Filters in Radiography
At x-ray energies, filters consist of material placed in the useful beam to absorb, preferentially, radiation based on energy level or to modify the spatial distribution of the beam. Filtration is required to absorb the lower-energy x-ray photons emitted by the tube before they reach the target. The use of filters produce a cleaner image by absorbing the lower energy x-ray photons that tend to scatter more.
The total filtration of the beam includes the inherent filtration (composed of part of the x-ray tube and tube housing) and the added filtration (thin sheets of a metal inserted in the x-ray beam). Filters are typically placed at or near the x-ray port in the direct path of the x-ray beam. Placing a thin sheet of copper between the part and the film cassette has also proven an effective method of filtration.
For industrial radiography, the filters added to the x-ray beam are most often constructed of high atomic number materials such as lead, copper, or brass. Filters for medical radiography are usually made of aluminum (Al). The amount of both the inherent and the added filtration are stated in mm of Al or mm of Al equivalent. The amount of filtration of the x-ray beam is specified by and based on the voltage potential (keV) used to produce the beam. The thickness of filter materials is dependent on atomic numbers, kilovoltage settings, and the desired filtration factor.
Gamma radiography produces relatively high energy levels at essentially monochromatic radiation, therefore filtration is not a useful technique and is seldom used.
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Secondary (Scatter) Radiation and Undercut Control
Secondary (Scatter) Radiation
Secondary or scatter radiation must often be taken into consideration when producing a radiograph. The scattered photons create a loss of contrast and definition. Often secondary radiation is thought of as radiation striking the film reflected from an object in the immediate area, such as a wall, or from the table or floor where the part is resting. Side scatter originates from walls, or objects on the source side of the film. Control of side scatter can be achieved by moving objects in the room away from the film, moving the x-ray tube to the center of the vault, or placing a collimator at the exit port, thus reducing the diverging radiation surrounding the central beam.
It is often called backscatter when it comes from objects behind the film. Industry codes and standards often require that a lead letter "B" be placed on the back of the cassette to verify the control of backscatter. If the letter "B" shows as a "ghost" image on the film, a significant amount of backscatter radiation is reaching the film. The image of the "B" is often very nondistinct as shown in the image to the right. The arrow points to the area of backscatter radiation from the lead "B" located on the back side of the film. The control of backscatter radiation is achieved by backing the film in the cassette with a sheet of lead that is at least 0.010 inch thick. It is a common practice in industry to place a 0.005" lead screen in front and a 0.010" screen behind the film.
Undercut
Another condition that must often be controlled when producing a radiograph is called undercut. Parts with holes, hollow areas, or abrupt thickness changes are likely to suffer from undercut if controls are not put in place. Undercut appears as a darkening of the radiograph in the area of the thickness transition. This results in a loss of resolution or blurring at the transition area. Undercut occurs due to scattering within the film. At the edges of a part or areas where the part transitions from thick to thin, the intensity of the radiation reaching the film is much greater than in the thicker areas of the part. The high level of radiation intensity reaching the film results in a high level of scattering within the film. It should also be noted that the faster the film speed, the more undercut that is likely to occur. Scattering from within the walls of the part also contributes to undercut, but research has shown that scattering within the film is the primary cause. Masks are used to control undercut. Sheets of lead cut to fill holes or surround the part and metallic shot and liquid absorbers are often used as masks.
Radiation Safety
Ionizing radiation is an extremely important NDT tool but it can pose a hazard to human health. For this reason, special precautions must be observed when using and working around ionizing radiation. The possession of radioactive materials and use of radiation producing devices in the United States is governed by strict regulatory controls. The primary regulatory authority for most types and uses of radioactive materials is the federal Nuclear Regulatory Commission (NRC). However, more than half of the states in the US have entered into "agreement" with the NRC to assume regulatory control of radioactive material use within their borders. As part of the agreement process, the states must adopt and enforce regulations comparable to those found in Title 10 of the Code of Federal Regulations. Regulations for control of radioactive material used in Iowa are found in Chapter 136C of the Iowa Code.
For most situations, the types and maximum quantities of radioactive materials possessed, the manner in which they may be used, and the individuals authorized to use radioactive materials are stipulated in the form of a "specific" license from the appropriate regulatory authority. In Iowa, this authority is the Iowa Department of Public Health. However, for certain institutions which routinely use large quantities of numerous types of radioactive materials, the exact quantities of materials and details of use may not be specified in the license. Instead, the license grants the institution the authority and responsibility for setting the specific requirements for radioactive material use within its facilities. These licensees are termed "broadscope" and require a Radiation Safety Committee and usually a full-time Radiation Safety Officer.
