Filament in X-ray machine is made up of: March 2005

Ans. B: Tungsten The earliest material used was barium oxide; it forms a monoatomic layer of barium with an extremely low work function. More modern formulations utilize a mixture of barium oxide, strontium oxide and calcium oxide. Another standard formulation is barium oxide, calcium oxide, and aluminium oxide in a 5:3:2 ratio. Thorium oxide is used as well. Oxide-coated cathodes operate at about 800-1000 degC, orange-hot. They are used in most small glass vacuum tubes, but are rarely used in high-power tubes since they are vulnerable to high voltages and oxygen ions, and undergo rapid degradation under such conditions. Hot cathodes may be either directly heated, where the filament itself is the source of electrons, or indirectly heated, where the filament is electrically insulated from the cathode; this configuration minimizes the introduction of hum when the filament is energized with alternating current. The filament is most often made of tungsten. With indirectly heated cathodes, the filament is usually called the heater instead. The cathode for indirect heating is usually realized as a nickel tube which surrounds the heater.