RU2046558C1 - X-ray radiation source - Google Patents

Abstract

FIELD: X-ray devices. SUBSTANCE: device has powerful pulse generator 1 of electronic beam 2. Target is designed as carbon base 4 covered with metal or carbide layer 3, which is applied on side of electronic beam generator. Metal layer maybe applied as powder mixed with carbon in equal volume. Metal layer may be protected on side of beam generator with carbon cover 5. In this target metal layer serves as converter of beam power to radiation caused by stopping. Carbon base which is highly resistant to temperature impacts carries mechanical stress. Carbon cover 5 protects metal layer from sputter. Addition of carbon in metal layer protects from fusion of metal. Thickness of carbon cover conforms to condition of beam transmission. Thickness of base conforms to condition of transparency of X-ray radiation. EFFECT: increased service life. 2 cl, 2 dwg

Description

The invention relates to x-ray technology and can be applied in radiation technologies, preferably in those where high pulsed dose rates of bremsstrahlung with quanta energies of up to 10 MeV are required, which exceed 10 ⁶ times the average source dose, which will be widely used in food, chemical and medical industry.

A known source of x-ray radiation containing a high-energy electron accelerator, a device for releasing electrons into the atmosphere and a target cooled by an air stream [1]
The disadvantages of this source are the small resource of the target and the complexity of the design due to the use of the exhaust device.

A known X-ray source containing a powerful pulsed frequency generator of an electron beam and a metal target with a large core charge Z _{m of} thickness l _m not exceeding the mean free path of the beam electrons in the target material [2] The target is located in the vacuum chamber of the accelerator and is cooled by thermal radiation .

The disadvantages of such a source are the small value of the allowable density of the average beam power released on the target (less than 20 W / cm ² ) and the small resource of the target, due to the high peak power of the beam exceeding the average by 10 ⁶ times or more. With such a high pulse power, the tantalum target “floats”, the tungsten target crumbles.

 The technical result of the invention is to increase the allowable density allocated to the target average beam power, and the resource of its work.

The technical result is achieved in that in an x-ray source containing a powerful pulsed frequency generator of an electron beam and a metal target with a large core charge Z _{m of} thickness l _m not exceeding the mean free path of the electron beam in the target material, the target is made in the form of a carbon substrate coated with the side of the electron beam generator with a working layer of the same metal, the substrate thickness l _p selected from the condition l _p <l _m ˙ ρ _m / ρ _p , where ρ _m , ρ _{p the} density of the target metal and the substrate (carbon), respectively about. In addition, the working layer is made of carbide of the same metal or a mixture of the same metal with carbon and / or a carbon coating is applied to the working layer of the metal from the side of the electron beam generator, the thickness of which is l _coat selected from the condition, l _coat <l _m .

 In this design of the target, the working metal layer, as in the prototype, performs the function of a converter of electron energy into x-ray radiation, and the graphite substrate, which has a higher resistance to pulsed thermal effects, carries a mechanical load. Mixing metal with graphite prevents the "fluidity" of the metal as a whole. The presence of a carbon coating prevents metal spraying. The high transparency of carbon for electrons and x-ray radiation allows you to get almost the same yield of x-ray radiation, as in the case of a purely metal target. Thus, the use of the carbon component in the target can significantly increase the allowable density of the energy released in it and its resource both due to the high resistance of the carbon component to pulsed effects, and due to its higher degree of thermal radiation.

 Figure 1 and 2 presents the source of x-ray radiation, options.

The source contains a pulsed frequency generator 1, accelerating the electron beam 2 and the target, consisting of a working layer 3 of the metal, a carbon substrate 4 and a carbon coating 5 (figure 2). The electron beam source 2 and the target are located in a vacuum chamber. The target is the anode of the electron beam generator. The metal layer 3, which acts as a converter of the energy of the beam into x-rays, is located on the substrate 4 from the side of the generator 1, its thickness is selected from the condition of maximum x-ray output. Typically, tungsten or tantalum is used, which have a large Z _m and a high melting point, while the layer thickness l _m is 0.5 1 electron free path, which is determined by the electron energy. The thickness of the carbon substrate l _p made of the most durable grades of graphite or carbon fabrics is selected from the condition of providing mechanical strength with minimal absorption of x-ray radiation:
l _p <l _m ˙ ρ _m / ρ _p , in the case of tantalum (ρ = 16.6 g / cm ³ ) or tungsten (ρ = 19.3 g / cm ³ ) l _p <17 l _m .

Currently, there are a number of technologies for applying metals to the surface of graphite, for example, impregnation of graphite, as is done in the manufacture of electric motor brushes or using a plasma torch, etc. The metal can be deposited on a substrate in the form of a powder mixed with carbon, taken in approximately equal volumes. In this case, carbon prevents fusion of the metal and changes in the shape of the working layer of the metal, as occurs in the case of a clean tantalum plate. The graphite component of the mixture has practically no effect on the x-ray yield. Work metal layer can be protected from an electron beam such as a carbon-coated carbon cloth, the thickness of which does not exceed the thickness of the metal _dormancy l <l _met. The carbon coating suppresses the erosion of the metal and at this thickness is almost transparent to the beam electrons.

 The source works as follows.

 When you turn on the pulse generator 1, the electron beam 2, the bombarding target, is absorbed by the working layer 3 of the metal, which is the source of hard bremsstrahlung. At high electron energies (greater than 1 MeV), the bremsstrahlung radiation is directed mainly towards the propagation of the electron beam and passes almost unhindered through the carbon substrate 4. The vacuum chamber has an exhaust window located opposite the substrate 4. Depending on the scheme of the electron beam generator, the target may have zero or high positive potential.

Let us consider a specific example of the operation of an x-ray source, in which a pulsed frequency generator of an electron beam is used based on a plasma current chopper with parameters: pulse frequency 2 Hz, beam electron energy 3 MeV, beam current 20 kA, duration 100 ns, pulse power 6 ˙ 10 ¹⁰ W, average 12 ˙ 10 ³ W. The mean free path of electrons for tungsten or tantalum is 2.3 g / cm ² . The optimum tantalum layer thickness from the point of view of the x-ray yield is 0.8 mm (1.3 g / cm ² ).

As the substrate 4, three layers of graphite fabric with a total thickness of 1.5 mm were used, which provides the necessary mechanical strength and with a large margin satisfies the condition for transmitting x-ray radiation. As the working layer of the metal, tantalum carbide powder 1.5 g / cm ² thick or a "washcloth" made of tungsten wire with a diameter of 0.04 mm and a mass thickness of 1.3 g / cm ^{2 was} used (defective incandescent lamp was used). As the carbon coating was used carbon fabric with a thickness of 0.5 mm (0.3 g / cm ² ). The dose rate of x-ray radiation at a distance of 0.5 m from the target was the same value of 0.25 kGy / h both in the case of the prototype (tantalum or tungsten plate 0.8 mm), and in the case of the above options for the target according to the scheme of FIG. .2. The target resource of pure metal at an average power density of 20 W / cm ² is about 1 hour: the tantalum plate “fuses”, the tungsten crumbles. The proposed target worked for 200 hours at an average power density of 50 W / cm ² without visible traces of destruction.

 Thus, the proposed target scheme makes it possible to double the average density of the power released on it and at least 200 times increase its resource.

Claims (2)

1. SOURCE OF X-RAY RADIATION, containing a powerful pulsed frequency generator of an electron beam and a target made of a material with a large atomic number z _m and a high melting point l _{m thick} , not exceeding the mean free path of the electron beam in the target material, characterized in that the target is made in the form of a working a layer placed on a carbon substrate, the thickness of which is selected from the condition
l _p <l _m · ρ _m / ρ _p ,
where ρ _m , ρ _{p the} density of the material of the working layer of the target and the carbon substrate, respectively,
in this case, metal, or metal carbide, or a powder mixture of metal with carbon in equal volumes is used as the material of the working layer of the target. 2. The source according to claim 1, characterized in that the working layer of the target by the electron beam incidence deposited carbon coating, the thickness l _{of n for} which the condition is selected from l _{to about n} <l _m.

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