Monday, May 21, 2012

Real-world tests for Polaris radiation detector

An invention from the College of Engineering promises to make radiation imaging more portable, with the potential to catch nuclear terrorists, improve safety in nuclear power plants, and map out radiation contamination following accidents like Fukushima.

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.
Just over a year ago, Professor Zhong He and members of his research group in the Department of Nuclear Engineering and Radiological Sciences formed the company H3D to commercialize their radiation imager. Unlike other radiation detectors on the market, theirs can run at room temperature – rather than -321°F – and it can also show where radiation is coming from.

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.
The plant workers have two particular tasks in mind for trying out Polaris technology. The first is finding radiation in cooling pipes. Radioactive particles that get into the cooling water can build up in places like elbow bends, says Wang. The radioactive areas pose an invisible risk to plant workers, and they also cause corrosion.

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.

Starting small

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.
H3D can install as many crystals as desired in their imagers, from basic single-crystal models up to proposed panels, large enough to scan a semi-truck as it passes through a border crossing. Yet H3D will be starting small with one- to four-crystal designs. Prototypes of this size are already in production for testing in nuclear power plants, says Wang. They are hand-held models, roughly the size of a rectangular Kleenex box. Provided that the tests go well, H3D could sell the first of these imagers in late summer.

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.”

Future directions

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.


  1. Great invention! Best wishes getting the attention of DOD and other agencies charged to protect us from radioactive terrorism. You've got a great advocate, spokesman, and persuasive salesman in Mike Hopkins. I should know, he sold me my first Blockbuster Video bonus card fifteen years ago! We're all rooting for you!

  2. That's a very interesting development. It's amazing how it can pinpoint the type of material emitted. A worth while innovation not just for nuclear safety but as well as for medical tech. Good luck!