A gamma radiation scan
A gamma scan was a scan for gamma radiation.
Doctor Clendenning, a colonist on Omicron Theta, was the first scientist who experimented with gamma scan detectors before the destruction of the planet in 2338.
With a gamma radiation scan, based on Dr. Clendenning's work, a trail of antiproton and gamma radiation left behind by the Crystalline Entity was detected, thereby making it possible to track the movements of the creature. (TNG: "Silicon Avatar")