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coolidge tube

X-ray tube

An X-ray tube is a vacuum tube that produces X-rays. They are part of X-ray machines. X-rays are part of the electromagnetic spectrum, an ionizing radiation with wavelength just shorter than ultraviolet light. X-ray tubes evolved from experimental Crookes tubes with which X-rays were first discovered in the late 1800s, and the availability of this controllable source of X-rays created the field of radiography, the imaging of opaque objects with penetrating radiation. X-ray tubes are also used in CAT scanners, airport luggage scanners, X-ray crystallography, and for industrial inspection.

X-ray tube function

As with any vacuum tube, there is an emitter, either a filament or cathode, which emits electrons into the vacuum and an anode to collect the electrons, thus establishing a flow of electrical current, known as the beam, through the tube. A high voltage power source, for example 30 to 150 kilovolts (kV), is connected across cathode and anode to accelerate the electrons. The X-ray spectrum depends on the anode material and the accelerating voltage .

In many applications, the current flow (typically in the range 1mA to 1A) is able to be pulsed on for between about 1ms to 1s. This enables consistent doses of x-rays, and taking snapshots of motion. Until the late 1980s, X-ray generators were merely high-voltage, AC to DC variable power supplies. In the late 1980s a different method of control was emerging, called high speed switching. This followed the electronics technology of switching power supplies (aka switch mode power supply), and allowed for more accurate control of the X-ray unit, higher quality results, and reduced X-ray exposures.

Electrons from the cathode collide with the tungsten (and sometimes molybdenum or copper) target deposited on the anode and accelerate other electrons, ions and nuclei within the deposited material. About 1% of the energy generated is emitted/radiated, perpendicular to the path of the electron beam, as X-rays. Over time, tungsten will be deposited from the target onto the interior surface of the tube, including the glass surface. This will slowly darken the tube and was thought to degrade the quality of the X-ray beam, but research has suggested there is no effect(cf., Half-Value-Layer Increase Owing to Tungsten Buildup in the X-ray Tube: Fact or Fiction, John G. Stears, Joel P. Felmlee, and Joel E. Gray; Radiology, Vol 160, Number 3, pp 837 - 838, Sept 86). Eventually, the tungsten deposit may become sufficiently conductive that at high enough voltages, arcing occurs. The arc will jump from the cathode to the tungsten deposit, and then to the anode. This arcing causes an effect called "crazing" on the interior glass of the X-ray window. As time goes on, the tube becomes unstable even at lower voltages, and must be replaced. At this point, the tube assembly (also called the "tube head") is removed from the X-ray system, and replaced with a new tube assembly. The old tube assembly is shipped to a company that reloads it with a new X-ray tube.

The X-Ray photon-generating effect is generally called the Bremsstrahlung effect, a contraction of the German brems for braking, and strahlung for radiation.

The range of photonic energies emitted by the system can be adjusted by changing the applied voltage, and installing aluminum filters of varying thicknesses. Aluminum filters are installed in the path of the X-ray beam to remove "soft" (non-penetrating) radiation. The number of emitted X-ray photons, or dose, are adjusted by controlling the current flow and exposure time.

Simply put, the high voltage controls X-ray penetration, and thus the contrast of the image. The tube current and exposure time affect the dose and therefore the darkness of the image.

Crookes tube

Historically, x-rays were discovered radiating from experimental discharge tubes called Crookes tubes invented by British physicist William Crookes and others. As the medical and other uses of x-rays became apparent, workshops began to manufacture specialized Crookes tubes to produce x-rays. These were the first x-ray tubes. These first generation cold cathode or Crookes x-ray tubes were used until the 1920s.

Crookes tubes generated the electrons needed to create x-rays by ionization of the residual air in the tube, instead of a heated filament, so they were partially but not completely evacuated. They consisted of a glass bulb with around 10-6 to 5×10-8 atmospheric pressure of air (0.1 to 0.005 Pa). An aluminum cathode plate at one end of the tube created a beam of electrons, which struck a platinum anode target or anticathode at the center generating x-rays. The anode surface was angled so that the x-rays would radiate through the side of the tube. The cathode was concave so that the electrons were focussed on a small (~1 mm) spot on the anode, approximating a point source of x-rays, which resulted in sharper images. The tubes had a third electrode, an 'auxiliary anode' connected to the first anode, but its usefulness was doubtful.

To operate, a DC voltage of a few kilovolts to as much as 100 kV was applied between the anodes and the cathode, usually generated by an induction coil or sometimes an electrostatic machine. This accelerated the small number of ions present in the gas, created by natural processes like radioactivity. These struck gas atoms, knocking electrons off them, creating more positive ions in a chain reaction. All the positive ions were attracted to the cathode. When they struck it, they knocked electrons out of the metal, which were accelerated toward the anode target. When these high speed electrons struck the atoms of the anode, they created x-rays by one of two processes, either Bremsstrahlung or x-ray fluorescence.

Crookes tubes were unreliable. As time passed, the residual air would be absorbed by the walls of the tube, reducing the pressure. This increased the voltage across the tube, generating 'harder' x-rays, until eventually the tube stopped working. To prevent this, 'softener' devices were used (see picture). A small tube attached to the side of the main tube contained a mica sleeve or chemical that released a small amount of gas when heated, restoring the correct pressure.

Coolidge tube

The Crookes tube was improved by William Coolidge in 1913. The Coolidge tube, also called hot cathode tube, is the most widely used. It works with a very good quality vacuum (about 10-4 Pa, or 10-6 Torr).

In the Coolidge tube, the electrons are produced by thermionic effect from a tungsten filament heated by an electric current. The filament is the cathode of the tube. The high voltage potential is between the cathode and the anode, the electrons are thus accelerated, and then hit the anode.

There are two designs: end-window tubes and side-window tubes.

In the end-window tubes, the filament is around the anode, the electrons have a curved path.

What is special about side-window tubes is:

  • An Electrostatic Lens to focus the beam onto a very small spot on the anode
  • The anode is specially designed to dissipate the heat and wear resulting from this intense focused barrage of electrons:
    • Mechanically spun to increase the area heated by the beam.
    • Cooled by circulating coolant.
  • The anode is precisely angled at 1-20 degrees off perpendicular to the electron current so as to allow escape of some of the X-ray photons which are emitted essentially perpendicular to the direction of the electron current.
  • The anode is usually made out of tungsten or molybdenum.
  • The tube has a window designed for escape of the generated X-ray photons.

The power of a Coolidge tube usually ranges from 1 to 4 kW.

Rotating anode tube

The rotating anode tube is an improvement of the Coolidge tube. Because X-ray production is very inefficient (99% of incident energy is converted to heat) the dissipation of heat at the focal spot is one of the main limitations on the power which can be applied. By sweeping the anode past the focal spot the heat load can be spread over a larger area, greatly increasing the power rating. With the exception of dental X-ray tubes, almost all medical X-ray tubes are of this type.

The anode consists of a disc with an annular target close to the edge. The anode disc is supported on a long stem which is supported by bearings within the tube. The anode can then be rotated by electromagnetic induction from a series of stator windings outside the evacuated tube.

Because the entire anode assembly has to be contained within the evacuated tube, heat removal is a serious problem, further exacerbated by the higher power rating available. Direct cooling by conduction or convection, as in the Coolidge tube, is difficult. In most tubes, the anode is suspended on ball bearings with silver powder lubrication which provide almost negligible cooling by conduction.

A recent development has been liquid gallium lubricated fluid dynamic bearings which can withstand very high temperatures without contaminating the tube vacuum. The large bearing contact surface and metal lubricant provide an effective method for conduction of heat from the anode.

The anode must be constructed of high temperature materials. The focal spot temperature can reach 2500°C during an exposure, and the anode assembly can reach 1000°C following a series of large exposures. Typical materials are a tungsten-rhenium target on a molybdenum core, backed with graphite. The rhenium makes the tungsten more ductile and resistant to wear from impact of the electron beams. The molybdenum conducts heat from the target. The graphite provides thermal storage for the anode, and minimizes the rotating mass of the anode.

Increasing demand for high-performance CT scanning and angiography systems has driven development of very high performance medical X-ray tubes. Contemporary CT tubes have power ratings of up to 100 kW and anode heat capacity of 6 MJ, yet retain an effective focal spot area of less than 1 mm2.

Hazards of x-ray production from vacuum tubes

Any vacuum tube operating at several thousand volts or more can produce x-rays as an unwanted byproduct, raising safety issues. The higher the voltage, the more penetrating the resulting radiation and the more the hazard. Color televisions and computer CRT displays operate at 30-40 kilovolts, making them the main concern among household appliances. Historically, concern has focused less on the cathode ray tube, since its thick glass envelope is impregnated with several pounds of lead for shielding, than on high voltage (HV) rectifier and voltage regulator tubes inside. In the 1970s it was found that a failure in the HV supply circuit of some GE TVs could leave excessive voltages on the regulator tube, causing it to emit X-rays. The models were recalled and the ensuing scandal caused the US agency responsible for regulating this hazard, the Center for Devices and Radiological Health of the Food and Drug Administration (FDA), to require that all TVs include circuits to prevent excessive voltages in the event of failure. This hazard was eliminated with the advent of all solid state TVs, which have no tubes beside the CRT. Since 1969 the FDA has limited TV X-ray emission to 0.5 milliRoentgens per hour.

More recently, concern has been expressed about the magnetron tubes in microwave ovens, although with operating voltages of only 4-5 kilovolts it is doubtful whether any x-rays produced could penetrate the tube envelope, typically constructed of ceramic and metal.

See also

Patents

  • Coolidge, , "X-ray tube"
  • Langmuir, , "Method of and apparatus for controlling X-ray tubes
  • Coolidge, , "X-ray tube"
  • Coolidge, , "X-ray tube"

References

External links

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