1. Technical Field
The present invention relates generally to photomultiplier tubes having a photosensitive cathode ("photocathode") for: i) absorbing light energy (i.e., photon energy); ii) using the absorbed photon energy to cause emission of electrons from a photocathode; iii) multiplying the number of electrons; and iv), outputting a signal proportional to, but greatly larger than, the magnitude of the absorbed photon energy from the light event. More particularly, the present invention relates to a generally toroidal or annular photon detector/electron multiplier having a 360.degree. surround, semi-transparent, cylindrical photocathode deposited on, or positioned adjacent, the vacuum side of the inner wall of a generally toroidal vacuum tube having an inner annular or cylindrical wall formed of thin-walled glass or other suitable thin-walled light-transmissive material and defining a centrally located coaxial detection chamber and an annular evacuated envelope for housing: i) the photocathode; ii) the particular electron multiplier structure employed, including focusing electrodes if desirable; and iii), a plurality of anodes and/or other electron multiplier output terminals.
In carrying out the invention, the photomultiplier comprises a multiple-section device having a single continuous cylindrical photocathode deposited on, or positioned adjacent, the vacuum side of the inner annular wall of a generally toroidal evacuated envelope, with the annular space within the torus-shaped envelope surrounding the photocathode being subdivided into a plurality of separate, adjacent, arcuate sections each housing an electron multiplier including an output anode or other output terminal for processing electrons emitted from a specific subtended arc of the cylindrical photocathode. In one form of the invention, the annular or generally toroidal envelope surrounding the cylindrical photocathode is subdivided into a preferably even-numbered plurality of adjacent arcuate sections so as to facilitate use of coincidence circuitry requiring time-coincident detection of light events in different sections of the photomultiplier tube in order to eliminate certain spurious signals from the photomultiplier tube which are substantially devoid of meaning and are unwanted such, for example, as thermal electron emissions from the photocathode.
The invention will herein be initially described in connection with liquid scintillation spectrometers--viz., scientific measuring instruments well known to persons skilled in the art which are used to detect scintillations occurring in samples containing one or more radioactive isotopes and a scintillation material which produces light scintillations when struck by radiation(s) emanating from the isotope(s)--since that is an environment wherein head-on photomultipliers have, for many years, found particularly advantageous use.
However, as the ensuing description proceeds, it will become apparent to persons skilled in the art that the invention is not limited to use with conventional liquid scintillation spectrometers but, rather, it finds equally advantageous use in a wide range of other environments including, merely by way of example and not by way of limitation, the detection and measurement of photons emanating from: i) samples external to the detection chamber which contain a liquid, crystal or plastic scintillator such, for example, as a beta emitter or other suitable radioactive isotope(s); ii) samples external to the detection chamber containing a luminescent material such, for example, as a fluorescent or a phosphorescent material used in luminescent spectroscopy and the like; and/or iii), light sources external to the immediate environment of the photomultiplier such as might be detected during astronomical observations or observation of other external sources including, but not limited to, samples or specimens being viewed through microscopes, telescopes, light collimators, or the like.
Moreover, because of the unique configuration of the photomultiplier of the present invention wherein different discrete adjacent sections of a single continuous cylindrical photocathode are associated with their own individual electron multiplier/anode (or other output terminal) combinations, all of such environments can employ photon energy detection and processing taking advantage of the benefits of coincidence counting where desirable.
2. Background Art
The prior art, including both the patented and non-patented art, is replete with publications relating in one form or another to photomultiplier tubes and uses thereof wherein incident light impinging upon a photocathode is absorbed thereby, causing emission of one or more primary electrons proportional to the number of impinging incident light photons, which primary electron(s) is(are) then multiplied using any of a wide variety of different types of conventional electron multipliers so as to produce an output signal which is proportional to the incident light energy; but, which is greatly amplified with respect thereto. Such conventional photomultiplier tubes have heretofore generally all employed a photocathode in either a head-on or a side-on configuration--i.e., in a head-on photomultiplier tube, incident light impinges on a photocathode located at one end of a generally cylindrical evacuated envelope with the photocathode material being deposited on, or positioned adjacent, the vacuum side of the light-transmissive end face of the envelope which lies in a flat or rounded plane intersecting the longitudinal axis of the envelope at substantially right angles thereto; whereas, in a side-on photomultiplier tube, the photocathode generally extends longitudinally along the internal light-transmissive sidewall of the evacuated envelope and parallel to the envelope's longitudinal axis.
Merely by way of example, in the field of liquid scintillation spectrometers, the head-on type of photomultiplier tube has, for four or more decades, been the photomultiplier tube of choice. Thus, U.S. Pat. Nos. 3,188,468-Packard and 4,002,909-Packard et al, both of which have been assigned to Packard Instrument Company, Inc. of Downers Grove, Ill., are representative of numerous patents relating to liquid scintillation spectrometers; and, both disclose a conventional liquid scintillation spectrometer of the type employing a pair of spaced apart, flat-faced, head-on photomultiplier tubes disposed on diametrically opposite sides of a cylindrical sample chamber into which discrete samples are inserted. Such discrete samples are generally contained in a vial, although continuous flow-through systems are well known and commonly employed. To improve the collection efficiency, the walls defining the sample chamber between the two photomultiplier tubes are generally coated or otherwise polished or mirrored so as to provide highly reflective surfaces, thereby attempting to reflect to the photocathodes some of the light energy from the sample which is directed in directions other than towards the photocathodes.
The exemplary and completely conventional equipment described in the foregoing Packard and Packard et al patents generally includes: i) an elevator assembly for shifting samples into and out of the sample chamber; ii) a lead shield to protect against external radiation; iii) light shields to exclude all light from sources other than the sample; iv) a sample transfer mechanism to deliver samples to, and/or remove samples from, the elevator assembly in seriatim order; and v), suitable and generally conventional circuitry for processing the signals output from the photomultiplier tube anodes. Generally, such circuitry includes: a) coincidence circuitry for excluding signals not detected by both photomultipliers--e.g., for excluding signals resulting from random thermal electron emissions from the photocathodes and/or other spurious random noise pulses; b) discriminators for passing only signals within a desired band of interest; c) gates; d) scalers; and e), similar electronic components.
Those interested in a more detailed description of liquid scintillation spectrometers and/or head-on photomultiplier tubes of the type commonly used therein are referred to the aforesaid Packard and Packard et al patents, as well as to an article entitled "Instrumentation For Internal Sample Liquid Scintillation Counting" authored by Lyle E. Packard and appearing at pages 50 through 66 of LIQUID SCINTILLATION COUNTING, Proceedings of a Conference held at Northwestern University, Aug. 20-22, 1957, edited by Carlos G. Bell, Jr. and F. Newton Hayes, and published by Pergamon Press (1958).
Because a conventional photomultiplier tube is an evacuated tube, severe constraints have been placed on the configuration of the tube so as to preclude implosion. Such constraints have, for example, required either that the light-transmissive face of the tube--i.e., the tube end in the case of a head-on photomultiplier tube-be of rounded or generally semi-spherical shape (as opposed to flat) or, alternatively, that the material of the envelope's light-transmissive face be relatively thick. These constraints have created problems with respect to collection geometry insofar as rounded tube ends are concerned; and, moreover, they have increased light absorption problems and increased unwanted light from Cerenkov radiation in the case of tubes having relatively thick faces. With the advent of envelopes having quartz or low potassium glass end walls, these problems were somewhat alleviated; but, nonetheless, absorption problems and problems with Cerenkov radiation emissions and resultant poor signal-to-noise ratios have continued to be encountered. And, of course, the fact that two photomultipliers directly view the sample only on opposite sides thereof has always created a collection problem in respect of light originally directed in other directions. Thus, the need for a photomultiplier capable of viewing a sample from all side aspects simultaneously has continued.
A) Side-on Photomultiplier Tubes
U.S. Pat. No. 4,347,458-Tomasetti et al assigned to RCA Corporation is of interest for its disclosure of a typical side-on photomultiplier tube of the type employing a photocathode generally parallel to the axis of the evacuated tube, a circular-cage arrangement of dynodes, and an output anode. In this type of conventional photomultiplier tube, the photocathode is generally opaque wherein incident light impinging on the photocathode is absorbed thereby, with the absorbed photons causing emission of primary electrons from the photocathode which are attracted by the first stage dynode. Each primary electron impinging on the first stage dynode produces emission of multiple secondary electrons from the first stage dynode which are then attracted to the next dynode stage where the electron multiplication process is repeated. More specifically, the photocathode, successive dynode stages and anode are each maintained at progressively higher voltage levels so as to attract and accelerate all electrons emitted from each preceding stage during the electron multiplication process.
B) Head-on Photomultiplier Tubes
As previously indicated, a conventional head-on photomultiplier tube--viz., a tube that may be differentiated from a side-on photomultiplier tube by, inter alia, having a photocathode adjacent the end of the evacuated envelope remote from the anode--is, and has for a long time been, the photomultiplier tube of choice in most conventional scintillation spectrometers. Generally the photosensitive cathode material is deposited on, or positioned adjacent, the inner face or vacuum side of the tube's envelope at one light-transmissive end of the envelope; and, therefore, it can be differentiated from the photocathode in a side-on photomultiplier tube by constituting a transmission-type device--e.g., incident light impinges on the non-vacuum side of the photocathode and is absorbed thereby, causing emission of primary electrons from the vacuum side of the photocathode which are then attracted towards the downstream dynode chain and cause the emission of multiple secondary electrons from each dynode stage for each impinging electron--as contrasted with a side-on photomultiplier tube where the incident light impinges on one face of the photocathode, is absorbed thereby, and primary electrons are emitted from that face. Such head-on photomultiplier tubes are commonly available in any of a variety of different conventional configurations.
U.S. Pat. No. 2,234,801 issued in 1941 to Paul Goorlich discloses an early version of a head-on photomultiplier tube employing a transparent flat-faced photocathode.
U.S. Pat. No. 5,363,014-Nakamura, assigned to Hamamatsu Photonics K.K., discloses what is generally known as a head-on photomultiplier tube having a linear-focused-type dynode structure characterized by its extremely fast response time. Such head-on photomultiplier tubes are commonly employed in those instances where time resolution and pulse linearity are significant considerations.
Watson U.S. Pat. No. 3,415,990 and Morales U.S. Pat. No. 4,143,291 are of interest for their disclosures of venetian blind-type photomultiplier tubes. In the Watson patent, the venetian blind dynode structure is quite conventional, comprising a plurality of dynode stages disposed in an array of stacked planar dynode elements closely simulating the structure of a venetian blind, with each such dynode stage being maintained at a progressively higher voltage. In the Morales patent, on the other hand, the dynode structure is modified with the dynodes being disposed in circular arrays. Generally, venetian blind-type dynode structures are not employed where fast time response is an important consideration.
Yet another type of conventional head-on photomultiplier tube is one employing a mesh-type dynode structure wherein a series of mesh-type dynodes are stacked in closely spaced proximity. Such a photomultiplier tube is disclosed in Kimura et al U.S. Pat. No. 4,937,506 assigned to Hamamatsu Photonics Kabushiki Kiasha. Such mesh-type dynodes are characterized by their compactness and are, therefore, highly desirable where space is a limitation. Moreover, such mesh-type dynode structures are characterized by high position sensitive capabilities resulting in excellent spatial resolution. However, while the characteristic of good spatial resolution is not considered to be of primary importance to the present invention which is particularly concerned with obtaining useable output voltage pulses of maximum amplitude, spatial resolution can, in some instances, be a desirable characteristic even when using the unique photomultiplier tube configuration of the present invention.
Another type of dynode structure commonly found in conventional head-on photomultiplier tubes is the box-and-grid type structure which, although commonly used because of its uniformity and simple dynode design, is, nevertheless, generally not acceptable where fast time response is a significant consideration.
Kyushima U.S. Pat. No. 5,180,943 assigned to Hamamatsu Photonics K.K. is of interest for its disclosure of a head-on photomultiplier tube employing a combination of a venetian blind-type dynode structure interleaved with a mesh-type dynode structure. A plurality of anodes are provided to insure improved spatial resolution.
C) Microchannel Plates ("MCP")
During the 1940's and/or 1950's, a somewhat different type of electron multiplier design was developed--a design which has come to be known as a microchannel plate ("MCP") and which is most notably, but not exclusively, employed in night vision devices. One fairly early patent relating to such an electron multiplier is Manley et al U.S. Pat. No. 3,260,876 assigned to North American Philips Company, Inc.--a patent which discloses an electron multiplier comprising a body formed of glass through which a plurality of generally parallel, spaced apart passages are formed. The front and rear faces of the glass body are provided with conductive coatings respectively coupled to first and second high voltage sources wherein the second source is at a higher voltage level than the first source, while the passages are coated with a suitable electron-emissive material of the type commonly employed with conventional dynodes. As a consequence, an exciting primary electron is attracted towards the front face of the glass body; and, when it impinges against a coated wall at or near the front end of a given passage, multiple secondary electrons are emitted which, in turn, are attracted to a downstream portion of the coated wall and produce still more secondary electrons for each impinging electron. In short, successive downstream portions of the passage or channel structure function as successive dynode stages in a conventional dynode chain, resulting in electron multiplication.
Other patents of interest pertaining to MCPs are Yin U.S. Pat. No. 4,142,101, Saito et al U.S. Pat. No. 4,780,395 and Beauvais et al U.S. Pat. No. 5,319,189. Yin discloses a low intensity x-ray and gamma-ray imaging device employing a fiber optic plate and photocathode for converting light photons to electrons which are amplified by an MCP. In the Yin imaging device, the amplified output from the MCP is then reconverted to photon energy by an output phosphor. Saito et al is of interest for its disclosure of a microchannel plate having a glass substrate and a plurality of parallel microchannels formed therein which are disposed at an angle to the longitudinal axis passing through the MCP, as well as a method of manufacture thereof. Beauvais et al discloses an x-ray image intensifier having a scintillator screen and photocathode positioned on the front face of an MCP.
It will be understood by persons skilled in the art that MCPs are commonly provided in a cylindrical disk-shaped form having a disk diameter ranging from on the order of about 18 millimeters ("mm") or somewhat less up to about 50 mm or more; and, wherein each disk ranges from approximately 0.5 mm to about 1.0 mm in thickness. However, as those skilled in the art will appreciate, MCPs are also available as off-the-shelf items having other than a circular disk-shaped configuration such, merely by way of example, as rectilinear or other shapes. Each microchannel will generally range from about 12 microns in diameter to about 20 microns in diameter; and, therefore, the length-to-diameter ratio of the microchannels will generally be on the order of about 40. Dependent upon the effective area of the disk, such MCP disks can have upwards of a million or more microchannels formed therein with each microchannel functioning as an electron multiplier.
D) Apertured Plate Electron Multipliers
A variation of the conventional microchannel plate design disclosed in the foregoing Yin, Saito et al and Beauvais et al patents comprises an apertured plate electron multiplier such as disclosed in Eschard U.S. Pat. Nos. 4,649,314 and 4,806,827, and in Boutot et al U.S. Pat. No. 5,043,628. Such designs generally comprise a series of transverse, spaced apart, parallel plates having "multiplier holes" formed therein, with each successive plate being maintained at a progressively higher voltage level as disclosed in the aforesaid Eschard patents. In the Boutot et al patent, two spaced apart apertured plates are employed in a head-on photomultiplier in combination with a conventional electron multiplier structure of the linear-focused variety.
E) Well-Type Radiation Counters
Luitwieler, Jr. et al U.S. Pat. No. 3,859,528, although disclosing a sample counting apparatus for detecting gamma radiation while employing a single head-on photomultiplier tube, is of interest primarily for its disclosure of a well-type counter employing sodium iodide (thallium activated) [NaI(T1)] crystals defining a cylindrical scintillating crystal well for reception of a gamma emitter. In other words, Luitweiler, Jr. et al, rather than providing a cylindrical photocathode to produce a 360.degree. surround device for using absorbed light photons to cause emission of electrons from the photocathode, contemplate a 360.degree. surround crystal formed of scintillating material for generating scintillations which are then detected by a single, flat-faced, head-on photomultiplier tube. A somewhat similar arrangement is disclosed in Kalish U.S. Pat. No. 3,944,832, which also provides a pair of sodium iodide (thallium activated) [NaI(T1)] crystals defining a central well for receiving a sample such as a sample containing a liquid scintillator and a beta emitter along with a gamma emitter. The well-defining crystals are photo-optically coupled to respective ones of a pair of conventional, spaced apart, flat-faced, head-on photomultiplier tubes for conveying scintillations generated in the sample, as well as in the crystals, to the photocathodes of the photomultiplier tubes.
Yet another well-type detector, here comprising a Geiger counter, is disclosed in Rogers et al U.S. Pat. No. 4,420,689. In this device, inner and outer cylindrical cathodes--not photocathodes--are positioned concentrically about a vertical axis; and, a plurality of anodes are positioned between the two concentric cathodes. The anode/cathode assembly is then positioned within a housing containing a conversion gas; and, a radioactive sample comprising a gamma emitter is positioned within the well defined by the innermost cathode with the gamma radiation interacting with the conversion gas to produce free electrons.
F) Hybrid Photodiode Electron Multiplier Tubes
Another conventional approach to electron multiplication in photomultiplier tubes known to persons skilled in the art for the past twenty years or more is the hybrid photomultiplier tube or "HPMT", also known scientifically as a "hybrid photodiode". Such photon detector/electron multipliers are described in a paper entitled "The DEP Hybrid Photomultiplier Tube" presented by L. Boskma, R. Glazenborg and R. Schomaker in the Proceedings of the 5th International Conference on Calorimetry at Brookhaven, N.Y. (September, 1994); and, are commercially available from Delft Electronische Producten (DEP) of Roden, Holland. The hybrid photodiode or HPMT basically comprises a vacuum tube having a photocathode spaced slightly from a silicon PIN diode. Incident light impinging upon the photocathode is absorbed thereby, with the absorbed photons causing emission of primary electrons in a conventional manner. The primary electrons are then accelerated towards the PIN diode, bombard the diode, and generate a plurality of electron-hole-pairs--typically, 3,500 electron-hole-pairs per primary electron at a photocathode voltage of -15 kV. Consequently, upon reverse biasing of the PIN diode, the electron-hole-pairs cause an electric current to flow which is then further amplified. The hybrid photodiode is characterized by its compactness, fast time response and excellent photo-electron resolution.
G) Prior Art of Miscellaneous Interest
Ehrfeld et al U.S. Pat. No. 4,990,827, is of interest for its disclosure of a micro secondary electron multiplier employing discrete dynodes which are microstructured and applied to an insulating substrate plate. In one of the disclosed embodiments, the micro secondary electron arrays are mounted on a flat annular base plate having a pair of sector-shaped arrays of such micro secondary electron multipliers. A semiconductor laser is provided with suitable optical lenses for establishing a laser beam which scatters light from a material disposed at the center of the radiometer array.
Helvy U.S. Pat. No. 5,077,504, discloses a multiple-section photomultiplier tube having a single evacuated envelope of the flat-faced, head-on variety with a plurality of closely adjacent, parallel, tubular sections of square cross-section disposed in a 4.times.4 array with each section having its own photocathode, its own linear-focused dynode array, and its own anode so that, effectively, sixteen (16) separate conventional photomultiplier tubes of rectangular cross-section are disposed within a single evacuated envelope. The patentee states that while all of the dynodes in most conventional multiple-section photomultiplier tubes are normally interconnected, in this disclosure one dynode in each of the sixteen (16) dynode arrays is electrically isolated from all other dynodes, thus enabling each of the isolated dynodes to be supplied with an independent voltage source enabling each of the sixteen (16) sections to be independently adjusted so that each channel has the same characteristics as all other channels.
The use of a multi-section, multi-anode photomultiplier tube employing an MCP electron multiplier for fluorescence spectroscopy is disclosed in an article entitled "Multiplexing Expands Yield from Fluorescence Analysis" (anonymous) appearing in Biophotonics International, pages 18 and 20 (March/April 1995). The device illustrated diagrammatically in the foregoing article employs an application specific integrated circuit or ASIC-based multiplexing and routing module developed by IBH Consultants in Glasgow, Scotland to couple a single-photon-timing multichannel detector to standard analysis electronics, with data output from each detector anode reflecting the fluorescence intensity detected in that section of the multi-section, multi-anode photon detector/electron multiplier.
Schmidt et al U.S. Pat. No. 5,097,173, is of interest for its disclosure of what is termed a "Channel Electron Multiplier Phototube"--e.g., apparently a variation of an MCP device--generally characterized by having non-linear channel shapes. However, in FIGS. 5 and 6 of the Schmidt et al patent, there is disclosed a structure which appears to be somewhat similar to a single MCP disk in that the device has hollow passageways formed in a unitary or monolithic ceramic body wherein the passageways are said to be straight, curved in two dimensions, or curved in three dimensions. The passageways do not appear to be microchannels--i.e., channels having a diameter of only a few microns and a length of up to approximately 1.0 mm. However, the process of operation appears to be quite similar to that of conventional MCPs.
Other prior art patents of miscellaneous interest include the following: i) Thompson U.S. Pat. No. 2,141,322 [a cascaded secondary electron emitter amplifier]; ii) Teal U.S. Pat. No. 2,160,798 [an electron discharge apparatus having cylindrical or frusto-conical shaped secondary electrodes or dynodes]; iii) Garin et al U.S. Pat. No. 4,330,731 [a particle detector employing thin planar amplifying plates defining an electron multiplier]; and iv), L'hermite U.S. Pat. No. 4,999,540 [a photomultiplier employing a stackable dynode structure comprising multiple sheets or venetian blinds].
The foregoing conventional photomultiplier tube designs--viz., i) side-on photomultiplier tubes employing circular-cage dynode chains; ii) head-on photomultiplier tubes employing box-and-grid, linear-focused, venetian blind and mesh-type dynode chains, together with combinations thereof; iii) microchannel plates; iv) apertured plates; v) multiple-section photomultiplier tubes; and vi), hybrid photodiodes-in addition to well-type detection chambers, have received widespread acceptance in the scientific community and have been used in a wide range of differing applications for periods of up to forty years or more. Notwithstanding the foregoing, such conventional prior art approaches to the absorption of photon energy, use of the absorbed photon energy to cause emission of electrons from photocathodes, and subsequent multiplication of the emitted electrons, have simply not addressed many of the concerns which continue to pose problems for the scientific instrument community.
Typical of such concerns are: i) the need for a photomultiplier tube capable of detecting light photons on a 360.degree. surround basis so as to maximize collection efficiency; ii) a photomultiplier tube of the foregoing character having a continuous cylindrical photocathode which is uniformly and closely spaced at all points from the axis of a detection chamber and, therefore, which is characterized by significantly improved collection geometry and efficiencies; iii) an evacuated envelope for a photomultiplier tube characterized by having its light-transmissive face formed of relatively thin-walled material, thereby reducing spurious random noise pulses and providing improved signal-to-noise ratios, yet which is resistant to implosion; and iv), a single compact photomultiplier tube suitable for detecting light photons emanating from a sample or other light source--regardless of whether that sample and/or light source is disposed internally of the detection chamber, externally of the detection chamber but immediately adjacent thereto, or remote from the detection chamber--and processing the detected signals using conventional coincidence counting techniques where appropriate.
In short, the foregoing needs which have persisted for decades continue to persist today despite the commercial acceptance and extensive use of the aforesaid conventional photomultiplier tube detectors and electron multipliers.
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