Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J35/00—X-ray tubes
  • H01J35/02—Details
  • H01J35/16—Vessels; Containers; Shields associated therewith
  • H01J35/18—Windows

Definitions

• the invention relates to x-ray tubes, and in particular to x-ray generating vacuum tubes. It will be described with regard to these generating tubes although it can also be applied to x-ray detector vacuum tubes (type radiographic image intensifier IIR). Vacuum tubes serving as an X-ray source are used, for example, in the non-destructive testing of material objects (metal structures, composites, luggage, etc.). They are also used in medical imaging.
• An X-ray generating vacuum tube essentially comprises a cathode emitting a high energy electron beam, and a metal target placed in the path of the electron beam.
• the target thus bombarded by the electrons emits X-rays in a privileged angular sector depending on the angle of incidence of the electrons on the surface of the target.
• the surface of the target at the point of impact of the beam is located in a plane inclined at about 70 ° relative to the direction of the incident beam, and it then returns X-rays into an angular cone of a few tens of degrees centered roughly on a beam normal.
• X-rays must be able to exit the tube.
• the walls of the tube are metallic, which is most often the case, an outlet window is preferably provided, made of a material which is the least absorbent for X-rays, facing the angular sector of emission of the X-rays.
• the rest of the wall is metallic and moreover protects the environment against the emission of X-rays in undesirable directions, that is to say outside the preferred angular sector of emission.
• the outlet window participates, like the rest of the wall of the tube, in maintaining the vacuum tightness which exists inside the tube.
• the difficulty lies in the realization of this window, since it is necessary to use a material having both good transparency to X-rays and on the other hand mechanical, chemical and thermal properties which make it suitable for the behavior of the vacuum tightness and simple industrial production of the tube.
• One of the most commonly used materials is beryllium. It has good X-ray transparency for thicknesses up to a millimeter and more; it has sufficient mechanical strength characteristics to produce windows of approximately 20 millimeters in diameter (with a thickness of one millimeter), which is sufficient for a certain number of applications.
• the invention provides an X-ray vacuum tube comprising a wall provided with an X-ray passage window, characterized in that this window is made of pyrolytic graphite and is shaped bell, that is to say curved surface.
• pyrolytic graphite is understood to mean carbon with a crystalline structure of graphite (notably different from the crystal structure of diamonds), deposited by progressive growth on an intermediate substrate from chemical decomposition of hydrocarbons at very high temperature, then separated from this. intermediate substrate by a demolding operation.
• the intermediate substrate is only there to determine the final shape of the graphite piece.
• This type of material can be shaped as desired (preferably, however, in a circular symmetry). It has good mechanical resistance to forces directed perpendicular to its surface. It has a lower mechanical resistance to tensile forces directed parallel to the surface, but the window is preferably given a shape such that the forces parallel to the surface are minimized. In particular, rather than giving it a flat disc shape like we did for beryllium, we will give it a bell (or crucible) shape.
• the bell is preferably brazed on a copper collar, the collar then being brazed on the copper wall of the tube.
• the graphite / copper brazing preferably uses an active brazing composed of silver, copper, and titanium.
• the thickness of the graphite window is preferably between 0.5 and 1.5 millimeters so that the absorption of X-rays by the window is sufficiently limited, the absorption coefficient of graphite being greater than that of beryllium.
• FIG. 1 shows an X-ray tube of the prior art, with a beryllium outlet window
• FIG. 2 shows a partial view of a tube according to the invention, with a window in pyrolytic graphite
• FIG. 3 shows an enlarged view of the window itself, mounted on a copper support before welding on the tube.
• a generally cylindrical X-ray vacuum tube is shown schematically, comprising a wall 10, in principle made of copper, and, inside the wall, essentially an electron gun 20 and a metal target 30.
• the tube is shown with its wall partially open to reveal these elements.
• the electron gun emits an electron beam 22 in the axis of the tube.
• the beam is focused on its axis on the one hand thanks to the shape of the cathode of the barrel and the electrodes which surround it (in particular a wehnelt serving to focus the beam), and on the other hand possibly by other distributed electrodes along the length of the tube.
• the high energy electron beam is directed towards the metal target 30.
• This target is preferably made of tungsten. Its surface is plane. In this example, the target is fixed, but we could consider that it is rotating to rotate the point of impact and therefore limit the heating of the target.
• the target can be constituted by a tungsten plate embedded in a copper block 32 promoting the dissipation of the heat generated by the impact of electrons on the target. Cooling by circulation of water is preferably provided in channels 34 formed in the copper block.
• the surface of the target forms an angle of about 70 ° with the axis of the electron beam.
• the impact of the beam generates the emission of X photons from the target. Most of the photons are emitted in a cone with an apex angle of approximately 45 °, starting from the point of impact of the electrons.
• the axis of this cone is substantially perpendicular to the axis of the electron beam, and it is located in a plane containing both the axis of the beam and the normal to the plane of the target.
• An X-ray passage window 40 is provided in the wall of the tube, facing this X-ray emission cone.
• the window is generally circular; its dimension can be approximately 20 millimeters in diameter, and it is then placed approximately 30 millimeters from the point of impact of the beam if 0 it is desired to allow the X-rays emitted through the entire cone to pass through approximately 45 °.
• the window 40 is a flat beryllium disc, mounted on one or two nickel collars (not shown) so that the beryllium is not assembled directly on the copper which constitutes the internal walls of the tube and which supports the window.
• the window is brazed around the edge of an opening formed in the wall of the tube and it provides vacuum tightness at the point of exit from the X-rays.
• the beryllium window 40 is replaced by a window 50 made of pyrolytic graphite.
• This window 50 has a bell shape and is preferably mounted on a cylindrical copper collar 60 which is sealed or brazed on the periphery of the opening of the wall of the tube.
• bell shape is meant a surface whose edges are not in the plane of the central part: the surface has a central part which is substantially perpendicular to the central axis of emission of the X-rays, while the edges of the surface tend to approach at least in part the direction of this axis.
• Pyrolytic graphite is a crystal structure of carbon, with a hexagonal mesh (unlike diamond carbon which is cubic), obtained by decomposition of hydrocarbons (in practice a mixture of methane and hydrogen) in an oven at very high temperature , and obtained by progressive growth atomic layer by atomic layer on a mandrel serving as an intermediate substrate for the deposition.
• the deposition temperature is preferably about 2270 ° K.
• the support mandrel must withstand this temperature. It can be made of graphite (not pyrolytic), obtained by heat treatment of a carbon block. It has the bell shape of the window to be produced. When the desired part thickness is obtained (approximately 0.5 to 1.5 millimeters), the growth is stopped and the part is demolded. The simple difference in expansion coefficients of the mandrel (non-pyrolytic graphite) and of the part produced (pyrolytic graphite) ensures easy release of the part. A layer of soot can also be deposited on the mandrel before deposition of the pyrolytic graphite, to facilitate demolding.
• the demoulded, bell-shaped graphite window may have a diameter of about 20 millimeters and a height of 10 to 15 millimeters.
• the bell shape gives the piece elasticity in all directions.
• the structure of pyrolytic graphite is such that it does not withstand tensile forces parallel to the plane of deposition of the layers but that it does resist bending forces perpendicular to this plane.
• a pyrolytic graphite disc purely and simply replacing the beryllium disc of the prior art would be less resistant than the bell window.
• the window maintains the vacuum tightness of the tube.
• Figure 3 shows in detail the window 50, mounted on a copper collar 60 to be brazed to this collar.
• the inside diameter of the skirt 52 of the bell window is equal to the outside diameter of the copper collar.
• the skirt is threaded over the collar and is brazed at the level of the collar surface in contact with the skirt.
• the solder, represented by a peripheral rod 54 is preferably an active solder composed of silver, copper, and titanium (ABA cusil solder rod).
• the collar preferably has a rim 62 on which the base of the skirt 52 rests.
• the collar thus linked to the graphite window is itself brazed on the wall of the X-ray tube. This is a copper on copper brazing which is not a problem and which holds the seal well. empty.
• Graphite is about twice as absorbent for x-rays as beryllium, but the resistance of graphite (especially with the bell shape of the window) allows the choice of a thickness of graphite about twice as thin as l thickness of beryllium that would be required for a window of the same diameter.
• the invention is also applicable to tubes which can be used for detecting X-rays, for letting X-rays coming from the outside and which are desired, for example, pass through the window, towards the inside of the detector tube. measure the intensity or intensity distribution.

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• 2001
  • 2001-05-04 FR FR0106023A patent/FR2824422B1/fr not_active Expired - Fee Related
• 2002
  • 2002-04-26 JP JP2002588586A patent/JP2004531860A/ja active Pending
  • 2002-04-26 WO PCT/FR2002/001470 patent/WO2002091420A1/fr not_active Application Discontinuation
  • 2002-04-26 US US10/476,576 patent/US7035378B2/en not_active Expired - Fee Related
  • 2002-04-26 EP EP02735511A patent/EP1399943A1/de not_active Withdrawn

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