Scintillation detectors use crystals that emit light when gamma rays interact with the atoms in the crystals. The intensity of the light produced is usually proportional to the energy deposited in the crystal by the gamma ray; a well known situation where this relationship fails is the absorption of < 200 keV radiation by intrinsic and doped sodium iodide detectors. The mechanism is similar to that of a thermoluminescent dosimeter. The detectors are joined to photomultipliers ; a photocathode converts the light into electrons; and then by using dynodes to generate electron cascades through delta ray production, the signal is amplified. Common scintillators include thallium - doped sodium iodide (NaI(Tl))—often simplified to sodium iodide (NaI) detectors—and bismuth germanate (BGO). Because photomultipliers are also sensitive to ambient light, scintillators are encased in light-tight coverings.