|Weiyi Wang points out a gamma ray interaction in the 18-crystal Polaris detector, shown on the screen of the attached computer almost in real time. Polaris software narrows down the origin of a gamma ray to a cone.|
The imager sees the world in gamma rays, which are like rays of visible light but with up to about a million times more energy. It contains crystals of very pure semiconductor that absorb the gamma rays, measuring their energies. The energies of the gamma rays emitted by a radioactive material are like a bar code, giving away its identity.
To figure out where a gamma ray is coming from, the detector looks at the way it bounces off an atom before being absorbed. The software can work out the angle of the bounce, which narrows down the gamma ray’s origin to a cone. After measuring several gamma rays, an intersection point appears – or perhaps more than one intersection, if gamma rays are coming from multiple places.
Wanted: radiation imager
While a gamma ray imager might not be this summer’s must-have accessory, He’s team is confident that the Department of Defense (DOD), Department of Energy (DOE) and Department of Homeland Security are all interested. The DOD has supported the development of semiconductor detectors and high performance electronics critical to these imagers since 2006, and the DOE has supported a group of graduate students. The ability to identify and locate nuclear materials could help head off would-be nuclear terrorists.
In order to get a feel for the potential security market, He’s team invited agents of law enforcement, emergency services, and the FBI to their lab for a demonstration of Polaris. “The overwhelming response was really positive,” says Michael Hopkins, an MBA candidate in U-M’s Ross Business School who joined He’s team in December.
|The red spot on the computer screen shows where in the room the gamma rays are coming from, while Weiyi Wang reveals the size of the spot in real space. In the background, James Berry upgrades an older model of the Polaris detector.|
Earlier that month, the team also welcomed engineers from the Fermi 2 reactor near Monroe, MI, and the Cook nuclear power plant near St. Joseph, MI, to their laboratory. When the contingents from the power plants saw what the new detectors could do, Weiyi Wang of He’s group says, “They got really excited.” The plants are presently equipped with detectors that operate at -321°F and only identify radiation – they can’t locate the sources. The plant engineers were intrigued enough to invite He’s team to the reactors in order to test the prototypes this month and next.
|A prototype radiation detector for use in nuclear power plants. The black protrusion at the top is a USB port for plugging in a laptop computer, which turns the data into images.|
In order to find these areas, Wang says, “the detector needs to be pushed against the pipe to survey it point by point,” making the task dull and time-consuming. An imager could pinpoint all hot spots in a snapshot. Likewise, the plant workers would like to image hot spots in reactor cavities, making sure that the cavity walls are clean after refueling operations.
|Prototype from the other side. The radiation image taken by the detector is layered over the image from the phone camera with a fish-eye lens, showing He's team where in the room the radiation is coming from.|
However, He’s lab has also produced a briefcase-sized 18-crystal model to demonstrate how the sensitivity of the detector improves with additional crystals. That model can pick up radiation that is even weaker than the natural background radiation from sources like the sun and Earth, says He. This version is more appealing for defense purposes. Alternatively, in the event of a nuclear accident, He says, “the large system would be an excellent choice to detect fallout and nuclear contamination inside and outside of the power plant.”
Beyond nuclear safety, medicine and space science need radiation detectors as well. Doctors can give patients small amounts of radioactive molecules that are taken up by specific kinds of tissue. Because the molecules collect in their target locations, tumors or organs show up as bright spots in the gamma ray image. Scientists also study rocks and water on planets such as Mars with the help of gamma rays. The solar wind can make atoms on planetary surfaces temporarily radioactive, allowing researchers to identify them by their gamma ray emissions.