Abstract:
A microchannel plate assembly includes a plurality of microchannel plates that are aligned along a common axis and coupled together. The microchannel plates each have an object-side surface and an image-side surface and the assembly has respective interfaces between the image-side surface and the object-side surface of adjacent microchannel plates. At least one ion barrier film is disposed on at least one of the microchannel plates, but only on the object-side surfaces in the interfaces.

Description:
BACKGROUND 
     A microchannel plate (MCP) is a planar component used for detection of particles that cross the boundary of its surface to enter one of thousands if not millions of hollow channels distributed across the MCP. Each channel is an electron multiplier that produces an electrical current generated by the multiplication of electrons via secondary emission. The currents from respective channels of the MCP emerge as localized streams of electrons that, unlike other electron multipliers, retain a spatial distribution of the particle impingement patterns across its surface. For this reason, MCPs are widely used in image intensifiers. 
     In a typical image intensifier configuration, a photocathode, an assembly of one or more MCPs, and a cathodoluminescent element, such as a phosphor screen, are enclosed within a vacuum. An electron is generated from an impinging photon by the photocathode and is multiplied by the MCPs, and the electrons that emerge from the MCPs are converted into photons by the phosphor screen. The photocathode is constructed from a wavelength-selective material, typically in a very thin layer, that is exposed in the chamber of the device. A major drawback of these types of image intensifying devices is that the electrostatic fields that transport the electrons from the photocathode coating to the MCP assembly also transport positive ions generated in the electron multiplication back towards the photocathode. Because these positive ions may have considerable mass, irreparable damage is done when such an ion strikes the photocathode. Efforts to mitigate this ionic transport are ongoing in the MCP field. 
     Depositing a thin ion barrier film (IBF) on the input side of the MCP is a conventional technique by which ions are prohibited from reaching the photocathode. There are several drawbacks to the use of the ion barrier film, one of which is a reduction in the signal-to-noise ratio (SNR) owing to absorption of electrons by the ion barrier film. Another drawback is the formation of a halo around objects in the image due to photoelectrons being incapable of initially penetrating the IBF and instead bouncing to another location and penetrating there. Yet another drawback is that higher voltage must be applied between the photocathode and the MCP in order to overcome the electron barrier established by the IBF. 
     Despite the recognized advantages of using ion barrier films, particularly where the useable lifetime of the photocathode is extended, poor imaging performance continues to frustrate consumers and designers alike. 
     SUMMARY 
     Described herein is microchannel plate assembly incorporating a particular IBF arrangement to mitigate recognized performance shortcomings in the art. A plurality of microchannel plates are aligned along a common axis and coupled together in an assembly. The microchannel plates each have an object-side surface and an image-side surface and the assembly has respective interfaces between the image-side surface and the object-side surface of adjacent microchannel plates. An ion barrier film is disposed on one of the object side surfaces in the interfaces. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an image intensifier tube in which the present general inventive concept may be embodied. 
         FIG. 2  is a diagram illustrating details of a multichannel plate assembly in which the present general inventive concept may be embodied. 
         FIG. 3  is a diagram illustrating another configuration of a multichannel plate assembly in which the present general inventive concept can be embodied. 
     
    
    
     DETAILED DESCRIPTION 
     The present inventive concept is best described through certain embodiments thereof, which are described in detail herein with reference to the accompanying drawings, wherein like reference numerals refer to like features throughout. It is to be understood that the term invention, when used herein, is intended to connote the inventive concept underlying the embodiments described below and not merely the embodiments themselves. It is to be understood further that the general inventive concept is not limited to the illustrative embodiments described below and the following descriptions should be read in such light. 
     Additionally, the word exemplary is used herein to mean, “serving as an example, instance or illustration.” Any embodiment of construction, process, design, technique, etc., designated herein as exemplary is not necessarily to be construed as preferred or advantageous over other such embodiments. Particular quality or fitness of the examples indicated herein as exemplary is neither intended nor should be inferred. 
       FIG. 1  is schematic diagram of an exemplary image intensifier tube (I2T)  100  in which the present invention may be embodied. Briefly, I2T  100  is constructed from a housing  110 , a photocathode element  120 , a microchannel plate (MCP) assembly  130  and cathodoluminescent (CL) element (CLE)  140 . Photocathode element  120 , MCP assembly  130  and CLE  140  are aligned on an optical axis  105  and relatively spaced apart so as to be in proximity focus with the adjacent element in the electron path. For purposes of orientation, I2T  100  has an object side  101  at which photons are accepted, and an image side  109  at which photons are emitted. By this defined geometry, photocathode element  120  is located at the object side  101  of image intensifier  100  and CLE  140  is located at the image side  109 . MCP assembly  130  thus also has an object side  132  at a surface of object-side MCP  136  and an image side  134  at a surface of image-side MCP  138 . The interior of I2T  100  is evacuated to form a vacuum chamber  107  of about 10 −9  to 10 −10  Torr to protect the photocathode from oxidation and rapid destruction. 
     Photocathode element  120  may be constructed from an optical window  122  and a photocathode  124  disposed on window  122 . Optical window  122  may be an optical flat of a material that is optically transmissive to photons having wavelengths of interest and for which I2T  100  is designed to detect. Photocathode  124  is a very thin, light-sensitive layer deposited on the inside of window  122  that converts impinging photons into electrons and releases them into the vacuum of the tube. In certain embodiments, photocathode  124  is formed from gallium arsenide (GaAs) bulk material with a negative electron affinity activation layer, such as cesium oxide (CsO). However, the CsO layer is fragile and can be severely damaged by ions fed back by to photocathode  124  by electric field  154 . While embodiments of the present invention ameliorate this issue, the present invention is not limited to a particular photocathode element  120 . 
     CLE  140  may have an optical window  146  constructed of a material that is optically transmissive at wavelengths matching the CL material, e.g., phosphor, in layer  144 . CL layer  144  may have additional materials such as aluminum and forms the anode of I2T  100 . 
     MCP assembly  130 , which is described in detail with reference to  FIG. 2 , may be disposed in vacuum chamber  107  in an electric field  152  generated between photocathode  124  and anode  142 . While electric field  152  is illustrated as being continuous, it may be formed piecewise by apply voltages V A -V E  at terminals  115  and may be discontinuous at MCP assembly  130 . In the illustrated embodiment, electric field  152  is a sum of object-side electric field  154  and image-side electric field  156 , which may be separately generated by applying suitable potential differences between terminals  115   a  and  115   b , and  115   e  and  115   f , respectively. The net effect of the electric fields  154  and  156  is to drive electrons from photocathode element  120  towards CLE  140 . 
     In operation, photons of an input wavelength enter the tube through window  122  and strike photocathode  124  to generate photoelectrons. The photoelectrons are accelerated by electric field  154  towards MCP assembly  130  where they are multiplied by cascaded secondary emission. For each electron that enters MCP assembly  130 , hundreds of electrons are generated. The generated electrons emerge from MCP assembly  130  in localized groups at the exit aperture of each microchannel, where they are accelerated by electric field  156  towards CLE  140 . The MCP assembly  130  and CLE  140  are spaced in proximity focus so that the localized groups of electrons arrive at the phosphor coating layer  144  with minimal dispersion. At CLE  140 , the electrons are converted into photons of an output wavelength by the material in CL layer  144 . For every photon striking photocathode  124 , tens of thousands of photons are generated by CLE  140 , thus “intensifying” the original image. 
     As illustrated in  FIG. 2 , MCP assembly  130  is constructed from object-side MCP  136  and image-side MCP  138  separated by a spacer  205 . Each microchannel plate  136 ,  138  may be manufactured from a highly resistive material of, for example, 2 mm thickness. A regular array of densely-distributed tiny tubes or slots, i.e., the microchannels  210 , traverse the thickness of the microchannel plate  136 ,  138  from one face to the opposite. The diameter d of microchannel  210  may be approximately 5 micrometers and the microchannels  210  may spaced apart at approximately 6 micrometers intervals. Microchannels  210  may be parallel to one other and may be formed in the plate at a small angle θ with respect to the optical axis  105 , e.g., approximately 3°-5°. The length-to-diameter ratio L/d, the material in the channel walls and the electric field strength across the microchannel establish the electron gain of the MCP. In certain embodiments, microchannels  210  have an L/d of between 60 to 70. 
     The object-side and image-side surfaces of MCP  136 ,  138  may be suitably coated with a metal electrode layer  222   o  and  222   i , respectively, such as NICHROME, although the present invention is not so limited. Electrode layers  222   o  and  222   i  may be deposited by evaporation to uniformly penetrate into microchannels  210 . This penetration, referred to as end-spoiling, affects the angular distribution and kinetic energy of exiting electrons. In certain embodiments, the penetration depth is approximately 0.5-3.5 times the diameter d of microchannel  210 , with deeper penetrations providing higher collimation of the exiting electron groups. Additionally, in certain embodiments, spacer  205  is conductive and serves as an electrical connection between facing electrode layers  222   o ,  222   i  in the interfaces. When so embodied, a single potential difference Vpp may be applied to the outermost electrode layers  222   o ,  222   i  by which electrical fields  223  and  227  are generated and terminate on the conductive boundaries of the inner electrode layers  222   o ,  222   i . Accordingly, the electric field in the interface region  230  is zero. 
     The interior surface of channel  210  may have a high secondary electron emission coefficient from lead/alkali content in the glass fibers from which MCP  136 ,  138  are manufactured. A firing procedure may be used to bring this content to the surface and this surface layer is electrically connected to the object-side electrode  222   o  and image-side electrodes  222   i  of each MCP  136 ,  138  to form an independent, continuous-dynode electron multiplier, in which electron multiplication takes place under the presence of a strong electric field  233  or  237 . The angle θ is established to increase the likelihood that an electron moving in a direction roughly parallel with optical axis  105 , i.e., the direction of the electric field, will strike the wall of one of the microchannels  210 . Such impact initiates cascaded secondary emissions of electrons, whereby the number of electrons increases exponentially along the length L of microchannel  210 . 
     The dual MCPs  136  and  138  of MCP assembly  130  can have an order of magnitude greater gain than a single MCP if both MCPs were operated at their typical single plate voltage levels. An electron arriving at object-side MCP  136  enters microchannel  210  and, upon striking layer  240 , initiates cascaded secondary emissions under electric field  233 . The multiplied electrons  242  emerge from the image side of object-side MCP  136  and enter the inter-plate interface  230  When the electric field in interface  230  is zero, the electrons from MCP  136  traverse the interface by way of kinetic energy. The electrons  242  enter microchannels  210  of image-side MCP  138  and initiate additional cascaded secondary emissions therein. These multiplied electrons then emerge from image side of MCP  138  and are directed toward CLE  140 . 
     An unavoidable consequence of such cascaded secondary emissions is the desorption of ions in microchannels  210 . The electric fields  233  and  237 , respectively, accelerate these positive ions toward the object side of MCPs  136  and  138 , respectively. If these ions are allowed to escape and come under the influence of electric field  154 , they are accelerated toward and strike photocathode  124 , causing irreversible damage to the CsO activation layer, thereby reducing its photo-responsiveness and shortening the lifespan of the device. Alternating the angular directions of microchannels  210  into the illustrated “chevron” condition is one measure that is taken to impede the progression of ions into electric field  154 . In addition, embodiments of the invention employ an ion barrier film (IBF)  225  over the object-side surface of image-side MCP  138 . In certain embodiments, IBF  225  is 0.5-0.10 nm aluminum oxide Al 2 O 3  layer deposited over the object-side surface of MCP  138  in a manner by which the layer covers the input apertures of microchannels  210  on this surface. Al 2 O 3  is minimally penetrable by ions and maximally penetrable by electrons, has high mechanical strength and is chemically stable. Other materials and structures, including multilayer structures may be used in IBF  225  without departing from the spirit and intended scope of the present invention. 
     In accordance with the present invention, object-side MCP  136  is filmless, i.e., free of an IBF. The application of an IBF to the MCP introduces a scattering center for impinging electrons. Introducing this scattering center at the object side, where only photoelectrons from photocathode  124  are present significantly reduces image quality. When the IBF is behind the object-side MCP, the backscattered electrons from the surface of the IBF are completely captured by the image-side surface of the object-side MCP  136 , since there is no electric field in interface region  230  to accelerate the electrons to the object-side surface of image-side MCP  138 . Moreover, without an IBF on the outermost object-side surface, a 30-50% less intense electric field across MCP  136  can produce approximately the same electron gain as that achieved with the IBF on the outermost object-side surface at full electric field strength. Alternatively, a full strength electric field would allow increasing the distance between photocathode  124  and object-side MCP  136 , which reduce field effect artifacts in the resulting image. And, while ions from object-side MCP  136  may still escape into electric field  154 , these ions are in number far less than the number of ions generated in image-side MCP  138  and depositing IBF on the image-side MCP  138  thus serves to greater effect, particularly in light of image quality achieved by keeping the object-side MCP  136  barrier free. 
       FIG. 3  is an illustration of a three plate MCP assembly, a so-called “Z-stack” by which the present invention can be embodied. MCP assembly  410  comprises an object-side MCP  420 , and image-side MCP  430  and an intermediate MCP  425 . The MCPs  420 ,  425  and  430  may be constructed in a manner described above. However, in MCP assembly  410 , only intermediate MCP  425  has an IBF  450  disposed on its object-side surface. In configuration  340 , IBF  350  is disposed on both intermediate MCP  425  and image-side MCP  430 . In both of these cases, object-side MCP  420  remains barrier free. Other configurations may be implemented in accordance with the present invention without departing from the spirit and intended scope thereof. 
     The descriptions above are intended to illustrate possible implementations of the present inventive concept and are not restrictive. Many variations, modifications and alternatives will become apparent to the skilled artisan upon review of this disclosure. For example, components equivalent to those shown and described may be substituted therefore, elements and methods individually described may be combined, and elements described as discrete may be distributed across many components. The scope of the invention should therefore be determined not with reference to the description above, but with reference to the appended claims, along with their full range of equivalents.