CRT's, high power applications such as transmitter final stages, photomultipliers, etc.

Compton Scattering.

Photons with energies toward the lower end of the gamma ray spectrum have a pretty decent probability to undergo this interaction, which is basically inelastic scattering of the gamma ray off an electron. When this scattering effect occurs near the photomultipliers, these less energetic, scattered gammas show up on your data plot.

Fortunately, pair-production becomes the dominant effect at around 100MeV and greater so all you have to account for then are spikes at multiples of 511 MeV.

There are many different devices used to detect nuclear radiation, though the most famous is probably the Geiger counter.

A Geiger counter uses a tube filled with an inert gas (e.g. helium, neon, or argon) which becomes briefly conductive when struck by beta particles or gamma rays. The tube amplifies the resulting current pulse and displays it, typically as needle movement, lamp light, or an audible click.

Other instruments for detecting radiation include ionization chambers, cloud chambers, bubble chambers, photomultipliers and dosimeters.

That depends on what range of wavelengths (frequencies) you want to detect.

It's almost impossible to build a detector that responds to all wavelengths, so

you select the band you're interested in, and build a detector optimized for that

range of the spectrum.

Here are a few examples of detectors, by wavelength:

-- very longest, down to 1 millimeter . . . . . a radio receiver

-- 1 millimeter down to 750 nanometers . . . . . absorbent material and a thermometer

-- 750 nanometers down to 350 nanometers . . . . . your eyes, camera, photo-film, etc

-- 350 nanometers down to 10 nanometers . . . . . ultraviolet techniques

-- 10 nanometers down to 0.01 nanometer . . . . . X-ray film or crystallography

-- less than 0.01 nanometer . . . . . photomultipliers detect light produced

when gamma rays impact crystalline material.