Comparison of X-ray sources and applications: conventional generators/synchrotron/others

Introduction

Initially the identity of x-rays was mysterious, their connotations including morbidity and the otherworldly. Gradually they became routine, finding a place in the high-energy end of the electromagnetic spectrum. X-Ray photons are electromagnetic radiation with wavelengths in the range 0.1 - 100 Å that can be produced by conventional generators, by synchrotrons, and by plasma sources. Modern laboratory X-rays sources are based on the same principle that was used at the beginning when the X-rays were discovered: energetic electrons impact a metal target generating Brehmsstrahlung and characteristic X-rays. While the method used to generate X-rays is still the same, source technology has advanced significantly. Because of the rapid improvements, the laboratory sources can now rival the intensity of second generation synchrotron beam lines. The user has more options than ever before, but this can also lead to confusion about how different sources perform compared to one another and which is the most appropriate for different applications. This note compares various sources and their applications.

1. What are X-Ray sources?

X-ray sources are vacuum tubes that use an electrostatic field to produce X-rays. They accelerate electrons to a high velocity and then suddenly stop them. X-ray tubes, as X-ray sources are sometimes called, are devices in which energy conversion takes place, the kinetic energy of fast moving electrons is converted into heat and X-ray energy.

To produce X-radiation, large amounts of electrical energy must be transferred to the X-ray tube. Typically less than 1% of the energy deposited in the tube is converted into X-rays the other 99% appears in the form of heat. Consequently, this limits the use of X-ray apparatus. If excessive heat is produced in the X-ray tube, the temperature will rise above critical values, and the tube can be damaged.

There are various sources of X-ray radiation, but X-rays can be generated by an X-ray tube when a charged particle can be accelerated or decelerated. Usually, in laboratories X-rays are produced from sealed tubes or rotating anodes. In medical X-ray tubes the target is usually tungsten or a more crack-resistant alloy of rhenium (5%) and tungsten (95%). Sometimes molybdenum is used for more specialized applications, such as

1

when soft X-rays are needed as in mammography. In crystallography, a copper target is most common. But cobalt is often used when fluorescence from iron content in the sample might otherwise present a problem.

2. Conventional generators

2.1. Crookes tube

X-rays tubes evolved from the experimental Crookes tubes which were used for the discovery of the X-rays when the medical and other uses of X-rays became apparent. They are used in radiography, CAT scanners, airport luggage scanners, X-ray crystallography and for industrial inspections.

The earliest x-ray tubes were of the cold cathode variety which means that they didn’t contain a heated filament in them like the later electronic vacuum tubes do. These tubes, referred to as Crookes tube after the inventor’s name, the English physicist William Crookes and other around 1869 – 1875 are a development of the Geissler tubes. These first generation cold cathode or Crookes X-ray tubes were used until the 1920s. In the Crookes tube the electrons are generated by the ionization of the residual air in the tube by a DC voltage applied between the electrodes, usually by an induction coil. Crookes tubes were of the general class of gas tubes since the pressure had to be in the ‘soft’ vacuum range (about 10-3 to 10-4 Torr) to permit the passage of electrons from the cathode to the x-ray producing target, the anode, in a so-called ‘dark’ discharge. If high voltage is applied to the tube, the small number of the electrically charged ions present in the gas is accelerated by the electric field and collides with other gas molecules knocking electrons off them and creating positive ions in a chain reaction. The positive ions are attracted to the cathode which is the negative electrode and they produce a large number of electrons when they strike the cathode. These electrons are attracted by the anode which is the positive electrode. This seemed to be an invisible ray, but when it hit the end of the tube, for some reason it made the glass glow green. These rays were called cathode rays by Eugen Goldstein, because at that time they didn’t know if the rays came from the cathode or from the anode, but if the anode was moved ‘round the corner’ then it was clear that the green glow was opposite the negative electrode.  The high speed electrons which struck the atoms of the anode create X-rays by one of the two processes: Bremsstrahlung or X-ray fluorescence.

In order to make the glow more visible, later on, researchers painted the inside back wall of the tube with phosphor which is a fluorescent chemical. After striking the wall, the electrons eventually make their way to the anode, flow through the anode wire, the power supply, and back to the cathode.

2

Crookes tubes were unreliable and temperamental. Both the energy and the quantity of cathode rays produced depended on the pressure of residual gas in the tube. As time passed the gas was absorbed by the walls of the tube, reducing the pressure. This reduced the amount of cathode rays produced and caused the voltage across the tube to increase, creating 'harder' more energetic cathode rays. Soon the pressure got so low the tube stopped working. To prevent this, 'softener' devices were used (Figure 1). 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. The glass envelope of the tube would blacken in use due to the X-rays affecting its structure, but the glow could be seen.

Figure 1 Crookes tube

2.2. Coolidge tube

The cold cathode tube went out of use shortly after 1913 when W. D. Coolidge introduced a tube with a hot cathode in which the electrons are produced by thermionic effect from a tungsten filament heated by an electric current. This is a more reliable source of electrons. The Coolidge tube, which uses high vacuum (typically below 10-6

Torr), has a number of advantages over the gas tube, but despite its superiority it did not immediately replaced the cold cathode tubes. The cold cathode tubes were being manufactured into the 1920s and were employed for instances in radiology as late as the 1960s.

The Coolidge Tube (figure 2) is the most popular X-ray source because of the high vacuum and its use of a heated filament as the source of the electrons. The fact that the Coolidge tube contains very little gas inside which is not involved in the production of X-rays differentiates it from previous X-rays sources.  In order to operate the device, the

3

cathode filament is heated and the Coolidge tube emits electrons. The hotter the filament gets, the greater the emission of electrons. The electrons are accelerated towards the positively charged anode and upon their collision they change direction and emit X-rays with a continuous range of energies. Some undesirable X-rays called stray radiation are produced by electrons striking other tube components, in addition to the x-rays produced at the focal spot of the anode. The fact that the production of x-rays was controllable by heating the cathode instead of varying the vacuum rendered the x-ray quality much more stable.

The advantages of the Coolidge tube are its stability, and the fact that the intensity and energy of the X-rays can be controlled independently. If the current to the cathode increases, then its temperature also increases. This will lead to an increased number of electrons emitted by the cathode, and as a result, the intensity of the x-rays. Increasing the high voltage potential difference between the anode and the cathode increases the velocity of the electrons striking the anode, and this increases the energy of the emitted x-rays. The opposite effects would be given by the decreasing of the current or the high voltage. The high degree of control over the tube output meant that the early radiologists could do with one Coolidge tube what before had required a stable of finicky cold cathode tubes and the Coolidge tube could function almost indefinitely unless broken or badly abused.

Figure 2 Coolidge tube

In certain Coolidge tubes there is actually a third electrode in front of the heater and much like a triode, different voltages applied to this grid will allow different amounts of current through. However for hobbyist purposes this grid can be left unconnected and the tube can be controlled via heater voltage, which is much easier to adjust.

Coolidge tubes are formed as either end-window tubes or side-window tubes.4

End-window tubes usually have a transmission target which is thin enough to allow X-rays to pass through the target which means that the X-rays are emitted in the same direction as the electrons are moving. In a common type of end-window tube the filament is around the anode, so the electrons have a curved path.

Figure 3 End-window Coolidge tube

Side-window tubes have an electrostatic lens which focuses the beam on a very small spot on the anode. The anode usually made out of tungsten or molybdenum is also specially designed to dissipate the heat and wear by either rotating the anode or by circulating coolant. The side window allows for escape of the generated X-ray photons.

Figure 4 Side-window Coolidge tube5

2.3. Rotating anode tube

The rotating anode tube is an improvement of the Coolidge tube and is the most common X-rays source. In a rotating anode tube, the anode target disc rotates on a highly specialized ball bearing system. The target is subjected to a focused stream of electrons emitting from the cathode and accelerated by a high potential difference between the target disc and the cathode. The X-ray beam is produced when the electron beam hits the anode. The cathode provides a controlled source of electrons and the filament is a constructed tungsten wire coil of precise pitch and length. A motor rotates the anode disc at a high speed up to 10,000 rpm and to temperatures of 2000° C. The anode assembly is mounted on bearings and actually forms the rotor of the electric motor. The disc has a tungsten rhenium target area and faced onto a molybdenum disc 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. The disc is cooled by radiation to the glass then oil.

The advantage of using a rotating anode tube is it permits selection of higher electrical load without the risk of overheating. It can be used in almost every radiography application with the exception of dental X-ray.

Constant advances are being made to the rotating anode tube and recent discoveries have led to the use of new anode material, reduced target angle, increased speed of anode rotation, and new styles of tubes including the grid controlled X-ray tube and metal/ceramic X-ray tube. 

Figure 5 Rotating anode tube

6

3. Microfocus X-ray tubes

X-ray radiation is caused by fast-moving electrons that collide with a solid body. The electrons are generated in a vacuum by a glowing filament (cathode). In the case of open microfocus tubes, this filament can be replaced. As a result, the lifespan for this type of tube is unlimited. The freely moving electrons are accelerated within the vacuum by an electrical field. This field lies between the filament and the centering coils (alignment unit).

The special features of microfocus X-ray tubes in comparison to conventional X-ray tubes are the centering coils and the focusing coils (objective lens). The centering coils center the beam of electrons so that it hits the target in the center. The focusing coils focus the electrons on the target. As a result, the beam source measures only a few micrometers. The fact that this beam source is so extremely small means that a several thousandfold geometric magnification can be achieved.

Figure 6 Microfocus X-ray tube

There are two basic types of microfocus x-ray tubes: solid-anode x-ray tubes and metal-jet-anode x-ray tubes.

7

Solid-anode microfocus X-ray tubes which are in principle very similar to the Coolidge tube, but the electron beam has to be focused into a very small spot on the anode. Usually the focus spots is in the range 5-20 µm for the microfocus X-ray sources, but spots smaller than 1 µm may be produced in some cases. The fact that this solid-anode microfocus X-ray tubes operate at very low power the electron-beam power must be set below a maximum value to avoid melting of the anode and it depend on the anode material.

On the other hand, in metal-jet-anode microfocus X-ray tubes the solid metal anode is replaced with a jet of liquid metal, which acts as the electron-beam target. This increases the power density significantly and allows the operation with a smaller focal spot, for example 5 µm which increases the image resolution at the same time acquire the image faster than for solid-anode tubes with 10 µm focal spots. The values of the power depend on the anode material.

4. Applications of X-ray tubes

While medicine and dentistry remains the most common use of X-rays, they are also widely used in other applications from airport security to industrial inspection and quality control systems. The arrangement of atoms in molecules is routinely determined using X-ray crystallography like the double helix structure of the DNA molecule determined by Crick and Watson.

Radiotheraphy is a non-imaging medical application of X-rays used for the treatment of cancer.

X-rays are best suited to imaging bones and have a very high resolution. For imaging soft tissue however, there is very little contrast and so a contrast medium is needed. Contrast mediums are substances given to the patient that absorb X-rays and produce an image of the area under investigation when X-rayed. Usually CAT (Computer Axial Tomography) scans, in which a series of X-rays are taken from various angles and interpreted by a computer, are better for imaging soft tissue.

The two main fields in which X-rays are used in medicine are radiography and fluoroscopy. Radiography includes orthopantomogram, mammography and tomography and the medical applications of fluoroscopy are angiography, barium swallow and biopsy.

X-rays can be used to study the structure of a material without actually destroying it. One approach is based on the usual method of producing X-rays. A sample of unknown material is used as the target in an X-ray machine and bombarded with high energy electrons. The X-ray pattern produced by the sample can be compared with the X-ray patterns for all known elements. Based on this comparison, the elements present in the unknown sample can be identified. A typical application of this technique is the analysis

8

of hair or blood samples or some other material being used as evidence in a criminal investigation.

X-rays are used for nondestructive testing in business and industry in many other ways. For example, X-ray pictures of whole engines or engine parts can be taken to look for defects without having to take an engine apart. Similarly, sections of oil and natural gas pipelines can be examined for cracks or defective welds. Airlines also use X-ray detectors to check the baggage of passengers for guns or other illegal objects.

5. Synchrotron

A synchrotron is a particle accelerator. Charged particles such electrons are accelerated at almost the speed of light and when they are deflected through magnetic fields they create bright light which is million times brighter than sunlight and a billion times greater than the radiation from a typical laboratory X-ray source. This makes synchrotron radiation possibly the most powerful light produced by humans. Powerful magnets and radio frequency waves are used to accelerate negatively charged electron along a stainless steel tube, where they reach high speed.  As the magnets are turned on and off, electrons get pulled along the ring of tubes. Since the fast-moving electrons emit a continuous spectrum of light, with various wavelengths and strength, scientists can pick whatever wavelength they need. Synchrotron are very expensive to built and to maintain, so there are only a few in the world ( approximatively 1 000).

Figure 7 Sketch of the synchrotron’s principle

The first particle accelerators were built in the 1930s by physicists who were interested in finding out about the elementary particles that make up atoms. The fact that when particles were forced to move around the circle they lost energy by emitting a beam of synchrotron radiation made difficult the use of the machines. The synchrotron radiation

9

was a valuable tool, so the machines were used as a source of radiation and they became known as first-generation synchrotrons.

Second-generation synchrotrons were built solely for their ability to generate synchrotron radiation. They were basically circular rings with ports arranged around the outside through which the intense synchrotron radiation passed.

In the last two decades, synchrotron technology has reached new heights. Magnetic devices such as undulators and wigglers are now installed within the ring, causing the stream of speeding electrons to oscillate as it passes by which greatly increases the intensity of the beam. This is known as a third-generation synchrotron.

Conventional X-rays can only be used to look at hard tissue (such as bones or teeth). Synchrotron X-ray images have a much higher resolution than conventional X-rays which means they have the advantage of also being able to reveal fine details of soft tissue. Scientists can also use the very bright light and X-rays from a synchrotron to determine the structure of atoms, molecules and many other materials.

The uses of synchrotron X-rays are many and range from designing drugs to making micromachines.

Currently the most important use of synchrotron X-rays is protein crystallography. Studies using synchrotron X-rays for protein crystallography can help scientists understand and imitate complex structures.

Using synchrotron X-rays for the analysis of viral proteins is an advantage because the analysis can now be completed in hours or days instead of the months or years needed for the conventional sources.

Different forms of synchrotron light are being used to capture images of internal cell features that are up to a thousand times smaller than was previously possible. Synchrotron light is also being used to construct high precision 3-D cell maps and to monitor cellular processes as biochemical reactions are taking place.

X-rays can be used in lithography for computer chips by focusing to such a fine point that they can cut out tiny machine components from silicon or plastic with incredible precision – on a scale of thousandths of a millimetre. They can also be used to etch patterns in microchips.

Synchrotron studies are a major foundation of modern materials science. Synchrotron light is being used to develop ceramics, structural composites and a wide range of plastics. It can be used to determine structural and chemical change at the level of individual atoms and molecules in processes such as corrosion and metal fatigue.

10

The powerful penetrating characteristics of synchrotron light also allow researchers to probe below the surface of electronic devices or to check the integrity of metal joining processes such as welding.

6. Carbon nanotubes

Because of numerous potential applications in industry and medical field, carbon nanotubes were developed. Carbon nanotubes have several advantages over other field-emitting materials, because in contrast to commonly used emitters such as tungsten, a nanotube is not a metal,but a structure built by covalent bonds.

Conventional x-ray machines have a long tube with an electron emitter, typically a tungsten filament at one end and a metal electrode at the other. The tungsten filament emits electrons when it is heated to 1,000 degrees Celsius which are accelerated along the tube striking the metal and creating x-rays. In comparison with those the carbon nanotubes allow the operation at room temperature rather than at 1,000 degrees Celsius and they can be used also as a high speed X-ray camera to capture clear images of objects moving at high speed for X-ray imaging and cancer therapy. A better quality of the image could increase the accuracy of radiotherapy so it doesn't harm normal tissue.

The most important characteristic of the carbon nanotubes consist in the ability to generate electrons. When a voltage is applied to them, a quantum effect known as field emissions allows for one electron to be generated, which amplifies the electric field around the tip of the structure and particles will be emitted easier. But the real innovation is the layout of the machine which has a 3D array of carbon nanotubes that do not fire in unison.

Conclusion

Since X-rays were discovered by Wilhelm Conrad Röntgen the sources have been developed continuously and were applied to a wide range of uses. In the last few years the X-ray technology was developed to have high resolution, high speed imaging and now the X-ray machines are shrinked to have portable X-ray sources for industrial and medical applications.

11

Bibliography

1. Shih-Lin Chang, X-ray multiple-wave diffraction: theory and application, Berlin ; New York : Springer, ©2004

2. Robert A. Meyers, Proteins: From Analytics to Structural Genomics, Vol. 1, Weinheim : Wiley-VCH, 2007

3. Gwyn P. Williams, Synchrotron Radiation, National Synchrotron Light Source, Brookhaven National Laboratory

4. Carbon nanotubes electron sources and applications, The Royal Society, 10.1098/rsta.2004.1438

5. http://en.wikipedia.org/wiki/X-ray

6. http://www.arpansa.gov.au/radiationprotection/basics/xrays.cfm

7. http://used-medicalequipmentblog.blogspot.com/2011/08/function-of-x-ray- tubes.html

8. http://www.focalspot.com/10-xray-takeaway-points.htm

9. http://en.wikibooks.org/wiki/Basic_Physics_of_Digital_Radiography/The_Source

10.http://en.wikipedia.org/wiki/X-ray#Sources

11.http://en.wikipedia.org/wiki/X-ray_generator#X-ray_sources

12.http://www.xradia.com/technology/basic-technology/sources.php

13.http://used-medicalequipmentblog.blogspot.com/2011/05/advances-in-ct- scanner-x-ray-tubes.html

14.http://en.wikipedia.org/wiki/Crookes_tube

15.http://en.wikipedia.org/wiki/X-ray_tube

16.http://www.crtsite.com/page5-2.html

17.http://www.orau.org/ptp/collection/xraytubescoolidge/coolidgeinformation.htm

18.http://teravolt.org/coolidge-tubes/

19.http://viscom.de/en/microfocus_x-ray_tubes_.php?cc=enuk&id_mnu=56

20.http://en.wikibooks.org/wiki/Xray_Crystallography/Equipment

12

21.http://www.radiologyinfo.org/en/info.cfm?pg=linac

22.http://medgadget.com/2008/06/ xray_linear_accelerator_a_new_ultramicroscope_into_life.html

23.http://www.ruppweb.org/Xray/x-ray_sources.html

24.http://www.physorg.com/news4073.html

25.http://news.softpedia.com/news/Carbon-Nanotubes-to-Allow-Real-Time-3D-X- ray-Scanning-117776.shtml

26.http://nanotechweb.org/cws/article/tech/9262

27.http://www.nature.com/news/2009/090728/full/news.2009.744.html

28.http://rdmag.com/News/2009/07/Materials-Carbon-Nanotubes-Supercharge-X- Ray-Devices/

29.http://asistm.duit.uwa.edu.au/synchrotron/downloads/pdfs/chapter02_3.pdf

30.http://xrayweb.chem.ou.edu/notes/xray.html

13