Gadolinium stands out among the rare earths for two strengths: magnetism and neutron absorption. Its thermal-neutron absorption cross-section is among the highest of all elements, so it is used directly as reactor control rods and burnable poisons, and in the ionization chambers and dosimeters of neutron detectors. In medical imaging, gadolinium is the core component of the main positive contrast agents used in magnetic resonance imaging: gadolinium ions are chelated and injected into the body, and their strong paramagnetism shortens the relaxation time of surrounding water molecules, so that tumors and vascular lesions show up clearly on T1-weighted images. In materials research, metallic gadolinium has a very strong magnetocaloric effect and is the core working material for room-temperature magnetic-refrigeration prototypes and room-temperature magnetic refrigerators. In addition, gadolinium gallium garnet (GGG) single-crystal wafers were once used as substrates for magnetic-bubble memories and remain a substrate material for some magneto-optical devices and laser gyros.
Gd2O3 (gadolinium(III) oxide) is the starting point for preparing gadolinium metal, gadolinium chelate contrast agents, and various gadolinium ceramics; it is also used in X-ray intensifying-screen phosphors and as an optical-glass additive, taking advantage of its high atomic number to absorb X-rays. GdCl3 (gadolinium trichloride) is a hydrometallurgical intermediate used in NMR and coordination-chemistry research to prepare paramagnetic reagents and organometallic precursors. Gd2O2S:Tb is the terbium-activated gadolinium oxysulfide scintillator: high-purity gadolinium oxysulfide powder is doped with trace terbium as the luminescence center, and under X-ray irradiation it emits green light. It is the most widely used scintillator in medical fluoroscopy, computed tomography, and digital radiography flat-panel detectors, and directly determines image clarity and the dose received by the patient.
