|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.