X-ray filters

X-ray filters - metal plates used to produce almost homogeneous x-ray radiation. The braking x-radiation (see) contains photons of all energies of the maximum determined applied to the x-ray tube voltage to zero. When passing through the x-ray filters radiation is attenuated unevenly: the number of low-energy photons (long-wavelength part of the spectrum) is reduced to a greater extent than the number of photons high energy (short wavelength spectrum). Irregularity of attenuation depends on the material and thickness of the x-ray filter. Filtered radiation contains relatively more photons, high energy becomes more rigid.
The material and thickness of the x-ray filters are chosen in such a way that the rigidity of the x-ray emission in further filter changed insignificantly. Such radiation is called almost homogeneous energy. It is widely used in radiotherapy and avoids radiation burns of the skin. When deep x-ray therapy uses x-ray filters made of copper, tin, lead thickness of 0.5-2 mm as these x-ray filters emit more soft, characteristic x-ray radiation, after such a filter (in the direction of the beam) put aluminum filter in 1-3 mm For surface therapy are applied x-ray filters aluminium 1-4 mm X-ray filters are not used in the treatment rays Bucky tray. In diagnostics using x-ray filters from aluminum 0.5-1 mm

X-ray filters are plates of homogeneous material, intended for stronger absorption soft part radiation and for obtaining monochromatic radiation.
Absorption capacity is directly proportional to the specific weight of the material x-ray filters, which are put in the way of working of the radiation beam, usually around the output window protective casing of the x-ray tube (see). As a rule, provides for the possibility to install different R. F.
In x-rays used x-ray filters made of aluminium. They absorb far infrared radiation, which, highly relaxing the body, does not reach the screen to x-ray or film and increases the radial load on the body. Thickness of the applied R. F. depends on the voltage of the tube (Fig. 1, 1). When correctly selected thickness R. F. radiation exposure is reduced (Fig. 1, 2). In enclosures x-ray tubes filled with oil, equivalent to aluminum R. F. thickness of 1-1.5 mm
In radiotherapy, depending on the voltage applied R. F. copper, aluminum or plastic. In copper x-ray filters occurs soft characteristic radiation that could lead to the x-ray skin burns. Therefore copper R. F. always cover aluminum, absorbing radiation copper. R. F. absorb far infrared radiation, thus increasing its rigidity and relative deep dose.
In Fig. 2 shows a weakening in the body radiation, the rigidity is characterized by a layer of half weakening 0.5 mm (Fig. 2, 1) and 2 mm copper (Fig. 2,2).
Radiation therapy with radioactive cobalt, giving almost monochromatic radiation, R. F. only lead to decrease of intensity of radiation without changing the nature of its distribution in the body. However, there is a wide application is found with the wedge-shaped R. F., that "distortions" of dose field, created in irradiated environment (Fig. 3). When multi-field exposure wedge-shaped x-ray filters expand opportunities for the creation of dose fields of the required configuration. The degree of bias field is determined by the angle of descent of the wedge. These R. F. made of heavy metals, including lead. Wedge R. F. currently used in radiotherapy.
When sources, giving radiation with large variations across the field (see Accelerators of charged particles), apply countervailing R. F. uneven thickness: thicker in the center and thinner at the edges. These filters are counting so that after passing through them the flux has acquired the necessary uniformity in the field.
Compensation of x-ray filters is used for a separate research in x-rays, for example to align blackening the image of the lung fields and the middle shade. Part of the radiation flux, forming the image of the lung fields, forced to go through the thickened areas of special R. F., more thin on the area of radiation flux, forming the image of the middle shade.

Fig. 1. The thickness of the aluminium filter depending on the voltage.
Fig. 2. The relative depth dose depending on the depth of the body.
Fig. 3. Dose field for wedge filter.