Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-08-25
2001-10-16
Allen, Stephone B. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S2140VT, C313S1030CM
Reexamination Certificate
active
06303918
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to the field of electro-optics and, more specifically, to a method and system for detecting radiation incorporating a photocathode that is protected from damage by collisions with positive ions.
BACKGROUND OF THE INVENTION
There are numerous methods and systems for detecting radiation. In one type of detector, photocathodes are used in conjunction with microchannel plates (MCPs) to detect low levels of electromagnetic radiation. Photocathodes emit electrons in response to exposure to photons. The electrons can then be accelerated by electrostatic fields toward a microchannel plate. A microchannel plate is typically manufactured from lead glass and has a multitude of channels, each one operable to produce cascades of secondary electrons in response to incident electrons. A receiving device then receives the secondary electrons and sends out a signal responsive to the electrons. Since the number of electrons emitted from the microchannel plate is much larger than the number of incident electrons, the signal produced by the device is stronger than it would have been without the microchannel plate.
One example of the use of a photocathode with a microchannel plate is an image intensifier tube. The image intensifier tube is used in night vision devices to amplify low light levels so that the user can see even in very dark conditions. In the image intensifier tube, a photocathode produces electrons in response to photons from an image. The electrons are then accelerated to the microchannel plate, which produces secondary emission electrons in response. The secondary emission electrons are received at a phosphor screen or, alternatively, a charge coupled device (CCD), thus producing a representation of the original image.
Another example of a device that uses a photocathode with a microchannel plate is a scintillation counter used to detect particles. High-energy particles pass through a scintillating material, thereby generating photons. Depending on the type of material used and the energy of the particles, these photons can be small in number. A photocathode in conjunction with a microchannel plate can be used to amplify the photon signal in similar fashion to an image intensifier tube. The detector can thus be used to detect faint particle signals and to transmit a signal to a device, e.g., a counter, that records the particle's presence.
One problem with the use of photocathodes in conjunction with microchannel plates is that the electrostatic fields that accelerate electrons toward the microchannel plate also accelerate positive ions toward the photocathode. Positive ions are common in most image intensifier tubes due to impurities in the tube, including the microchannel plate and the phosphor screen. These impurities can include positive ions and chemically active neutral atoms that can become positively charged. When the positive ions collide with the photocathode, they can cause both physical and chemical damage. This greatly shortens the useful life of the photocathode and the device in which it resides.
The problem of positive ions can be overcome to some extent by placing an ion barrier film on the input side of the microchannel plate. The film serves to block the positive ions from reaching the photocathode. The barrier has the unfortunate side effect of reducing the transmission of electrons. This interference reduces the signal to noise ratio of the detector, e.g., an image intensifier tube.
An alternative method of overcoming the problem removes impurities from the components of an image intensifier tube in order to reduce the number of positive ions impinging on the photocathode. Less positive ions equates to less damage to the photocathode and a longer life for the image intensifier tube.
The aforementioned methods do not provide a means of “hardening” the photocathode itself in order to increase the photocathode's resistance to damage from positive ion collisions. A hardened photocathode would be valuable in that it would have a longer lifetime than a normal photocathode. The hardened photocathode could be used alone or in combination with the impurity removal procedures mentioned above to greatly prolong the lifetime of an image intensifier tube or other device. Consequently, what is needed is a method and system for detecting radiation that incorporates a hardened photocathode and that increases the resistance of the photocathode to ions.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method for producing hardened photocathodes is provided that substantially eliminates or reduces disadvantages and problems associated with using photocathodes and microchannel plates in combination. A method and device for detecting radiation using a hardened photocathode is also provided. The method and device for detecting radiation overcome drawbacks, such as shorter lifespan, associated with similar devices used previously.
A method for detecting radiation is disclosed. The method comprises nine steps. Step one calls for forming a detector having a photocathode with a protective layer of cesium, oxygen and fluorine; a microchannel plate (MCP); and an electron receiver. Step two requires receiving radiation at the photocathode. Step three provides for the photocathode discharging electrons in response to the received photons. In step four, the method provides for accelerating discharged electrons from the photocathode to the input face of the microchannel plate. The next step calls for receiving the electrons at the input face of the microchannel plate. Step six calls for generating a cascade of secondary emission electrons in the microchannel plate in response to the received electrons. The seventh step calls for emitting the secondary emission electrons from the output face of the microchannel plate. In the eighth step, the method provides for receiving secondary emission electrons at the electron receiver. The last step calls for producing an output characteristic of the secondary emission electrons.
A device for detecting radiation is disclosed. The device comprises a photocathode, a microchannel plate and an electron receiver The photocathode is operable to receive radiation on an input side and to discharge electrons from its output side in response. The output side of the photocathode has a protective layer comprising cesium, oxygen and fluorine. The microchannel plate serves to receive electrons on its input face from the photocathode, to produce a cascade of secondary emission electrons and to discharge those electrons from its output face. The electron receiver is operable to receive secondary emissions electrons from the microchannel plate and to produce an output characteristic of those electrons.
A method for manufacturing a hardened photocathode is also disclosed. The method comprises four steps. The first step requires forming a photocathode having an input side for receiving radiation and an output side for discharging electrons. The second through fourth steps require exposing the output side of the photocathode to cesium, oxygen and fluorine respectively.
A technical advantage of the present invention is that the hardened photocathode has a longer usable lifespan than previous photocathodes. This greatly increases the value of devices such as image intensifier tubes that need to be useful for as many hours as possible without requiring replacement. Another technical advantage of the present invention is that a microchannel plate used in conjunction with the hardened photocathode does not need to have an ion barrier film on the microchannel plate. This allows a detector employing the hardened photocathode to have a higher signal to noise ratio.
Still another technical advantage of the present invention is that the hardened photocathode can be used in conjunction with other methods of increasing the lifespan of a detector, such as removing impurities from components of the detector. In addition, other technical advantages may be apparent to one skilled in the art of
Estrera Joseph P.
Giordana Adriana
Glesener John W.
Allen Stephone B.
Baker & Botts L.L.P.
Litton Systems Inc.
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