Hard X-rays can penetrate solid objects, and their largest use is to take images of the inside of objects in diagnostic radiography and crystallography.
The roentgen (R) is an obsolete traditional unit of exposure, which represented the amount of radiation required to create one electrostatic unit of charge of each polarity in one cubic centimeter of dry air.
The rad is the (obsolete) corresponding traditional unit, equal to 10 millijoules of energy deposited per kilogram. The sievert (Sv) is the SI unit of equivalent dose, which for X-rays is numerically equal to the gray (Gy).
X-rays are generated by an X-ray tube, a vacuum tube that uses a high voltage to accelerate the electrons released by a hot cathode to a high velocity. In crystallography, a copper target is most common, with cobalt often being used when fluorescence from iron content in the sample might otherwise present a problem. X-ray fluorescence: If the electron has enough energy it can knock an orbital electron out of the inner electron shell of a metal atom, and as a result electrons from higher energy levels then fill up the vacancy and X-ray photons are emitted.
So the resulting output of a tube consists of a continuous bremsstrahlung spectrum falling off to zero at the tube voltage, plus several spikes at the characteristic lines. In medical diagnostic applications, the low energy (soft) X-rays are unwanted, since they are totally absorbed by the body, increasing the dose. To generate an image of the cardiovascular system, including the arteries and veins (angiography) an initial image is taken of the anatomical region of interest. A specialized source of X-rays which is becoming widely used in research is synchrotron radiation, which is generated by particle accelerators. The most commonly known methods are photographic plates, photographic film in cassettes, and rare earth screens.
Before the advent of the digital computer and before invention of digital imaging, photographic plates were used to produce most radiographic images.
Since photographic plates are sensitive to X-rays, they provide a means of recording the image, but they also required much X-ray exposure (to the patient), hence intensifying screens were devised. Areas where the X-rays strike darken when developed, causing bones to appear lighter than the surrounding soft tissue.
Contrast compounds containing barium or iodine, which are radiopaque, can be ingested in the gastrointestinal tract (barium) or injected in the artery or veins to highlight these vessels. An increasingly common method is the use of photostimulated luminescence (PSL), pioneered by Fuji in the 1980s.
The PSP plate can be reused, and existing X-ray equipment requires no modification to use them. For many applications, counters are not sealed but are constantly fed with purified gas, thus reducing problems of contamination or gas aging. Some materials such as sodium iodide (NaI) can "convert" an X-ray photon to a visible photon; an electronic detector can be built by adding a photomultiplier.
Note that the voltages across the two halves of the center-tapped secondary are equal in amplitude but opposite in phase. During the negative half-cycle of the input waveform, diodes D2 and D4 conduct and charge capacitors C1 and C3 toward the peak voltage of 18.95 V with indicated polarities. Finally, the size of the filter capacitor depends on the secondary current rating of the transformer. Therefore, for the proper operation the regulators must be heat-sinked in order to keep their temperature down.
Besides the ±15 and ±5-V regulated supply voltages, there is often a need for a 60-Hz square-wave signal, which is used as a time base in scanning the digital displays and as a trigger for sequential and timing circuits. Full Featured Touch Control Programmable Power Supply Full touch control variable power supply.


This process produces an emission spectrum of X-ray frequencies, sometimes referred to as the spectral lines. The intensity of the X-rays increases linearly with decreasing frequency, from zero at the energy of the incident electrons, the voltage on the X-ray tube. A second image is then taken of the same region after iodinated contrast material has been injected into the blood vessels within this area. The contrast compounds have high atomic numbered elements in them that (like bone) essentially block the X-rays and hence the once hollow organ or vessel can be more readily seen. In modern hospitals a photostimulable phosphor plate (PSP plate) is used in place of the photographic plate.
Most digital ICS, including microprocessor and memory ICS , operate on a ± 5-V supply, while almost all linear ICS (op-amps and special-purpose ICS) require ± 15-V supplies. During the positive half-cycle of the input voltage, diode D1 conducts and capacitor C1 charges toward a positive peak value =18.95 V.
Note, however, that the diode pair that conducts during either the positive or negative half-cycle does not do so for the entire half-cycle. The circuit arrangement of the ±5-V supply is identical to that of the ±15-V supply except that here the specifications for the transformer T2 secondary are different. If a regulator is a metal package (TO-3 type), the appropriate heat sink is mounted on the case of the package. If needed, a 1-Hz (1-s) signal for the real-time clock can be readily obtained from the 60-Hz signal by using a divide-by-60 network.
The spectral lines generated depend on the target (anode) element used and thus are called characteristic lines. These two images are then digitally subtracted, leaving an image of only the iodinated contrast outlining the blood vessels. Photographic film largely replaced these plates, and it was used in X-ray laboratories to produce medical images. In the pursuit of a non-toxic contrast material, many types of high atomic number elements were evaluated. After the plate is X-rayed, excited electrons in the phosphor material remain "trapped" in "colour centres" in the crystal lattice until stimulated by a laser beam passed over the plate surface. Electrons accelerate toward the anode, in the process causing further ionization along their trajectory. In this figure two separate transformers are used because they are readily available; however, it is possible to custom design a single transformer with the same specifications to replace the two. However, since the drop-out voltage (Vin – V0) is 2 V, the input voltage for the 7815 must be at least +17 V and that for the 7915 must be at least — 17 V. The diodes conduct only during the time when the anodes are positive with respect to the cathodes. The working voltage rating (WVDC) of the capacitor, on the other hand, depends on the peak rectified output voltage and must be at least 20% higher than the peak value of the voltage it is expected to charge to.
Therefore, the operation and considerations for the ±5-V supply are the same as those presented for the ±15-V supply. However, if the regulator is an epoxy package, a silicon grease may be used on the back of the package, and then the package can be bolted to the chassis of the power supply cabinet with insulating hardware. Although not commonly done, a higher-frequency Signal can also be obtained from the 60-Hz signal by using e multiplier. The radiologist or surgeon then compares the image obtained to normal anatomical images to determine if there is any damage or blockage of the vessel. In more recent years, computerized and digital radiography has been replacing photographic film in medical and dental applications, though film technology remains in widespread use in industrial radiography processes (e.g.


For example, the first time the forefathers used contrast it was chalk, and was used on a cadaver's vessels. This process, known as a Townsend avalanche, is detected as a sudden current, called a "count" or "event".
This means that the rectified peak voltage must be greater than +17 V and — 17 V, which in turn implies that the secondary voltage must be larger than 34 V peak or 24 V rms. In other words, when the diodes are forward biased, the capacitors are charged by current pulses. When the film is developed, the parts of the image corresponding to higher X-ray exposure are dark, leaving a white shadow of bones on the film. The output of these secondaries is then applied to the bridge rectifiers, which convert the sinusoidal inputs into full-wave rectified outputs.
The voltage across the center-tapped secondary in Figure 1-2 is 26.8 V rms, thus satisfying the minimum voltage requirement of 24 V rms. The PRV rating of the bridge rectifier diodes, also known as a working inverse voltage (WIV), is specified on the data sheets. Data sheets give the maximum average rectified current I0max that the diode can safely handle. Capacitors C2 and C4 at the output of 7815 and 7915 regulators, respectively, help to improve the transient response and should be in the range of 1µF.
Photographic plates are mostly things of history, and their replacement, the "intensifying screen", is also fading into history. The filter capacitors at the output of the bridge rectifiers are charged to the peak value of the rectified output voltage whenever the diodes are forward biased.
Also, the peak voltage between either of the secondary terminals and the center-tap (ground terminal) is 18.95 V peak, which is less than the maximum peak voltages of +35 V and -35 V for the 7815 and 7915, respectively. The bridge rectifier, MDA200 (Mot0r0la’s rectifier) in Figure 1-2, has a PRV rating of 50 V, which is higher than needed. The metal silver (formerly necessary to the radiographic & photographic industries) is a non-renewable resource.
Since the diodes are not forward biased during the entire positive and negative half-cycle of the input waveform, the voltage across the filter capacitors is a pulsating dc that is a combination of do and a ripple voltage. This bridge rectifier is, in fact, used here because it is readily available and more commonly used. In addition, when the power supply is first turned on, the initial charging of the capacitor causes a large transient current called the surge current to pass through the diodes. Thus it is beneficial that this is now being replaced by digital (DR) and computed (CR) technology.
The surge current IFS flows only briefly and is therefore much larger than the maximum average current I0max. Where photographic films required wet processing facilities, these new technologies do not. The maximum surge-current IFSM is normally included on the data sheets; it is 60 A for the MDA200.



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Comments

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  2. TM_087
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