Since the late 1960s we’ve come up with three basic ways of detecting high-energy particles. Well-detectors and coded-aperture masks use solid-state electronics made from semiconducting materials, new technologies that replaced the earlier detectors, like Geiger counters. The most recent development is focusing optics, which I will explain in a moment.

Detectors work by converting light particles (photons) directly into electrical pulses. A popular method for the detection of gamma rays involved the use of crystal scintillators. A scintillator is a material that emits low-energy photons (usually in the visible range) when they are struck by gamma radiation. When used as a gamma-ray detector, the scintillator doesn’t directly detect the gamma rays. Instead, the gamma rays produce charged particles in the scintillator crystals, which interact with the crystal and emit photons. These lower-energy photons are then collected by photomultiplier tubes, which amplify their signal so that they can be counted. When a gamma ray hits one of these detectors it makes a little pulse – we actually detect little blip blip blip blips – which are individual photons being counted by the detector.

Before the 1990s most people used this kind of well-detector. We’d use a blocking material around the detector on all sides but one. Then we’d point that one side toward the astronomical source. For example, we’d point the detector in the direction of a supernova and see how many gamma rays we got. Then we’d point it somewhere else where there’s no source, but still has the background particles, and we’d just subtract the two to figure out how much gamma radiation was coming from the supernova. But we weren’t doing imaging with these detectors, just detecting how much signal there was.