The inert, noble gases xenon (Xe), krypton (Kr), and argon (Ar) have several industrial applications, such as: filler gas for lamps (Xe, Kr, Ar), insulation in multiple-pane windows (Xe, Kr, Ar), anaesthetic and imaging agent in medicine (Xe), propellant for ion thrusters on spacecraft (Xe), gain medium for lasers (Xe, Kr, Ar), and medium to detect and measure ionizing radiation (Ar, Xe). Ar is used in bulk quantities, as a protective gas, during the production of metals and semiconductors. Xe, Kr, and Ar naturally occur in the atmosphere of the earth at . mol %, . mol %, and .