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In page Gadolinium:

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Gadolinium is used as a phosphor in medical imaging. It is contained in the phosphor layer of X-ray detectors, suspended in a polymer matrix. Terbium-doped gadolinium oxysulfide (Gd2O2S:Tb) at the phosphor layer converts the X-rays released from the source into light. This material emits green light at 540 nm because of the presence of Tb3+, which is very useful for enhancing the imaging quality. The energy conversion of Gd is up to 20%, which means that one fifth of the X-ray energy striking the phosphor layer can be converted into visible photons.[citation needed] Gadolinium oxyorthosilicate (Gd2SiO5, GSO; usually doped by 0.1–1.0% of Ce) is a single crystal that is used as a scintillator in medical imaging such as positron emission tomography, and for detecting neutrons.[1]