RU2676672C1 - X-ray acute-focus radiator with rod anode - Google Patents

Abstract

FIELD: physics.SUBSTANCE: invention relates to miniature x-ray emitters. X-ray acute-focus radiator with a rod anode has a cylindrical vacuum cylinder made of heat-resistant glass with a window for outputting radiation. Field radiating cathode is made of tungsten wire filaments with a diameter of 20–25 microns with nanopowder of narrow-band semiconductor deposited on its surface AB(InAs, InSb, GaSb) and installed on the inside of cover 1 in the form of four intersecting filaments located in a plane parallel to the cover, separated from it at a distance of 1–3 mm, and forming a cathode square. Anode is made of a tungsten rod-cone with an angle of 15–25°, 2–3 mm high, the base is attached to the heat sink, and the apex is directed to the center of the window and passes through the center of the cathode square by about 2/3 of its height; the distance between the cathode filaments and the anode-cone is 2–3 mm. Heat sink is made of copper in the form of a truncated cone with selectable dimensions: height H, radii of bases from the side of the attached anode, smaller – r, larger – r.EFFECT: technical result is stable operation during operation, an increase in the specific power dissipation, an increase in the resolution for cases of compact performance of devices.1 cl, 2 dwg, 1 ex

Description

The invention relates to x-ray technology, in particular, to miniature x-ray emitters, and can be used to create compact devices for local local control - in medicine, technology, inspection systems, everyday life.

Analogs and prototype.

In X-ray tubes used to control and measure high spatial resolution, control electrode systems — electron guns — are used to obtain the smallest sizes of the electron spot on the anode (sharp focal spot). Common disadvantages of these options are design complexity and large dimensions, gigantic values of the energy density on the surface of the anode, which sharply reduce the efficiency of heat removal and the reliability of the anode.

Known are “gunless” designs of miniature X-ray emitters, for example: a sharp-focus two-electrode pulsed X-ray tube with a pointed rod anode (patent RU 2479883); pulsed x-ray tubes with a sharp focus anode and a ring cathode (patents RU 2521433, 2521436, 2524351); X-ray tube with a ring cathode and a sharp focus anode (patents RU 2479883); an x-ray tube with an anode in the form of a pointed rod and an annular cathode assembly (patent RU 2328790); an x-ray tube with a microchannel plate as an electron source and locally modulated radiation (patent RU 2507627); miniature x-ray emitter with a microchannel element and a controlled cathode (patent RU 2563879).

The main solution for these options is the use of a rod pointed anode and around it an annular cathode, creating a circular flow of electrons, with an opening for the passage of x-ray radiation. Their common drawbacks are ineffective heat removal from the anode tip and the large distance from the anode to the cathode (and, accordingly, the window), which is necessary to ensure maximum sharpness (minimum area) of electron anode seeding.

Of the listed analog options, the closest in structure to the claimed (prototype) is a pulsed X-ray tube [1] containing a tungsten flat cathode with explosive emission, having an axisymmetric hole (window) relative to the anode made of a tantalum rod with a pointed apex in the form of a cone with angle not more than 60 °, located below the plane of the cathode at a distance of not more than 2 mm. The rod anode is mounted on a holder attached through a rod attached to the base of the housing.

The disadvantages of the prototype are:

Amplitude and spatial instability of x-ray radiation due to the large instability of the electron current of explosive emission from the metal cathode and the formation of one emission center, the position of which migrates from shot to shot.

- The use of an explosive emission cathode leads to the need to protect the elements inside the tube from the actions of the inevitably atomized cathode metal, which greatly complicates the design and increases the dimensions of the tube. In addition, the reliability and durability of the operation are reduced, and the power modes of the pulsed version are limited.

The rod pointed anode and the used version of its location relative to the cathode do not allow to obtain a focal spot with a diameter of less than 0.6 mm.

- The location of the anode relative to the cathode with the minimum possible angle to the axis of the tube, necessary to reduce the size of the electron spot on the conical surface of the anode, leads to a significant deviation of the axis of the cone of x-ray radiation from the axis of the tube and, consequently, to a decrease in the radiation power at the object under study.

- A large distance from the tip of the anode to the object is at least 10 mm, which reduces the efficiency of using x-ray radiation.

- A low-efficiency heat sink in the form of a rod with a large ratio of length to diameter and thermal resistance, determined by the cylindrical design of the tube.

Description of the design and operation of the emitter.

These drawbacks of analogues and prototype are to some extent eliminated in the claimed embodiment due to the use of a field emission cathode (autocathode) in the form of a piece of wire of super small diameter, an anode in the form of a needle rod of heat-resistant metal, connected firmly to a conical radiator with low thermal resistance, which should be through the foil connected loosely to the radiator of the application device.

The claimed emitter is as follows - FIG. 1, a and c. FIG. 1, c is a bottom view in section AA.

1 - output cathode,

2 - cathode holder,

3 - cathode,

4 - anode,

5 - window-foil,

6 - backup

7 - cathode cover,

8 - cylinder body,

9 - heat sink,

10 - spring holder

11 - glass cement,

12 - anode cover,

13 - output anode,

14 - plate

15 - FEM-getter.

The claimed emitter operates as follows.

When a high-voltage voltage is applied between the autocathode 3 and the anode 4, to the electrodes 1 and 13, there is auto-emission from segments of four wire cathodes 3, the ends of which are mounted on spring holders 2 attached by glass-cement to the cathode cover 7. Emission electrons bombard the side surface of the anode 4, creating X-ray radiation through a window-foil 5 attached to a support 6 on the cathode cover 7 of the cylinder body 8. The rod anode 4 is firmly attached to the cone-shaped heat sink 9, held by three spring holders 10, 11 connected to the anode fiberglass cover 12. Tapered heat sink 9 is attached to the plate 14 freely, connected vacuum-tight to the anode cap 12, through which the emitter must be installed through the heat conductive adhesive layer to the radiator viscous application of the device. Maintaining a vacuum inside the emitter is provided by a microchannel element (FEM) 15.

Determination of properties of structural elements.

X-ray emitters consist of a vacuum cylinder having a window for outputting radiation, a thermionic or field-emission cathode, an electrode structure for controlling the flow of electrons from the cathode (gun); anode for braking electrons and creating x-rays; element of maintaining a vacuum in the cylinder (getter); heat sink; protective cover.

A specific design of the miniature X-ray emitter is the absence of a gun in them. In the case when the cathode is point-like and located as close to the anode as possible, the role of the gun is practically reduced to zero. The role of the protective casing is performed by the housing of the application device.

Cathode. Anode.

The determining parameters when using an autocathode are the shape, dimensions, location of the cathode and anode. They are specified taking into account the emission ability, depending on the material [2]. The field values for field emission in cases of an emitter-metal should be of the order of 10 ⁷ V / mm. When narrow-band A ₃ B ₅ semiconductors (InSb, InAs, GaSb) nanoparticles (InSb, InAs, GaSb) are used as the emitter material, the field values necessary for emission can be reduced by an order of magnitude [3].

The cathode consists of four tungsten wires located in a cross manner (Fig. 1, c) so that they frame the anode with a square on four sides. The diameter of the wire is chosen to be minimal, but taking into account its manufacturability and the necessary strength under operating conditions in the device. Based on the experience of the applicant of this application for an invention, a tungsten wire with a diameter of 20-25 microns was selected.

The anode is a rod-cone. To reduce the size of the focal spot, it is necessary that the dimensions of the anode are minimal, and to increase the voltage and power - maximum. A compromise is established by determining the acceptable dimensions and material of the anode.

An important parameter for determining the optimum is the angle at the apex of the anode cone, which is determined taking into account the highest yield of x-ray radiation for a given size of the focal spot.

In the case of bremsstrahlung x-rays, its direction is a vector (Umov vector) perpendicular to the vectors of change in speed (braking vector) and changes in magnetic field strength, which, in turn, are perpendicular to each other. The direction of the Umov vector determines the indicatrix of radiation and depends on the field parameters between the cathode and the anode, the depth and direction of electron penetration into the anode material. The axis of the x-ray indicatrix lies in a plane perpendicular to the line of electron motion and is randomly oppositely directed within 360 ° [4].

The radiation directivity angle is further blurred when electrons flying out of the cathode can have randomly directed velocity vectors. This is facilitated by the "hot" cathode and the medium in the path of the electrons. In this regard, the field emission cathode is preferable in comparison with thermal, plasma, and explosive emission.

Electron deceleration occurs in the surface layers of the anode metal. Radiation occurs at a certain depth and leaves at certain angles to the surface of the anode. Taking into account the distribution (blurring) of the values of these angles, the angle at the apex of the anode cone is determined. The shorter the electron penetration depth into the material, the sharper the cone angle. Based on this, the anode material must be selected as heavy as possible. Moreover, it should have the highest possible melting point and stable characteristics with strong heating. All of these properties are most consistent with tungsten and tantalum, of which tungsten is preferred as the most widely used metal in practice. The penetration depth of electrons into tungsten does not exceed one micron (http://www.ngpedia.ru/id644445p2.html), which in our case means that there is no influence of the near-surface absorption thickness on the radiation and cone angles. The angle at the apex of the cone is then set taking into account the projection of its lateral surface onto the plane of the window, which determines the size of the focal spot, and a reasonable choice of the size of the anode for reasons of obtaining maximum power dissipation. In addition, to ensure the value of the form factor during field emission at least at the level of 100, it is necessary to fulfill the relation h = z: d _k = 100: 1, where h is the height of the anode cone, z is the distance from the cathode to the anode, d _k is the diameter cathode wire. With the chosen option, d _k = (20-25) μm h = z = (2-3) mm, the angle of sharpening of the rod-anode is (15-25) °.

Thus, according to the conditions of the proposed option, a tungsten wire with a diameter of 20–25 μm coated with nanoparticles of a narrow-gap semiconductor A ₃ B ₅ (InSb, InAs, GaAs) is selected as the cathode, and a tungsten pointed rod with a length of (2-3) mm and an angle of sharpening is selected as the anode (15-25) °. The distance between the cathode filaments and the anode rod is ~ (2-3) mm.

Window-foil. Backup

Usually in X-ray tubes, the lightest metal, beryllium, is used as the window material. However, due to the need to maintain atmospheric pressure and to avoid damage to the window due to mechanical deformation during heating, the window should have a relatively large thickness, which is undesirable. Thermophysical calculations performed in [5] show that aluminum or carbon (in the form of glassy carbon or polycrystalline artificial diamond) can be used instead of beryllium as the material of the exit window due to their high thermal conductivity. For reasons of technological simplicity, we choose aluminum. In order to minimize radiation losses, the thickness of the aluminum foil is chosen to be minimal, but taking into account the long-term preservation of vacuum in the device. Calculation according to the data of [Handbook of a chemist 21. www.ngpedia.ru/pic/ 042DQoH2C7K1k5a5K0x80012113777.gif] for the diffusion of nitrogen (the main component of air) through an aluminum membrane 50 μm thick leads to a value of the time of the beginning of vacuum deterioration (10 ⁴ -10 ⁵ ) hours, which is acceptable for real-world applications. A decrease in membrane thickness by 2 times leads to a decrease in leakage time by 4 times.

To determine the optimal window size, we used the program for calculating stresses in the membrane with edges fixed along the contour (http://al-vo.ru/mekhanika/raschet-progiba-plastiny.html). The initial data for the calculation:

The foil material is Al, thickness h = 50 μm;

Yield strength (strength) σ = 100 N / mm ² ;

The modulus of elasticity E = 66000 N / mm ² ;

Poisson's ratio μ = 0.34;

Distributed uniform load corresponding to normal atmospheric pressure q = 0.1 N / mm ² .

The calculation results show that when the hole size is 2 mm, the maximum tensile stresses in the aluminum foil are 38 N / mm ² , i.e. there is a 2-3-fold safety factor (relative to yield strength). With a window diameter of more than 2 mm, it is necessary either to increase the thickness of aluminum, or to create a support in the form of a mesh of more durable material.

When the window opening diameter is more than 2 mm, it is advisable to attach aluminum foil to the support in the form of a mesh made in a thicker foil plate with a relatively strong material joined by thermal expansion coefficient (KTR) to the body material. To calculate the thickness of the support supporting the window-foil and representing a grid in the area of the window-foil, we will use the same program taking into account the decrease in the strength of the plate due to the presence of holes. According to the recommendations (Norms for calculating the strength of equipment and pipelines ... PNAE G - 7-002-86, Moscow, Energoatomizdat, 1989) for flat bottoms and covers with several holes, the minimum value of the coefficient of strength reduction is calculated by the formula:

Where Σd _i - is the sum of the lengths of the chords of the holes in the most weakened diametrical section; D _r . - diameter of the window. For this case, ϕ _i = 1 / (1 + 0.8 + 0.64) = 0.4. Other data for calculation:

Plate (foil) material - carpet, thickness 0.5 mm;

Yield strength (strength) σ = 500 N / mm ² ;

Elastic modulus E = 140,000 N / mm ² ;

Poisson's ratio μ = 0.3;

Distributed uniform load corresponding to normal atmospheric pressure q = 0.1 N / mm ² .

The calculation results show that with a window diameter of 20 mm, the maximum stresses are 60 N / mm ² , which, taking into account the attenuation coefficient of 0.4, gives a three-fold safety margin.

Thus, aluminum with a thickness of 25-30 μm, a diameter of 3-5 mm, which is vacuum tightly attached with a support of a thickness of 0.4-05 mm and a diameter of up to 20 mm, is chosen as a foil window. The support has a mesh structure in the region of the window-foil with a transparency of ~ 0.8, and outside this region it is vacuum-tightly attached to the cover of the emitter housing.

Heat sink Plate.

The choice of the shape, size and material of the elements allows us to calculate their thermal parameters [6]. Anode is a tungsten rod-cone connected by a base to a heat sink. The best option for heat removal is a truncated cone made of well heat-conducting material, the best of which is copper.

The calculation is based on the determination of the thermal resistance R _T (m ² K / W) of the truncated cone (Fig. 2):

- элемент высоты конуса, находящийся на расстоянии

от верхнего основания конуса и имеющий площадь поперечного сечения

, причем тепловой поток равномерно распределен по площадке

.

где

- радиус текущего сечения конуса. λ - коэффициент теплопроводности (Вт/мК). Из рисунка видно, что

.Where

- a cone height element located at a distance

from the upper base of the cone and having a cross-sectional area

moreover, the heat flux is evenly distributed over the site

.

Where

- radius of the current section of the cone. λ is the coefficient of thermal conductivity (W / mK). The figure shows that

.

The calculations lead to the formula for R _T : R _T = L (πλr ₁ r ₂ ) ^-1 .

The temperature difference on the cone is calculated by the formula: ΔТ = Р / R _T.

Given thermal radiation, the heat balance equation:

where - σ = 5.7 (10 ^-8 ) W / m ² K is the Stefan-Boltzmann constant.

The second term in formula (1) in most cases gives an insignificant addition and has a small share of influence.

For the case of copper heat sink (λ ~ 400 W / m ² K), the temperature drop ΔT [K] on it with dissipated heat power P [W] - ΔT ~ Pr ₁ r ₂ / N (all dimensions in millimeters, H = L - height cone).

Of fundamental importance for X-ray emitters is the presence of highly heated elements in the vacuum volume. This leads to heating of the case and the appearance of large temperature gradients, which can lead to mechanical damage to the device and its elements. In this embodiment, a relatively massive heat sink made of copper having a coefficient of thermal expansion (CTE), significantly different from the values for glass, must be mechanically disconnected from the case. That is, the heat sink must be attached to the plate freely. Another element with thermal resistance will appear in the system, which, as shown by the calculation, is more than 10 times less than the thermal resistance of the heat sink.

Thus, the dimensions of the heat sink are calculated depending on the values of power consumption. The plate is selected in the form of a relatively thick foil of material joined along the KTP with the glass of the case.

Getter. FEM.

To maintain the vacuum in electronic tubes, a getter getter is used (gettering. Http://www.ngpedia.ru/ id640968p2. Html).

There are various options for introducing getters into the design of x-ray tubes, a common drawback of which is the need to maintain the working temperature of the getter, which complicates the design of the tube and worsens its characteristics. In our version, it is proposed to introduce a special getter element with a developed surface into the tube structure, onto which the film of the sprayed getter is sprayed. This structural element can be made in the form of a microchannel element (FEM) with channels closed on one side. A source of getter material in the form of a tablet or ring is located on the side of the open channel openings. When the source is activated by induction heating, getter material pairs are deposited on the inner surface of the channels of the microchannel plate, thus creating a getter mirror of a considerable area. The sorption capacity of such a getter element is determined by the total area of the MCP channels, which is many times greater than the area of the getter mirror in conventional vacuum tubes.

An example of execution and application.

The dimensions of the heat sink: N = 30 mm, r ₁ = 3 mm, r ₂ = 10 mm. Overheating ΔТ ~ Pr ₁ r ₂ / Н ~ (50-100) К for the case of dissipated heat power Р ~ 50 W.

The size of the focal spot (0.2-0.3) mm.

Dimensions of the emitter: cylinder height (35-40) mm and diameter (45-50) mm.

The emitter must be installed on the massive metal bottom of the device. The body of the device may be in the form of a flattened parallelepid, located on the table.

In this design, the device can be used for local control of the internal structure of an object of a flattened shape with a resolution of no worse than 0.2 mm. The advantage of the device in this case is the possibility of operational, manually controlled local control of the structure of the object. The device can be used in medical institutions, emergency rooms, in production, in domestic conditions, during the inspection of small objects.

Information Sources Used

1. RF patent 2524351. Pulse x-ray tube. Priority - November 1, 2012. Patent holder: JSC "Plasma". Authors: Merkulov B.P., Makhanko D.S., Teterin D.E.

2. Egorov N.V., Sheshin E.P. Electronic emission. M .: Intellect. 2011.

3. N.D. Zhukov, D.S. Mosiyash, A.A. Khazanov, N.P. Abanshin. Optimization of the structure and material of the cathode. Applied Physics, 2015, No. 3, p. from. 93-97.

4. Blokhin M.A. X-ray Physics, https://www.twirpx.com/file/1067543

5. Podymsky A. A. Powerful x-ray tubes for projection radiography. Abstract. thesis for a job. student senior candidate those. n St. Petersburg, 2016.

6. Heat transfer: Textbook for high schools / V.P. Isachenko, V.A. Osipova, A.S. Sukomel. M .: Energoizdat, 1981. - 416 p.

Claims (1)

X-ray sharp focus emitter with a rod anode, having a vacuum cylindrical cylinder made of heat-resistant glass with caps 1 and 2 at the ends; a rod anode pointed with an angle of not more than 60 °, surrounded by a cathode and connected to a heat sink connected to the radiator of the application device; a window in the lid 1 for outputting x-ray radiation; a getter in the form of a microchannel element (FEM), characterized in that the cathode is made of a tungsten filament wire with a diameter of 20-25 μm with a nanopowder of narrow-band A ₃ B ₅ semiconductor deposited on its surface (InAs, InSb, GaSb) and mounted on the inner side of the cap 1 in the form of four intersecting filaments located in a plane parallel to the lid, spaced from it at a distance of 1-3 mm, and forming a cathode square; the anode is made of a tungsten rod-cone with an angle of 15-25 °, a height of 2-3 mm, the base is attached to the heat sink, and with the top directed to the center of the window and passes through the center of the cathode square at about 2/3 of its height; the distance between the cathode filaments and the cone anode is 2-3 mm; the heat sink is made of copper in the form of a truncated cone with the selected dimensions: height H, radii of the bases from the side of the attached anode, smaller - r ₁ , larger - r ₂ ; with a large base, the heat sink joins freely (with the possibility of sliding) through the opening of the lid 2 to the plate attached vacuum-tightly to the lid 2 from its outer side; the sizes are selected taking into account the dependence of the temperature difference ΔT [K] on the heat sink on the dissipated heat power P [W]: ΔT ~ Pr ₁ r ₂ / N (all dimensions in millimeters).

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