Image Converter Having Serial Arrangement of Microchannel Plate, Input Electrode, Phosphor, and Photocathode

This invention relates to image converters and more particularly to radiant energy image converters employing microchannel plates (MCP).

The term "radiant energy" employed herein includes radiant energy in the X-ray spectral region, the visible spectral region, the ultraviolet spectral region and high energy radiation, such as .alpha. particles, .beta. particles, .gamma. particles and neutrons.

Previously known X-ray image converters employed a phosphor screen/photocathode sandwich upon which the X-rays impinged to produce photoelectrons which were then focused onto an electron target, such as a phosphor screen, an MCP, etc. The disadvantage of this prior image converter is relatively low quantum efficiency and image-quality degrading properties.

An object of the present invention is to provide a radiant energy image converter that has relatively high quantum efficiency and improved image-quality while retaining relatively high limiting resolution and high gain of MCPs as compared to the previously known radiant energy image converters.

A feature of the present invention is the provision of a radiant energy image converter comprising a vacuum envelope having a longitudinal axis; input means for radiant energy images disposed coaxial of the axis at one end of the envelope; a microchannel plate disposed coaxial of the axis within the envelope in a spaced relationship with the input means, the microchannel plate including an input electrode adjacent to the input means; a radiant energy sensitive phosphor means applied directly to the input electrode; a photocathode applied to the phosphor means; a radiant energy transparent mesh electrode disposed within the envelope coaxial of the axis between the input means and the photocathode; means coupled between the mesh electrode and the input electrode to provide a retarding field therebetween to reflect photoelectrons produced at the photocathode into the plate; a readout means disposed coaxial of the axis at the other end of the envelope; and an electron lens means disposed within the envelope coaxial of the axis between an output of the plate remote from the input means and the readout means to focus photoelectrons emanating from the output of the plate onto the readout means to provide a discernible output related to the radiant energy image.

Above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a schematic diagram partially in cross-section illustrating the radiant energy image converter in accordance with the principles of the present invention; and

FIG. 2 is an enlarged view of the MCP phosphor screen/photocathode sandwich of FIG. 1 illustrating the conversion of incident radiant energy to photoelectrons in accordance with the principles of the present invention.

Referring to FIG. 1, the image converter includes a vacuum envelope 1 having a longitudinal axis 2. A radiant energy input window 3 is disposed at one end of envelope 1 coaxial of axis 2 having the property of passing radiant energy incident thereon. Input window 3 may be eliminated if the converter of this invention is employed in a vacuum environment such as outer space. An MCP 4 is disposed coaxial of axis 2 within envelope 1 spaced along axis 2 from input window 3. MCP 4 includes on the input end thereof an input electrode 5. A phosphor screen, or scintillator, 6 is applied directly to electrode 5 and a photocathode 7 is applied directly to phosphor screen 6 or applied directly to a protective layer on phosphor screen 6. Intermediate photocathode 7 and input window 3 is disposed a radiant energy transparent mesh or ring electrode 8 disposed within envelope 1 and coaxial of axis 2. A potential is provided between electrode 8 and electrode 5 to provide a retarding field between these two electrodes to reflect photoelectrons produced at photocathode 7 into MCP 4. A readout device 9 is disposed coaxial of axis 2 and at the other end of envelope 1. Readout device 9 may be a phosphor screen for direct viewing, a video camera tube, a gain/storage target of the type used in television camera tubes for video electronic readout/display/magnetic tape recording, or self-scan array readouts, such as charge coupled devices, charge injection devices and Reticon chips. Alternatively, the output image for MCP 4 may be focused onto a second MCP before readout of the types mentioned above. An electron lens 10 is disposed within envelope 1 coaxial axis 2 between the output of MCP 4 and readout device 9 to focus photoelectrons emanating from the output of MCP 4 onto readout device 9 to provide a discernible output such as a visual image or a video signal which is related to the radiant energy image input.

The input radiant energy image passes through the radiant energy transparent input window 3 and impinges upon photocathode 7 with certain portions of the input image passing through photocathode 7 and impinging upon phosphor screen 6 and other portions of the image input going directly to MCP 4. The structure employed in the converter of the present invention ensures high quantum efficiency by producing photoelectrons by (1) direct conversion of radiant energy quanta to photoelectrons at photocathode 7, (2) by conversion of radiant quanta which pass through photocathode 7 to light quanta in phosphor screen 6 and the subsequent conversion of these light quanta into photoelectrons in photocathode 7 and (3) by conversion of radiant energy quanta into photoelectrons in MCP 4. The three different photoelectrons that are produced in the converter of the present invention are illustrated in FIG. 2 and identified by (1), (2) and (3) corresponding to the similarly identified conventions above.

High gain is achieved and high resolution is produced by reflecting the photoelectrons produced at photocathode 7 into MCP 4 by electronically biasing mesh electrode 8 negative with respect to input electrode 5. A single MCP provides a useful electrical gain of 10.sup.3 -10.sup.6. The photoelectrons at the output of MCP 4 are focused by electron lens 10, which may be a proximity electrostatic or electromagnet electron lens, onto readout device 9.

It has been determined that the image converter of the present invention will operate with the above advantages in various spectral regions provided photocathode 7 and phosphor screen 6 are made of appropriate material. For instance, in the X-ray spectral region, photocathode 7 may be made of cesium antimony and phosphor screen, or scintillator 6 may be composed of cesium iodide. In order to detect .alpha., .beta. and .gamma. particle and neutron radiation, photocathode 7 may be made of cesium antimony and phosphor screen or scintillator, 6 may be made of cesium iodide. In the ultraviolet spectral region, photocathode 7 may be made of a bialkali compound and phosphor screen 7 may be made of sodium iodide. An example of a successfully employed bialkali compound includes cesium, potassium and antimony. In the visible spectral region, photocathode 7 may be made of a multialkali and phosphor screen 6 may be composed of a phosphor compound known as P22R. An example of a successfully employed multialkali includes sodium, potassium, cesium and antimony.

While I have described above the principles of my invention in connection with specific apparatus it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.