EP0253561A1

EP0253561A1 - Image intensifier tubes

Info

Publication number: EP0253561A1
Application number: EP87306027A
Authority: EP
Inventors: Richard Stark Enck
Original assignee: Picker International Inc
Current assignee: Philips Medical Systems Cleveland Inc
Priority date: 1986-07-17
Filing date: 1987-07-08
Publication date: 1988-01-20

Abstract

An image intensifier tube (10) having an evacuated tube envelope (12) and an output window (16) mounted in association with one end of the envelope (12) by a sealing arrangement (30, 36, 38, 40, 42) comprising ductile material (40) compressed between two opposing sealing surfaces (36, 42) one (42) forming a portion of the window (16) and the other (36) forming a portion of the envelope (12), the sealing arrangement effecting a substantially vacuum tight seal while permitting relaxation of stresses by allowing relative movement between the window (16) and envelope (12). The ductile material (40) is suitably indium although other ductile material such as gallium, lead, platinum, tin, gold and silver can be used as well.

To form the seal the sealing surfaces (36, 42) are first metallized with a suitable wetting material, such as copper, a predetermined thickness of the ductile material (40) is then applied to a first one (36) of the metallized sealing surfaces (36, 42), the second metallized sealing surface (42) is then positioned in opposing coplanar relationship with the first (36) such that the second metallized surface (42) contacts the ductile material (40), the ductile material (40) is then compressed between the sealing surfaces (36, 42) thereby reducing the predetermined thickness of the ductile material (40), and finally the sealing surfaces (36, 42) along with the compressed ductile material (40) are heated to cause the ductile material (40) to wet the metallized sealing surfaces (36, 42).

Description

This invention relates to image intensifier tubes, more especially such tubes of a kind suitable for medical diagnostic use having a rectangular image format. The invention also relates to methods of making vacuum seals suitable for use in the manufacture of such tubes.
In US Patent No. 4,255,666 a two-stage, proximity type image intensifier is described. This device incorporates two stages of amplification in an effort to provide improved gain over that of a single stage device described in US Patent No. 4,140,900. Even further improvements have been described in co-pending European patent application No. 87301241.3.
Generally, these devices comprise an evacuated, electrically conductive tube envelope having a circular cross-section; a stainless steel input window in the tube envelope; a circular glass output window, an output display screen on the output window and a high voltage scintillator/photocathode assembly mounted between the input window and the output display screen. The difference between the three above described devices resides primarily in the configuration of the scintillator/photocathode assemblies.
Such devices produce high quality images that compete directly with conventional glass bottle, inverter type x-ray image intensifiers while also providing a flat panel format. However, a disadvantage of the conventional, inverter type intensifier is shared with these flat panel devices. This disadvantage is the cylindrical geometry used in the construction of such intensifiers.
Conventional, inverter-type intensifiers have always been utilized a cylindrical construction format for three basic reasons: 1) the electron optics utilized in such devices require cylindrical electrodes to create cylindrically shaped electron lenses to perform the critical electron focussing task; 2) the high mechanical stresses created by the pressure differential compressing the body wall is best accommodated by using cylindrical or spherical body surfaces that are inherently strong under such vacuum loads; and 3) sealing interfaces that are required for mounting windows and the like, are more practical to manufacture in circular shapes since the stresses on such seals are radially uniform thereby reducing the incidence of seal fractures thus destroying the vacuum integrity. As a result, a cylindrical vacuum tube bodies with circular image formats have dominated virtually all image intensifier designs.
The two US patents and one co-pending European patent application cited above, disclose the fact that the panel type image intensifier tubes described therein can more readily be constructed in a rectangular format since they do not utilize electron focussing. Thus restriction 1) referenced above does not apply. Also disclosed is their use of a stainless steel input window and a metal tube envelope. Such structure also makes rectangular shapes possible since it provides a solution to restriction 2) referenced above. However, until the present invention, no practical solution to the problem of making a large, rectangular viewing seal on panel type image intensifier tubes has been suggested and hence restriction 3) referenced above, has forced the use of cylindrical geometry in all panel type image intensifier tubes since their inception. This is true even for certain existing rectangular image format intensifiers as described in US Patent No. 4,426,721 wherein the rectangular image screens are housed in a glass cylinder instead of a rectangular formatted housing.
The circular image format of the panel type image intensifiers described above is presented via a circular viewing output window that is mounted on one end of the cylindrical metal tube body or envelope. The circular structure results from the need to mount the viewing window onto a metal adapter flanger that is in turn welded to the metal body. The glass output window is secured to the metal adapter flange by a process known in the art as "fritting". Fritting results in a very strong and rigid seal, but demands a near perfect expansion match between the window and flange materials. Additionally, any residual stress caused by the slightest mismatch must be distributed in a uniform manner to prohibit subsequent fracturing of the seal. Accordingly, a circular format for the window and adapter flange is always used in preference to a rectangular shape since the rectangular shapes create high residual stress patterns at their corners.
A circular image is normally presented when a circular output window is chosen since a rectangular image presented on a circular output wastes available space creating what is known as a poor "filling factor". However, the preferred system image format is rectangular. Compromises are therefore made that waste substantial amounts of image storage and processing space in typical electronic imaging systems.
Accordingly, a rectangular image and housing configuration with a good filling factor is clearly a system preference.
It is an object of this invention to overcome the above referenced problems and others by providing an improved image intensifier tube which may have a rectangular image format and which has improved means to seal the output window to the tube envelope.
According to one aspect of the invention there is provided an image intensifier tube comprising: an evacuated tube envelope defining an output end; and an output window mounted in association with the output end of said tube envelope; and characterised by sealing means comprising malleable and ductile material and affixed in a region between said output end and said output window to effect a floating substantially vacuum tight seal between the output window and the tube envelope.
According to a second aspect of the invention there is provided an image intensifier tube comprising: an evacuated, electrically conductive envelope; an input window; an output window; an output display screen on the output window; a photocathode carrying assembly mounted between the input window and the output display screen; and a structure for mounting the output window to the tube envelope and for providing a vacuum tight seal between the envelope and the output window; characterised in that said structure comprises a flange portion proximate the output end of the tube envelope extending outwardly from the exterior of the tube envelope, said flange portion defining a planar surface parallel to the output window; a lip comprising a portion of the tube envelope extending beyond and substantially perpendicular to the planar surface formed by said flange portion; and a malleable seal means affixed to the planar surface of said flange portion and compressed between the output window and the flange portion.
According to a third aspect of the invention there is provided an image intensifier tube having a vacuum seal between a tube envelope having a rectangular cross-section and a rectangular output window, characterised in that said seal comprises; a pair of opposing, coplanar seal surfaces one of said surfaces forming a portion of the tube envelope and the other forming a portion of the output window; and ductile seal means mounted for compression between said seal surfaces.
According to a fourth aspect of the invention there is provided a seal arrangement for flexibly mounting an output window of an image intensifier tube to one end of the tube envelope while maintaining a vacuum tight seal between the output window and the tube envelope characterised in that the seal arrangement comprises; seal support means mounted on the external surface of the tube envelope proximate the end of the tube envelope defining a support surface facing the output window; and malleable seal material applied to the support surface, said seal material being compressed under vacuum between the output window and the seal support means.
According to a fifth aspect of the invention there is provided a method of making a vacuum seal comprising the steps of metallizing a pair of seal surfaces with a wetting material; applying a predetermined thickness of ductile seal material to one of the metallized seal surfaces; positioning the other metallized seal surface in opposing coplanar relationship with said one metallized surface such that said other metallized surface contacts the seal material; compressing the seal material between the seal surfaces thereby reducing the predetermined thickness of the seal material; and heating the seal surfaces and compressed seal material to cause the seal material to wet the metallized seal surfaces.
According to a sixth aspect of the invention there is provided a method of making a vacuum seal between the end of an evacuated envelope and window closing off said end comprising the steps of: metallizing a pair of seal surfaces each associated with one of the envelope and window; applying a predetermined thickness of seal material to the seal surface associated with the envelope; positioning the window over the end of the envelope such that the seal surface associated with the window contacts the seal material; drawing a vacuum on the envelope and window assembly thereby compressing the seal material between the seal surfaces to a thickness less than the predetermined thickness; and heating the envelope and window assembly to cause the seal material to wet the metallized seal surfaces.
The use of indium or other soft metal as a vacuum tube seal material is known, as is described in US Patents 2,730,637 and 3,334,256. However, these patents utilize the soft metal to aid or improve the overall integrity of an already existing vacuum seal. Also, neither patent describes a "floating" seal as is described herein which allows for relaxation and release of residual stresses. Further, neither patent shows or suggests the use of soft metal for use as a seal material in a rectangular format tube assembly where redidual stress at the corner is most severe.
One image intensifier tube and a method of making a vacuum seal of the tube in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings in which:-

Figure 1 is a diagrammatic illustration of the tube;
Figure 2 is a sectional view of the tube as seen from the plane indicated by line A-A in Figure 1; and
Figure 3 is a detail view of a portion of Figure 2 showing the seal arrangement of the tube.

Referring to Figures 1 and 2, a rectangular shaped, proximity type radiation image intensifier tube 10 according to the present invention is illustrated. These Figures reveal the same fundamental components as in conventional panel type image intensifier tubes with the two exceptions being the conversion from circular to rectangular components and the use of a special vacuum seal means. It should be noted at the outset, that although the invention is described in terms of rectangular geometry, the present invention has utility in any image intensifier geometry, e.g. circular, square or some arbitrary shape as required for specialized applications.
The image intensifier tube 10 comprises a metallic, typically type 304 stainless steel, vacuum tube envelope 12. In the preferred embodiment, the tube envelope has a generally rectangular cross-section.
The envelope is closed at one end by a metallic, inwardly concave input window 14. The window 14 is made of a specially chosen metal foil or alloy metal foil in the family of iron, chromium, and nickel, and in some embodiments, additional combinations of iron or nickel together with cobalt or vanadium. By making the window thin, down to 0.1mm in thickness, high x-ray transmission is achieved while maintaining desired tensile strength. In particular, a foil made of 17-7 PH type of precipitation hardened chromium-nickel stainless steel is utilized in the preferred embodiment. This alloy is vaccum tight, high in tensile strngth and has very attractive x-ray properties, e.g. high transmission to primary x-rays, low self-scattering, and reasonably absorbing with respect to patient scattered x-rays. The window 14 is concaved into the tube like a drum head.
One purpose of having a metallic window 14 is that it can be quite large in dimension with respect to inverter type tubes with a convex, glass window. The shape of the window corresponds to the cross-sectional shape of the tube envelope 12.
The envelope 12 is closed at the other end by a glass output window 16 upon which an output phosphor display screen 18 is deposited. The shape of the output window corresponds to the cross-sectional shape of the tube envelope 12. The manner in which the output window 16 is mounted to the envelope 12 is described in reference to Figure 3 below.
In between the windows 14 and 16, a scintillator/phosphor/photocathode assembly, shown generally at 20, is suspended. The operation of the various scintillator/phosphor/photocathode assemblies useful in panel type image intensifier tubes is described in detail in the above mentioned US patents and co-pending European patent application and therefore will not be described in further detail herein other than to note that a high voltage is applied between the scintillator/phosphor/photocathode assembly 20 and the output phosphor display screen 18 on the output window 16.
Because the tube envelope 12 is metallic, it is necessary that the scintillator/phosphor/photocathode assembly 20 be suspended from the envelope by insulators 22. Suitable suspension means is fully described in US Patent No. 4,315,183.
In operation, a radiation source 24 generates a beam of penetrative radiation 26 which passes through a patient's body 28 and casts a shadow or image onto the face of the tube 10. The radiation image passes through the input window 14 and impinges upon the scintillator/phosphor/photocathode assembly 20. The scintillator/phosphor/photocathode assembly 20 converts the radiation emanating from the patient's body to an intensified electron stream which impinges the output display screen 18. The display screen 18 in turns converts the electron pattern to a corresponding light pattern which is viewable at the output window 16.
Referring to Figure 3, the manner in which the output window is mounted to the tube envelope in accordance with the present invention is shown in detail. The metal tube envelope 12 is a generally box-shaped structure defining external and internal wall structures 32, 34, respectively. The output end of metal tube envelope 12 is fashioned to form a flange portion 30 and lip portion 38. The flange 30 extends outwardly from the external wall 32 and surrounds the periphery of envelope 12 defining a flat, continuous, seal support surface 36 about the perimeter of the envelope 12. The width of the flange portion 30 is typically 6.35mms may vary provided a reasonable width to accommodate error is furnished.
The flange 30 is comprised of the same material as the tube envelope 12 and is typically secured to the external wall 32 by brazing. The flange 30 is positioned relative the end of the envelope 12 such that the seal support surface 36 is spaced from the envelope end by typically 0.25mm. Placement of the flange proximate to, but spaced from, the output end of the envelope defines a lip portion 38 which forms a reference for output window 16. The function of lip portion 38 is defined in more detail below.
While the above description describes the preferred embodiment, it is to be realized that flange 30, seal support surface 36 and lip portion 38 can be formed in many other ways. For example, one could form the envelope 12 to include flange 30 and machine surface 36 leaving lip 38.
The glass output windown 16 is sealed to the output end of the metal envelope 12 using sealing means 40. The sealing means is preferably comprised of indium. The output window 16 comes into contact with the sealing means 40 along an output window seal surface, shown generally at 42. The shape of the seal surface 42 corresponds to the shape of seal support surface 36. The respective surfaces oppose and are substantially parallel to one another.
The use of indium as a seal material has proved advantageous due to its ductility and malleability. The indium seal as shown will cold flow when compressed such that the glass to metal interface is essentially a floating seal. Therefore, stresses that are common with hard or rigid seals are effectively eliminated since the indium is sufficiently ductile to move so as to allow any forces that would cause undo stress in the seal area to equalize. The indium is preferably configured as a preformed unitary washer whose shape generally corresponds to seal support surface 36 and seal surface 42. The washer thickness, before envelope evacuation, is thicker than the height of lip portion 38.
Besides its malleability and ductility, indium exhibits a very low vapour pressure, i.e. remains a solid under low pressure. This is important should material find its way into the evacuated envelope. If the material would vaporize under the vacuum typically encountered in proximity type tubes (10⁻⁷Torr) contamination of the internal electrodes, such as the scintillator/phosphor/photocathode assembly, could result.
It is to be noted that other sufficiently ductile, low vapour pressure materials could be used in place in indium. Examples of such materials are the following: gallium, tin, platinum, gold and silver. The critical factor in choosing a suitable material is that the resulting seal must be sufficiently malleable and ductile to allow force equalization while forming a vacuum-tight seal and having a vapour pressure substantially below the vacuum pressure of the evacuated assembly.
The vacuum seal is fabricated as follows. First, both seal surface 42 on output window 16 and seal support surface 36 on flange 30 are metallized, by vacuum deposition, with a material that indium (or other chosen seal material) will readily wet. The preferred metallization material is copper. The thickness of the deposited copper is not critical and is typically a few microns.
After metallization, the preformed washer of indium is placed on the seal support surface 36 of flange 30 and is registered against lip portion 38. The output window 16 with its output display screen 18 already in place, is then placed in position onto the sealing means 40. The output window seal surface 42 is aligned onto the indium washer. Vacuum is then pumped on the entire tube assembly 10 and the resulting force on the output window 16 compresses the sealing means 40 until the internal surface of the output window 16 comes into contact with lip portion 38. The compression of the indium onto the metallized surfaces creates in part a molecular bond (wetting) between the material. At this point the seal is essentially complete and the vacuum tube can now proceed through the rest of the conventional vacuum processing steps.
In an alternative method of manufacturing, after the sealing means is placed on the seal support surface 36 but before the output window is put in place, the envelope 12 with flange 30 and sealing means 40 are baked at high temperatures (200°-400°C) to out-gas the assembly. This process removes surface gasses which may otherwise impair the integrity of the vacuum seal.
The vacuum baking steps referred to above may also occur after assembly of the output window to the tube envelope.
These vacuum baking steps also aid in completing the wetting process. During the baking process however, one may be concerned that the sealing material might become liquidous and seep into the tube envelope. This possibility is substantially eliminated in the above disclosed embodiment through the provision of lip portion 38. The lip portion is located on the inner side (vacuum side) of the seal. During evacuation, the inner surface of the output window 16 comes to rest on the lip portion 38, forming a tight seam at their interface. The sealing means, even in the liquid state, retains sufficient viscosity not to seep through the seam. This combination thus provides a barrier and prevents seepage into the tube interior.
The lip portion 38 also provides rigid support for the output window 16. During evacuation, the output window 16 compresses the sealing means 40 until contact is made with lip 38 at which point further compression is prevented. The rigid support controls the thickness of the sealing means 40 from becoming too thin by the compression force exerted by the atmosphere on the output window 16.

Claims

1. An image intensifier tube (10) comprising: an evacuated tube envelope (12) defining an output end; and an output window (16) mounted in association with the output end of said tube envelope (12); characterised by sealing means (40) comprising malleable and ductile material and affixed in a region between said output end and said output window (16) to effect a floating substantially vacuum tight seal between the output window (16) and the tube envelope (12).

2. An image intensifier tube (10) according to Claim 1 wherein said malleable and ductile material (40) comprises material selected from the group consisting on indium, gallium, platinum, tin, gold and silver.

3. An image intensifier tube (10) according to Claim 2 wherein said malleable and ductile material (40) comprises indium.

4. An image intensifier tube (10) according to Claim 3 wherein said malleable and ductile material (40) comprises a preformed washer comprising indium.

5. An image intensifier (10) according to any one of the preceding claims wherein the tube envelope (12) is rectangular.

6. An image intensifier tube (10) according to any one of the preceding claims wherein the output window (16) is rectangular,

7. An image intensifier tube (10) according to any one of the preceding claims wherein said output end of the tube envelope (12) comprises: an inner lip portion (38) defining a rigid interface between said output window (16) and said output end of the tube envelope (12); an outer flange portion (30) affixed to the tube envelope (12) proximate the output end and extending outward from the tube envelope (12), said outer flange portion (30) defining a seal support surface (36); and said sealing means (40) is affixed on the seal support surface (36) between the outer flange portion (30) and the output window (16) to effect a substantially vacuum tight seal between the output window (16) and the tube envelope (12).

8. An image intensifier tube (10) according to Claim 7 wherein said inner lip portion (38) defines the minimum seal thickness.

9. An image intensifier tube (10) according to Claim 7 or Claim 8 wherein said inner lip portion (38) defines a barrier for preventing the sealing means (40) from entering the interior of the tube envelope (12).

10. An image intensifier tube (10) according to any one of Claims 7 to 9 wherein said seal means (40) is compressed between the output window (16) and the seal support surface (36).

11. An image intensifier tube (10) according to Claim 10 wherein said lip portion (38) defines a gauge for limiting the compression of the seal means (40).

12. An image intensifier tube (10) comprising: an evacuated, electrically conductive envelope (12); an input window (14); an output window (16); an output display screen (18) on the output window (16); a photocathode carrying assembly (20) mounted between the input window (14) and the output display screen (18); and a structure for mounting the output window (16) to the tube envelope (12) and for providing a vacuum tight seal between the envelope (12) and the output window; characterised in that said structure comprises; a flange portion (30) proximate the output end of the tube envelope (12) extending outwardly from the exterior of the tube envelope (12), said flange portion (30) defining a planar surface (36) parallel to the output window (16); a lip (38) comprising a portion of the tube envelope (12) extending beyond and substantially perpendicular to the planar surface (36) formed by said flange portion (30); and a malleable seal means (40) affixed to the planar surface (36) of said flange portion (30) and compressed between the output window (16) and the flange portion (30).

13. A seal arrangement for flexibly mounting an output window (16) of an image intensifier tube (10) to one end of the tube envelope (12) while maintaining a vacuum tight seal between the output window (16) and the tube envelope (12) characterised in that the seal arrangement comprises; seal support means (30) mounted on the external surface (32) of the tube envelope (12) proximate the end of the tube envelope (12) defining a support surface (36) facing the output window (16); and malleable seal material (40) applied to the support surface (36); said seal material (40) being compressed under vacuum between the output window (16) and the seal support means (30).

14. A seal arrangement according to Claim 13 further comprising a lip (38) forming a portion of the end of the tube envelope (12), said lip (38) extending beyond the support surface (36) of the seal support means (30) for providing a barrier to seepage of seal material into the interior of the tube envelope (12).

15. A seal arrangement according to Claim 13 further comprising a lip (38) forming a portion of the end of the tube envelope (12), said lip (38) extending beyond the support surface (36) of the seal support means (30) for controlling the minimum compressed thickness of the seal material (40).

16. A radiation image intensifier tube (10) having a vacuum seal between a tube envelope (12) having a rectangular cross-section and a rectangular output window (16), characterised in that said seal comprises; a pair of opposing, coplanar seal surfaces (36, 42) one (36) of said surfaces forming a portion of the tube envelope (12) and the other (42) forming a portion of the output window (16); and ductile seal means (40) mounted for compression between said seal surfaces (36, 42).

17. An image intensifier tube (10) according to Claim 16 additionally comprising wetting means deposited on the seal surfaces (36, 42) for adhering said seal means (40) to said seal surfaces (36, 42).

18. An image intensifier tube (10) according to Claim 17 wherein said seal means (40) comprises material selected from the group consisting of indium, gallium, platinum, tin, gold and silver.

19. A method of making a vacuum seal characterised by the steps of; metallizing a pair of seal surfaces (36, 42) with a wetting material; applying a predetermined thickness of ductile seal material (40) to one (36) of the metallized seal surfaces (36, 42); positioning the other metallized seal surface (42) in opposing coplanar relationship with said one metallized surface (36) such that said other metallized surface (42) contacts the seal material (40); compressing the seal material (40) between the seal surfaces (36, 42) thereby reducing the predetermined thickness of the seal material (40); and heating the seal surfaces (36, 42) and compressed seal material (42) to cause the seal material (40) to wet the metallized seal surfaces (36, 42).

20. A method of making a vacuum seal between an end of an evacuated envelope (12) and a window (16) closing off said end characterised by the steps of; metallizing a pair of seal surfaces (36, 42) each associated with one of the envelope (12) and window (16); applying a predetermined thickness of seal material (40) to the seal surface (36) associated with the envelope (12); positioning the window (16) over the end of the envelope (12) such that the seal surface (42) associated with the window (16) contacts the seal material (40); drawing a vacuum on the envelope and window assembly (12, 16) thereby compressing the seal material (40) between the seal surfaces (36, 42) to a thickness less than the predetermined thickness; and heating the envelope and window assembly (12, 16) to cause the seal material (40) to wet the metallized seal surfaces (36, 42).