarri.

A similar achievement occurred at the Shanghai Superintense Ultrafast Laser Facility, where muons of comparable energies were produced, as reported in Nature Physics on May 6. This study mainly investigated the production mechanisms of muons at those energy levels rather than focusing on creating a tight beam suitable for imaging, notes Wentao Wang, a physicist at the Shanghai Institute of Optics and Fine Mechanics.

The recent strides in laser-driven accelerators have made this progress possible, according to Pattathil. “It is essential to accelerate electrons to near-light speeds to produce a significant number of muons. Only recently have we achieved sufficient quality with very energetic electron beams,” he explains. Although current laser accelerators are not compact enough for portability, advancements in laser technology could eventually make handheld muon sources feasible.

Researchers have initiated some experimental applications. At Colorado State University, a muon detector and lead object were placed within a truck just outside a facility where another muon beam is generated from a laser-powered accelerator. The lead object created a shadow in muons detectable by the muon detector, a proof-of-concept demonstration shared in March during an online workshop titled Laser-Driven GeV Muon Sources at ELI. “This marks an initial step towards practical applications for these laser-generated muon beams,” Shrock adds.