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All About X-Rays and X-Ray Tubes

14 August, 2011 | X-Ray Tubes

The History of X-Ray

X-rays are capable of penetrating some thickness of matter. Medical x-rays are produced by letting a stream of fast electrons come to a sudden stop at a metal plate. It is believed that X-rays emitted by the sun or stars also come from fast electrons.

The images produced by X-rays are due to the different absorption rates of different tissues. Calcium in bones absorbs X-rays the most, so bones look white on a film recording of the X-ray image, which is called a radiograph. Fat and other soft tissues absorb less, and look gray. Air absorbs the least, so lungs look black on a radiograph.

Wilhelm Conrad Röntgen discovered an image cast from his cathode ray generator, projected far beyond possible range of the cathode rays. Further investigation showed that the rays were generated at the point of contact of the cathode ray beam on the interior of the vacuum tube, that they were not deflected by magnetic fields, and they penetrated many kinds of matter.

A week after his discovery, in 1895, Rontgen took an X-ray photograph of his wife’s hand which clearly revealed her wedding ring and her bones. The photograph electrified the general public and aroused great scientific interest in the new form of radiation. Röntgen named the new form of radiation “X-radiation”, therefore the term X-rays.

What is X-Ray?

X-rays are waves of electromagnetic energy. They behave in much the same way as light rays, but at much shorter wavelengths. When directed at a target, X-rays can often pass through the substance uninterrupted, especially when it is of low density. Higher density targets (like the human body) will reflect or absorb the X-rays. They do this because there is less space between the atoms for the short waves to pass through. Thus, an X-ray image shows dark areas where the rays traveled completely through the target (such as with flesh). It shows light areas where the rays were blocked by dense material (such as bone).

X-Ray Applications

The most important application of the X-ray has been its use in medicine. This importance was recognized almost immediately after Roentgen’s findings were published in 1895. Within weeks of its first demonstration, an X-ray machine was used in America to diagnose bone fractures.

Thomas Alva Edison invented an X-ray fluoroscope in 1896. American physiologist Walter Cannon used Edison’s device to observe the movement of barium sulfate through the digestive system of animals and eventually, humans. In 1913, the first X-ray tube designed specifically for medical purpose was developed by American chemist William Coolidge. X-rays have since become the most reliable method for diagnosing internal problems.

What an X-Ray Tube is Comprised of

Inside the X-ray tube, as with any electronic vacuum tube, there is a cathode, which emits electrons into the vacuum and an anode to collect the electrons, thus establishing a flow of electrical current, knows as the beam, through the tube. A high voltage power source, 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, 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.
Electrons from the cathode collide with the anode material, usually tungsten, molybdenum or copper, and accelerate other electrons, ions and nuclei within the anode material. About 1% of the energy generated is emitted/radiated, usually perpendicular to the path of the electron beam, as X-rays. The rest of the energy is released as heat. Over time, tungsten will be deposited from the target onto the interior surface of the tube, including the glass surface.

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 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 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” radiation. The number of emitted X-ray photons, or doses, is adjusted by controlling the current flow and exposure time.

Rotating Anode System

To avoid the order heating of a fixed point on the anode surface, a rotating anode system was introduced. 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 cooling 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 2,500 ºC during an exposure, and the anode assembly can reach 1,000 ºC following a series of large exposures. Typical materials are a tungsten-rhenium target on a molybdenum core, with graphite. The rhenium makes the tungsten more ductile and resistant to wear from the 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.

Current High-Performance X-Ray Tubes

Increasing demand for high-performance computed tomography scanning and angiography systems has driven development of very high performance medical X-ray tubes. In these X-ray tubes power dissipation can be up to several KW’S.

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