The focal spot size is affected by: surface area of the wire, helix pitch (the number of turns per inch), helix diameter/length, proudness of the filament in its focus cup, and the shape of the cup itself. Accelerated Filament Burn Out: X-ray tube characteristics are affected by several factors including: tube current, tube voltage, anode to cathode spacing, target angle and the focal spot size (electron beam size). 1956, 15 March 1969) (This reference also has a good picture of a notched filament run under direct current conditions and an unnotched wire run under alternating current conditions.) A 5 or 6% decrease in diameter is considered end of life by many manufacturers.ī. (Tungsten Filament Life Under Constant-Current Heating, A. This represents a reduction of 5.13% in the wire diameter and the filament has attained about 98% of its life. Additionally if high inrush currents are allowed with a cold filament this accelerates burn out by overheating the thinned spots.įor filament life, a reduction of about 10% of the wire mass is considered to be the end of life. The higher the filament temperature the more the tungsten grains grow with time and the quicker the notching progresses. Hot spots evaporate tungsten more readily and the wire thins more at these locations, ultimately burning open. Ideally tungsten evaporates uniformly but in practice it begins to form hot spots at crystal grain boundaries which are visible as “notches”. When hot tungsten slowly evaporates from its surface, the higher the temperature the greater the evaporation rate. Recrystallization is accomplished by heating the wire very rapidly to about 2600 Celsius in a few seconds and holding it there for a very short time.Ī common parameter for filaments is the filament life. This changes the raw fibrous wire microstructure into one which the crystal structure has a length to diameter ratio in the range of 3 to 6. X-ray tube manufacturers stabilize and strengthen the filaments with a process called recrystallization. The wire is relatively strong, rugged and keeps its shape when stresses such as vibration and shock are controlled. Tungsten wire is readily available and processed into useable forms. The helix serves to strengthen the filament and provides increased surface area to maximize electron emission. The filament is made from wire which is wound into a helix and inserted into a cup which acts as a focusing element to form the necessary rectangular electron beam. Despite experimentation with other emitters: dispenser cathodes, Lanthanum and Cerium hexaboride, thorium and rhenium doped tungsten, pure tungsten has remained the best filament material. Normal Filament Burn Out: The electron beam in an X-ray tube is supplied by a tungsten filament which has been used since the inception of electron tubes and also in incandescent light bulbs. X-ray tubes age and have a limited life because the characteristics and materials used begin a gradual degradation and are consumed so that performance gradually decreases until they no longer perform satisfactory.Ī. G) Filament Limit/Filament Preheat Settings The integration and control of the X-ray tube and generator is critical to producing anticipated technical results and long tube life. These include: thermodynamics, heat transfer, materials science, vacuum technology, high voltage, electronics, atomic/radiation disciplines, manufacturing processes, and many lesser but important technologies. Many scientific disciplines are required and must be controlled to produce a quality product. This factor limits the useful life of the X-ray tube. In the production of X-rays less than 1% of the energy produces useful X-rays while the remaining 99% is transformed into heat. Rotating anode tubes operate in excess of 1000 milliamperes but are used primarily in a pulsed mode of about 1 millisecond to 10 seconds. Stationary anode tubes typically operate at 1-20 milliamperes in nearly continuous duty and can be on for many hours at a time. Today two types of tubes dominate: rotating anode tubes used primarily for medical purposes from 25 kilovolts (kV) to 150 kV, and stationary anode tubes used in the inspection industry from 25 kV to over 400 kV with some in the million volt range. For over 100 years X-ray tubes have made advances owing to new applications, materials, processing equipment and design. X-ray tubes are a proven, cost effective way to produce X-radiation useful in the medical, inspection and scientific fields.