See scintillator and scintillation counter for practical applications. The central electromagnetic calorimeter uses alternating sheets of lead and scintillator. BSE detectors are usually either of scintillator or of semiconductor types. When an ionizing particle passes into the scintillator material, atoms are excited along a track. Terbium-activated gadolinium oxysulfide is frequently used as a scintillator for x-ray imaging. Sodium iodide systems, as with all scintillator systems, are sensitive to changes in temperature. ORNL has also determined uranium and thorium in organic scintillator at the 1e-15 g/g level. Particle energy deposited in the material of a scintillator is proportional to the scintillator's response. The radiation is seen by a scintillator device. Radiation that is absorbed by the mass is not seen by the scintillator device. Bismuth germanate, BiGeO, (BGO) is used as a scintillator. A single module consists of alternating layers of iron and a plastic scintillator. OSO 5 carried a CsI crystal scintillator. XMASS is an underground liquid scintillator experiment in Kamioka. It is the most common oxide-based scintillator.