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, i.e. 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.

X-Ray Tube and Anode Materials

Typically, the tubes used by X-ray sources are made of glass, surrounded by metal, and sealed by a vacuum. The cathode is located in the dome-shaped part of the tube.

Because X-ray sources produce large amounts of heat during the beam-formation process system designs must account for these temperature considerations. The intensity of the produced photons depends upon the atomic number of the anode material, and the number of electrons that hit the anode. For this reason, the anode is usually made of tungsten, due to its high atomic number, which increases the intensity of the X-rays. Tungsten's high melting point is an additional selection consideration.

X-ray source anodes are also made of molybdenum or copper. Modern anode discs are made of a combination of metals. An alloy of rhenium and tungsten is used for the face of the anode disc because it retains its smoothness better than tungsten as the tube ages. Molybdenum is also commonly used because is it not as dense as tungsten and can accept a given amount of heat without a rise in temperature. Often used as the anode disc is a molybdenum disc with a coating of 10% rhenium and 90% tungsten over the target back.

Buyers of X-ray sources should be aware of recent advances in X-ray tube materials. There are several advantages of using a metal/ceramic tube versus a glass tube. These materials allow for higher tube currents to be used because the anode has a higher heat capacity. Another advantage is longer tube life, since the deposition of tungsten on the glass acts as an electrode and shortens the tube life; the addition of a metal enclosure prevents the deposition from altering the ground (and increases tube life). The metal enclosure also decreases off-focus radiation by attracting off-focus electrons to the ground wall, preventing electron back scatter.

How X-Ray Sources Work

X-ray tubes have several basic components. These include: a source of electrons such as a filament with a heating source, a system capable of accelerating electrons across a space where there is nothing to impede them, i.e., a vacuum glass tube with high tension (HT) transformer, and a target structure where the electrons strike, also known as the anode. Modern X-ray tubes have modifications depending on the functionality of the device.

 

Applying a high voltage to the X-ray tube releases electrons from the filament cathode. These electrons then race towards and collide into the anode. The high-speed collision of electrons produces X-ray photons. The electrons continue towards a beryllium foil that absorbs the scattered electrons in order to allow the X-rays to pass through the tube. The passing of the electrons from the cathode to the anode establishes a flow of electrical current, known as a beam, through the tube.

X-ray sources use an X-ray generator to produce a flow of electrons. Electrical current is then transformed to a higher voltage.   The energy of the X-ray depends on the electric potential difference of the cathode and anode.

 

Watch a video by ionactiveconsulting which explains how an X-ray tube works

 

Watch how X-ray systems work

Types of X-Ray Source Tubes

The GlobalSpec SpecSearch database provides information about these types of X-ray tubes.

Coolidge Tube

The Coolidge Tube, also known as a hot cathode tube, is the most popular X-ray source.  The characteristics of this device are its high vacuum and its use of a heated filament as the source of the electrons. There is very little gas in the Coolidge tube, which differentiates it from previous X-ray sources.  In order to operate the device, the cathode filament is heated. As it is heated, the Coolidge tube emits 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.

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

coolidge xray tube

Coolidge X-ray Tube. Image Credit: Oak Ridge Associated Universities

There are two designs for the Coolidge tube.

  • End-window tubes usually have a transmission target which is thin enough to allow X-rays to pass through the target. This means that the X-rays are emitted in the same direction as the electrons are moving

Description: http://www.bruker-axs.de/fileadmin/user_upload/xrfintro/images/Abb5_e.jpg

The principle of the end-window tube. Image Credit: Bruker axs

  • Side-window tubes have an electrostatic lens which focuses the beam on a very small spot on the anode. The anode 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.

Side window tube

The principle of the side-window tube. Image Credit: Bruker Axs

http://www.bruker-axs.de/fileadmin/user_upload/xrfintro/images/Abb4_e.jpg

 

Rotating Anode Tube

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. When the electron beam hits the anode, it produces the X-ray beam. 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. 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. 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. 

rotating anode tube

X-ray Source Tube. Image Credit: University of Illinois

Application Specific X-Ray Tubes

There are several types of application-specific X-ray tubes.

  • Radiotherapy tubes

Tubes for deep therapy have KVp in the range of 200-300 kV and usually work at 15-20mA. Single focus tubes size (6-8mm) have a hooded anode and target angle of about 35o. The glass envelope is about 60 cm in length to prevent external arcing due to high voltage. These are oil cooled stationary anode tubes. To allow emission of the required primary X-ray beam, a hole is cut in the hood below the target.

These are not being used today.

  • Stereographic X-ray tubes

These are similar to conventional rotating anode X-rays except the rotating anode is bombarded simultaneously by two beams of electrons from two independent cathode assemblies.
These are used for stereoradiographic and stereofluoroscopic X-ray examinations.

  • Mammography X-ray tube

For maximum visualization of soft tissues of the breast having similar ability to absorb X-rays, a beam of soft radiation (longer wavelength) is required. Longer wavelength can be produced by selecting an X-ray tube which operates at low KVp (20-40). Features of a mammography tube include the use of target made of molybdenum, closer spacing of cathode and anode, a Beryllium window (thinned glass window), and use of molybdenum filter in place of aluminum filter.

  •  Tubes for computed tomography (CT)

CT requires longer exposure times at higher KV than needed for general radiography. They have a heavy-duty rotating anode tube with higher thermal capacity and a smaller focal spot (up to 0.6 mm). These tubes are air cooled with current value up to 600 mA. Some CT tubes are grid controlled for pulsed radiations to reduce the radiation dose.

  • Field emission X-ray tubes (also called cold cathode tubes)

In this style, in place of electron emission by thermionic-effect, the electrons are extracted from the cathode by a high potential difference. Since less electrons are emitted these tubes can be used only for neonatal radiography. If a higher voltage (up to 350Kv) is applied, these tubes can be used for high Kv chest radiography. Field emission X-ray tubes are not useful for general purpose radiography

 

Selection Criteria for X-Ray Tubes

When selecting X-ray tubes, buyers should be aware of some common variables in X-ray tube design and selection: operating heat produced, operating heat capacity, target angle, focal spot size, duty cycle, operating wattage, application for which the tube will be used, physical dimension considerations, and many others.

Heat Produced

Heat is produced in the focal spot area by the bombarding electrons from the cathode.

Since only a small fraction of the electronic energy is converted in X-radiation, it can be ignored and assumed that all of the electron energy is converted into heat. In a single exposure, the quantity of heat produced in the focal spot area is given by

Heat (J) = w x KVp x MAS.

Where KV is the effective KV value and KVp is the peak KV value. When used to mean the quantity, voltage or potential, it is written KV or KVp. When used as the unit, it is written as kV or kVp. MAS is the total quantity of electrons passing a point. It is the product of the current (MA) and the time in seconds (S).

In this relationship, w is the waveform factor; its value is determined by the waveform of the voltage applied to the X-ray tube. Values for most waveforms encountered in diagnostic X-ray machines are: constant potential, 1.0; three-phase, 12 pulse, 0.99; three-phase, 6-pulse, 0.96; single-phase, 0.71.

Although the joule is the basic unit for energy and heat, it is not always used to express X-ray tube heat. The special heat unit (HU) was introduced when single-phase equipment was common to make it easy to calculate heat. The heat unit (HU) is a smaller quantity of heat than a joule since one joule is equal to 1.4 heat units.

Since the product of the joules-to-heat unit conversion factor (1.4) and the waveform factor for single-phase (0.71) is equal to 1, the following relationship is obtained:

Heat (HU) = KVp x MAS.

Here it is seen that for single-phase operation, the heat produced in heat units is the product of the KVp and MAS.

The rate at which heat is produced in a tube is equivalent to the electrical power and is given by

Power (watts) = w x KVp x MA.

The total heat delivered during an exposure, in joules or watt-seconds, is the product of the power and the exposure time.

Heat Capacity

For a given object, the relationship between temperature and heat content involves, heat capacity, which is a characteristic of the object. The general relationship can be expressed as follows:

Temperature = heat / heat capacity.

The heat capacity of an object is more or less proportional to its size, or mass, and the material's specific heat. In an object with a large heat capacity, the temperature rise is smaller than in one with a small heat capacity.

 

 Design Tip: In X-ray tube operation, the goal is never to exceed specific critical temperatures that produce damage. This is achieved by keeping the heat content below specified critical values related to the tube's heat capacity.

In most X-ray tubes, there are three distinct areas with critical heat capacities, as shown in the diagram below. The area with the smallest capacity is the focal spot area, and is the point at which heat is produced within the tube. From this area, the heat moves by conduction throughout the anode body and by radiation to the tube housing. Heat is removed from the tube housing by transfer to the surrounding atmosphere. When the tube is in operation, heat generally flows into and out of the three areas shown. Damage can occur if the heat content of any area exceeds its maximum heat capacity.

Heat Capacities

The Three Critical Heat Capacities in an X-ray Tube. Image Credit: Sprawls.

 Design Tip: The maximum heat capacity of the focal spot area, is the major limiting factor with single exposures. If the quantity of heat delivered during an individual exposure exceeds the track capacity, the anode surface can melt.

Target Angle

The anode angle determines the actual relationship between the focal spot width, focal spot heat capacity and the size of the of the projected focal spot. The angles generally range from about 7° to 20°. For an effective focal spot size, the track width and heat capacity are inversely related to the anode angle. Anodes with small angles give maximum heat capacity but they are limited with respect to the area that can be covered by the X-ray beam. When specifying an x-ray tube for purchase, the anode angle should be selected by a compromise between heat capacity, especially for smaller focal spots, and field of coverage.

Focal Spot Size  

Focal spot size is a measurement of the resolution that will be afforded by a particular X-ray tube. In general, the smaller the focal spot size, the better the resolution.  The size of the focal spot that is possible is contingent upon the mA level for the application, the kV for the application, duty cycle, necessary beam coverage, and target angle of the tube.  Improved resolution is afforded by small focal spot sizes but reducing focal spot size, will make it necessary to run at lower mA and/or kV levels relative to focal spot size.

 Design Tip: From the standpoint of producing X-ray images with minimum blur, a small focal spot is desired. However, a small focal spot tends to concentrate heat and give the focal spot track a lower heat capacity. The only advantage of a larger focal spot is increased heat capacity.

Applications

X-ray tubes are used in a variety of applications, such as:

  • Electro-medical imaging - Tubes are used in many types of medical imaging equipment, including dental apparatus, podiatry applications and traditional medical imaging apparatus as well as advanced applications such as bone densitometry equipment and in-vitro tissue analyzers.
  • Security control systems and equipment - Tubes are used in a variety of security applications that require imaging, such as general package inspection, airport security checkpoints and airline baggage inspection.
  • Non destructive testing (NDT) - A variety of X-ray tubes commonly used in the rapidly growing field of NDT.
  • Production inspection systems - X-ray tubes are used widely forapplications that perform X-ray inspection as a part of the production process.
  • Metal treating/metallurgical applications - Tubes are used for industrial applications that require composition analysis, stress testing or a host of other imaging requirements.
  • Metrology- Beryllium window X-ray tubes are commonly used in X-ray metrology applications.
  • Laboratory analytical instruments - X-raytubes currently operate in a variety of laboratory applications including particle analysis and water pollution analysis devices.
  • Fresh fruit and vegetable inspection - X-ray tubes cab be used to inspect produce to detect whether or not damage or insect infestation is in the produce supplied to the general population for consumption.

Considerations for choosing an X-ray tube  Chart credit: IART.org

Purpose

Heat Storage Capacity

Target Angle

Focal Spot Size

Neuroradiography

3,00,000 H..U.

7o

0.3, 6.0 mm

Angiography

4,00,000 H..U. to 1.35 MHU

12o

0.6, 1.2 mm

Mammography

3,00,000 H..U.

10o

0.6 mm

General Radiography

3,00,000 H..U.

12o

0.3, 0.6, 1.2 mm

General Radiography

3,00,000 H..U.

15o

0.6, 0.1, 2.0 mm

CT Conventional

1 MHU - 2 MHU

12o

0.6mm

CT Spiral

3.5 - 6.3 MHU

12o

0.5mm X 0.7mm & 1.0 X 1.0mm

References

An Overview of XRF Basics

Coolidge X-ray Tubes

Developments in the X-ray Tube

Major Components of CT Scanner

The Rotating Anode X-ray Tube

X-ray Tube Heating and Cooling

X-ray Tubes Manufacturer-Brand X-ray Tube Co., Inc.