X-ray image intensifier tube

Abstract: In an X-ray image intensifier tube an entrance section is optimized in respect of image quality, optical aberrations and efficiency. To achieve this, notably in order to avoid photocathode charging phenomena, a separating layer having an adapted electrical transverse conduction is provided. In order to reduce scattered radiation, an edge portion of the entrance screen is deactivated for relevant examinations. In order to increase efficiency, use is made of a double phosphor layer having different X-ray absorption properties. In order to compensate for vignetting a radial variation of the thickness or of the radiation properties of a separating layer or of the luminescent layer itself is used.

Description: The invention relates to an x-ray image intensifier tube comprising an exit window with an exit screen, an entrance section provided with an entrance window and an entrance screen which comprises a luminescent layer, a photocathode and a chemically separating layer, the chemically separating layer being positioned between the photocathode and the luminescent layer and disposed on a carrier layer, the chemically separating layer establishing electrical contact between the photocathode and the carrier layer, An x-ray image intensifier tube of said kind is known from the US-patent US 3 838 273. The entrance screen of the known x-ray image intensifier tube is provided with a barrier layer which chemically insulates the photocathode from the luminescent layer in order to prevent mutual contamination. Moreover the barrier layer, which is notably an indium-oxide layer has an electrical resistivity that is substantially lower than that of the photocathode, so that a lateral variation of the electrical potential of the photocathode is reduced in order to mitigate electron-optical image distortions. Both chemical insulation and providing a low electrical resisitivity are provided by the material of the barrier layer. Therefore, a drawback of the known x-ray image intensifier is that a reduction of the variation of the potential is achieved at the cost of a lesser chemical separation. An object of the invention is to provide an x-ray image intensifier tube having an entrance screen in which variation of the electrical potential of the photocathode is reduced and at the same time adequate chemical separation of the photocathode and the luminescent layer is achieved. This object is achieved by an x-ray image intensifier tube according to the invention which is characterized in that the carrier layer is substantially electrically conductive and the chemically separating layer is provided with a pattern of holes or thinned portions enabling electron tunnelling. The holes or thinned portions in the chemically separating layer ensure adequate electrical conductivity between the photocathode and the carrier so that variation of the electrical potential across the entrance screen is reduced. Chemical interaction between the photocathode and the luminescent layer is still adequately reduced to prevent mutual contamination because the surface are of the holes or thinned portions amounts to a small fraction of the surface area of the entrance screen. Electrical contact between the photocathode and the carrier layer through the thinned portions is provided via electron tunnelling. The carrier layer can be formed by a chemically insulating and electrically conductive layer, such as an aluminium oxide (Al 2 O 3 ) which is separately deposited. The carrier layer may also be formed as a top layer of the luminescent layer which top layer has a dense packing so that adequate electrical conductivity is ensured thereacross. The luminescent layer notably consists of a comparatively thick structured layer of caesium-iodide (CsI) as described in US 3 825 763 on which there is provided a comparatively thin top layer of CsI as described in US 4 820 926. The adaptation of deposition techniques will enable the carrier layer and the photocathode to extend further into recesses in the luminescent layer than the chemically insulating layer. The holes or thinned portions cover no more than about 1% of the surface are of the entrance screen are preferably distributed reasonably homogeneously over the surface. A surface of the luminescent layer which is remote from the carrier layer is mechanically smoothed in a preferred embodiment. This can be achieved, for example, by rubbing, grinding or pressing; notably in the case of rolling pressing of the luminescent layer on such a smooth surface, it suffices to use a comparatively thin chemically separating layer as the carrier for the photocathode and the photocathode itself can be deposited uniformly and with an increased electrical conductivity. The image equality can thus be enhanced as a result of lower x-ray absorption of scattering in the chemically separating layer and of improved homogeneity, also as regards layer thickness, of the photocathode. For example, in the case of vapour-deposition of the luminescent layer, the surface topology may also differ locally, for example it may vary with the radial position on the screen. These differences can also be mitigated by mechanical smoothing, thus improving local homogeneity in the photo-electron beam. In a preferred embodiment, the homogeneity or a desired variation in the photo-electron beam is adapted by imparting a radially varying thickness to the separating layer. A photo-electron beam which precompensates for vignetting can thus be realized, for example with a current density which increases towards the image periphery. Such a layer can be formed with a high degree of precision in a screen in accordance with the invention notably because of the smooth carrier surface. For the selection of materials for such an intermediate layer, reference is made to US-A-4,831,249, but known ITO aluminium oxide layers can also be used. In a further preferred embodiment the local intensity adaptation is realized by utilizing a luminescent layer with a degree of doping which varies radially. As is known, vapour-deposited CsI (Na) is preferably used for an entrance screen of an X-ray image intensifier tube. A radial variation, for example a concentration which increases with the radius, can be comparatively easily imparted to the Na concentration, for example by using adapted vapour deposition techniques. A photo-electron beam exhibiting a current density which increases towards the image periphery can thus again be realized. A substantial advantage is now obtained in that loss of image resolution at the image periphery, due to the known locally thicker construction of the luminescent layer, for example as described in EP-A- 282.089, is now avoided. In a further preferred embodiment, the X-ray image intensifier tube has an effective entrance surface area which is smaller, due to shielding, than the surface area for which the electron-optical system of the tube is conceived. As a result, improved electron-optical imaging of the photo-electron beam on an exit screen can be realized without any loss of efficiency of the entrance screen. Notably a circular entrance screen is reduced from approximately 25 cm to approximately from 15 to 20 cm. The latter dimension is preferably adapted to a desired exit surface area for specific diagnostic examinations. By abstaining from depositing phosphor outside an effective entrance screen surface thus obtained, it can be ensured, better than in the case of external shielding, that no disturbing scattered radiation is generated at that area. In a further preferred embodiment, a round entrance screen of, for example 25 cm is reduced to a rectangle of, for example 15 x 20 cm. The entrance image format can thus be simply adapted to a customary format for, for example a subsequent television chain. In another preferred embodiment, the entrance luminescent layer comprises two sub-layers, a first sub-layer which is remote from the photocathode exhibiting a comparatively low absorption for medical X-rays (radiation up to, for example 60 keV), but a comparatively high absorption for secondary radiation to be generated in a second layer which is situated near the photocathode. Thus, on the one hand the efficiency is increased because the K radiation from the second layer, preferably consisting of CsI (K edge approximately 35 keV), is at least partly converted in the first layer into luminescent light to be effectively used. On the other hand, the resolution is improved because a comparatively large part of the primary X-rays is converted into luminescent light in the second layer, thus reducing light dispersion. It is to be noted that a luminescent screen comprising two different phosphors is known per se from US-A-4,712,011, but therein the phosphors are mixed or provided in different volume parts, transversely of the layer thickness, so that a loss of resolution is liable to occur. The second layer notably consists of CsI and the first layer consists of a phosphor having a comparatively high absorption for the 35 keV K radiation of the CsI, such as Ca W O 4 , bismuth germanate or combinations thereof. Some preferred embodiments in accordance with the invention will be described in detail hereinafter with reference to the drawing. Therein: Fig. 1 shows an X-ray image intensifier tube in accordance with the invention as well as a sectional view of its entrance screen. Fig. 2 shows such a tube having a reduced entrance screen as well as a sectional view of its entrance screen. An X-ray image intensifier tube 1 as shown in Fig. 1 comprises an electron-optical system 2 which in this case comprises a shielding electrode 4, a focusing electrode 6 and an anode 8. In the tube there are also provided an entrance screen 10 and an exit screen 12. In the present case the entrance screen 10 comprises a carrier 14, a luminescent layer 16, a separating layer 18 and a photocathode 20. Via the electron-optical system 2, an image-carrying photo-electron beam 22 emerging from the photocathode 20 is imaged on the exit screen 12. In the exit screen 12 there is formed a luminescent image which can be studied, photographed, converted into a video signal, etc. via an exit window 24. The tube envelope contains, in addition to the exit window 24, a preferably metal entrance window 26, metal wall portions 28 and an insulating ring 32. The entrance screen 10 is accommodated as a separate component in the tube in the present embodiment, but may alternatively be provided directly on the entrance window 26 instead of on the carrier 14. Separation of vacuum window and substrate for the entrance window offers the advantage that the substrate can be optimized in respect of the electron-optical requirements etc., without it being necessary to take into account the vacuum-atmospheric pressure transition. In a practical case the carrier 14 is formed by an aluminium foil, the luminescent layer 16 is a layer of CsI.Na having a thickness of from approximately 300 to 500 µm, and the photocathode is a S9 or S20 photocathode having a layer thickness of approximately 0.01 µm. The separating layer 18 serves notably to prevent mutual contamination of the luminescent layer and the photocathode and to constitute a suitably defined supporting surface for the comparatively thin photocathode layer. An entrance section 30 of an X-ray image intensifier tube, assumed to comprise the entrance window 26, the entrance screen 10 with the substrate 14, the luminescent layer 16, the intermediate layer 18, the photocathode 20 and possible additions to the entrance window or the entrance screen in the present embodiment, is required to convert a incident image carrying X-ray beam 21 into a photoelectron beam 22 with a high yield and a high resolution, said photoelectron beam 22 having an optimum geometry and structure for the imaging on the exit screen 12 by the electron-optical system. Negative effects exerted thereon by the separating layer 18 are avoided in the screen shown by depositing this usually electrically insulating layer, for example consisting of Al 2 O 3 , in such a manner, in order to prevent charging phenomena on the photocathode, that adequate electrical conductivity between the photocathode and a carrier for the separating layer, substantially homogeneously across the layer, remains ensured. The carrier for the separating layer can be formed by a top layer of the luminescent layer 16 as well as by a electrically conductive additional layer provided thereon, for example by making the surface of the luminescent layer smoother, or by improved optical matching between the luminescent layer 16 and the photocathode 20. Adapted sputtering techniques can be applied, for example to ensure that the separating layer covers cavities in the substrate less deeply or that comparatively uniformly distributed openings or thin locations occur across the surface of the separating layer. The occurrence of charging phenomena can thus be avoided, without giving rise to a disturbing reduction of chemical separation. An X-ray image intensifier tube as shown in Fig. 2 comprises a diaphragm 40 which is exchangeable or not and which ensures that an edge portion of the entrance screen is not exposed to radiation so as to obtain a image which is disturbed less by scattered radiation. This is attractive notably for, for example vascular examinations where an optimum, disturbance-free image of a comparatively small object is desired. The diaphragm 40 forms an active entrance screen having a dimension Φ = 15 to 20 cm from an entrance screen having a dimension Φ = 25 cm, or forms a square image (CCD camera) adapted to the video chain or a rectangular image (television pickup tube) dimensioned, for example 20 x 20 cm 2 or, for example 25 x 17 cm 2 (monitor image geometry). If the tube is intended exclusively for said specific examination, from a point of view of scattered radiation reduction, a diaphragm is preferably dispensed with and the desired reduced geometry is imparted to the entrance screen itself, i.e. to the luminescent layer and the photocathode. Scattered radiation due to X-ray scattering to the environment or light scattering to the photocathode at that area is also avoided. A gain in efficiency is also achieved by constructing the luminescent layer 16 as a first layer 16-a which has a high absorption for secondary X-rays to be generated in its second layer 16-b. The first layer then preferably has a comparatively low absorption for the primary X-rays of, for example 60 KeV. The secondary radiation of CsI is approximately 35 KeV.

Claims: An x-ray image intensifier tube (1) comprising an exit window (24) with an exit screen (12) an entrance section (30) provided with an entrance window (26) and an entrance screen (10) which comprises a luminescent layer (16), a photocathode (20) and a chemically separating layer (18), the chemically separating layer (18) being positioned between the photocathode (20) and the luminescent layer (16) and disposed on a carrier layer, the chemically separating layer (18) establishing electrical contact between the photocathode (20) and the carrier layer, characterized in that the carrier layer is substantially electrically conductive and the chemically separating layer (18) is provided with a pattern of holes or thinned portions enabling electron tunnelling. An x-ray image intensifier tube as claimed in Claim 1, characterized in that said holes or thinned portions are substantially uniformly distributed across the surface of the chemically separating layer (18). An x-ray image intensifier tube as claimed in Claim 1 or 2, characterized in that a combined surface area of said holes and/or thinned portions amounts to at most approximately 1% of the overall surface of the chemically separating layer (18). An x-ray image intensifier tube as claimed in Claim 1,2 or 3, characterized in that the carrier layer is formed by a comparatively thin CsI-layer of a dense packing. An x-ray image intensifier tube as claimed in any one of the preceding Claims, characterized in that the chemically separating layer (18) consists of sputtered indium-tin oxide (ITO) and aluminium-oxide (Al 2 O 3 ). An x-ray image intensifier tube as claimed in any one of the preceding Claims, and which is provided with an electron-optical system (2) for imaging the entrance screen (10) on the exit screen (12), characterized in that the entrance screen (10) comprises a shielding portion that is substantially insensitive for x-rays so that the x-ray sensitive surface of the entrance section (10) is smaller than the surface area of the entrance section that can be electron-optically imaged on the exit screen (12). An x-ray image intensifier tube as claimed in Claim 6, characterized in that the shielding portions consists of a material absorbing x-rays. An x-ray image intensifier tube as claimed in Claim 6 or 7, characterized in that the surface area of the entrance screen (10) is substantially circular and the x-ray sensitive surface area is substantially rectangular. An x-ray image intensifier tube as claimed in any one of the preceding Claims, characterized in that the luminescent layer (16) comprises two sub-layers, a first sub-layer (16a) which is remote from the photocathode (20) exhibiting a high absorption for secondary radiation from a secondary sub-layer (16b) adjacent to the photocathode (20). An x-ray image intensifier tube as claimed in Claim 9, characterized in that the second sub-layer (16b) exhibits an absorption for primary x-rays which is higher than that of the first sub-layer (16a). An x-ray image intensifier tube as claimed in Claim 9 or 10, characterized in that the second sub-layer (16b) consists of caesium-iodide (CsI) and the first sub-layer (16a) consists of a material having a high absorption for K α -radiation of CsI. An x-ray image intensifier tube as claimed in anyone of Claims 9, 10 or 11, characterized in that the first sub-layer (16a) consists of CsI, the second layer (16b) being chosen form calcium-tungstate (CaWO 4 ), bismuth-germanate or compositions thereof. An x-ray image intensifier tube as claimed in any one of the preceding Claims, characterized in that the thickness of the chemically separating layer (18) varies radially from the centre to the periphery of the entrance screen (10). An x-ray image intensifier as claimed in any one of the preceding Claims, characterized in that the doping concentration of the luminescent layer (16) varies radially from the centre to the periphery of the entrance screen (10).

Inventors: COLDITZ JOHANNES KARL EWALD, DIEBELS HENRICUS FRANSICA CORN, POORTER TIEMEN, SIMONS AUGUST LEONARD HERMAN, VAN DER VELDEN JOHNNY WILHELMU, COLDITZ, JOHANNES KARL EWALD, DIEBELS, HENRICUS FRANSICA CORNELIS, POORTER, TIEMEN, SIMONS, AUGUST LEONARD HERMAN, VAN DER VELDEN, JOHNNY WILHELMUS
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1992
Main classification: H01J29
IPC codes: G21K4, H01J31, H01J29