Electric lamp and discharge devices – Photosensitive – Secondary emitter type
Reexamination Certificate
1999-11-22
2001-04-10
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
Photosensitive
Secondary emitter type
C313S1050CM
Reexamination Certificate
active
06215232
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to an improved microchannel plate and method for its manufacture. More particularly, the present invention relates to a microchannel plate which provides for low positive ion flow through the microchannel plate. This microchannel plate provides for increased gain, reduced noise, and improved signal-to-noise ratio. Still more particularly, the present invention relates to devices, such as an image intensifier tube or photomultiplier tube, which includes a photocathode sensitive to ion feedback and which uses a microchannel plate according to the present invention to reduce ion feedback and impact on the photocathode.
BACKGROUND OF THE INVENTION
The channel electron multiplier (CEM) is a well known device. The CEM consists of an elongate tube of material which is a secondary emitter of electrons. Conventionally, this secondary electron emitter material is carried on the inner surface of a structural tube formed of insulative material. An electric current, and associated electrostatic field in the secondary electron emitter material, is maintained along the length of the CEM from an inlet end to an outlet end of the tubular CEM structure. Because the performance of the conventional CEM depends on length-to-diameter ratio rather than physical size of the structure, the channel size can be reduced to very small dimensions.
Accordingly, arrays of plural channel electron multipliers have been fabricated. A conventional device which provides such an array of channel electron multipliers is the microchannel plate. Conventionally, microchannel plates are made by drawing a number of fine-dimension glass tubes of either a hollow configuration or of a configuration with a removable core fiber. The glass tubes are joined in bundles and further drawn under pressure causing the tubes to bond to one another at their outer surface, thus forming a boule or elongate rod-like structure of multiple fine-dimension glass tubes in parallel. Next, a pair of spaced apart parallel transverse cuts across this boule of glass tubes defines between the cut lines a comparatively thin plate having perhaps a million glass tubes extending between its opposite faces. If the glass tubes are of fiber core construction, the core fiber is etched out using chemicals. For example, an acid or a base may be used to etch the glass.
The inner surface of each of the multitude of glass tubes (or microchannels) is then activated to make this glass surface a practical secondary emitter of electrons. This activation of the glass surface is effected by reducing this surface at elevated temperature in a hydrogen atmosphere. The glass of the tubes is made of a material which is doped with selected materials, such as lead and antimony. After reduction of the glass, this doped glass leaves metal atoms or metal oxide molecules exposed on and close to the inner surface of the microchannels, and provides a thin coating of glass semiconductor material extending along the inner surface of the microchannels between the opposite faces of the plate.
A metallic electrode is applied to each of the opposite faces of the plate, and the microchannel plate is operated with an applied electrostatic potential applied across these electrodes. As a result, a current flow between the electrodes takes place in the surface layer of reduced semiconductor glass, which current is referred to as the “strip current” of the microchannel plate. Because the glass of the tubes is itself an insulator with a bulk-resistivity in the range of 10
17
to 10
22
ohm-cm, substantially no practical electric current flows in the body of the microchannel plate itself—other than in the semiconductor reduced-glass coating each of the microchannels. The microchannel is operated in an evacuated environment of reduced pressure to allow electron flow along the channels with amplification by secondary emission of electrons from the inner surfaces of the microchannels.
The microchannels in a conventional microchannel plate are straight, and hence are subject to ion feedback. The ion feedback occurs because molecules and atoms of residual gas and other materials in the operating environment of the microchannel plate, and which become positively charged, are accelerated by the applied electrostatic field in a direction opposite to the electron flow. Because these ions are both very massive compared to an electron, and are accelerated to a high potential energy by the applied electrostatic field, they can be destructive to surfaces which they impact, and the impacting ions can cause unwanted emissions of electrons from the microchannel walls and/or the photocathode. As is known in the technologies using microchannel plates, these ions flowing toward a photocathode, for example, can both erode the photocathode by their dynamic impact, and also may imbed into the cathode, thus changing the crystalline structure and chemistry with resulting loss of performance of the photocathode to liberate photoelectrons in response to incident photons of radiation.
For these reasons, conventional microchannel plates have been operated in pairs with the microchannels of the paired plates forming a chevron shape to trap ions feeding back toward the inlet end of the first microchannel plate. Unfortunately, it is impossible to precisely align the microchannels of one plate to those of the other, so that resolution of paired microchannel plates is always less than one plate alone could provide. Alternatively, a few microchannel plates have been formed with curved channels in order to impact the ions with the walls of the channels, and thereby recombine the ions with an electron to produce neutral particles. However, microchannel plates with curved channels are very expensive and difficult to manufacture.
Conventional devices which use microchannel plates are image intensifier tubes of night vision systems, and photomultiplier tubes. Photomultiplier tubes are used for such purposes as scintillation detectors in particle accelerators and fluoroscopic detectors of chemical analyzers. A night vision system converts available low intensity ambient light to a visible image. Such night vision systems require some residual light, such as moon or star light, in which to operate. The star-lighted sky of the night is generally rich in infrared radiation, which is invisible to the human eye. The infrared ambient light is intensified by the night vision scope to produce an output image in light which is visible to the human eye. The present generation of night vision scopes use image intensification technology with a photocathode responsive to both visible and infrared photons to release photoelectrons. One or more microchannel plates are used to amplify the low level of photoelectrons to render a shower of secondary-emission electrons in a pattern replicating the invisible infrared image. These electrons are directed onto a phosphorescent screen to provide a visible image.
Alternatively, a microchannel plate can be used as a “gain block” in a device having a free-space flow of electrons. That is, the microchannel plate provides a spatial output pattern of electrons which replicates an input pattern, and at a considerably higher electron density than the input pattern. Such a device is useful as a particle counter to detect high energy particle interactions which produce electrons.
Regardless of the data output format selected, the sensitivity of the image intensifier or other device utilizing a microchannel plate is directly related to the amount of electron amplification or “gain” imparted by the microchannel plate. That is, as each photoelectron enters a microchannel and strikes the wall, secondary electrons are knocked off or are emitted from the area where the photoelectron initially impacted. The physical properties of the walls of the microchannel are such that, generally and statistically speaking, a plurality electrons are emitted each time these walls are contacted by one energetic electron. In other words, the material of the walls has a high
Johnson Charles Bruce
Lin Po-Ping
Pierle Robert L.
Litton Systems Inc.
Marsteller & Associates, P.C.
Patel Ashok
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