thermionic emission in radiology

High-voltage cables going into the x-ray tube. The primary purpose of the rectifier unit (15) is to change the alternating current (AC) into direct current (DC). 19. which of the following supplies and controls the heat required by the xray tube filament for thermionic emission of electrons? Current Electricity. This results in a doubling of the x-ray output. heterogeneous 6-5). The wavelength and energy of the x-ray beam is said to be, Characteristics of the Cathode and the Anode, Although it is essential to have at least one filament for x-ray production, modern multipurpose x-ray tubes are, Each filament is situated in a hollow area in the cathode called a, Introduction to Anatomy, Positioning, and Pathology, Radiography Essentials for Limited Practice. anatomically programmed radiography (APR) X-rays created by this interaction are called bremsstrahlung radiation. As more and more electrons build up in the area of the filament, their negative charges oppose the emission of additional electrons. What type of radiation is composed of photons not produced at the actual focal spot? The various components of each section are numbered so that you can easily refer to them in the discussion that follows. The autotransformer, step-down transformer, and high-voltage transformer are all located in a tank near the x-ray machine (Fig. 1. 5-2 Coolidge hot cathode tube, 1913. Only one filament is used at a time. High-Voltage Circuit The control console is the unit where the operator sets all of the exposure techniques, such as kilovolts peak (kVp), milliamperes (mA), and exposure time. Note that the arrow direction of the diode symbol indicates the direction of current flow permitted by the diode. Single-phase generators produce a pulsating current that alternates from positive to negative during each electric cycle (Fig. All of the following are related except one. A disposable plastic barrier is in place, and film packets, film holders, and supplies have been assembled before the procedure. FIG. 2004 Sep;32(3):139-47. Thermionic emission is defined as the electron emission caused by a sufficiently high level of thermal energy. 3. It is expanded in Fig. Above 70kVp, about 85% of the beam is bremsstrahlung. There are four essential requirements for the production of x-rays: (1) a vacuum, (2) a source of electrons, (3) a target, and (4) a high potential difference (voltage) between the electron source and the target. An incoming electron removes an inner orbital electron from the tungsten atom, creating a hole in the K-shell. Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat ). In the X-ray tube, the purpose of the filament is to provide the free electrons necessary for X-ray production. How many heat units are generated by an exposure of 70 kVp, 400 mA, and 0.1 second on a 3 rectified unit? In half-wave rectification the negative phase of the electric cycle is totally eliminated and a gap remains. rectification exposure timer tungsten X-rays are produced when an incoming electron misses all the electrons in the tungsten atom, gets very close to the nucleus, and then suddenly slows down and abruptly changes direction. 5-6). 6-8 Full-wave rectification. Never hold the client, the image receptor, or the tube head during exposure, Use personnel radiation-monitoring devices, Specialized film packet sandwiched between metal filters contained inside a plastic holder or badge, Advantageinexpensive, reliable technology, Limitationcannot be worn for periods longer than 1 month, Plastic badge containing lithium fluoride crystals that absorb x-ray energy, Advantagemore sensitive and accurate than film monitors, Limitationmore costly than film monitors, Advantageinstant reading with real time results, Disadvantagerequires Internet connection and fee for service, Documentation of clients radiographic exposure, The clients record should include appropriate space for listing radiographic services, All radiographic exposures, both diagnostic and undiagnostic, should be entered sequentially and cumulatively, Essential information for radiographic services, Documentation of clients refusal of radiographic services, The client must be informed of possible diagnostic information and possible risks from radiographic procedure, The client should be informed of all protection procedures used (e.g., equipment standards, quality assurance, standard darkroom procedures, protective shielding, and operator competence), Having the client sign a refusal-of-radiographic-services form does not release the oral health care provider from responsibility, See the section on Prevention of Disease Transmission During Radiographic Procedures in, Infection control procedures established for the oral health care setting must include guidelines for dental radiographic procedures, The clients health history must be reviewed and an oral examination performed before radiographic services, All clients should be considered potentially infectious, Clean and disinfect the radiographic equipment (e.g., tube head and PID, tube-head support arm, control panel, countertop, treatment chair), Use the barrier technique and disposable plastic wrap to cover items likely to become contaminated (e.g., tube head and PID, tube-head support arm, control panel, countertop, treatment chair), Obtain radiographic supplies (e.g., image receptors, holding devices), Disposable intraoral film packet barrier wraps may be used to minimize the possibility of disease transmission during the radiographic procedure and processing, Digital sensors should be wiped with an Environmental Protection Agency (EPA)accepted disinfectant before placing the protective sheath barrier and following removal; consult the manufacturers recommendations, While exposing intraoral radiographs, wear disposable client-care gloves, Wipe the film packet with an EPA-accepted disinfectant after removal of the film from the clients mouth and drop the film into a containment cup. 5-6). Do not attempt to assimilate it all at once. 6-7. 5-4 Thermionic emission. 5-4). Characteristic radiation: created when an incoming electron interacts with the K-shell electron and knocks it out of orbit. This process is called thermionic emission. Roentgen discovered x-rays while working with a Crookes tube (Fig. control console Electrostatic focal spot correction for x-ray tubes operating in strong magnetic fields. Thermionic energy conversion (TEC) is the direct conversion of heat into electricity by the mechanism of thermionic emission, the spontaneous ejection of hot electrons from a surface. 25 terms . Federal and State Bureaus of Radiological Health, 5. This means that it is made up of many different wavelengths and energies. Before the discovery of the principle, gas tubes were used for x-ray production. Repeated exposure may lead to some unrepaired effects that accumulate in exposed tissues, c.Accumulated radiation effects accelerate and increase the probability of inducing cancer and the normal aging process; high doses result in a more rapid expression of the effects, ARegulatory and recommending agencies (see the unnumbered table Web Site Information and Resources at the end of this chapter), 1.International Commission on Radiological Protection (ICRP), 2. Simplified diagram of an x-ray circuit. Illustration of the production of general and characteristic radiations. Below 70kVp, 100% of the photons in the x-ray beam are from bremsstrahlung interactions. The filament and the focusing cup determine this path of acceleration. The negative impulses are made positive during full-wave rectification rather than being eliminated (Fig. It is expanded in Fig. A high-voltage electric source provides acceleration of the electrons. See also field emission. Radiographic image formation depends on the differential attenuation of x-ray photons from the primary beam by the clients tissues, 2. Med Phys. actual focal spot X-ray energy is measured in kiloelectron volts (keV). Recommended THERMIONIC EMMISION AND RADIOACTIVITY Himanshu Singh Ch 02 electron emission shakilmishi Term Paper - Field Assisted Thermionic Emission, Field Emission, and Applicat. Thermionic emission is a process where thermal energy causes a broadening of the electron distribution such that some higher energy electrons will emit into vacuum. Full-Wave Rectification The process of rectification prepares the current for x-ray production by ensuring that it flows in the right direction, in this case from the filament to target. effective focal spot An incoming electron removes an inner orbital electron from the tungsten atom, creating a hole in the K-shell. 5-1), a cathode ray tube that was the forerunner of the fluorescent tube and the neon light. The source of electrons is a filament at one end of the tube. The primary x-ray beam is made up of both bremsstrahlung and characteristic radiation. Characteristic radiation (see Figure 6-4), (1)Can only occur at 70kVp or higher with a tungsten target;2 accounts for a small portion of x-rays produced by dental x-ray machines, (2)Occurs when an electron removes an orbital electron from the target atom, (3)During restabilization of the ionized atom, the hole created by the ejected orbital electron is filled by an outer-shell electron; the movement of the outer-shell electron results in the transference of electron-binding energy into x-ray energy, (4)The energy of the resulting characteristic x-ray photon occurs from the difference in binding energies of the orbital electrons involved, 1. Before the discovery of the principle, gas tubes were used for x-ray production. Refers to the passing of x-ray photons through a material without any alteration of the photon or the material; accounts for approximately 9% of interactions, b.Photons proceed to strike the image receptor, a. 6-5 X-ray transformer tank containing the autotransformer, filament transformer, high-voltage transformer, and rectifier. exposure switch 2005 Mar;33(1):3-18. Although the physical mechanism has been known for over a century, it has yet to be consistently realized in a manner practical for large-scale deployment. ARadiation is the emission or movement of energy through space in the form of particles or waves, b. Exam 2 [grids, radiographic quality, technique adjustment, cassettes] Terms in this set (87) What is the function of a grid? The physical area of a rotating anode focal track, affected by the cathode electron beam, is the, 19. Filament Circuit The heating of the filament leads to the release of electrons in a process called thermionic emission. Rectifiers use diodes to convert the circuit from AC to DC. 5-9). focal spot -, McCollough CH. The primary x-ray beam is made up of both bremsstrahlung and characteristic radiation. Federal regulations limit the size of the intraoral x-ray beam to 7cm (2.75 inches) in diameter, c.Rectangular PIDs have an exit size of 3.5 4.4cm (1.375 1.75 inches), d.Available lengths of PIDs are 20cm (8 inches), 30cm (12 inches), and 40cm (16 inches), e.The longer PID produces an x-ray beam that is less divergent, (1)Decreases radiation exposure to client, a.Descriptionexposure factors (mA, kVp, and exposure time) are set, and electrical circuits are activated by using the controls located there, (1)Most intraoral machines available today have preset mA and kVp, (2)Usually, settings of 5, 7, 10, or 15mA are available; kVp settings range from 60 to 100, (3)Depending on the machine type, the automatic timer is adjusted in impulses ( of a second) or in fractions of a second, (4)The exposure switch is depressed to initiate the emission of x-radiation, (5)The emission of x-radiation from the machine is indicated by an audible chirping sound and the glow of an exposure indicator light, (6)The control panel must be located in a protected area and still allow for observation of the client during exposure, 1. Bookshelf Can only occur at 70kVp or higher with a tungsten target; Occurs when an electron removes an orbital electron from the target atom, During restabilization of the ionized atom, the hole created by the ejected orbital electron is filled by an outer-shell electron; the movement of the outer-shell electron results in the transference of electron-binding energy into x-ray energy, The energy of the resulting characteristic x-ray photon occurs from the difference in binding energies of the orbital electrons involved, Represent a portion of the electromagnetic spectrum, Exist as pure energy without mass or charge, Travel at the speed of light (186,000 miles per second), When x-ray photons interact with matter, they may be either absorbed or scattered, If an incident (initial) photon is absorbed, it ceases to exist, If an incident photon is scattered, its direction of travel is altered; scattered photons do not aid in the formation of the radiographic image and only increase image density; scattered radiation causes fog (or noise) that degrades image quality, Attenuation (removal) of x-ray photons from the beam as they travel through matter (tissue) is determined by the intensity (energy) of the radiation and the density, atomic number, and electrons per gram of the matter, As the energy of the radiation increases, the number of photons passing through the matter increases, As the density, atomic number, or electrons per gram of the material increase, the number of photons passing through the matter decreases, Primary radiationphotons coming directly from the target of the x-ray tube, Secondary radiationradiation resulting from the interaction of primary radiation and matter, Scatter radiationone form of secondary radiation in which the direction of travel has been altered, Refers to the passing of x-ray photons through a material without any alteration of the photon or the material; accounts for approximately 9% of interactions, Photons proceed to strike the image receptor, Results from an incident photon colliding with a tightly bound inner-shell electron, The incident photon ceases to exist; the electron is ejected as a recoil electron (or photoelectron), Ionization of the atom occurs; the ejected electron leaves a vacancy in the shell that must be filled, Low-energy characteristic radiation is produced by the shifting of an outer-shell electron to the inner vacancy, In the energy range used for diagnostic applications, the photoelectric effect accounts for approximately 30% of interactions, Results from an incident photon colliding with a loosely bound outer-shell electron, Incident photon gives up part of its energy in the ejection of the orbiting electron, The direction of travel of the incident photon is changed; its energy is reduced, The scattered photon and the ejected electron may have sufficient energy to undergo many more ionizing interactions before losing their entire energy, Compton-scattered x-radiations may exit the patients tissues and cause image fogging, In the diagnostic energy range, Compton scattering accounts for approximately 60% of interactions, The interaction of an incident photon passing near an outer-shell electron and being scattered without losing energy, The incident photon ceases to exist, and a new photon of identical energy is released from the electron, In the diagnostic energy range, coherent scattering accounts for approximately 8% of total interactions, Image formation and differential attenuation, Radiographic image formation depends on the differential attenuation of x-ray photons from the primary beam by the clients tissues, If all photons exited the clients tissues (no absorption), the image receptor would be totally exposed; if all photons were attenuated (absorbed), the image receptor would be unexposed, As a result of photoelectric, Compton, and coherent interactions, x-ray photons are removed from the beam, Variances in the ability of tissues to absorb x-rays produce radiographic contrast, Generally, as density, atomic number, and electrons per gram of tissue increase, the number of absorbed photons increases; metallic dental restorative materials, enamel, dentin, cementum, and bone absorb photons to a great extent and produce radiopaque images; bone marrow spaces, sinuses, pulp chambers, and periodontal ligament spaces do not attenuate photons and produce radiolucent images, Interactions of Ionizing Radiation with Cells, Tissues, and Organs, Whole-body exposure (total body)each gram of tissue in the entire body absorbs equal amounts of radiation, Specific-area exposure (localized)each gram of body tissue irradiated in the specific area absorbs equal amounts of radiation (e.g., skin exposure from four bitewing radiographs), Direct effecttransfer of energy by ionization mechanism from an x-ray photon to a biologically critical molecule such as deoxyribonucleic acid (DNA), Indirect effecttransfer of energy by the ionization mechanism from an x-ray photon to a noncritical molecule, which, in turn, delivers the energy to the biologically critical molecule, Genetic effectcauses mutations in future generations; results from the exposure of reproductive cells, yielding alterations in genetic coding, Somatic effectinjury in the person being irradiated, Latent periodtime between the exposure and development of the biologic effect, Deterministic effect when the severity of a biological response is dependent on the dose; for example: erythema (redness of the tissue) would be expected to increase in direct proportion to exposure to damaging radiation, Stochastic effecta biologic response that is based on the probability of occurrence rather than its severity; for example: cancer may or may not occur with exposure to damaging radiation, Acute effects (short-term or early)effects that may occur minutes, hours, or weeks after exposure; usually result from high doses of whole-body exposure, Chronic effects (long-term or late)effects observed years after original exposure. Specialized film packet sandwiched between metal filters contained inside a plastic holder or badge, b.Advantageinexpensive, reliable technology, c.Limitationcannot be worn for periods longer than 1 month, a. The great majority of this kinetic energy (>99%) is converted into heat, and only a small amount is converted into the energy form that we know as x-rays (Fig. Represent a portion of the electromagnetic spectrum, 2. Hospitals with more powerful equipment may have a larger supply. The electron binding energy is measured in electronvolts (eV), where 1 eV = 1.6 x 10 -19 J. X-rays created by this interaction are called bremsstrahlung radiation. 6-9. FIG. The emission of electrons from a metal surface when heat energy is imparted to it is called the thermionic emission. 6-2, A. Part 2: X-ray interactions and image formation. 21. Note the cup used to contain contaminated film packets. The negative impulses are made positive during full-wave rectification rather than being eliminated (Fig. 5-3 illustrates a simple x-ray tube with its principal parts labeled. 6-7). Thermionic emission from a metal can be explained in terms of Fig. This circuit is divided into two parts by the step-down transformer (11 and 12). Study of increased radiation when an x-ray tube is placed in a strong magnetic field. Hospitals with more powerful equipment may have a larger supply. PMC State Bureau of Health and other local agencies, BAssessment of need for radiographic procedures, 1. The function of the cathode is to produce a. a focal point. 6-1, the x-ray circuit is divided into three sections or subcircuits: the low-voltage circuit, the filament circuit, and the high-voltage circuit. Thermionic emission definition. Due to the lowered work function and low tunneling barriers, the heterostructure shows enhanced emission and could be operated at much lower temperatures than the pure hexaboride (indicated by the arrows). A licensed physician or dentist must prescribe radiographic services, 2. The process is called thermionic emission (or Edison effect). The AC power supply (1) is wired into the building, providing electric power from the local power company. The kinetic energy of the electron is converted into an x-ray photon. The kVp range for most dental x-rays is 65-100 kV A higher kVp produces x-rays with more penetrating ability Radiation produced with high kVp results in short wavelengths increasing mA results in an increase in Temp of filament and number of x-rays produced the mA range for dental radiography is 7 to 15 mA Thermionic emission occurs in the As explained in Chapter 4, it increases the incoming voltage by the value of the transformer ratio. 6-6). In terms of producing x-ray images, there is no difference between a bremsstrahlung and characteristic photon. Do not attempt to assimilate it all at once. The https:// ensures that you are connecting to the In order to expel the electrons from the system, they need to be given the energy. Essential components of an X-ray tube include a cathode, and an anode separated a short distance from each other, a vacuum enclosure, and high voltage cables forming the X-ray generator attached to the cathode and anode components. Oxide of A b <j> in volts = kb/e. The x-rays are pulsating on, off, on, off, and so forth at a rate of 60 pulses per second. The cathode is the negative end of the tube; the filament is part of the cathode. When this occurs, current is induced to flow through the secondary side of the transformer, creating voltage across the x-ray tube. Factors that determine tissue sensitivity, a. As explained in Chapter 4, it increases the incoming voltage by the value of the transformer ratio. General (braking) radiation . density control Bremsstrahlung is created when an incoming electron slows suddenly near the nucleus (N) of the tungsten atom and abruptly changes direction, resulting in the creation of an x-ray photon. Although the thermionic emission in traditional materials, such as copper and silicon, has been well-explained by a theoretical model put forward by British physicist O. W. Richardson in 1901 . Radiology. The step-up transformer is also referred to as the high-voltage or high-tension transformer.

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