Advancing hidden nuclear material detection.

• Author: Jean, Grace V.
• Position: INSIDESCIENCE+TECHNOLOGY

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...