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X-RAYS
X-ray are a form of Electromagnetic radiation, having essentially the same nature as light rays, but having a relatively shorter wavelength. X-rays have a wavelength of less than 100 angstrom units as compared to a wavelength of 10,000 angstrom units with visible light. Because of the short wavelength of x-rays and gamma rays they are capable of penetrating materials that reflect or absorb light rays. Unlike light years, However x-rays are not refracted to any easily measured degree by glass and therefore it is impractical to focus them.
X-rays are generated using an x-rays tube. When electrons traveling at high speed collide with some material or when they have an abrupt change of direction, x-rays are generated. In an x-ray tube a high voltage on the target(anode) of the tube results in electrons being “boiled”off the filament of the tube (cathode) and striking the target material. When the electrons strike the target they are stopped abruptly, resulting in the production of x-rays.


X-RAY TUBE
As previously stated, x-rays are generated using a high voltage source and an x-ray tube. Figure #1 shows the component of a typical x-ray tube used in the micro focus system. This tube uses a tungsten target(anode).
The anode or target area must be cooled on this particular type of tube because most of the energy is dissipated in the form of heat when the electrons strike the tungsten target.
The filament of the tube is heated by current passing through it of several amperes from a low voltage source.
This results in electrons being boiled off the filament. The greater the current passing through the filament, the greater the number of electrons emitted from the filament. This electron emission is called the tube current.
X-ray output will be proportional to tube current.
The electron optics of the x-ray tube in the x-ray systems are designed to focus these electrons down into a very small beam, which is, in turn, focused onto the tungsten target.



A MICROFOCUS TUBE HAS A GREATER BEAM DENSITY, REQUIRING LESS EXPOSURE TIME TO GET A GOOD IMAGE
Because the electrons are striking a smaller target, more x-rays will be emitted at the smaller focal point, Creating greater beam density.

Micro-focus X-ray tube refers to the way the electron beam in an x-ray tube Is focused down to a very small area on the target of the x-ray tube, because The electrons are focused down to this small size, the diameter of the area On the target emitting X-rays is smaller. This area is called the focal spot. If the focal spot is small, in the case of the X-ray systems 10 microns, the System is able to resolve items at least to that size.
( 15 microns would be About 6 mils). micron is equal to about .000059 inch, therefore 10 microns would equal about .00039 inch or about 4 mils.


THE RANGE OF THE HIGH VOLTAGE WILL DETERMINE THE RANGE OF THE WAVELENGTHS OF THE X-RAYS?
As the High Voltage is increased, shorter wavelengths are added to the X-ray frequencies. Shorter wavelengths penetrate denser samples.


PENETRATION
When x-rays are generated from the x-ray tube, they are radiated in all directions. Because of the beryllium Window and lead casing around the x-ray tube only The x-rays passing through the window penetrate the samples. When the x-rays penetrate the sample some of the x-rays are absorbed in the sample and some Are passed through the sample creating the shadow image of the sample. The amount of absorption is dependent upon the sample thickness,the Density of the material and the atomic number of the material.
Shorter wavelengths of x-rays will penetrate denser samples.


X-RAY BEAM INTENSITY
The x-ray beam intensity is the function of magnitude and energy of the x-ray. Both increased tube current and increased tube voltage will result in an increase In x-ray beam intensity.


X-RAY BEAM DENSITY
X-ray beam density is a function of area and tube current. This refers to the number of x-rays passing through an area at any one time. If the tube current (mill ampere) is increased, the number of electrons striking the focal spot of the tungsten anode increases. This in turn increases the number of x-rays being generated at that point in time.
Greater beam density allows for a shorter exposure time for obtaining a radiographic image. High beam density of the Micro focus systems enhances viewing a sample in real time.


SHORTER WAVELENGTHS HAVE MORE PENETRATING POWER
At lower voltage levels, x-rays of longer wavelengths are produced. At Higher voltage levels both long wavelength x-rays and shorter wavelength x-rays are produced.
This means that at higher voltages, there will be more penetration power.


FOR MOST CASES, WE HAVE FOUND THAT X-RAY WAVELENGTHS PRODUCED WITH 100KV AND A TUNGSTEN ANODE ARE ABLE TO PENETRATE MOST SAMPLE IN THE ELECTRONICS INDUSTRY X-RAY TERMS
The following terms describe some of the major characteristics of x-ray generation.


TUBE VOLTAGE
This is the voltage that is applied to the anode (target) of the x-ray tube. The amount of tube voltage is stated in terms of kilovolts. An increase in tube voltage results in x-rays of a shorter wavelength x-rays being generated.
The advantage of shorter wavelength x-rays being generated is an increase in the ability to penetrate thicker and denser samples.
There are disadvantages to increased tube voltage. Increased voltage can cause “with-out” where no detail is detected because the x-rays are of a wavelength which completely penetrated the sample. Higher tube voltage also results in more heat being generated on the anode of the x-ray tube and greater x-ray shielding requirements.


HIGH VOLTAGE
The high voltage applied to the x-ray tube will determine the speed with which the electrons will travel to the target. Are considered to be low voltage x-ray machines using an 80-120kv x-ray tubes. Because of the small sizes of the focal spot a much lower tube current is required for the required x-ray beam density because the sample can be moved close to the source.

An increase in the high voltage applied to the x-ray tube will also have a relative effect on increasing the intensity and penetration of the X-ray beam.

Produced at 120 kilovolts contain all the wavelengths that would be produced at 80 kilovolts, however, x-rays produced at 120 kilovolts Would also contain some shorter wavelengths that would not be present in the 80 kilovolt x-rays. Therefore, there would be a relative increase in the penetration of the x-rays because of the shorter wavelengths present at the higher kilo voltage.


TUBE CURRENT
Tube current refers to the flow of electrons present in the electron beam in the x-ray tube. Tube current is stated In terms a mill ampere When tube current is Increased there is a relative increase in the number of electrons Striking the tube target. Therefore, there will be a resulting increase in x-ray beam density improving the quality of the x-ray image by giving a wider ranger of grays scale in a shorter exposure time.


SPOT SIZE
The spot size is generally measured in microns “one micron equals approximately. 000039”. In the micro focus systems, the electron beam is focused Down to a very small focal spot (approximately 10 microns ). This results In being able to resolve very small detail. The smaller the spot size, the smaller the detail that can be resolved.


RADIOGRAPHY
The Micro product family employs micro-focus real-time radiography to accomplish non-destructive testing of electronic components, hybrid circuitry, multi-layer circuit boards and surface mounted device assemblies.

Radiography is essentially, the process used to create and image an x-ray or gamma ray sensitive surface.
Using the highly penetrating X-rays or gamma rays which are non-destructive, an image can be created showing the physical internal parts of an object. The information this image or radiograph supplies can be used to determine hidden defects or internal irregularities.


MILLIAMPERE
The amount of radiation emitted by an x-ray tube is determined by the amount of current passing through
the tube, the kilo voltage and the time the tube is energized.
When the current(mill ampere)is increased there will be a corresponding increase in the intensity of the emitted x-rays.
There is no increase in x-ray penetration because no additional wavelengths are added to the beams shown in Figure 4.


GEOMETRIC PRINCIPLES
X-ray and Gamma rays exhibit properties which are similar to light rays. This is especially true when we think of the radiograph as a shadow picture of the sample produced by x-rays. It is therefore important to be familiar with the principles of shadow formation. The geometric principles will be explained using light rays as being analogous to x-rays, but it should be noted that x-rays and gamma rays Have other properties which will affect imaging such as the Scattered radiation that light rays do not have. These principles of Shadow formation are, enlargement, sharpness and distortion

The target material of an x-ray tube is selected because of the melting point of the material and the atomic number.
The high melting point of tungsten is desirable because of the large number of electrons striking the target and the resultant transformation of heat at the focal spot. The efficiency of the material to generate x-rays is dependent on the atomic number, therefore tungsten is doubly desirable as a target material because of its high atomic number.


COOLING
When the electrons strike the target (anode) of the x-ray tube most of the energy is transformed Into heat. To prolong the life of the target material, the heat must be dissipated from the target area. This is accomplished on the X-ray systems by circulating air through the inside of the x-ray tube enclosure to reduce the temperature to a suitable operating level.


FOCAL SPOT
The x-ray system normally use a micro focus x-ray source or a “point source’ x-ray tube. This refers to the size of he area onto which the electron stream is being focused and the size of the effective focal spot. The target of the x-ray tube is placed at an angle of 20’ to the electron beam as shown in Figure 7.
This creates an effective focal spot much smaller than the actual area of the focal spot on the target. The focal area shown in Figure 5 is of a line-focus tube to better illustrate this cripple. The X-ray systems use a spot-focus tube which would result In a similar smaller “effective” spot size.


X-RAY TUBE CURRENT AFFECTS BEAM DENSITY AND THEREFORE, IMAGE CONTRAST
As the tube current increases, the number of electrons hitting a specific Target area on the x-ray tube anode Increase This means that more X- Rays are being generated per spot, or there is increase in beam density.


THE MICRO SYSTEMS HAVE A FIXED TUBE CURRENT. APPROXIMATELY 0L.125 MILLIAMPS AT 80KV / POWER(WATTS) EQUALS CURRENT TIMES VOLTAGE
The Micro Focus penetrate at 10 watts of power. Therefore: 10 Watts = 80,000 volts times 0.125 milliamps. ( P=VI )


THE HIGHER THE X-RAY TUBE TARGET VOLTAGE, THE GREATER THE PROBLEM WITH SCATTERED RADIATION
Scattered radiation causes a degradation of the x-ray image.


SEALED X-RAY TUBES SUCH AS THE ONE USED IN THE MICROFOCUS SYSTEMS, AVOID THE CONTINUED MAINTENACE OF THE VACUUM PUMP SYSTEM USED ON THE VACUUM-PUMP TYPE TUBES. HIGH ATOMIC NUMBERED ENCAPSULATIONS PREVENT THE IDENTIFICATION OF LESS DENSE INTERNAL COMPONENTS
Because the x-ray image is on time of the density of a component, and The density of an object is directly related to its atomic number, Covering a component with a material that is of a higher atomic Number than the internal parts of a component will prevent the x-rays from detailing the internal component.


THEREFORE : OUR 10 MICRON SPOT SIZE EQUALS APPROXIMATELY 0.0004 INCHES
10 X 0.03937 = 0.00039 = 0.0004 INCH



IMAGE QUALITY
Because of the short wavelength of X-rays, they are able to penetrate solid object. X-rays are both absorbed by the objects and pass through the objects. When the x-rays pass through an object they are projected onto an x-rays sensitive surface and a “shadow” image of the object can be obtained.
When the image is projected onto x-ray sensitive film it is called a radiograph. There are a number of factors which will affect the quality of the recorded image or radiograph. The amount of x-rays emitted from the source, the size of the effective focal spot, the amount of radiation which is absorbed by the sample, the proportion of the radiation which passes through the sample and the affect of the recording surface or intensifying screen.
The intensity of the x-ray beam in the X-Ray tube will be determined by the setting of the Kilovolts control.
Therefore, the penetration of the radiation and intensity are increased when the Kilo voltage setting is increased.


IF THE FOCAL SPOT STAYED THE SAME SIZE, AN INCREASE IN HIGH VOLTAGE WOULD RESULT IN MUCH MORE HEAT BEING GENERATED ONTHE TARGET OF THE X-RAY TUBE
Heat causes degradation of the target area of an x-ray tube.
Most of the Power is dissipated in the form of heat on the target of the tube.


THE SMALLER THE FOCAL SPOT SIZE, THE BETTER THE RESOLUTION OF THE SYSTEM
If the focal spot size is 10 microns, as with the micro focus X-ray systems, Then you should be able to resolve detail to at least 4 mils or less.


THE SMALL FOCAL SPOT SIZE PERMITS REAL-TIME MAGNIFICATION OF THE SAMPLE
Because the parallax problem is minimized due to the small spot size, The sample does not have to be in contact with the x-ray film or the Image camera. Therefore, the distance of the sample to the camera can Be varied resulting In magnification of the video image.


X-RAYS REACT A MANNER SIMILAR TO LIGHT RAYS EXCEPT, PRACTICALLY SPEAKING, THEY CANNOT BE FOCUSED
The image created by the x-rays penetrating a sample is similar to a Shadow.
The image will not be three dimensional, just as a shadow will Not be three dimensional.
The shadow principles can be applied to x-ray Images as well as light images.

The larger the effective focal spot or area of the source the further the source must be from the sample to Minimize the parallax effect. This is illustrated in Figure 9.

It is also important in the formation of shadows that the sample and the surface upon which the shadow is Projected be as parallel to each other as is possible and that the center of the light ray beam or x-ray beam be perpendicular to the sample and recording surface. This will ensure that the shadow is a true image representation of the sample.


ENLARGEMENT
It is possible to magnify the sample image on the X-Ray systems by moving the image camera of film away from the sample and source. It is also possible to determine the degree of Magnification by using the following formula:

Where B is the Sample diameter, S is the shadow diameter, D0 is the distance from the source to the sample and D1 is the distance from the source to the shadow. Figure 9 illustrates this relationship.


SHARPNESS
Another characteristic of shadow formation is the feature of sharpness of the image. As illustrated in Figure 10, the size of the light source,(or the effective focal spot on the target), will affect the sharpness of the shadow image. If the source is more than a single spot, there will be multiple shadows cast of a single sample which will be slightly displace d, creating an image area which is not sharp, the penumbra.
This condition is referred to as a parallax condition. The larger the effective focal spot, the greater the penumbra area. This unsharpness is also affected by the distance of the source from the sample as shown in Figure 10.


CONTRAST
When a sample is placed in the path of the X-ray beam, some of the x-rays will be absorbed by the sample and some of the x-rays will be transmitted through the sample. The x-rays which pass through the sample are projected onto a recording surface which is sensitive to the x-rays. In the case of the X-ray systems, the x-rays are projected onto a phosphor target which in turn projects visible light photons onto the surface of a Image Intensifier camera tube. There are a number of factors which determine how many x-rays will pass through the sample. First, the wavelength of the x-rays will determine their penetration, shorter wave-length X-ray are able to penetrate denser sample. As the Kilo voltage is increased on the Anode of the X-ray tube, shorter wavelengths are added to the continuous spectrum of the x-rays, increasing the effective penetration of the X-rays.
Another factor which influences the penetration of the x-rays is the relative density of the sample. Sample that Have a high atomic number tend to absorb more of the x-rays, therefore, fewer x-rays will strike the recording surface.
Finally, the exposure time will have an effect on the recorded image or radiograph. The greater the exposure time, the more x-rays that will be accumulated on the recording surface. Therefore, an extremely dense sample can still have a radiograph done of it by increasing the exposure time. The advantage of the and is that the micro focus tube gives a beam density that, with most sample materials inspected, is sufficient to generate a radiographic Image in real-time.
The contrast of the radiographic image is related to the amount of x-rays which penetrate each thickness of the sample. The density differences in the sample are displayed as gray level difference on the radiograph.
At the lower kilo voltage, fewer x-rays penetrate the two thick nesses or densities is large. However, when the kilo voltage is increased, adding x-rays penetrating the thick and thin sections effect decrease the contrast between the two thick nesses.


SENSITIVITY
When a sample is being inspected with X-rays and the sample has minute variations of densities, The lowest possible voltage should be used to increase the contrast between slight density differences.

When a sample which is extremely dense is inspected, it may be necessary to use a higher voltage to penetrate The sample, thus, minute variations of density within the sample would not be detectable. It would be possible however to increase the exposure time of the sample to detect minor flaws.



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