Advancing Hidden Nuclear Material Detection

By Grace V. Jean
On any given day, ships and trucks deliver cargo containers filled with tons of imported goods. Homeland security officials have long warned that terrorists may use them to smuggle nuclear materials into the United States.

Detecting radioactive substances at ports and border crossings has remained problematic for two reasons. First, most smuggled nuclear materials would not emit much energy in the first place. Picking up those signals is a fundamental physics problem, said Andrew Wiedlea, deputy branch chief of the Defense Threats Reduction Agency’s innovation and systems engineering office. Because of the low energy emissions, available portal technologies that attempt to detect them often have high false alarm rates. Moreover the systems cannot find heavily shielded nuclear materials and some detectors are simply too large to fit existing border inspection lanes.

To help solve the problem, military scientists are pursuing solid-state materials that may one day yield detectors that can accurately locate radioactive substances and also fit into devices small enough for troops to pin onto their collars.

There are several ways to detect radioactive materials. One method is to sense gamma rays, or high-energy photons emitted by nuclear compounds. When gamma rays collide with certain materials, such as plastic, they give off lower-energy photons of visible light. The photons can be converted into electrons to generate a measurable electrical pulse.

Another way to find nuclear reactive materials is by detecting the emission of neutrons, or the non-charged particles found in atoms. Uranium and plutonium — the two radioactive compounds used in modern nuclear weapons — emit neutrons through their natural decay process.

Conventional neutron detectors consist of metal tubes containing helium-3 gas. When a high voltage is applied to a fine wire running the length of the tube, any passing neutrons create a nuclear reaction with the helium-3 atoms. The atoms split into two particles, a proton and a triton. A triton consists of a single proton and two neutrons. Those particles zoom through the rest of the gas and collide with other helium-3 atoms. The collisions knock loose electrons, which are attracted to the tube or the wire depending on polarity and cause a sudden jump in the electrical current.

The problem with the existing gas-based detector technology lies in its unwieldy baseball bat-size and the decreasing supply of helium-3. Scientists are pursuing solid compounds, including boron nitride, to replace the gas.

“You get a lot more boron atoms per volume in this solid boron nitride than you get in the helium-3 atoms per unit volume in a tube,” said Wayne McGinnis, the scientist leading the research at the Navy’s Space and Naval Warfare Systems Center in San Diego. The resulting solid-state device is much smaller than its tubular gas counterpart, but just as effective, he said.

One configuration of a commercially available helium-3 gas detector comes in a tube about 50 centimeters long with a 1.27-centimeter diameter. For the same sensitivity to neutrons, the solid-state detector would contain a 4-centimeter by 4-centimeter square film of boron nitride that is 100 microns thick. There are 1,000 microns in a millimeter.

“For the same neutron flux, you can have a much smaller detector yet still count the same number of neutrons coming in because of the high density” of boron atoms for nuclear reactions, said McGinnis.

The scientists sandwich a thin layer of the boron nitride between several electrodes. To generate an electric field to attract electrons that are freed up by the neutron reactions, only tens or hundreds of volts need to be applied, compared to thousands or more volts in the helium gas-based system.

One challenge in using boron nitride as a neutron detector is that the material itself has to be of high quality. Boron nitride often comes in polycrystalline form, where there are multiple tiny crystals all crunched together. With the naked eye, the material looks like white silt. But a closer inspection reveals grain boundaries that can trap freed electrons, which means one would not see a current pulse to indicate a nuclear reaction in the detector, explained McGinnis.

With a team of scientists at the University of Michigan, McGinnis is working to improve the material. Researchers are growing films in a hexagonal crystal structure and testing them in the neutron detector.

“The focus is on getting a high enough quality material grown so that it will work as a detector to see those current pulses,” said McGinnis. “As we do that, we will be testing the bench shop prototype detectors we’ve made with neutron sources that we have access to,” he said.

When scientists placed two of the detectors back to back, they could tell from which direction neutrons were emanating because the concentration of colliding particles was higher at the closer device.  

Scientists believe that fitting six parallel detector units in a cube shape will further identify the location of a neutron source. The team recently received a patent for this concept.  

Once the test results are published, McGinnis said researchers will seek industrial partners to produce the system as a deployable detector.

Topics: Science and Engineering Technology

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