Electric lamp and discharge devices – Photosensitive – Secondary emitter type
Reexamination Certificate
1997-10-30
2002-05-07
Day, Michael H. (Department: 2879)
Electric lamp and discharge devices
Photosensitive
Secondary emitter type
C313S1050CM, C313S534000
Reexamination Certificate
active
06384519
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to electron multiplier devices, and more specifically relates to dynode arrays, methods of making the same and components incorporating the same.
Photomultiplier tubes (PMT) are versatile, sensitive detectors of radiant energy in the ultraviolet, visible, and near infrared regions of the electromagnetic spectrum. A photomultiplier tube consists of a photoemissive photocathode, an electron multiplier device based on secondary electron emission and an anode to collect the signal electrons, all housed inside a vacuum envelope. Radiant energy such as light incident on the photocathode causes the photocathode to emit electrons. In the electron multiplier device, these electrons are accelerated by an electric field towards an electrode referred to as a “dynode”. As the electrons impinge on the dynode, they cause the dynode to emit a larger number of secondary electrons which are in turn accelerated to another dynode producing more secondary electrons. This process continues for several stages, with progressively larger numbers of electrons being emitted at each successive stage. The electrons from the last dynode stage are collected on an anode which is connected to an external circuit, outside of the vacuum envelope. The dynodes may be arranged to provide a tortuous path which changes direction at each dynode. This helps to assure that electrons from each dynode will impinge on the next dynode, and also protects the photocathode against positive ions which may be emitted from the anode or from the dynodes. PMT's are used in industrial and scientific apparatus as detectors in systems for measuring the intensity of a beam of radiant energy. For a large number of applications, the PMT is the most sensitive detector available. The superiority of the PMT arises from the secondary electron emission amplification, which makes it possible for the device to approach “ideal” device performance limited only by the statistics of photoemission. The electron gain of a PMT—the ratio of the number of electrons provided by the last stage to the number of electrons provided by the photocathode—typically ranges from 10
3
to as high as 10
8
. Thus, even when the radiant energy to be detected is extremely weak, a PMT can provide output signals at levels which are easily measured by auxiliary electronic equipment. PMTs can also have extremely fast time response (~100 ps), which provides the capability for measuring radiant energy varying at a rapid rate. Stated another way, the combination of gain and bandwidth provided by PMTs is unmatched by any other detector. PMTs have very low quiescent power when the individual dynodes are powered separate from an active power supply circuit. A dynode set can also be used to amplify a stream of electrons or ions from a source other than a photocathode, and can provide similar advantages in these applications.
The dynode sets used in measurement devices typically provide only one channel or set of cascaded dynode stages, and amplify only one stream of electrons. Thus, in a light-sensing PMT, the electrons emitted by the entire photocathode are amplified together in one stream of electrons so that the device provides a single output signal representing the light incident on the entire photocathode.
Imaging devices typically must process a separate signal for each of many picture elements or “pixels” in a two-dimensional array of pixels constituting an image. For example, a monochrome (black and white) image can be represented by a set of signals, each representing the brightness of the image within a pixel at a particular position. Many common imaging devices, such as the charge-coupled-device or “CCD” imaging devices in home video cameras and in electronic still cameras incorporate a two-dimensional array of detectors incorporating a separate detector for each pixel in the image. A lens focuses the image onto the array, and each detector provides a signal representing the brightness of one pixel in the image. These signals can be reconstructed to provide an image, such as a television or still picture representing the original image. To provide reasonable resolution in the resulting image, the imaging device should include a large number of detectors. Even a medium-quality imaging system such as a consumer video camera requires tens of thousands of pixels; high quality imaging requires hundreds of thousands of pixels. However, with common CCD technology, there is a direct relationship between the size of each detector and the sensitivity of the device, and a similar relationship between the size of each detector and immunity to random electronic noise. Thus, the spatial resolution of the device—the number of detectors which can be provided in a device of a given size—is limited. CCD technology has branched into two major classes. One class provides low cost sensors for large consumer markets such as camcorders, line scanners, etc. whereas the other class provides very high quality CCDs for scientific imaging. The low cost sensors are capable of achieving high data rates (~60-100 MHz for certain line scanners), they suffer in image quality and are not satisfactory for high frame rate scanning arrays. The high quality CCD sensors, while providing excellent low noise performance, cannot provide that performance at high frame rates. Thus, while Si based CCD technology has made great progress, there is still a large gap between what is desired for high quality imaging and performance of the present generation of CCD sensors. The CCD devices do not provide the high gain, bandwidth and response time of dynode devices.
Attempts have been made to fabricate plural-channel dynode arrays heretofore. Ehrfeld et al., U.S. Pat. No. 4,990,827 and Shimabukuro et al. U.S. Pat. No. 5,329,110 propose making arrays of small electron multipliers by certain microfabrication techniques. However, the techniques and structures taught by these references are suitable for making linear arrays of electron multipliers; they are not well suited to fabrication of a two-dimensional array of dynode channels.
Comby et al., Nuc. Inst. Meth. Phys. Res. A 343, 263 describe an all ceramic multichannel electron multiplier in a PMT having four imaging pixels of 0.6 mm diameter employing a five stage dynode structure. The dynodes are provided as metallic plates arranged along a channel. Openings in the plates are offset from one another to form a tortuous path. According to the reference, the results of gain measurements from these devices demonstrated that machined channels can be built with high gain. Using a Ag—O—Cs coated dynode material, they were able to achieve gains of about 100 for the five stage multiplier, amplifying photoelectrons from a CS
3
Sb photocathode. As set forth in Comby et al, Proceedings. International Conference On Inorganic Scintillators and Their Applications, SCINT95, DELFT Univ. of Tech. The Netherlands, September (1995), by treating Au dynodes with Sb—Cs, gains in excess of 10
3
were demonstrated in an all ceramic PMT with 0.6 mm pixels in a 4×4 array. These articles propose that it may be possible to fabricate a 256-pixel device. Thus, dynode array devices available heretofore do not provide the spatial resolution needed for high-quality imaging.
Another electron multiplying device is known as a microchannel plate or “MCP”. MCP's typically have numerous continuous channels extending through an insulating layer. A coating of a material having high electron emissivity is applied on the interior of each channel. The coating has a high electrical resistance. A voltage applied through electrically conductive layers extending on opposite side of the insulating layer creates a potential gradient between opposite ends of the coating. Electrons entering each channel are accelerated along the channel by the potential gradient, and impinge on the walls of the channel. Such collisions yield secondary electrons which are also accelerated and provide further collisions. Although MCP's provide advantages such
Beetz, Jr. Charles P.
Boertsler Robert W.
Steinbeck John
Winn David R.
Day Michael H.
Lerner David Littenberg Krumholz & Mentlik LLP
NanoSciences Corporation
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