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Citation, DOI & article dataCitation: Mudgal, P., Moore, C. Characteristic radiation. Reference article, Radiopaedia.org. (accessed on 07 Nov 2022) https://doi.org/10.53347/rID-25429 Characteristic radiation is a type of energy emission relevant for X-ray production. This energy emission happens when a fast-moving electron collides with a K-shell electron, the electron in the K-shell is ejected (provided the energy of the incident electron is greater than the binding energy of K-shell electron) leaving behind a 'hole'. An outer shell electron fills this hole (from the L-shell, M-shell, etc. ) with an emission of a single x-ray photon, sometimes called a characteristic photon, with an energy level equivalent to the energy level difference between the outer and inner shell electron involved in the transition. As opposed to the continuous spectrum of bremsstrahlung radiation, characteristic radiation is represented by a line spectrum. As each element has a specific arrangement of electrons at discrete energy level, then it can be appreciated that the radiation produced from such interactions is 'characteristic' of the element involved. For example, in a tungsten target electron transitions from the L-shell to the K-shell produce x-rays photons of 57.98 and 59.32 keV. The two energy levels are as a result of the Pauli exclusion principle which states that no two particles of half-integer spin (such as electrons) in an atom can occupy exactly the same energy state at the same time; therefore the K-shell represents two different energy states, the L-shell eight states and so on. When an electron falls (cascades) from the L-shell to the K-shell, the x-ray emitted is called a K-alpha x-ray. Similarly, when an electron falls from the M-shell to the K-shell, the x-ray emitted is called a K-beta x-ray1. However, it is possible to have M-L transitions and so on but their likelihood is so low they can be safely ignored. Each element differs in nuclear binding energies, and characteristic radiation depends on the binding energy of particular element. Characteristic radiation never exists in isolation and the line spectra is usually superimposed on the continuous spectra of bremsstrahlung radiation. Quiz questionsReferences
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Equipment
CathodeFilament
Focusing cup
Anode
Definitions
Stationary anode: these are generally limited to dental radiology and radiotherapy systems. Consists of an anode fixed in position with the electron beam constantly streaming onto one small area. Rotating anode: used in most radiography, including mobile sets and fluoroscopy. Consists of a disc with a thin bevelled rim of tungsten around the circumference that rotates at 50 Hz. Because it rotates it overcomes heating by having different areas exposed to the electron stream over time. It consists of:
Heating of the anodeThis is the major limitation of x-ray production. Heat (J) = kVe x mAs or Heat (J) = w x kVp x mAs key: kVe = effective kV Heat is normally removed from the anode by radiation through the vacuum and into the conducting oil outside the glass envelope. The molybdenum stem conducts very little heat to prevent damage to the metal bearings. Heat capacity A higher heat capacity means the temperature of the material rises only a small amount with a large increase in heat input. Temperature rise = energy applied / heat capacity Tube rating Each machine has a different capacity for dissipating heat before damage is caused. The capacity for each focal spot on a machine is given in tube rating graphs provided by the manufacturer. These display the maximum power (kV and mA) that can be used for a given exposure time before the system overloads. The maximum allowable power decreases with:
Other factors to take into consideration are:
Anode cooling chart As well as withstanding high temperatures an anode must be able to release the heat quickly too. This ability is represented in the anode cooling chart. It shows how long it takes for the anode to cool down from its maximum level of heat and is used to prevent damage to the anode by giving sufficient time to cool between exposures. Anode heel effect
An x-ray beam gets attenuated on the way out by the target material itself causing a decrease in intensity gradually from the cathode to anode direction as there is more of the target material to travel through. Therefore, the cathode side should be placed over the area of greatest density as this is the side with the most penetrating beam. Decreasing the anode angle gives a smaller effective focal spot size, which is useful in imaging, but a larger anode heel effect. This results in a less uniform and more attenuated beam. ** smaller angle = smaller focal spot size but larger anode heel effect ** OthersWindow: made of beryllium with aluminium or copper to filter out the soft x-rays. Softer (lower energy) x-ray photons contribute to patient dose but not to the image production as they do not have enough energy to pass through the patient to the detector. To reduce this redundant radiation dose to the patient these x-ray photons are removed. Glass envelope: contains vacuum so that electrons do not collide with anything other than target. Insulating oil: carries heat produced by the anode away via conduction. Filter: Total filtration must be >2.5 mm aluminium equivalent (meaning that the material provides the same amount of filtration as a >2.5 mm thickness of aluminium) for a >110 kV generator Total filtration = inherent filtration + additional filtration (removable filter) Written by radiologists, for radiologists with plenty of easy-to-follow diagrams to explain complicated concepts. An excellent resource for radiology physics revision.
Producing an x-ray beam1. Electrons produced: thermionic emissionA current is applied through the cathode filament, which heats up and releases electrons via thermionic emission. The electrons are accelerated towards the positive anode by a tube voltage applied across the tube. At the anode, 99% of energy from the electrons is converted into heat and only 1% is converted into x-ray photons. Accelerating potential
The accelerating potential is the voltage applied across the tube to create the negative to positive gradient across the tube and accelerate the electrons across the anode. It is normally 50-150 kV for radiography, 25-40 kV for mammography and 40-110 kV for fluoroscopy. UK mains supply is 230 V and 50 Hz of alternating current. When the charge is negative the accelerating potential is reversed (the cathode becomes positive and the anode becomes negative). This means that the electrons are not accelerated towards the anode to produce an x-ray beam. The ideal waveform for imaging is a positive constant square wave so that the electron flow is continuously towards the anode. We can convert the standard sinusoidal wave into a square wave by rectification. Full wave rectification: the use of a rectification circuit to convert negative into positive voltage. However, there are still points at which the voltage is zero and most of the time it is less than the maximum kV (kVp). This would lead to a lot of lower energy photons. There are two rectification mechanisms that prevent too many lower energy photons:
Effect of rectification on spectrum
Filament currentThe current (usually 10 A) heats up the filament to impart enough energy to the electrons to be released i.e. it affects the number of electrons released. Tube currentThis is the flow of electrons to the anode and is usually 0.5 – 1000 mA. Summary
2. X-ray production at the anodeThe electrons hit the anode with a maximum kinetic energy of the kVp and interact with the anode by losing energy via:
InteractionsAt the anode, electrons can interact with the atoms of the anode in several ways to produce x-ray photons.
1. Characteristic radiation
It is called “characteristic” as energy of emitted electrons is dependent upon the anode material, not on the tube voltage. Energy is released in characteristic values corresponding to the binding energies of different shells. For tungsten: 2. Bremsstrahlung
Bremsstrahlung causes a spectrum of photon energies to be released. 80% of x-rays are emitted via Bremsstrahlung. Rarely, the electron is stopped completely and gives up all its energy as a photon. More commonly, a series of interactions happen in which the electron loses energy through several steps.
Summary of steps
X-ray spectrum
The resulting spectrum of x-ray photon energies released is shown in the graph. At a specific photoenergy there are peaks where more x-rays are released. These are at the characteristic radiation energies and are different for different materials. The rest of the graph is mainly Bremsstrahlung, in which photons with a range of energies are produced. Bremsstrahlung accounts for the majority of x-ray photon production. Beam quality: the ability of the beam to penetrate an object or the energy of the beam. Beam quantity: the number of x-ray photons in the beam Altering the x-ray spectrumIncreasing the Tube Potential (kV) Increased :
If kV great enough, characteristic energy produced
Increasing the Tube Current (mA) Increased quantity of x-ray photons No change in:
Filtration Fewer lower energy photons Increased:
Decreased:
Waveform of Current Having a more uniform current (rectified) results in increased:
Increasing Atomic Number of Target Increased:
How are characteristic XWhen electrons change from one atomic orbit to another, characteristic X-rays are produced. The individual photon energies are characteristic of the type of atom and can be used to identify very small quantities of a particular element.
Which type of xWhen an electron passes near the nucleus it is slowed and its path is deflected. Energy lost is emitted as a bremsstrahlung X-ray photon.
What type of XTwo types of interactions produce radiation. An interaction with electron shells produces characteristic x-ray photons; interactions with the atomic nucleus produce Bremsstrahlung x-ray photons.
How does increasing kVp affect the energy of K shell characteristic XAn increase in kVp extends and intensifies the x-ray emission spectrum, such that the maximal and average/effective energies are higher and the photon number/intensity is higher.
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