Detecting Dirty Bombs from a Safe Distance

Tracking and locating hazardous radioactive materials has become an immense concern for the United States Department of Homeland Security. In the wrong hands, materials snatched from a hospital or a nuclear plant could be used to build a “dirty bomb” that might cause major harm to human health and the environment.

Researchers at the University of New Hampshire (UNH) have now built a highly sensitive device for detecting radioactive materials remotely, using spare detectors from NASA’s Compton Gamma-Ray Observatory. The detectors were originally intended to measure gamma rays in space. “It is a very robust, reliable, and precise piece of instrumentation that has already been proven to work in space,” says James Ryan, the lead researcher for the telescope and a professor of physics at UNH. “If it works in space, you bet it will work on the ground.”

Security personnel currently use handheld detectors to locate radioactive materials, but these instruments are often affected by background radiation and cannot always pinpoint the source of radiation accurately. “This is an important area that needs new detectors,” says Neil Gehrels, chief of the astroparticle physics laboratory at NASA’s Goddard Space Flight Center, in Greenbelt, MD. “The challenge is to build a system that is sensitive and detects the kinds of gamma rays that would be of security concern, and two, to build a robust system that can be used in the field.”

The new telescope, called Gamma-Ray Experimental Telescope Assembly (GRETA), can fit into the back of a truck, allowing security personnel to spot high-energy gamma rays simply by scanning an area. “Our instrument can detect different radioactive isotopes in different places with one exposure,” says Ryan. The researchers also developed software to operate the telescope and analyze data in real time.

“Ryan’s work is really outstanding, and he has one of the best systems for sensitive, wide-field gamma-ray detection,” says NASA’s Gehrels. “The detectors can sense sources from afar and use a technique that allows them to see exactly where the source is located.” While there are other groups working on similar systems, Gehrels says that the UNH group is leading the field.

The telescope comprises two different cylinder-shaped “scintillation” detectors that work in tandem, surrounded by large glass photomultiplier tubes. When a gamma ray hits the front detector, the energy is scattered and absorbed by the rear detector. The energy is then converted into visible light and detected by the highly sensitive photomultiplier tubes. The specific color and intensity of the light can identify the radioactive material involved, says Gerald Share, a visiting senior research scientist in the department of astronomy at the University of Maryland, who was not involved in the work.

“The instrument measures how much energy is deposited in each detector to calculate the total energy of the gamma ray,” says Ryan. “This tells you what the radioactive isotope is.” The researchers have hooked up the telescope to a laptop and developed software that is easy to control.

The researchers will present their work at the 2009 IEEE International Conference on Technologies for Homeland Security. “Since the 9/11 terrorist attacks, we realize we are in a dangerous world, and it is essential to put money into technologies like this,” says Share. “We have no other choice.”

Ryan says that the telescope could be used in a variety of national-security settings, such as for cargo inspections and to search for rogue nuclear weapons.