WO2007098493A2 - Large-area flat-panel photon detector with hemispherical pixels and full area coverage - Google Patents

Abstract

A photo detector with a flat panel vacuum enclosure with a window including hemispherical cavities coated with photocathode material to convert photons into photoelectrons. The electrons are focused from the hemispherical photocathodes to their centers where they are detected in small electron sensors. Reflectors in dead areas between the hemispheres direct light to the active hemispherical photocathodes which leads to full area coverage. The result is that the entire panel surface is active in photon detection, and the information is concentrated to a very small area of the electron sensors. A voltage plate simultaneously distributes the anode and cathode potentials to all of the pixels in the panel. In one embodiment, the photoelectrons are detected with a scintillator and Geiger-mode avalanche photodiodes (G-APD).

Description

LARGE-AREA FLAT-PANEL PHOTON DETECTOR WITH HEMISPHERICAL PIXELS AND FULL AREA COVERAGE

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims priority from U.S. provisional application serial number 60/776,054 filed on February 22, 2006, incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not Applicable

INCORPORATION-BY-REFERENCE OF MATERIAL

SUBMITTED ON A COMPACT DISC [0003] Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0004] This invention pertains generally to photosensors, and more particularly to a monolithic flat-panel photon detector including a matrix of hollowed hemispherical cells and interstitial reflectors.

2. Description of Related Art

[0005] Photon detectors are indispensable in many areas of fundamental physics research, particularly in the emerging field of particle astrophysics, as well as in medical imaging and nuclear nonproliferation monitoring. Research in the area of high-energy physics and high-energy astrophysics is often based on the detection of Cherenkov or fluorescence photons emitted by charged particles in a wide variety of transparent media. For example, the interactions of neutrinos in transparent media create charged particles, which in turn radiate Cherenkov or scintillation photons that are detected as an indication of a captured neutrino. Therefore, efficient and sensitive photon detectors are the most important component of a neutrino telescope. [0006] Similarly, the study of very energetic or extremely rare phenomena will be the topic of many future experiments within the field of particle astrophysics including studies of proton decay, neutrino oscillations, geoneutrinos, supernova explosions, cosmic neutrinos, cosmic gamma rays, ultra-high energy cosmic rays, gamma-ray bursts, active galactic nuclei, dark matter, neutrinoless double beta decay, etc. These elusive phenomena require special means of observation, with detectors whose sizes will greatly exceed the dimensions of the largest current devices. In the construction of detectors on such a large scale, no other option remains than to use natural media — the atmosphere, deep packs of ice, and water. In these transparent media, charged particles that originate in impacts or decays of primary particles radiate Cherenkov or fluorescence light. [0007] Photosensors play an equally important role in other diagnostic areas such as medical imaging, nuclear radiation monitoring and nuclear proliferation control. Photosensors are the only well proven active detector component to convert photons into electrical signals. The best known photon detector in the art is the photomultiplier tube (PMT) developed by the 1960's. The photomultiplier is a sensitive detector of radiant energy in the ultraviolet, visible and near infrared portions of the electromagnetic spectrum. Generally, the photomultiplier tube consists of a photocathode, several dynodes and an anode within a glass vacuum tube. Photons striking the photocathode produce photoelectrons as a consequence of the photoelectric effect. The photoelectrons are directed by an electric field to an electrode or series of dynodes, each having a positive voltage greater that the previous one.

Secondary electrons are emitted by the first dynode and directed to a second dynode accelerated by an electric field. A greater number of electrons are produced from dynode to dynode and the electrons are eventually collected by an anode which provides a signal current pulse that is read indicating the presence of a photon.

[0008] However, vacuum photomultiplier tubes have shown some significant deficiencies in performance, have a limited capability of forming arrays, and are labor intensive and expensive to manufacture. For example, the PMT has a low photoelectron collection efficiency (at most -70% in large area PMTs) and a low quantum efficiency (20-25%), limited only to a narrow spectral region. Additionally, the PMT is not capable of resolving the number of photons in a photosensor pixel i.e. it does not have a single-photon resolution. [0009] Photomultipliers also have a high sensitivity to magnetic fields, including the geomagnetic field of the Earth. Magnetic fields may divert the path of electrons and detection efficiency may depend on the orientation of the PMT in space. Not only must each PMT be shielded from magnetic fields, they must never be accidentally exposed to ambient light in order to avoid overexcitation and burnout, which is in practice very difficult to achieve. Readout lines must also be shielded and pre-amplifiers are often required to eliminate noise. [0010] Panels or arrays composed of hundreds of individual PMT tubes assembled together typically require a complex three dimensional structure and are limited by that structure. For example, PMT arrays composed of hundreds of shielded PMT tubes require that each PMT be individually supplied with 10-12 different levels of high voltage. That requires a dense network of high voltage and shielded readout cables (thousands), with a separate high voltage power supply and a separate voltage divider for each PMT. Connector sockets may be unreliable in environments with high levels of moisture or vibration. Furthermore, parasitic currents and oxidized contacts may lead to spurious effects that are very difficult to track down. [0011] Accordingly, there is an increasing need for photosensor arrays with large detection surfaces and high sensitivity that are capable of mass production at relatively low cost. There is also a need for a photosensor array that is insensitive to magnetic fields or accidental exposure to ambient light and is strong and durable. The present invention meets these needs as well as others and is a significant improvement over the art. BRIEF SUMMARY OF THE INVENTION [0012] The present invention is a photon detector that is suitable for applications where low cost, industrial mass-production, extremely large quantities, high sensitivity, reliability and durability are essential. The photon detector shown in FIG. 1 through FIG. 8 is particularly useful for photon detection in dense media like water, liquid scintillator, plastic scintillator, or ice. It provides excellent optical coupling in dense media, has low buoyancy, simple mounting, unique robustness, unprecedented stability in high ambient pressures (e.g. deep ocean deployment), virtual insensitivity to accidental exposure to high levels of light, and insensitivity to the geomagnetic field.

[0013] The preferred embodiment of the invention is a monolithic flat-panel vacuum enclosure with a transparent front plate having a matrix of hollow hemispherical cells cast into the vacuum side of the window plate. The back plate may have matching hemispherical cells or may be a plain flat plate forming the second part of the evacuated enclosure. The interior of the hemispherical cells is coated with a material to form a photocathode. An electron detector or a scintillator is disposed at the center of the hemisphere preferably equidistant from all of the edges of the hemisphere. [0014] Hollow reflectors are placed between the active hemispherical cells in the plane of the top plate to direct the light from dead regions between the hemispheres to the active hemispherical area, which leads to full area coverage. In addition, each of the reflectors preferably provides space for placement of getter vacuum pumps to remove residual gasses. [0015] The panel preferably includes a voltage plate in the middle that simultaneously distributes the anode and the cathode potentials to all pixels in the panel, creating an optimal electron lens in each pixel, together with the hemispherical cavities.

[0016] In use, the 'vacuum part' of the detector transforms incoming photons into electrons in the photocathode layer, and compresses the signal to such a small area that a very small and inexpensive semiconductor sensor may be used for the electron detection. Alternatively, the sensor may detect photoelectrons through the detection of scintillation light that the photoelectron creates in a small scintillator. [0017] In the preferred embodiment, the incoming photons may also hit the hemispherical photocathodes indirectly after reflection from reflector cavities directing photons to the hemispherical cells and the photocathode. A photoelectron (e) released by the photon (ph) from the photocathode is focused and accelerated to a small electron detector such as a fiber-coupled scintillator. The amplified secondary light signal from the scintillator preferably travels via optic fiber out of the vacuum enclosure to a Geiger-mode Avalanche Photodiode (G-APD), where it is read-out and strongly amplified.

In another embodiment, the photoelectron is detected directly by an electron detector such as an APD.

[0018] According to one aspect of the invention, a photon detector is provided with a vacuum enclosure having a flat window plate with a matrix of hollow hemispherical-shaped cells cast into the vacuum side of the flat window plate, and a back plate. In one embodiment the plates are sealed together. In another embodiment the panel may have sides. A photocathode is deposited within the hemispherical cells and a photoelectron detector at the center of each hemispherical cell is provided. A voltage plate capable of holding an operational voltage across its thickness.

[0019] According to another aspect of the invention, the voltage plate has a cathode surface and an anode surface that extends through the plate through a number of openings and a cathode surface. The cathode surface has a resistor layer extending radially from the anode openings and the electric potential from anode to the cathode is gradually changed.

[0020] In a further aspect of the invention, a photon detector is provided that has a preferably symmetrical vacuum enclosure having a first flat window plate with a matrix of hollow hemispherical-shaped cells cast into the vacuum side of the flat window plate, sides, and a second flat window plate with a matrix of hollow hemispherical-shaped cells cast into the vacuum side of the second window plate. Both sides of the panel are capable of detecting photons. A photocathode is disposed within each of the cells. A photoelectron detector is located at the center of each hemispherical cell. A voltage plate capable of holding an operational voltage across its thickness is located under each window plate. [0021] According to another aspect of the invention, a photon detector is provided that is particularly suited for detection in optically dense media, such as water, liquid scintillator, plastic scintillator, or crystal scintillator. This is due the fact that light from such optically dense media may enter the panel through the glass window without significant losses or reflections, and proceed directly to the photosensitive photocathode area, or reach that area after one or more reflections from the mirror-coated cavities. In addition, the nearly perfect optical coupling of the panel to dense optical media permits full angular acceptance, i.e. all photons (created in the coupled dense medium) will be detected, irrespective of angle of incidence of the photon to the panel. [0022] Further aspects of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

[0023] The invention will be more fully understood by reference to the following drawings which are for illustrative purposes only: [0024] FIG. 1 is a top view of a panel of one embodiment of a photon detector according to the present invention. [0025] FIG. 2 is a cross-sectional view of the photon detector of FIG. 1 taken along the lines 2-2 of FIG. 1. [0026] FIG. 3 is a cross-sectional view of the photon detector of FIG. 1 taken along the lines 3-3 of FIG. 1.

[0027] FIG. 4A and FIG. 4B depict the anode side and the cathode side of the high voltage plate of the embodiment shown in FIG. 1.

[0028] FIG. 5 is cross-sectional view of a symmetrical double sided embodiment of a photon detector using a photodiode as a photoelectron detector according to the present invention. [0029] FIG. 6 is cross-sectional view of a symmetrical double sided embodiment of a photon detector using a scintillator as a photoelectron detector according to the present invention.

[0030] FIG. 7 is a cross-sectional view of FIG. 6 showing photon and electron paths. [0031] FIG. 8 is a cross-sectional view of a schematic of a single tube photoelectron detector showing scintillator, fiber plate and Geiger-mode APD array.

[0032] FIG. 9 is a flow chart of the method for photon detection for amplifying photons.

DETAILED DESCRIPTION OF THE INVENTION

[0033] Referring more specifically to the drawings, for illustrative purposes the present invention is embodied in the apparatus generally shown in FIG. 1 through FIG. 8 and the method shown in FIG 9. It will be appreciated that the apparatus may vary as to configuration and as to details of the parts, and that the methods may vary as to the specific steps and sequence, without departing from the basic concepts as disclosed herein. [0034] Turning first to FIG. 1 through FIG. 3, a single sided embodiment of the photon detector invention is shown. The photon detector 10 depicted is preferably a hermetically vacuum-sealed flat-panel box, similar to a flat-panel TV screen, with a top window plate 14, a voltage plate 16 and an bottom plate 18. The voltage plate 16 and the bottom plate 18 are separated by blocks 20 to create a thin chamber 22 that will permit the placement of wires or fiber optic cables beneath the voltage plate 16 and the top plate 14 and above bottom plate 18. Sidewalls 12 or seals on the edges of the plates form an evacuated chamber. Spacers 20 between the high-voltage plate 16 and the bottom plate 18 function to neutralize the outside mechanical force arising due to the atmospheric pressure on the window plate 14 and the bottom plate 18, or the higher ambient pressure in dense media. [0035] The window plate 14 is preferably made of transparent dielectric material such as glass or quartz which is flat on the exterior side 24 exposed to the environment, i.e. the source of incoming light, and is patterned with a matrix of hollow hemispheres 26 and reflector cavities 28 on the interior or vacuum side of the plate. The top plate 14 or bottom plate 18 are not limited in thickness so that the device can be adapted for use in pressure environments. Bottom Plate 18 is preferably a flat glass plate that closes the panel structure from the opposite side of the window plate 14, and provides (together with the top window plate 14 and the side walls 12) hermetic vacuum sealing.

[0036] The hemispherical cavities 26 within the top plate 14 may have dimensions of essentially any value; however hemispheres ranging from a few millimeters to a few centimeters are preferred. Each hemispherical cavity 26 forms a "Roman Dome" structure with strong "pillars" that give the panel a high mechanical stability. In higher ambient pressure applications, a rectangular, rater than a hexagonal pixel pattern, is preferred in that the rectangular pattern offers significantly stronger pillars.

[0037] It can be seen that all of the features in all of the components are open and accessible, which allows both: (i) the application of industrial glass- forming or milling techniques in the component production, and (ii) continuous production-line technology for the panel assembly.

[0038] A photocathode 30 is deposited on all hemispherical surfaces 26 of plate 14. In one embodiment, the photocathode 30 is backed up by a thin transparent conductive dielectric layer, such as ITO (Indium-Tin-Oxide), which establishes and maintains the photocathode potential. It will be seen that any known photoemissive material can be applied to the interior of the hemisphere 26 to form the photocathode 30. Alkali-antimonides materials have been shown to be suitable materials for the photocathode 30, for example. Photocathode 30 materials may also be selected based on the spectral response of the material and the wavelength of radiation to be detected.

Composite patterns of cells with alternating types of photocathode material may also be formed.

[0039] Referring also to FIG. 3, plate 14 also has reflectors 28 that are shaped to reflect the incoming light onto the nearby hemispheres 26. In the embodiment shown, the reflectors are suitably shaped cavities that are placed between the hollow hemispheres. The light that is reflected by the reflectors 28 would otherwise have fallen between the photosensitive hemispheres 26. The reflectors 28 therefore reduce dead area and provide high surface coverage. The reflective surfaces of reflector 28 may be either a naked glass surface, i.e. a total internal reflector, or a surface coated with a metallic or dielectric reflective material.

[0040] The actual shape of the reflector 28 can vary but should take into account mechanical stress due to environmental pressure. If mechanical stress is not an issue then a shape that maximizes the angles of reflection onto the hemispheres is preferred. [0041] The hemispheres 26 and the internal sections of the detector must have the air removed and the vacuum maintained within the panel. In the embodiment shown, optional "getter pumps" 32 are placed in the interior of the reflector cavities 28 as well as other areas and cavities except in the active photon detection volume of the hemispheres 26. For example, in one embodiment, getter pumps 32 are placed in the reflector cavities 28, on the anode surface of the voltage plate 16, and on the inner surface of the bottom plate 18. [0042] Getters 32 may be in the form of evaporable getters, pre-evaporated getter films (deposited in vacuum before the final panel sealing), or non- evaporable getters (NEG). In most cases, except the pre-evaporated getter films, getters may be activated (and sometimes multiply reactivated) from the outside by inductive or laser heating and do not need any electrical feedthroughs. Getter pumps are provided to eliminate residual gases and maintain a vacuum within the panel through the lifetime of the detector. [0043] In the embodiment shown, optional passages 34 between the hemispheres 26 and the reflector cavities 28 with getters pumps 32, as well as the holes 36 through the voltage plate 16 allow the residual gas to reach the getter surfaces.

[0044] The anode and cathode sides of voltage plate 16 are shown in FIG. 4A and FIG. 4B. High-voltage plate 16 is a dielectric flat plate preferably made of glass and capable of holding the operational voltage across its thickness. The plate 16 brings simultaneously both the cathode and the anode potential to every pixel in the panel. The cathode surface 38 of the high-voltage plate is the surface facing the photocathode 30 and the hollow hemispheres 26 of top plate 14. The anode surface 40 faces the bottom plate 18. The cathode surface 38 has two types of surfaces, a conductive surface and a resistive surface. [0045] In FIG. 4A, the high-voltage-transition area is a circular area underneath each hollow hemisphere which is covered with a resistive material 42 that gradually changes the electric potential from the anode potential 44 in the middle of each pixel to the cathode potential at the periphery 46 of each circular pixel. Electric contact is established between the resistive layer 42 and the conductive surface 46 that surrounds the resistive circular area. The area 46 around the circular high-voltage-transition islands is a conductive material that electrically connects the photocathodes 30 of all the individual pixels in the panel to a universal cathode potential (not shown).

[0046] The anode surface 40 shown in FIG. 4B on the opposite side of the high-voltage plate from the cathode surface 38 is covered with conductive material over its entire area in order to distribute the universal anode potential to all the pixels in the panel through the holes 48. The anode potential is transmitted through the holes 48 from the anode side 38 to the cathode side

40. Holes 48 in the high-voltage plate 16 are preferably located in the center of curvature of each hemisphere, which locally transmits the anode potential to the opposite side of the high-voltage plate in the middle of the high-voltage- transition area. [0047] Holes 48 also allow access of electron detectors as well as wires or optical cables for an electron sensor or scintillator through the high-voltage plate 16. Holes 36 enable residual gas circulation through the voltage plate 16 and access to getter pumps 32. [0048] An electron detector 50 is placed in the center of curvature of each hemisphere 26, in the middle of each pixel configured to receive photoelectrons from the photocathode 30. The electron detector 50 may be an avalanche photodiode (APD), or a PiN photodiode, or a scintillator, or any other detector, capable of detecting electrons of energies of approximately 5- 35 keV in a vacuum. Leads 52, may be wires if the electron detector 50 detects directly or a fiber optic cable if the electron detector 50 is a scintillator. [0049] In the preferred scintillator embodiment, the scintillator crystal is covered with a thin layer of Aluminum or similar conductive metal. The Aluminum coating is needed both to establish the anode potential, and to prevent light feedback from the scintillator to the photocathode. The coating may also provide a barrier against the evaporation of dopants from the scintillator, and absorption of gases from the environment by the crystal. In the scintillator embodiment of the photoelectron detector, it is preferred that holes 48 be non-orthogonal holes to allow smooth fiber transition with a large bending radius of curvature. (See FIG. 6). [0050] A readout feedthrough for the entire panel (not shown) is preferably placed on the side or the edge of the panel for exterior access, or in some other suitable place, depending on the application. A single, compact feedthrough containing output lines 52 of all the pixels 50 in a panel is preferred. The output signals are transmitted from each pixel to the feedthrough by a separate optical guide or electric conductor 52. Optical feedthroughs may be established either by direct hermetic placement of the optical guides into the panel wall, or indirectly, via transmission through a small prefabricated optical fiber coupling plate that is made as a part of the panel wall structure. Alternatively, output signals may be received by direct light transmission from the vacuum side of the panel, with or without the aid of some collimating devices (cones and lenses).

[0051] Preferably each panel is connected to a voltage source through a single pair of connectors per panel. The connector with the higher of the two potentials is connected to the anode side 38 of the high-voltage plate 16, and the lower potential connector is connected to the universal conductive surface of the cathode side 40 of the high-voltage plate 16. [0052] Turning now to FIG. 5, an alternative embodiment of the photon detector according to the present invention is shown. In this embodiment, the panel is fully symmetric, with identical dome-structured window plates on both sides, which provides ideal mechanical symmetry for applications in high- pressure environments. This is the preferred embodiment for applications requiring a high level of mechanical stability. In this configuration, either both sides of the panel may be active for photon detection, or one side may be passive, without a photocathode and readout. The passive side serves to establish mechanical symmetry, but it may also host additional getter pumps in that embodiment. [0053] In FIG. 5, the panel 100 is composed of a first exterior plate 102 and a second exterior plate 104. These exterior plates are separated by a first voltage plate 106 and a second voltage plate 108 and blocks 110. The cathode plane 112 of the first and second voltage plates are oriented under the hemispheres 114 of the vacuum side of the first exterior plate 102 and the second exterior plate 104. The anode planes 116 of the first and second voltage plates 106, 108 face each other and are separated by blocks 110. [0054] A photocathode 118 is disposed within each the hemisphere 114 of the vacuum side of the first and second exterior plates 102,104 and electrically coupled with the cathode side of the voltage plates 106,108. The anode potential is transmitted through a hole centered below the hemisphere and surrounded by a circular shaped electrically resistive material on the cathode side of each voltage plate. The high voltage plates 106, 108 simultaneously supply both the anode and the cathode potential to all of the pixels in the panel, and provide a continuous voltage divider within each pixel. The high voltage plates 106, 108 preferably have the same structure as shown in detail in FIG. 4A and FIG. 4B. [0055] Optical reflectors 120 are preferably formed from cavities in the exterior window plate 102 or 104 and are placed between the hemispheres 114 to reflect photons that would otherwise be lost in a dead area to the hemispheres 114 and provide nearly 100% coverage. Although cavities are preferred, other imbedded reflectors could also be used to reflect photons.

[0056] In the embodiment shown in FIG. 5 and FIG. 6, the optical reflector cavities 120 optionally include vacuum "getter" pumps 122 that help to remove residual gasses when the chamber, formed by sealing the exterior plates 102,104 and voltage plates 106, 108 and blocks 110, is evacuated. Residual gas or a compromised vacuum will result in inefficient or ineffective detection.

Some getter pumps may also be periodically recharged or reactivated during the life of the detector. [0057] Electron detectors 124 are disposed in center holes of the voltage plates 106, 108 forming a readout network consisting of large diameter optical fibers coupled to scintillators. Alternatively, photoelectrons can be directly detected by PiN photodiodes or avalanche photodiodes (APD) or similar detector 124 as shown in FIG. 5.

[0058] In the embodiment shown in FIG. 6, the electron detectors 124 are scintillators and the optical fibers 126 run through the space between the voltage plates 106, 108 created by blocks 110. The optical fiber 126 from each scintillator is preferably coupled to a Geiger-mode avalanche photodiode (G-APD) or to a G-APD matrix in the case of a large device. The secondary sensors are placed outside of the vacuum enclosure in one embodiment. This results in a particularly simple and robust construction, without any electronic components enclosed in the vacuum panel dramatically simplifying thermal processing and reducing cross-contamination between the photocathode and the semiconductor sensor. It also allows significant readout upgrades and modification of the actual panel resolution at any time, by remapping of the readout cells. [0059] Photoelectrons originating from the hemispherical photocathode 118 are accelerated within the hemispherical vacuum tube 114 to an energy of approximately 5 to approximately 35 keV, and focused to a small scintillator surface 124. The slightly amplified light signal from the scintillator is detected in a Geiger-mode APD array, placed outside the vacuum enclosure, on the other side of the fiber plate window that preserves the image configuration. [0060] It can be seen that the double-sided panel comprises a fully symmetric multi-dome structure, ideal for applications in high-pressure environments. It may be sensitive to light on both sides, which is potentially very useful in many applications. If only one side of the panel is designed to be sensitive, the other side may be passive, but still shaped in the same way in order to play its important pressure-supporting role.

[0061] In use, the incoming photons (ph) hit the hemispherical photocathodes

118 either directly, or after reflection from the reflector cavities 120 as seen in FIG. 7. A photoelectron (e) released by the photon (ph) from the photocathode 118 is focused and accelerated to a small electron detector 124 (in the present case, a fiber-coupled scintillator). The amplified secondary light signal from the scintillator 124 travels via fiber 126 out of the vacuum enclosure, to a Geiger- mode APD (not shown), where the signal is read-out and strongly amplified. [0062] One significant aspect of the invention is the effective amplification of the photons. However, light amplification should not be confused with the final signal amplification. It can be seen that the structure of the device shown in

FIG. 1 through FIG. 7, will amplify the detected photons. [0063] Referring also to FIG. 9, detection 130 of ambient photons can be improved by converting 132 the photons received by the apparatus to photoelectrons; focusing the photoelectrons on a scintillator 134 that creates many photons that can be transferred by an optical cable or directly detected by photodiodes or other detector, and detecting 136 the amplified light photon detector such as a photodiode. Electrical signals generated by the detector can be amplified if necessary at block 138. [0064] In general, the detected light is effectively amplified by the detection apparatus shown in FIG. 7 as follows:

[0065] (a) Photocathode 118 converts a received photon to an electron. [0066] (b) An electron "lens" accelerates and focuses all the photoelectrons to a very small focal area covered with a scintillator 124.

[0067] (c) The scintillator 124 emits many photons (hundreds to thousands) upon the electron impact. The scintillator is coated on its front side by a very thin layer of aluminum that acts both as a shield against light feedback to the photocathode, and as electrical conductor to set up the large potential between the cathode and the scintillator.

[0068] (d) These photons from the scintillator 124 are transported via optical coupling 126 from the scintillator 124 to a Geiger-mode avalanche photodiode (G-APD), preferably placed outside the vacuum enclosure.

[0069] As a result of strong electron focusing, the entire photosensitive area effectively maps to a very small active G-APD readout area (-1000 times smaller than the panel area). This strong, more than 1000-fold concentration of information from the irreducibly large photocathode area to the readout device, effectively bypasses Liouville's theorem by replacing a photon by a photoelectron, i.e. a charged particle that can be acted upon by an electrostatic field. To put this number into perspective, if it is assumed that the light-receiving area in the radiation detector is 1 ,500 m², a 1500-fold concentration would lead to an integral photoelectron readout area of only 1 m². Accordingly, the light amplifier receives light on its entire front area and subsequently amplifies and concentrates the signal to the scintillator-optical fiber combination.

[0070] Another important function is the amplification of the light signal. Use of a scintillator provides a light pulse that is significantly stronger than the intrinsic noise of the G-APD, i.e. at least 30 photons, when measured outside of the vacuum enclosure. This may be achieved by using a high light yield scintillator, and at least 15 kV of electron acceleration potential. The G-APD will then itself provide the real signal amplification, typically a ~10⁶ gain. Note that the light amplifier presents an ideal match between its vacuum part and the G-APD. The two main disadvantages of G-APDs - the high level of intrinsic noise, and the small G-APD pixel size, are fully compensated by light amplification, and electron concentration, respectively.

[0071] The placement of the G-APDs outside the vacuum enclosure leads to a simple and robust construction without any electronic components enclosed within the vacuum panel. This also prevents chemical cross-contamination between the photocathode and the silicon sensor. Since G-APDs are not an integral part of the flat panel, they could be upgraded at any time. More important, that would allow one to modify the position resolution of the panel, merely through remapping of G-APD pixels to a different number of primary pixels. This would allow production of a standard panel for all applications that require different resolutions.

[0072] The hemispherical photocathode configuration has many advantages over existing detectors. For example, the open hemispherical array architecture and the flat global structure are amenable to mass-production in continuous production lines techniques (like TV panels) including CMOS technology. The open architecture allows the deposition of the photocathode, the conductive layers, and the getter layers from one side of the window plate in a production line (in contrast to the internal processing of PMTs, where the photocathode and the getter surfaces are created by deposition within the tube, using evaporators that are enclosed in the tube and remain there). [0073] A second advantage is durability and safety against a catastrophic burnout with accidental exposures to strong light, since G-APDs may be safely exposed to ambient light while fully powered. Furthermore, the device is virtually insensitive to the earth magnetic field, both because the primary device, the vacuum Light Amplifier, is nearly unaffected by the earth magnetic field due to its high acceleration voltage, and G-APDs are completely insensitive to (even extremely large) magnetic fields. Flat panels also are perfectly suitable for high pressure exposure (deep ocean) - like in a classical dome architecture, the flat-to-hemispherical transition is ideal, particularly in a fully symmetric configuration. Virtually no danger from implosion, or massive chain-reaction implosion like in Super Kamiokande experiment in Japan, where the implosion of one single spherical PMT at the bottom of the 30 m deep-water container created a shock wave that destroyed around 7000 PMTs (practically all PMTS). No buoyancy in liquids (water or liquid scintillators) occurs.

[0074] A third advantage is a reduction in scale compared with the typical photomultiplier tube (PMT) permitting large arrays. The photoelectron detector

(APD, scintillator and G-APD), and its connection (cables, optical fibers, feedthroughs) may be very small compared to a conventional dynode column. Consequently, each pixel in the panel comprises one scintillator, coupled to an optical fiber, and one G-APD cell (1 x1 mm²) located outside the vacuum enclosure (or just one APD inside the vacuum enclosure) rather than a 10-12- stage dynode system with an electrical feedthrough for each dynode stage, support structure in the vacuum, multi-pin connectors outside, and a HV resistive divider for each conventional PMT. Timing jitter due to nearly perfect electron focusing cannot be minimized in large area PMT's with classical dynodes, and a low acceleration potential in the first stage.

[0075] The apparatus also has a low storage and transportation volume and the panels are easy to mount and hold in large detection applications and experiments. The cables for the electron detector readout, or optical fibers in the Light Amplifier configuration, are packed inside the panel and read out on the side. There is one small feedthrough for the entire panel and the readout coupling, either optical or electrical, is very small. Closely tiled flat panels may completely cover very large areas with very little dead area.

[0076] Another advantage is superior performance. The structure provides for nearly 100% photoelectron collection efficiency which is normally a big problem in large area PMT's with secondary dynodes that results in lower and non-uniform photoelectron collection efficiency (at most -70% in large area PMTs). There is greater than 95% coverage area with virtually no dead areas on the panel. Single-photon resolution, superb in the light amplifier configuration with use of G-APDs. The very strong signal amplification (several times 10⁶) eliminates the need for expensive electromagnetic shielding or expensive preamplifiers. [0077] The flat configuration is also good for production, spherical for time resolution, high voltage hold off, angular acceptance, uniform photocathode deposition, and collection efficiency. The evacuated hemispherical configuration, in conjunction with the continuous high-voltage divider, leads to spherical equipotential surfaces, without any edges or abrupt changes. No insulators are present in the hemispherical cavity, only conductors and resistive surfaces. Insulators may charge up to the point where a flashover happens, while conductors and resistors eventually conduct away the surplus electric charge. [0078] The invention may be better understood with reference to the accompanying examples, which are intended for purposes of illustration only and should not be construed as in any sense limiting the scope of the present invention as defined in the claims appended hereto. [0079] Example [0080] In order to demonstrate the light amplification function, a large single hemispherical vacuum enclosure was used rather than a flat panel with multiple cells. The enclosure used was a spherical vacuum tube made of glass, 370 mm in diameter, with a bi-alkali photocathode deposited on the inside surface, and a flat 18 mm diameter focal area covered with a thin scintillator layer (blue light emitting scintillator Y₂SiO₅(Ce)), which is covered on top with a thin layer of Aluminum to prevent light feedback to the photocathode, and to establish the anode electrical potential. [0081] However, the prototype had a 5 mm thick window which offered rather poor coupling efficiency, but still sufficiently strong for our proof-of-concept tests. The one square millimeter G-APD was estimated to have received less than 0.7% of the photons emitted by the scintillator, with an average input light signal of five photons. In spite of the weak amplification and strong photon dispersion, pulses corresponding to single photoelectrons were detected and individual photoelectrons were resolved within multi-photon events and groups of pulse amplitudes within well-defined multi-photon peaks.

[0082] As illustrated in the schematic of FIG. 8, one preferred scintillator design 150 within a single pixel vacuum tube includes a fiber plate 152 to couple the scintillator 154 to the G-APD array 156, both for an efficient coupling and for precise imaging. Photoelectrons (e) originating from the hemispherical photocathode (not shown) are accelerated within the hemispherical vacuum tube, preferably to an energy of -25 keV, and focused to a small scintillator surface 154. A thin layer of a conductive metal 158 is preferably applied to the scintillator 154. The slightly amplified light signal from the scintillator is detected in a Geiger-mode APD array 156, placed outside the vacuum enclosure, on the other side of the fiber plate 152 window that preserves the image configuration.

[0083] Finally, it will be seen that the single tube embodiment can have a large photocathode area greater than that covered by a hemisphere. In one embodiment, the cathode area covers approximately 270 degrees or more with a corresponding spherical shaped scintillator disposed in the center of the tube. It can be seen that the photocathode can be applied to a spherical enclosure and receive and detect photons arriving at the detector from almost any direction in this embodiment.

[0084] This single-pixel device would provide almost full angular acceptance, a very large photosensitive area, perfect timing resolution, and 100% photoelectron collection efficiency. Its shape would be ideally fitted for deep- sea or ice neutrino telescopes. Using a matrix of G-APDs of fine granulation will not only significantly improve the position resolution, but also reduce the effective background noise. In particular, this will allow for a major improvement in deep-sea neutrino telescopes with the ability to discriminate weak optical light flashes from the ubiquitous background from ⁴⁰K decays and from the strong intrinsic PMT noise.

[0085] Accordingly, the panels of the present invention may be used in the mass-production of new, inexpensive, medical whole-body scanning devices for widespread use in functional imaging and diagnostics such as the gamma camera, SPECT scanners and PET (Positron Emission Tomography).

[0086] Other applications may include devices for the massive screening of cargo of all sizes and kinds for nuclear materials, passive gamma-ray detection, passive neutron detection, detection of gamma rays following neutron activation, detection of neutrons following induced or spontaneous fission in fissionable materials, detection of gamma-rays following induced or spontaneous fission in fissionable materials, gamma-ray Imaging of containers and vessels, gamma-ray imaging from a large distance, e.g. in ports, or from a large helicopter, truck, or ship. Although the description above contains many details, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention.

Therefore, it will be appreciated that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the present invention is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean "one and only one" unless explicitly so stated, but rather "one or more." All structural, chemical, and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U. S. C. 1 12, sixth paragraph, unless the element is expressly recited using the phrase "means for."

Claims

CLAIMS What is claimed is:

1 . A photon detector, comprising: a monolithic flat-panel vacuum enclosure having a flat window plate with a vacuum side; a matrix of hollow hemispherical-shaped cells housed in said enclosure; said cells cast into the vacuum side of the flat window plate; a photocathode within said cells; and means for detecting photoelectrons.

2. A photon detector as recited in claim 1 , further comprising: reflectors positioned between said cells; wherein said reflectors direct light from locations between the cells to the hemispherical cells.

3. A photon detector as recited in claim 1 : wherein said means for detecting photoelectrons comprises a PiN photodiode.

4. A photon detector as recited in claim 1 : wherein said means for detecting photoelectrons comprises an avalanche photodiode.

5. A photon detector as recited in claim 1 , wherein said means for detecting photoelectrons comprises: a scintillator; a fiber optic cable coupled to said scintillator; and a Geiger-mode avalanche photodiode coupled with said optic cable.

6. A photon detector as recited in claim 1 , further comprising: a plurality of getter pumps within said vacuum enclosure.

7. A photon detector as recited in claim 6, wherein said getter pump comprises a pre-evaporated film.

8. A photon detector as recited in claim 6, wherein said getter pump comprises a nonevaporable film multiply activated by external heating.

9. A photon detector as recited in claim 1 , further comprising: a conductive transparent dielectric layer on the photocathode configured for maintaining photocathode electric potential.

10. A photon detector, comprising: a vacuum enclosure; said vacuum enclosure having a flat window plate; said flat window plate having a vacuum side; said flat window plate having a matrix of hollow hemispherical-shaped cells cast into said vacuum side; a photocathode within said cells; a voltage plate; said voltage plate having a thickness; said voltage plate configured for holding an operational voltage across its thickness; and a photoelectron detector at the center of each cell.

1 1 . A photon detector as recited in claim 10, wherein said voltage plate comprises: a dielectric plate with a plurality of openings and a cathode surface and an anode surface; said anode surface extending through said plurality of openings of said dielectric plate; said cathode surface having a resistor layer extending radially from said openings; wherein an electric potential from said anode to said cathode is gradually changed.

12. A photon detector as recited in claim 10, further comprising: reflectors positioned between said cells; wherein said reflectors direct light from locations between the cells to the cells.

13. A photon detector as recited in claim 10: wherein said photoelectron detector comprises a PiN photodiode.

14. A photon detector as recited in claim 10: wherein said photoelectron detector comprises an avalanche photodiode.

15. A photon detector as recited in claim 10, wherein said photoelectron detector comprises: a scintillator; a fiber optic cable coupled to said scintillator; and a Geiger-mode avalanche photodiode coupled with said optic cable.

16. A photon detector as recited in claim 15, wherein said scintillator further comprises: a layer of a conductive metal electrically coupled to an anode.

17. A photon detector as recited in claim 16, wherein said conductive metal comprises aluminum.

18. A photon detector as recited in claim 10, further comprising: a plurality of getter pumps within said vacuum enclosure.

19. A photon detector as recited in claim 10, further comprising: a conductive transparent dielectric layer on the photocathode maintaining photocathode electric potential.

20. A photon detector, comprising: a vacuum enclosure; said vacuum enclosure having a first flat window plate; said first flat window plate having a vacuum side; said first window plate having a matrix of hollow hemispherical-shaped cells cast into said vacuum side of said first window plate; said vacuum enclosure having a second flat window plate; said second flat window plate having a vacuum side; said second window plate having a matrix of hollow hemispherical-shaped cells cast into the vacuum side of said second window plate; a photocathode within said cells; a first voltage plate; said first voltage plate having a thickness; said first voltage plate configured for holding an operational voltage across its thickness; a second voltage plate; said second voltage plate having a thickness; said second voltage plate configured for holding an operational voltage across its thickness; and a photoelectron detector at the center of each cell.

21 . A photon detector as recited in claim 20, wherein said first and second voltage plates comprise: a dielectric plate with a plurality of openings and a cathode surface and an anode surface; said anode surface extends through said plurality of openings of said dielectric plate; said cathode surface having a resistor layer extending radially from said openings; wherein the electric potential from anode to the cathode is gradually changed.

22. A photon detector as recited in claim 20, further comprising: reflectors positioned between said cells; wherein said reflectors direct light from locations between the cells to the cells.

23. A photon detector as recited in claim 20: wherein said photoelectron detector comprises a PiN photodiode.

24. A photon detector as recited in claim 20: wherein said photoelectron detector comprises an avalanche photodiode.

25. A photon detector as recited in claim 20, wherein said photoelectron detector comprises: a scintillator; a fiber optic cable coupled to said scintillator; and a Geiger-mode avalanche photodiode coupled to said optic cable.

26. A photon detector as recited in claim 25, wherein said scintillator comprises: a layer of a conductive metal electrically coupled to an anode.

27. A photon detector as recited in claim 26, wherein said conductive metal comprises aluminum.

28. A photon detector as recited in claim 20, further comprising: a plurality of getter pumps within said vacuum enclosure.

29. A photon detector as recited in claim 20, further comprising: a conductive transparent dielectric layer on the photocathode configured for maintaining photocathode electric potential.

30. A method for detecting photons, comprising: exposing a photocathode to photons to produce photoelectrons within a vacuum chamber; focusing said photoelectrons to impact a scintillator; directing photons produced by the impact of said photoelectrons with said scintillator to a photon detector.

31. A method as recited in claim 30, further comprising: accelerating said photoelectrons to impact said scintillator.

32. A method as recited in claim 30, further comprising: amplifying an electronic signal produced by said photon detector.

33. A method as recited in claim 30, further comprising: directing photons produced by said scintillator through an optical cable to a photon detector outside of the vacuum chamber.

34. A method as recited in claim 30, wherein said photocathode comprises a hemispherical shape and the scintillator has a hemispherical shape.

35. A method as recited in claim 30, wherein said photon detector comprises a plurality of Geiger-mode avalanche photodiodes.