Meet the machines that matter: the portable neutron detector

Nuclear materials are rarely quiet. Even when hidden inside a sealed container, they emit a kind of heartbeat through streams of particles and radiation that carry critical information about its contents.

In a laboratory, scientists have the tools and expertise to read that heartbeat. But in an emergency, first responders need a way to interpret it quickly, without highly specialized training. That’s why officials at the U.S. Department of Energy initiated the development of a portable neutron multiplicity counter to translate those emissions into actionable information.

After several iterations — including the Short 14 and Fission Meter by Lawrence Livermore National Laboratory (LLNL) and the nPod by Los Alamos National Laboratory — scientists at the national laboratories, pushed by the DOE, converged on a single solution: the MC-15.

First responders need a point-and-shoot camera that takes blueprints instead of photographs. An as-built blueprint is crucial to assess a threat device’s viability and yield and to inform render-safe planning,” said Sean Walston, LLNL physicist. “The MC-15, along with gamma-ray spectroscopy and radiography, is one of the three pillars that constitutes such a point-and-shoot camera.”

Designed for field use, the MC-15 became the preeminent neutron multiplicity counter for response to nuclear-based threats in the U.S. and, increasingly, around the world.

The 2019 R&D 100 Award-winning machine operates by not just recording that a neutron was detected, but also when.

“The defining quality of a special nuclear material is the ability to sustain a fission chain,” said Walston. “A fission chain occurs when a neutron strikes a fissile atom, causing the nucleus to split and to release additional neutrons that trigger more fission in nearby atoms, and so on.”

Fifteen tubes filled with helium gas are embedded in polyethylene inside the MC-15 to record when a neutron is detected to within 100 nanoseconds. Hydrogen in the polyethylene slows down the neutrons so the helium-3 proportional counters can detect them, and a field programmable gate array records when the event occurred and in which tube.

“Neutrons from fission chains come in bursts separated by gaps,” said Walston. “By understanding the neutron time correlations, we can estimate the amount of special nuclear material from the sizes of the gaps, and its configuration from the lengths of the chains.”

A cursory data analysis is displayed on the detector’s touch-screen interface, ensuring accessibility for emergency responders without technical backgrounds. Multiple detectors can also be connected to function as a single, larger instrument for significantly higher data quality.

Rigorous testing and real-world deployments have demonstrated the device’s reliability in even the harshest environmental conditions. Scientists at LLNL conducted storage and transportation simulations by performing vibration and drop tests, as well as subjecting the detector to extreme temperature, humidity, moisture and dust conditions.

“Our goal was to create a neutron multiplicity counter that could be deployed quickly and provide clear, actionable information to responders in any scenario,” said Walston.

Since its development, the MC-15 has been deployed by the National Nuclear Security Administrations’ Nuclear Emergency Support Team (NEST) and federal partners to provide accurate results when it matters the most. The detector is behind-the-scenes, ready to support first responders in investigating a potential radiological threat.