US 3436550 A
Description (OCR text may contain errors)
April 1969 J. FINKLE ELECTRONIC PICK-UP TUBE FOR INCIDENT X-RAYS WITH IMAGE INTENSIFIER Filed Sept. 5, 1963 Attorney .LNHEWdS V31. N43 -X United States Patent US. Cl. 250213 8 Claims ABSTRACT OF THE DISCLOSURE Image intensifier for electronic pick-up tube in which X-rays impinge upon a fluorescent layer whose luminous output stimulates the emission of electrons from a photocathode to generate an electron beam having the pattern of the incident radiation, the intensifier including a photoelectro-luminescent phosphor layer and a photoconductive layer with an intervening barrier layer between a pair of conductive layers connected across an alternating-voltage source; the barrier layer is opaque to visible light but transparent to the incident X-rays so that the latter excite the adjoining phosphor layer, this excitation being reinforced by the concurrently applied alternating field which varies locally in conformity with the conductivity pattern developed in the photoconductive layer on the other side of the barrier.
My present invention relates to an electronic pick-up tube designed to produce an intensified visible picture of an incident X-ray image, either by direct conversion or upon interim preservation on a storage medium.
It is known to convert a pattern of incident X-rays into a correspondingly patterned electron beam by letting the X-rays impinge upon a fluorescent layer whose photons excite an electron-emissive layer; it is also known to convert an X-ray image into an intensified visible image by applying a voltage across a multilayer structure including a photoconductive layer and an electroluminescent layer separated by a high-resistance barrier, the photoconductive substance being stimulated by the incident radiation.
The general object of my invention is to provide, in a system for visualizing X-ray images by electronic means, an improved electronic image intensifier allowing the use of reduced doses of radiation while giving a clear picture for diagnostic purposes.
I have found, in conformity with my present invention, that this object can be realized by utilizing, as part of an image-intensifier structure of the general character referred to, a barrier layer which is substantially opaque to luminous radiation from a preceding fluorescent layer but is transparent to the same X-rays which excite the fluorescent layer to generate a conductivity pattern in a photoconductive layer on one side of the barrier, the combined effect of this conductivity pattern and of the penetrating incident radiation acting upon a photoelectroluminescent layer on the other side of the barrier to generate a bundle of photons directed onto an adjoining electron-emissive layer inside a tube envelope provided with output means (such as a viewing screen or a scanning system) for making the resulting beam pattern visible with or without intermediate storage.
According to a more specific feature of my invention, the photoconductive, barrier and photoelectroluminescent layers of an intensifier structure as described above are subjected to an alternating electric field from a voltage source connected across a pair of radiation-permeable conductive layers. The magnitude of this alternating field varies, in a direction perpendicular to the tube axis, according to the pattern of conductivity generated in the photoconductive layer by the incident radiation. Since the areas of locally increased field strength coincide with the regions of concentrated X-rays impinging directly upon the photoelectroluminescent layer, the rate of photon emission from the latter is greatly enhanced. Thus, the incident X-rays need only be strong enough to activate the irradiated phosphor of the photoelectroluminescent layer, the energy required for the amplification of the photon output being obtained from the applied electrostatic field. The adjacent barrier layer, acting as a mirror for light emitted by the photoelectroluminescent phosphor layer, insures that all of this light is directed toward an electron-emissive layer disposed beyond the phosphor layer.
The phenomenon of photoelectroluminescence manifests itself in the following manner:
When incident radiation falls upon a phosphor layer exhibiting this property, an emission of visible light occurs in the absence of an electric field. The application of a voltage across the phosphor layer reduces the emission of light, yet subsequent removal of this quenching voltage produces a sudden flash followed by a return to the original steady-state intensity. Thus, the application of an alternating voltage across the barrier layer results in repeated charges and discharges of the phosphor layer with emission of intense luminescent radiation, appearing to the eye as a continuous illumination, from the X-ray-' stimulated areas thereof.
The electron-emissive layer, together with the preceding fluorescent layer and the intervening intensifier structure, constitutes the photocathode of an otherwise conventional electron tube which may be equipped with an electronexcitable luminescent screen to reveal a visible picture of the radiation image. Alternatively, and in accordance with a preferred embodiment, the tube is provided with scanning means of the type used in television camera tubes, such as those of the Vidicon, image-orthicon or image dissector (Farnsworth) type. The output of such a tube may then be temporarily preserved in a storage tube for subsequent reproduction.
The invention will be described in greater detail hereinafter with reference to the accompanying drawing the sole figure of which is a somewhat diagrammatic illustration of a representative embodiment.
As shown in the drawing, a Farnsworth-type tube 2 having the usual evacuated envelope is provided with a composite photocathode positioned to receive an incident X-ray pattern 1. This photocathode comprises a multiplicity of layers transparent to X-rays, namely a lightreflecting supporting layer 3, a fluorescent layer 4 carried by reflector 3, a base plate 5, a conductive layer 6, a photoconductive layer 7, and a high-resistance barrier layer 8 opaque to visible light. Layer 8 carries a photoelectroluminescent phosphor layer 9 adjoining another conductive layer 10 which is transparent to luminescent radiation emitted by layer 9. Electrodes 6 and 10 may comprise an easily volatilizing metal, such as aluminum, silver or gold, deposited in vacuo with a thickness of about 5 to 10 mils. An electron-emissive layer 11, forwardly of the stack of layers 5-10, generates an electron beam 16 in response to light emitted by phosphor layer 9; layer 11 may consist of material also responsive to incident X-rays, e.g. bismuth, as disclosed in US. Patent No. 2,897,388, in which case the rate of electron emission is further increased.
The remaining, conventional elements of tube 2 include a focusing coil 12 surrounding its envelope, a set of accelerating anodes 13a, 13b, 13c, 13d of progressively increasing axial widths and progressively higher positive potential obtained from a common potentiometer 13, an electron-multiplier structure 17 with entrance orifice 17' for the beam 16, scanning electrodes (partly shown at 14, 15) for deflecting the beam across this orifice, and a target electrode 18 aligned with the orifice, the latter electrode forming part of an output circuit not further illustrated.
A generator 21 of alternating current is connected across the two layer electrodes 6, 10 by way of a transformer 31 in series with a switch 32, the secondary of this transformer being connected in series with a condenser 29 which bridges a biasing battery 23 and an associated circuit breaker 24. The polarity of battery 23 is so chosen that electrode 10, and therefore also the adjoining phosphor layer 9, is more highly positive than electrode 6 and adjoining photoconductive layer 7 upon closure of circuit breaker 24.
A source of variable biasing voltage 33 is shown connected to the barrier layer 8 which may thus be utilized in the manner of a control grid to vary the intensity of the image.
The magnitude of the alternating field developed across electrodes 6 and 10 may range between 600 and 1000 volts; if layer 9 has a thickness of about 100 microns, the average field strength within the layer can be between about 10 and 10 volts per centimeter. A preferred frequency is 800 cycles/sec.
The potential of the electron-emissive layer 11 may be suitably selected by conventional biasing means not illustrated.
The phosphor layer 9 may consist of organic or inorganic material, e.g. cesium iodide imbedded in a plastic coating; suitable organic materials include such polycyclic compounds as stilbene, anthracene, naphthalene and phenanthrene combined with calcium-titanium silicate, calcium-titanium stannate, co-crystallization products of lanthanum silicate, cerium oxide, calcium orthophosphate/stannate, and calcium orthophosphate/cerium oxide. Favorable emission spectra are obtained with calcium tungstate admixed with lead tungstate and a halogen. Other suitable' compounds are zinc silicate, zinc selenide, zinc sulphide, barium-lead sulphate, zinc-cadmium sulphide, magnesium tungstate and zinc-beryllium silicate, combined with a halogen and/or some other activator. The inclusion of a small amount of lead in the phosphor layer increases its sensitivity to X-rays. An admixture of one mole percent of copper with calcium tungstate will cause a 56% increase in the intensity of photon emission upon excitation by X-rays. With silicates there may be added a titanium activator in a molar ratio of 1:10. Advantageously, the radiation emitted by layer 9 should be near the blue region of the spectrum (4000 to 6000 angstroms) and should correspond to the band of greatest sensitivity of emissive layer 11. Layer 9 should be homogeneous, continuous and nongranular and should exhibit uniform electrical properties throughout its area; reference may be made to US. Patent No. 2,685,530 for the preparation of such layers.
If desired, the supporting layer 3 may be omitted, with fluorescent layer 4 carried directly on the envelope of tube 2. Layer 11 may be supported on a further transparent layer, not shown, which should be conductive for a removal of the charge built up by the emission of electrons during operation of the device. These and other modifications, readily apparent to persons skilled in the art, are intended to be embraced within the spirit and scope of my invention as defined in the appended claims.
Having described my invention, what I desire to claim by Letters Patent is:
1. In an electronic pick-up tube for incident X-rays comprising an evacuated envelope, an electron-emissive layer responsive to incident light in said envelope and output means in said envelope for converting a beam of electrons emitted by said layer into a visible picture, the combination therewith of a fluorescent layer in the path of incident X-rays within said envelope, and a multilayer image-intensifier structure between said electron-emissive layer and said fluorescent layer within said envelope, said structure including a photoconductive layer disposed to receive luminous radiation from said fluorescent layer, a photoelectroluminescent layer interposed between said electron-emissive layer and said photoconductive layer in close proximity to the latter, and a high-resistance barrier layer between said photoconductive and photoelectroluminescent layers, said barrier layer being substantially opaque to said luminous radiation but transparent to X- rays penetrating said fluorescent layer for enabling excitation of said photoelectroluminescent layer jointly by said incident X-rays and by an electric field emanating from said photoconductive layer, thereby directing upon said electron-emissive layer a bundle of photons in the pattern of said incident X-rays to generate a corresponding electron beam.
2. The combination defined in claim 1 wherein said structure includes a pair of radiation-permeable conductive layers bracketing said photoconductive and photoelectroluminescent layers, further comprising a source of alternating voltage connected across said conductive layers.
3. The combination defined in claim 2, further comprising a biasing circuit in parallel with said source for impressing upon said photoelectroluminescent layer a D-C potential more positive than that of said photoconductive layer.
4. The combination defined in claim 2 wherein said source has an output voltage suflicient to apply to said electron-emissive layer an alternating field of a magnitude of substantially 10 to 10 volts per cm. thickness.
5. The combination defined in claim 2 wherein said source has a frequency on the order of 800 cycles per second.
6. The combination defined in claim 1 wherein said barrierlayer is provided with means for applying thereto a biasing potential controlling the degree of excitation of said photoelectroluminescent layer by said photoconductive layer.
7. The combination defined in claim 1 wherein said electron-emissive layer consists of a material excitable by said incident X-rays and positioned to receive said X-rays concurrently with said bundle of photons.
8. The combination defined in claim 1 wherein said barrier layer is a reflector for luminous radiation gen erated in said photoelectroluminescent layer.
References Cited UNITED STATES PATENTS 3,064,133 11/1962 Murr et al 250-213 2,593,925 4/1952 Sheldon 250-213 2,929,935 3/1960 Lempert 250-213 3,073,989 1/1963 Amsterdam 250-213 3,210,551 10/1965 Vaughn et a1. 250-213 RALPH G. NILSON, Primary Examiner.
MARTIN ABRAMSON, Assistant Examiner.