James Popp and Kevin Lynch: Searching for a New Method To Detect Nuclear Material

James Popp and Kevin Lynch, York College

After the Sept. 11 terrorist attacks, Congress passed three laws to tighten security at U.S. ports, where hundreds of thousands of shipping containers arrived unscreened – the perfect scenario for smuggling nuclear materials and making dirty bombs.

In 2014, the U.S. General Accountability Office reported “substantial progress” in tightening security, including cargo inspections abroad, computer-guided inspections and 1,400 radiation monitors. Yet, GAO said, the threat remains, partly because Homeland Security’s Domestic Nuclear Detection Office had to scrap a $3 billion next-generation radiation-detector program after spending more than $280 million on development.

L-R: Dr. Kevin Lynch and Dr. James Popp, high energy physicists and professors in the Physics Department at CUNY's York College fix high vacuum equipment in the experimental high energy physics lab. © Audrey C. Tiernan
Two physicists at York College may have a partial – and far cheaper – solution, which they’re developing with a three-year, $300,000 grant from the Air Force Office of Scientific Research.

Professors James Popp and Kevin Lynch of York’s Department of Earth and Physical Science don’t rely on bulky, power-hungry X-ray equipment or, like the scuttled federal program and current port monitors, on the rare helium-3 isotope, which is used to detect the neutrons emitted by radioactive materials.

Rather, using off-the-shelf equipment, they will detect naturally occurring particles called muons, which form when cosmic rays collide with atoms in Earth’s atmosphere. Muons have most of the properties of electrons, but they penetrate deeper and deflect only slightly when they pass through an object – except if it is unusually dense, like the metallic shielding used to contain radioactive materials.

Muon probes were used in the 1950s and ’60s to look inside pyramids and, after the 2011 Fukushima disaster, to examine the damaged and highly radioactive power plants. “The twist we’re putting on it is: Rather than using specialized physics particle detectors, which are one-off specialty items, we’re using standard components in a device that’s the size of a breadbox,” Lynch said.

Their method leverages recent advances in smart phone electronics – literally the computer that runs your smart phone – and modern developments in detector technology in high-energy physics, they said.

They will place two detectors on opposite sides of a container, synchronize them and count the muons that pass through. The shifts in trajectory that occur when muons hit high-density shielding will collectively cast a shadow, revealing the shape of what’s inside slice by slice, vaguely like a medical MRI body scan. The equipment, housed in a sealed, all-weather, shock-resistant case, will be mobile and easily deployed, acquire data automatically and analyze it with an onboard, industrial-grade server; then it will tell the operator what to do next.

The professors are now building a proof-of-concept prototype, after spending a year designing the device and selecting components.

Separately, the Department of Energy and the Fermi National Accelerator Laboratory are funding their participation in the “Mu2e” experiment, a collaborative search by several hundred physicists and engineers for a theoretical quirk in muon behavior.

A muon typically decays into an electron, a neutrino and an antineutrino. However, certain theoretical models predict, in rare instances (less than one in 10 to the 16th events – or 1 followed by 16 zeroes) muons will decay into electrons without producing neutrinos and antineutrinos. The Mu2e experiment is designed for 10 to the 17th power sensitivity, 10,000 times more accurate than the current 10 to the 13th power state-of-the-art.

The experiment hinges on this fact: When a muon that is bound in an atomic-like orbit about a elemental nucleus converts directly into an electron, the resulting electron has a single energy that is characteristic of the nucleus to which it was bound. The Mu2e apparatus they are helping to develop will filter out electrons with normally occurring energies, allowing only those with this one unique energy to get through.

Popp and Lynch are assembling several critical components of that filtering system – a one-of-a-kind, high-resolution, particle-tracking detector to measure the energy of electrons created by muon decay. Their detector is “a 10-foot-long collection of 22,000, three-foot, metal-coated drinking straw-size tubes that are filled with gas and have a wire in the middle,” Lynch said. “As you shoot electrons through a magnetic field, they travel along a helical [screw-like] trajectory. The detector will measure the shape of that curve and reveal what the electrons’ energy is. This event will unambiguously identify the rare hypothetical muon decay we are searching for.”

Regardless of the results, Mu2e will have a major impact on the understanding of the fundamental laws of physics.

If the experiment fails to find neutrino-free decay, it would undercut scores of Grand Unified Theories – that is, theories that seek to merge three of the universe’s four fundamental forces (electromagnetic, weak and strong, but not gravity) into a single force, as may have existed when the universe was born in the Big Bang. That would be real science.

But if Mu2e does indeed find neutrino-free muon decays, “It would be spectacular, a major discovery,” Popp said.

As Lynch explained: “It’s as if you know that not a single one of the 100 billion people who have lived in the history of the planet had blue hair, and then you find one with blue hair. You’d have to rewrite the rules of genetics.”