Title:
INTERNAL COOLING FOR PHOTODETECTORS
United States Patent 3757151
Abstract:
A technique is disclosed for improving the performance of photodetectors such as the photocathode of a photomultiplier tube, by reducing thermal electron emission noise and thus increasing the signal to noise ratio. A thermoelectric cooler is positioned within the evacuated envelope of a photomultiplier tube in thermal communication with the photocathode, which is normally the predominant source of electronic noise. Location of the cooler within the tube envelope permits efficient cooling of the photocathode while at the same time reducing the expense and bulk of conventional cryostatic coolers.
Application Number:
05/279922
Publication Date:
09/04/1973
Filing Date:
08/11/1972
View Patent Images:
Export Citation:
Assignee:
Ace Sophisticates Inc. (Greenbelt, MD)
Primary Class:
Other Classes:
62/3.200, 62/3.600, 313/11
International Classes:
H01J43/04; H01L35/00; H01J43/00; H01J1/02
Field of Search:
62/3 136/203 313/11,39
Primary Examiner:
Lake, Roy
Assistant Examiner:
Hostetter, Darwin R.
Claims:
What is claimed is
1. Apparatus for maintaining the photocathode of a vacuum photomultiplier tube at a predetermined low temperature level, comprising:
2. The apparatus of claim 1, wherein said thermoelectric cooling means comprises a Peltier effect device, and means for applying electrical power to said Peltier device whereby heat is transferred away from said photocathode.
3. The apparatus of claim 1, further including an electrically insulating layer between said photocathode and said cool surface.
4. The apparatus of claim 3, wherein said electrically insulating layer is bonded to said photocathode to provide intimate heat conductive contact between said photocathode and said cool surface.
5. The apparatus of claim 4, wherein said thermoelectric cooling means comprises a Peltier device, and further including means for applying electrical power to said Peltier device whereby heat is transferred away from said cool surface of said thermoelectric device to maintain said photocathode at said predetermined temperature level to thereby reduce thermionic emission from said photocathode.
6. In a photomultiplier tube having an evacuated envelope, an anode and photocathode within the evacuated envelope, the improvement comprising:
7. The device of claim 6 wherein said thermoelectric cooling means comprises a Peltier effect device and means for applying a unidirectional current thereto to produce a cool surface and a warm surface.
8. The device of claim 7, further including an electrically insulating layer between said photocathode and said cooling surface.
9. The device of claim 8 wherein said photocathode comprises a coating on said insulating layer.
10. The device of claim 7, wherein said photocathode comprises a gallium arsenide or other type of photoemissive crystal in thermal contact with said cooling surface.
11. The device of claim 7, wherein said photocathode comprises a thin film photoemissive material on a substrate, said substrate being in thermal contact with said cooling surface.
12. The device of claim 7, wherein said photocathode comprises a photoemissive material supported on a base and in thermal contact with said cooling surface.
13. The device of claim 12, wherein said base is thermally conductive and electrically insulative, said base being secured to said cooling surface of said Peltier effect device.
14. The device of claim 7, further including means for thermally connecting said warm surface to said envelope to provide a heat conductive path from said photocathode to said envelope.
15. The device of claim 14, wherein said means for applying a current to said Peltier effect device comprises a shielded cable system within said evacuated envelope.
16. The device of claim 14 further including an electrically insulating but heat conductive layer between said photocathode and said cooling surface.
17. The device of claim 14, further including an electrically insulating but heat conductive layer between said warm surface and said envelope.
18. In an electron tube having an evacuated envelope and a photodetector having a photoemissive surface located within said envelope, means for reducing the thermal generation of free electrons by said photodetector, comprising:
1. Apparatus for maintaining the photocathode of a vacuum photomultiplier tube at a predetermined low temperature level, comprising:
2. The apparatus of claim 1, wherein said thermoelectric cooling means comprises a Peltier effect device, and means for applying electrical power to said Peltier device whereby heat is transferred away from said photocathode.
3. The apparatus of claim 1, further including an electrically insulating layer between said photocathode and said cool surface.
4. The apparatus of claim 3, wherein said electrically insulating layer is bonded to said photocathode to provide intimate heat conductive contact between said photocathode and said cool surface.
5. The apparatus of claim 4, wherein said thermoelectric cooling means comprises a Peltier device, and further including means for applying electrical power to said Peltier device whereby heat is transferred away from said cool surface of said thermoelectric device to maintain said photocathode at said predetermined temperature level to thereby reduce thermionic emission from said photocathode.
6. In a photomultiplier tube having an evacuated envelope, an anode and photocathode within the evacuated envelope, the improvement comprising:
7. The device of claim 6 wherein said thermoelectric cooling means comprises a Peltier effect device and means for applying a unidirectional current thereto to produce a cool surface and a warm surface.
8. The device of claim 7, further including an electrically insulating layer between said photocathode and said cooling surface.
9. The device of claim 8 wherein said photocathode comprises a coating on said insulating layer.
10. The device of claim 7, wherein said photocathode comprises a gallium arsenide or other type of photoemissive crystal in thermal contact with said cooling surface.
11. The device of claim 7, wherein said photocathode comprises a thin film photoemissive material on a substrate, said substrate being in thermal contact with said cooling surface.
12. The device of claim 7, wherein said photocathode comprises a photoemissive material supported on a base and in thermal contact with said cooling surface.
13. The device of claim 12, wherein said base is thermally conductive and electrically insulative, said base being secured to said cooling surface of said Peltier effect device.
14. The device of claim 7, further including means for thermally connecting said warm surface to said envelope to provide a heat conductive path from said photocathode to said envelope.
15. The device of claim 14, wherein said means for applying a current to said Peltier effect device comprises a shielded cable system within said evacuated envelope.
16. The device of claim 14 further including an electrically insulating but heat conductive layer between said photocathode and said cooling surface.
17. The device of claim 14, further including an electrically insulating but heat conductive layer between said warm surface and said envelope.
18. In an electron tube having an evacuated envelope and a photodetector having a photoemissive surface located within said envelope, means for reducing the thermal generation of free electrons by said photodetector, comprising:
Description:
BACKGROUND OF THE INVENTION
The present invention relates, in general, to a method and apparatus for improving the signal to noise ratio of photoemissive surfaces, and more particularly to a device for reducing the temperature of the photocathode in a photomultiplier tube by placing in direct thermal contact with the photocathode a thermoelectric cooler which is incorporated within the evacuated envelope of the tube.
It is presently common practice to cool photomultiplier tubes in order to reduce their temperatures and thus decrease the amount of "dark current" which occurs by reason of thermal emission of electrons from the photocathode material. Because the present invention is primarily applicable to such tubes, it will be described in that context; however, it will be understood that the present invention may be utilized in combination with other photoemissive surfaces where the detection of light and its conversion to corresponding electric signals is to be accomplished. Thus it will be seen that photodetectors in general may be cooled by the method and apparatus of the present invention, without departing from the scope of the present invention.
A photomultiplier tube, which is a device used to detect and measure very small quantities of light, or photons, is capable of measuring light of various wave lengths, ranging from beyond the X-ray region, through the visible wave lengths and into the infra-red regions, depending upon the active materials used on the photoemissive surface of the tube cathode. All photomultipliers take advantage of the fact that under certain conditions a given photocathode material will emit one or more free electrons per incident light photon. By means of this photoelectric effect the light photon is transduced into an equivalent number of photoelectrons, which constitute an equivalent electrical signal. In a photomultiplier tube the emitted photoelectrons undergo high multiplication, or amplification, in a series of dynode stages, each of which releases secondary electrons upon impact by a primary electron. By appropriate selection of the dynode material, numerous electrons are emitted upon each impact by a primary electron, whereby the small number of photoelectrons by the photocathode is multiplied. This increased number of secondary electrons is ultimately received at the output anode of the tube, to provide an electrical pulse, or count, whereby the light received by the photomultiplier tube produces an analogous output signal.
Although photomultiplier devices are remarkably sensitive, at their limit they are found to exhibit electrical noise which is indistinguishable from photon-produced signals, and such noise prevents accurate measurement of very small quantities of light. There are, of course, additional sources of noise within a photomultiplier tube, but noise from such sources can be obscured electronically. Since, however, the photocathode generated noise is indistinguishable from the desired photoelectron current, this noise cannot be filtered out or otherwise removed from the tube output signals. This is because photocathode noise and photoelectrons both occur naturally as single electron events, with the noise producing a "dark current" which renders the detection of weak light signals nearly impossible. The present invention recognizes the fact that if very minute quantities of light are to be measured, it is essential that thermal emission from the photocathode of a photomultiplier tube be eliminated and that this can be accomplished by maintaining a reduced temperature on the photocathode.
The prior art has recognized that spurious emissions, or "dark counts" can be reduced in photomultiplier tubes by reducing the temperature of the whole tube. This is usually accomplished by placing the tube within a refrigerated housing, or cryostat, whereby the photomultiplier tube is reduced from room temperature to a temperature sufficiently low to reduce substantially the photocathode noise events. The type of refrigerant used in a cryostat varies greatly, some using dry ice, others liquid nitrogen, while still others use water or air cooled thermoelectric piles. Some of the refrigerants used in these systems are costly and awkward to handle; others require continuous filter, pump or fan maintenace, and all these systems require constant attention.
The application of refrigerants to photomultiplier tubes has been accomplished in a number of different ways. For example, in some applications, a cold dry gas is flowed over the entire envelope, or over its protruding electrodes. More typically, the tube is simply placed in the motionless dry air of the cryostat and allowed to come to temperature equilibrium; however, this approach has the shortcoming that it may take many hours or even several days to bring the tube to thermal equilibrium. Further, the internal tube temperatures obtained by such methods cannot be measured accurately even after an equilibrium temperature is reached, and the exact photocathode temperature is subject to much uncertainty.
Cryostat cooling, in addition to the foregoing difficulties, creates a number of undesirable effects. For example, cryostats are usually ten or more times larger than the tubes which they cool, and where space or weight is a premium, the cryostat becomes very undesirable. Because of the low temperatures at which a photomultiplier tube operates when cooled by a cryostat, very dry air must be used within the system in order to avoid condensation on the tube which not only interferes with the optical properties of the system, but can also cause electrical problems. Elimination of water condensate in many systems is very difficult to accomplish. Further, the cooling of a cryostat can be hazardous to the photomultiplier tube itself, for too rapid a change in temperature can, and often does, fracture the glass envelope. The glass-to-metal seals near the pins of the tube are particularly susceptible to fracturing because of uneven thermal shock and resultant stress. This problem of temperature change is particularly difficult for where the multiplier tube must be exchanged periodically or where the tubes are cooled, allowed to return to room temperature and subsequently recooled, a very real breakage problem is encountered. To avoid thermal shock during the cooling of a tube, it may be necessary to reduce the temperature gradually and it is inconvenient to have to wait for many hours while the system reaches thermal equilibrium; however, it is equally inconvenient to have to leave the cryostat in operation all the time so that the tube can be used occasionally.
Because of the high cost of cryostats and the difficulties encountered in using them, the cryostat has fallen into the category of "last resort" usage in the efforts to improve light-measuring systems.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to overcome the difficulties inherent in prior art photomultiplier tube cooling systems by providing an inexpensive, easy to use cooling system which completely eliminates the bulk of prior art cryostats by locating a photocathode cooler entirely within the envelope of the photomultiplier tube.
It is a further object of the invention to provide means for cooling the photocathode of a photomultiplier tube quickly and efficiently while at the same time reducing the cost and time delays inherent in prior devices.
It is a further object of the present invention to provide a means for decreasing the cooling time required for photomultiplier tubes while at the same time reducing the danger of fractures due to temperature cycling.
It is another object of the invention to provide means for reducing the size and weight of a cooling system for a photomultiplier tube by cooling only the photocathode thereof.
Another object of the invention is to provide means for cooling a photomultiplier tube without the use of external refrigerants.
Another object of the present invention is to provide cooling means for a photomultiplier tube which has low power consumption and increased cooling efficiency.
It is another object of the present invention to provide means for inexpensively enhancing the signal to noise ratio of a photomultiplier tube by providing means for rapid and accurate temperature control of the photocathode.
It is a further object of the present invention to provide means for cooling the photocathode of a photomultiplier tube without producing a condensate on the optical surfaces of the tube.
It is another object of the present invention to provide means for reducing the temperature of a photoemissive surface by direct thermal contact between the surface and a thermoelectric cooling element.
Briefly, the present invention accomplishes the foregoing and other objects by positioning a thermoelectric device of the type exhibiting the Peltier effect inside the envelope of a photomultiplier tube, whereby the cooling device is located within the insulating vacuum of the tube. The thermoelectric device is secured in intimate heat transfer relationship with the photocathode of the tube, either directly or through the medium of thermally conductive material.
Because of the large variety of photoemissive materials which are suitable for use as photocathodes, and because of the variety of thermoelectric materials, the particular manner in which the cooling surface of the thermal electric cooler is secured to the photocathode will vary with the elements selected for a particular application. Accordingly, the thermal electric device may be clamped, soldered, welded, fastened by suitable adhesives, or otherwise mechanically secured to the photocathode, with the exact method selected being dependent upon the materials used. Some photocathods can be vapor deposited directly onto the thermoelectric cooler. The thermoelectric device may be directly secured to the photocathode, or a layer of electrically insulating but thermally conductive material may be interposed. However, with the thermal cooler located within the evacuated envelope of the tube and in thermal contact with the photo-emissive surface, the present invention takes advantage of the insulation provided by the tube vacuum to provide a decreased cooling time, and an increased efficiency of cooling, while at the same time providing reduced space requirements, and reduced maintenance, to overcome the disadvantages of prior art cooling systems and to improve the signal to noise ratios of the photomultiplier tube.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and additional objects, features and advantages of the present invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments thereof, taken with the accompanying drawing, in which:
FIG. 1 is a diagramatic view of a thermoelectric cooling device mounted in a conventional gallium arsenide photomultiplier tube;
FIG. 2 is a diagramatic view of a thermoelectric cooler in accordance with the present invention utilized in a window-coated "end on" photomultiplier; and
FIG. 3 is a diagrammatic view of the thermoelectric cooler of the present invention as applied to a conventional "side on" photomultiplier tube.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring now more particularly to the drawings, there is illustrated in FIG. 1 a photomultiplier tube 10 which may be one of the numerous photomultiplier tubes presently available on the market, but which for the purposes of this description may be of the type utilizing a gallium arsenide crystal 12 as the photocathode. The crystal in such tubes is held in position by a suitable metal foil strap 13 which extends over two edges of the crystal and which is spot welded or other-wise secured to a suitable base support 14. Alternatively, the crystal may be held in place by spring pressure arms or the like, but in any event is held sufficiently tightly to make good electrical contact with the substrate or base support 14 against which it is forced. In the present case, the base support is a block of material which exhibits a high thermal conductivity and which may be either electrically conductive or electrically insulating.
The gallium arsenide crystal 12 is photoemissive; that is, it emits an electron in response to each incident photon as a function of its characteristic efficiency so that when a photon of proper wavelength enters the photomultiplier tube, as by way of an optically transparent window 15 in the face of the tube, a corresponding electrical output will be produced.
The glass face plate 15 is mounted on and is hermetically sealed to the envelope wall 16 of the tube which is, in turn, hermetically sealed to or formed as a part of tube base 17. As is conventional in the tube art, the wall 16 and base 17, which may, for example, be of glass or metal, cooperate with the face plate 15 to provide an air tight chamber 18 which may be evacuated and which carries the active tube elements. Also in conventional manner, the photomultiplier tube includes a plurality of diodes 19 - 28 which are suitably supported within the evacuated envelope of the tube. Each dynode is connected to a suitable electrical source by means of corresponding leads which may extend through the base of the tube envelope in the form of conventional pins or the like. An anode 30 is also mounted in the evacuated chamber 18 to receive electrons emitted by the dynode series. As is known in the art, potentials of increasing positive magnitude are applied to each of the dynodes 19-28 in sequence so that electrons emitted by one dynode will be attracted to the next, and so on.
As is conventional in photomultiplier tubes, a photon 32 enters the tube through face plate 15 and strikes the surface of the photoemitter material 12. The nature of this material is such that the incident photon will cause the release of a photoelectron from the surface of the crystal. This photoelectron, indicated diagramatically at 34, is attracted by the potential on dynode 19, which potential is positive with respect to that of the photocathode. The photoelectron follows a line path 36 to dynode 19, striking the surface thereof and causing a secondary emission of the electrons. The electrons so emitted from dynode 19 are attracted by the relatively more positive voltage of dynode 20, and when these electrons strike that dynode each electron causes a secondary emission of a number of electrons. In this manner, the released electrons strike each of the remaining dynodes 21 -28 in sequence, each electron producing a secondary emission whereby the single photoelectron 34 is cascaded into an electron "packet" which is attracted to the anode 30. When this packet strikes the anode, there is produced on the output electrode of the tube a small rapid electrical pulse, or single count.
As had been indicated, there are numerous sources of noise within a photomultiplier tube, but most sources produce signals which can be identified and eliminated from the resultant output signal; however, any spurious electrons emitted by the photocathode in the absence of an incident light photon cannot be distinguished from a photoelectron emitted in response to an incident photon, for both will produce identical output signals. Accordingly, if operation of the tube is to be improved, the noise signals generated by spurious electrons emitted from the photocathode must be eliminated. This is accomplished in the present invention by means of a thermoelectric cooling device 38, which, in the illustrated embodiment is secured to the wall of the tube by means of an electrically insulating layer 40. The opposite side of the thermoelectric device 38 carries, and is in intimate heat exchange relationship with, the base support element 14.
The thermoelectric device 38 is any of a great variety of devices which exhibit the Peltier effect, all of which have a heat pumping ability and thus produce a temperature differential between two spaced surfaces. Peltier effect devices may be constructed in virtually any geometrical configuration and thus the exact size and shape of the device utilized in the photomultiplier tube 10 will depend upon the mechanical construction of the photocathode and its support elements and the space available within the tube.
In the illustrated embodiment of FIG. 1, the cooler 38 is arranged to have its cold surface in direct thermal and electrical contact with the base support element 14 while the warm surface of the thermoelectric device is in thermal but not electrical contact with the wall 16 of the tube. Layer 40 electrically insulates wall 16, which may be metal, from the cooler and photocathode electrical potentials which may be typically 2,000 volts. The unidirectional electric current which is required to produce the Peltier effect temperature differential across element 38 is applied by means of coaxial lines 42 and 44. The use of a coaxial line reduces the effect within the tube of the magnetic field produced by the power supplied to the cooling element and thus eliminates interference by this source with the path 36 followed by the electrons within the tube. If the power supply for the cooler 38 is floated electrically at the voltage required for operation of the photocathode, the leads 42 & 44 may be utilized to supply the cathode voltage. In this way, the cooled surface of the thermoelectric device, the base support element 14, and the photocathode 12 may be maintained at the same potential, thereby substantially reducing the need for electrically insulating the various elements from each other. On the other hand, if it is desired to operate the thermoelectric device at some potential other than that of the photocathode an electrically insulating but thermally conducting layer must be interposed between element 38 and the support 14.
It should be emphasized that the photomultiplier tube envelope wall 16 need not be metallic. Although this wall is in thermal contact with the warm surface of the thermal electric device 38, the heat pumped from the photocathode need not be dissipated to a metal wall. The internal surface of all commercially used photomultiplier tubes is sufficiently large to provide an adequate heat sink for cooler 38 if a proper thermal contact is effect, and a glass envelope will conduct sufficient heat to permit the temperature of the photocathode to be reduced the desired amount.
The thermal emissions which generate noise signals in a photomultiplier tube occur at temperatures even below room temperature, and accordingly the cooling apparatus must operate to reduce the temperature of the photocathode considerably in order to minimize this source of noise. A typical temperature for photocathode operation is -25° Centigrade, although some are operated at temperatures as low as -200° C; for each photocathode material there is an optimum temperature below which cathode efficiency begins to decline, and thus the temperature for optimum signal to noise ratio will vary.
Turning now to a consideration of FIG. 2 of the drawings, a modified version of a photomultiplier device utilizing the present invention is illustrated at 50. This photomultiplier tube is an example of the "end on" system of detecting light, wherein the photocathode is a thin semiconducting film deposited on the glass window through which light enters the tube.
Photomultiplier tube 50 includes an envelope formed by the sidewall 51, base 52 and face plate 53, the face plate being of an optically transparent material such as glass and the sidewall and base being of glass, metal, or other conventional materials. Mounted within the evacuated chamber 54 defined by the tube envelope is an optically transparent substrate 55 which is mounted parallel to face plate 53 and closely spaced thereto to receive photons 56 which pass through the face plate. A thin film coating of a photoemissive semiconductor 57 carried by substrate 55 forms a photocathode, whereby the entering photon 56 passes through substrate 55 and interacts with the photoemissive cathode to produce a free electron, diagramatically illustrated at 58. The photoelectron follows a substantially straight line path 59 to the first multiplier stage of the tube. In the illustration of FIG. 2, the multiplier section is illustrated as being a commercially available multiplier device known as a Channeltron, indicated at 60. The Channeltron, which is manufactured by the Bendix Corporation, is functionally similar to the array of FIG. 1, although it is of a smaller size and by reason of its configuration is relative immune to stray magnetic fields.
As with the dynode array, the Channeltron device 60 responds to the impact of a photoelectron 58 to produce at its output end a corresponding packet of secondarily emitted electrons which are collected on an output anode 62 and appear on an output pin 64 as a small electrical pulse, or single count, which is analogous to the input photon. Again, the anode output lead 64 and the power supply leads 65 and 66 to the Channeltron device 60 extend through the base 52 of the tube in the form of pins for easy connection to external circuitry, in known manner.
The substrate 55 which carries photocathode 57 may be secured within the tube envelope by means of an annular support base 68 which is formed with an interior peripheral channel to receive the edge of the substrate. The support base firmly clamps the edge of the substrate to provide not only strong mechanical support but a good thermal contact as well so that there will be adequate heat transfer between the substrate and the base support element. Support element 68 is constructed of a thermally conductive material, with its outer peripheral edge being secured to the cold surface of an annular thermoelectric cooler element 70. The support element 68 may be secured to cooler 70 by a pressure fit, by soldering or any other convenient means to provide a good thermal contact between the two elements. If the power supply to the photocathode is to be provided by the power leads of the thermoelectric cooler element, whereas the element 70 electrically floats at the high potential of the photocathode, it will be apparent that the base element 68 must be of an electrically conductive material. If, on the other hand, the thermoelectric cooler and photocathode are to be separately powered, then element 68 will be electrically insulative.
The warm outer peripheral surface of the thermoelectric device 70 is in thermal contact with the wall 51 of the tube envelope in order to dissipate the heat pumped from the photocathode by element 70. In this case, the element 70 is also shown to be in electrical contact with the metal wall 51 of the envelope, although it should be understood that, if desired, an insulating layer may separate the cooler from the envelope walls. In the illustration of FIG. 2, electrical power is supplied to the thermoelectric cooler element 70 and to the photocathode 57 by way of coaxial lines 71 and 72 which extend through base 52 for connection to a suitable source of power.
The manner in which the thermoelectric cooling of the present invention may be applied to a third type of photomultiplier tube is illustrated in FIG. 3, wherein the "side on" type of tube is illustrated. In this Figure, the photomultiplier tube 75 is shown in end view with the cylindrical side wall 76 being in section. The side wall is illustrated as being of glass, with the evacuated interior 77 of the tube envelope carrying an array of dynodes 78-87 in the conventional manner. The photocathode is illustrated herein at 90 as being a thin film of photoemissive material supported on a metal substrate 91, which is in turn mechanically supported in a suitable manner within the tube envelope, and serves to provide a uniform distribution of the supply voltage to the photocathode material. A photon of light, indicated diagramatically at 92, upon entering the photomultiplier tube 75 through the glass envelope, 76, passes through a grid 93 and impinges on the surface of photocathode 90. The photoemissive surface emits a photoelectron, diagramatically illustrated at 94, which is attracted by the potential on the first dynode 78, the photoelectron following a path 95 to the dynode. When the electron strikes element 78, it emits a number of secondary electrons which are, in turn, attracted to dynode 79, and so on through the dynode array. Electrons emitted by the last dynode 87 are attracted to an anode 96 to produce an output pulse which corresponds to the input photon. The anode 96 is partially surrounded by a trap 97 for shielding purposes.
Cooling of the photocathode 90 is accomplished in this embodiment by means of a thermoelectric cooler element 98 located within the envelope and in heat exchange relationship with the photocathode. Since tubes of the type illustrated in this Figure are generally very compact, it may be necessary in some designs to place the thermoelectric cooler at some location remote from the main photocathode structure. For example, in the illustrated configuration, the thermoelectric device is located below the photocathode, for example with its warm surface in contact with the base of the tube. The cool upper surface 99 of device 98 may be thermally connected to the photocathode by means of, for example, an L-shaped metal strip 100 which has its horizontal leg secured to the cool surface 99 and its vertical leg 102 in thermal contact with the photocathode. In the illustrated embodiment, the leg 102 is secured to the photocathode by means of an electrical insulator 103 which has been included to demonstrate the type of configuration that is required if it is found undesirable to float the cooler power supply at the voltage level of the photocathode. This insulator provides thermal contact between the vertical leg 102 of the heat conductor 100 and the photocathode substrate 91.
Where space is available the cool surface of the thermoelectric cooler may be secured directly to the substrate 91 with a thermal connection being provided between the warm surface of the thermoelectric device and the wall 76 of the tube envelope. The large surface area of the envelope serves as a heat sink and radiator of the energy pumped from the photocathode.
From the foregoing it can be seen that the provision of a thermoelectric cooler properly placed within the evacuated envelope of a photomultiplier tube provides an inexpensive and practical method of greatly improving the operation of present types of photomultiplier tubes. The use of thermoelectric cooling provides a substantial reduction in the size and weight of the photomultiplier system as compared to the cryostat apparatus previously used, and because of the reduced mass, the cooling time is reduced. Since the whole tube is not subjected to temperature extremes, but only the specific area of interest, the danger of fracture to temperature cycling is substantially eliminated. The present invention eliminates the need for costly and difficult to handle regrigerants, since a photocathode surrounded by an insulating vacuum presents only a small thermal load, the power consumption of the cooling apparatus is reduced considerably from that of prior systems. Internal cooling of the photocathode eliminates the exposure of cold external tube surfaces to moisture-filled air because of the insulating vacuum within the tube, and thus the problem of condensate accumulating on the surface of the tube is eliminated. Since fans, water cooling, and the like are eliminated, the size of the cooling apparatus and its accompanying maintenance problems are reduced, and because of the small thermal load encountered in cooling only the photocathode, normal air convection around the tube envelope is sufficient to remove any heat generated. The cooler element of the present invention permits rapid and accurate temperature control of the photocathode and by the use of suitable feedback systems, compensation for changes in temperature conditions, caused, for example, by absorption of radiant heat, can be obtained. Since the internal cooling technique of the present invention can be applied to all existing designs of photomultiplier tubes with resultant reduction in thermal noise and enhancement of tube sensitivity, the present invention provides a significant advance in the art. It will, of course, be recognized by those skilled in the art that numerous variations of the embodiments illustrated herein can be constructed without departing from the true spirit and scope of the invention as it is defined in the following claims.
The present invention relates, in general, to a method and apparatus for improving the signal to noise ratio of photoemissive surfaces, and more particularly to a device for reducing the temperature of the photocathode in a photomultiplier tube by placing in direct thermal contact with the photocathode a thermoelectric cooler which is incorporated within the evacuated envelope of the tube.
It is presently common practice to cool photomultiplier tubes in order to reduce their temperatures and thus decrease the amount of "dark current" which occurs by reason of thermal emission of electrons from the photocathode material. Because the present invention is primarily applicable to such tubes, it will be described in that context; however, it will be understood that the present invention may be utilized in combination with other photoemissive surfaces where the detection of light and its conversion to corresponding electric signals is to be accomplished. Thus it will be seen that photodetectors in general may be cooled by the method and apparatus of the present invention, without departing from the scope of the present invention.
A photomultiplier tube, which is a device used to detect and measure very small quantities of light, or photons, is capable of measuring light of various wave lengths, ranging from beyond the X-ray region, through the visible wave lengths and into the infra-red regions, depending upon the active materials used on the photoemissive surface of the tube cathode. All photomultipliers take advantage of the fact that under certain conditions a given photocathode material will emit one or more free electrons per incident light photon. By means of this photoelectric effect the light photon is transduced into an equivalent number of photoelectrons, which constitute an equivalent electrical signal. In a photomultiplier tube the emitted photoelectrons undergo high multiplication, or amplification, in a series of dynode stages, each of which releases secondary electrons upon impact by a primary electron. By appropriate selection of the dynode material, numerous electrons are emitted upon each impact by a primary electron, whereby the small number of photoelectrons by the photocathode is multiplied. This increased number of secondary electrons is ultimately received at the output anode of the tube, to provide an electrical pulse, or count, whereby the light received by the photomultiplier tube produces an analogous output signal.
Although photomultiplier devices are remarkably sensitive, at their limit they are found to exhibit electrical noise which is indistinguishable from photon-produced signals, and such noise prevents accurate measurement of very small quantities of light. There are, of course, additional sources of noise within a photomultiplier tube, but noise from such sources can be obscured electronically. Since, however, the photocathode generated noise is indistinguishable from the desired photoelectron current, this noise cannot be filtered out or otherwise removed from the tube output signals. This is because photocathode noise and photoelectrons both occur naturally as single electron events, with the noise producing a "dark current" which renders the detection of weak light signals nearly impossible. The present invention recognizes the fact that if very minute quantities of light are to be measured, it is essential that thermal emission from the photocathode of a photomultiplier tube be eliminated and that this can be accomplished by maintaining a reduced temperature on the photocathode.
The prior art has recognized that spurious emissions, or "dark counts" can be reduced in photomultiplier tubes by reducing the temperature of the whole tube. This is usually accomplished by placing the tube within a refrigerated housing, or cryostat, whereby the photomultiplier tube is reduced from room temperature to a temperature sufficiently low to reduce substantially the photocathode noise events. The type of refrigerant used in a cryostat varies greatly, some using dry ice, others liquid nitrogen, while still others use water or air cooled thermoelectric piles. Some of the refrigerants used in these systems are costly and awkward to handle; others require continuous filter, pump or fan maintenace, and all these systems require constant attention.
The application of refrigerants to photomultiplier tubes has been accomplished in a number of different ways. For example, in some applications, a cold dry gas is flowed over the entire envelope, or over its protruding electrodes. More typically, the tube is simply placed in the motionless dry air of the cryostat and allowed to come to temperature equilibrium; however, this approach has the shortcoming that it may take many hours or even several days to bring the tube to thermal equilibrium. Further, the internal tube temperatures obtained by such methods cannot be measured accurately even after an equilibrium temperature is reached, and the exact photocathode temperature is subject to much uncertainty.
Cryostat cooling, in addition to the foregoing difficulties, creates a number of undesirable effects. For example, cryostats are usually ten or more times larger than the tubes which they cool, and where space or weight is a premium, the cryostat becomes very undesirable. Because of the low temperatures at which a photomultiplier tube operates when cooled by a cryostat, very dry air must be used within the system in order to avoid condensation on the tube which not only interferes with the optical properties of the system, but can also cause electrical problems. Elimination of water condensate in many systems is very difficult to accomplish. Further, the cooling of a cryostat can be hazardous to the photomultiplier tube itself, for too rapid a change in temperature can, and often does, fracture the glass envelope. The glass-to-metal seals near the pins of the tube are particularly susceptible to fracturing because of uneven thermal shock and resultant stress. This problem of temperature change is particularly difficult for where the multiplier tube must be exchanged periodically or where the tubes are cooled, allowed to return to room temperature and subsequently recooled, a very real breakage problem is encountered. To avoid thermal shock during the cooling of a tube, it may be necessary to reduce the temperature gradually and it is inconvenient to have to wait for many hours while the system reaches thermal equilibrium; however, it is equally inconvenient to have to leave the cryostat in operation all the time so that the tube can be used occasionally.
Because of the high cost of cryostats and the difficulties encountered in using them, the cryostat has fallen into the category of "last resort" usage in the efforts to improve light-measuring systems.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to overcome the difficulties inherent in prior art photomultiplier tube cooling systems by providing an inexpensive, easy to use cooling system which completely eliminates the bulk of prior art cryostats by locating a photocathode cooler entirely within the envelope of the photomultiplier tube.
It is a further object of the invention to provide means for cooling the photocathode of a photomultiplier tube quickly and efficiently while at the same time reducing the cost and time delays inherent in prior devices.
It is a further object of the present invention to provide a means for decreasing the cooling time required for photomultiplier tubes while at the same time reducing the danger of fractures due to temperature cycling.
It is another object of the invention to provide means for reducing the size and weight of a cooling system for a photomultiplier tube by cooling only the photocathode thereof.
Another object of the invention is to provide means for cooling a photomultiplier tube without the use of external refrigerants.
Another object of the present invention is to provide cooling means for a photomultiplier tube which has low power consumption and increased cooling efficiency.
It is another object of the present invention to provide means for inexpensively enhancing the signal to noise ratio of a photomultiplier tube by providing means for rapid and accurate temperature control of the photocathode.
It is a further object of the present invention to provide means for cooling the photocathode of a photomultiplier tube without producing a condensate on the optical surfaces of the tube.
It is another object of the present invention to provide means for reducing the temperature of a photoemissive surface by direct thermal contact between the surface and a thermoelectric cooling element.
Briefly, the present invention accomplishes the foregoing and other objects by positioning a thermoelectric device of the type exhibiting the Peltier effect inside the envelope of a photomultiplier tube, whereby the cooling device is located within the insulating vacuum of the tube. The thermoelectric device is secured in intimate heat transfer relationship with the photocathode of the tube, either directly or through the medium of thermally conductive material.
Because of the large variety of photoemissive materials which are suitable for use as photocathodes, and because of the variety of thermoelectric materials, the particular manner in which the cooling surface of the thermal electric cooler is secured to the photocathode will vary with the elements selected for a particular application. Accordingly, the thermal electric device may be clamped, soldered, welded, fastened by suitable adhesives, or otherwise mechanically secured to the photocathode, with the exact method selected being dependent upon the materials used. Some photocathods can be vapor deposited directly onto the thermoelectric cooler. The thermoelectric device may be directly secured to the photocathode, or a layer of electrically insulating but thermally conductive material may be interposed. However, with the thermal cooler located within the evacuated envelope of the tube and in thermal contact with the photo-emissive surface, the present invention takes advantage of the insulation provided by the tube vacuum to provide a decreased cooling time, and an increased efficiency of cooling, while at the same time providing reduced space requirements, and reduced maintenance, to overcome the disadvantages of prior art cooling systems and to improve the signal to noise ratios of the photomultiplier tube.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and additional objects, features and advantages of the present invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments thereof, taken with the accompanying drawing, in which:
FIG. 1 is a diagramatic view of a thermoelectric cooling device mounted in a conventional gallium arsenide photomultiplier tube;
FIG. 2 is a diagramatic view of a thermoelectric cooler in accordance with the present invention utilized in a window-coated "end on" photomultiplier; and
FIG. 3 is a diagrammatic view of the thermoelectric cooler of the present invention as applied to a conventional "side on" photomultiplier tube.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring now more particularly to the drawings, there is illustrated in FIG. 1 a photomultiplier tube 10 which may be one of the numerous photomultiplier tubes presently available on the market, but which for the purposes of this description may be of the type utilizing a gallium arsenide crystal 12 as the photocathode. The crystal in such tubes is held in position by a suitable metal foil strap 13 which extends over two edges of the crystal and which is spot welded or other-wise secured to a suitable base support 14. Alternatively, the crystal may be held in place by spring pressure arms or the like, but in any event is held sufficiently tightly to make good electrical contact with the substrate or base support 14 against which it is forced. In the present case, the base support is a block of material which exhibits a high thermal conductivity and which may be either electrically conductive or electrically insulating.
The gallium arsenide crystal 12 is photoemissive; that is, it emits an electron in response to each incident photon as a function of its characteristic efficiency so that when a photon of proper wavelength enters the photomultiplier tube, as by way of an optically transparent window 15 in the face of the tube, a corresponding electrical output will be produced.
The glass face plate 15 is mounted on and is hermetically sealed to the envelope wall 16 of the tube which is, in turn, hermetically sealed to or formed as a part of tube base 17. As is conventional in the tube art, the wall 16 and base 17, which may, for example, be of glass or metal, cooperate with the face plate 15 to provide an air tight chamber 18 which may be evacuated and which carries the active tube elements. Also in conventional manner, the photomultiplier tube includes a plurality of diodes 19 - 28 which are suitably supported within the evacuated envelope of the tube. Each dynode is connected to a suitable electrical source by means of corresponding leads which may extend through the base of the tube envelope in the form of conventional pins or the like. An anode 30 is also mounted in the evacuated chamber 18 to receive electrons emitted by the dynode series. As is known in the art, potentials of increasing positive magnitude are applied to each of the dynodes 19-28 in sequence so that electrons emitted by one dynode will be attracted to the next, and so on.
As is conventional in photomultiplier tubes, a photon 32 enters the tube through face plate 15 and strikes the surface of the photoemitter material 12. The nature of this material is such that the incident photon will cause the release of a photoelectron from the surface of the crystal. This photoelectron, indicated diagramatically at 34, is attracted by the potential on dynode 19, which potential is positive with respect to that of the photocathode. The photoelectron follows a line path 36 to dynode 19, striking the surface thereof and causing a secondary emission of the electrons. The electrons so emitted from dynode 19 are attracted by the relatively more positive voltage of dynode 20, and when these electrons strike that dynode each electron causes a secondary emission of a number of electrons. In this manner, the released electrons strike each of the remaining dynodes 21 -28 in sequence, each electron producing a secondary emission whereby the single photoelectron 34 is cascaded into an electron "packet" which is attracted to the anode 30. When this packet strikes the anode, there is produced on the output electrode of the tube a small rapid electrical pulse, or single count.
As had been indicated, there are numerous sources of noise within a photomultiplier tube, but most sources produce signals which can be identified and eliminated from the resultant output signal; however, any spurious electrons emitted by the photocathode in the absence of an incident light photon cannot be distinguished from a photoelectron emitted in response to an incident photon, for both will produce identical output signals. Accordingly, if operation of the tube is to be improved, the noise signals generated by spurious electrons emitted from the photocathode must be eliminated. This is accomplished in the present invention by means of a thermoelectric cooling device 38, which, in the illustrated embodiment is secured to the wall of the tube by means of an electrically insulating layer 40. The opposite side of the thermoelectric device 38 carries, and is in intimate heat exchange relationship with, the base support element 14.
The thermoelectric device 38 is any of a great variety of devices which exhibit the Peltier effect, all of which have a heat pumping ability and thus produce a temperature differential between two spaced surfaces. Peltier effect devices may be constructed in virtually any geometrical configuration and thus the exact size and shape of the device utilized in the photomultiplier tube 10 will depend upon the mechanical construction of the photocathode and its support elements and the space available within the tube.
In the illustrated embodiment of FIG. 1, the cooler 38 is arranged to have its cold surface in direct thermal and electrical contact with the base support element 14 while the warm surface of the thermoelectric device is in thermal but not electrical contact with the wall 16 of the tube. Layer 40 electrically insulates wall 16, which may be metal, from the cooler and photocathode electrical potentials which may be typically 2,000 volts. The unidirectional electric current which is required to produce the Peltier effect temperature differential across element 38 is applied by means of coaxial lines 42 and 44. The use of a coaxial line reduces the effect within the tube of the magnetic field produced by the power supplied to the cooling element and thus eliminates interference by this source with the path 36 followed by the electrons within the tube. If the power supply for the cooler 38 is floated electrically at the voltage required for operation of the photocathode, the leads 42 & 44 may be utilized to supply the cathode voltage. In this way, the cooled surface of the thermoelectric device, the base support element 14, and the photocathode 12 may be maintained at the same potential, thereby substantially reducing the need for electrically insulating the various elements from each other. On the other hand, if it is desired to operate the thermoelectric device at some potential other than that of the photocathode an electrically insulating but thermally conducting layer must be interposed between element 38 and the support 14.
It should be emphasized that the photomultiplier tube envelope wall 16 need not be metallic. Although this wall is in thermal contact with the warm surface of the thermal electric device 38, the heat pumped from the photocathode need not be dissipated to a metal wall. The internal surface of all commercially used photomultiplier tubes is sufficiently large to provide an adequate heat sink for cooler 38 if a proper thermal contact is effect, and a glass envelope will conduct sufficient heat to permit the temperature of the photocathode to be reduced the desired amount.
The thermal emissions which generate noise signals in a photomultiplier tube occur at temperatures even below room temperature, and accordingly the cooling apparatus must operate to reduce the temperature of the photocathode considerably in order to minimize this source of noise. A typical temperature for photocathode operation is -25° Centigrade, although some are operated at temperatures as low as -200° C; for each photocathode material there is an optimum temperature below which cathode efficiency begins to decline, and thus the temperature for optimum signal to noise ratio will vary.
Turning now to a consideration of FIG. 2 of the drawings, a modified version of a photomultiplier device utilizing the present invention is illustrated at 50. This photomultiplier tube is an example of the "end on" system of detecting light, wherein the photocathode is a thin semiconducting film deposited on the glass window through which light enters the tube.
Photomultiplier tube 50 includes an envelope formed by the sidewall 51, base 52 and face plate 53, the face plate being of an optically transparent material such as glass and the sidewall and base being of glass, metal, or other conventional materials. Mounted within the evacuated chamber 54 defined by the tube envelope is an optically transparent substrate 55 which is mounted parallel to face plate 53 and closely spaced thereto to receive photons 56 which pass through the face plate. A thin film coating of a photoemissive semiconductor 57 carried by substrate 55 forms a photocathode, whereby the entering photon 56 passes through substrate 55 and interacts with the photoemissive cathode to produce a free electron, diagramatically illustrated at 58. The photoelectron follows a substantially straight line path 59 to the first multiplier stage of the tube. In the illustration of FIG. 2, the multiplier section is illustrated as being a commercially available multiplier device known as a Channeltron, indicated at 60. The Channeltron, which is manufactured by the Bendix Corporation, is functionally similar to the array of FIG. 1, although it is of a smaller size and by reason of its configuration is relative immune to stray magnetic fields.
As with the dynode array, the Channeltron device 60 responds to the impact of a photoelectron 58 to produce at its output end a corresponding packet of secondarily emitted electrons which are collected on an output anode 62 and appear on an output pin 64 as a small electrical pulse, or single count, which is analogous to the input photon. Again, the anode output lead 64 and the power supply leads 65 and 66 to the Channeltron device 60 extend through the base 52 of the tube in the form of pins for easy connection to external circuitry, in known manner.
The substrate 55 which carries photocathode 57 may be secured within the tube envelope by means of an annular support base 68 which is formed with an interior peripheral channel to receive the edge of the substrate. The support base firmly clamps the edge of the substrate to provide not only strong mechanical support but a good thermal contact as well so that there will be adequate heat transfer between the substrate and the base support element. Support element 68 is constructed of a thermally conductive material, with its outer peripheral edge being secured to the cold surface of an annular thermoelectric cooler element 70. The support element 68 may be secured to cooler 70 by a pressure fit, by soldering or any other convenient means to provide a good thermal contact between the two elements. If the power supply to the photocathode is to be provided by the power leads of the thermoelectric cooler element, whereas the element 70 electrically floats at the high potential of the photocathode, it will be apparent that the base element 68 must be of an electrically conductive material. If, on the other hand, the thermoelectric cooler and photocathode are to be separately powered, then element 68 will be electrically insulative.
The warm outer peripheral surface of the thermoelectric device 70 is in thermal contact with the wall 51 of the tube envelope in order to dissipate the heat pumped from the photocathode by element 70. In this case, the element 70 is also shown to be in electrical contact with the metal wall 51 of the envelope, although it should be understood that, if desired, an insulating layer may separate the cooler from the envelope walls. In the illustration of FIG. 2, electrical power is supplied to the thermoelectric cooler element 70 and to the photocathode 57 by way of coaxial lines 71 and 72 which extend through base 52 for connection to a suitable source of power.
The manner in which the thermoelectric cooling of the present invention may be applied to a third type of photomultiplier tube is illustrated in FIG. 3, wherein the "side on" type of tube is illustrated. In this Figure, the photomultiplier tube 75 is shown in end view with the cylindrical side wall 76 being in section. The side wall is illustrated as being of glass, with the evacuated interior 77 of the tube envelope carrying an array of dynodes 78-87 in the conventional manner. The photocathode is illustrated herein at 90 as being a thin film of photoemissive material supported on a metal substrate 91, which is in turn mechanically supported in a suitable manner within the tube envelope, and serves to provide a uniform distribution of the supply voltage to the photocathode material. A photon of light, indicated diagramatically at 92, upon entering the photomultiplier tube 75 through the glass envelope, 76, passes through a grid 93 and impinges on the surface of photocathode 90. The photoemissive surface emits a photoelectron, diagramatically illustrated at 94, which is attracted by the potential on the first dynode 78, the photoelectron following a path 95 to the dynode. When the electron strikes element 78, it emits a number of secondary electrons which are, in turn, attracted to dynode 79, and so on through the dynode array. Electrons emitted by the last dynode 87 are attracted to an anode 96 to produce an output pulse which corresponds to the input photon. The anode 96 is partially surrounded by a trap 97 for shielding purposes.
Cooling of the photocathode 90 is accomplished in this embodiment by means of a thermoelectric cooler element 98 located within the envelope and in heat exchange relationship with the photocathode. Since tubes of the type illustrated in this Figure are generally very compact, it may be necessary in some designs to place the thermoelectric cooler at some location remote from the main photocathode structure. For example, in the illustrated configuration, the thermoelectric device is located below the photocathode, for example with its warm surface in contact with the base of the tube. The cool upper surface 99 of device 98 may be thermally connected to the photocathode by means of, for example, an L-shaped metal strip 100 which has its horizontal leg secured to the cool surface 99 and its vertical leg 102 in thermal contact with the photocathode. In the illustrated embodiment, the leg 102 is secured to the photocathode by means of an electrical insulator 103 which has been included to demonstrate the type of configuration that is required if it is found undesirable to float the cooler power supply at the voltage level of the photocathode. This insulator provides thermal contact between the vertical leg 102 of the heat conductor 100 and the photocathode substrate 91.
Where space is available the cool surface of the thermoelectric cooler may be secured directly to the substrate 91 with a thermal connection being provided between the warm surface of the thermoelectric device and the wall 76 of the tube envelope. The large surface area of the envelope serves as a heat sink and radiator of the energy pumped from the photocathode.
From the foregoing it can be seen that the provision of a thermoelectric cooler properly placed within the evacuated envelope of a photomultiplier tube provides an inexpensive and practical method of greatly improving the operation of present types of photomultiplier tubes. The use of thermoelectric cooling provides a substantial reduction in the size and weight of the photomultiplier system as compared to the cryostat apparatus previously used, and because of the reduced mass, the cooling time is reduced. Since the whole tube is not subjected to temperature extremes, but only the specific area of interest, the danger of fracture to temperature cycling is substantially eliminated. The present invention eliminates the need for costly and difficult to handle regrigerants, since a photocathode surrounded by an insulating vacuum presents only a small thermal load, the power consumption of the cooling apparatus is reduced considerably from that of prior systems. Internal cooling of the photocathode eliminates the exposure of cold external tube surfaces to moisture-filled air because of the insulating vacuum within the tube, and thus the problem of condensate accumulating on the surface of the tube is eliminated. Since fans, water cooling, and the like are eliminated, the size of the cooling apparatus and its accompanying maintenance problems are reduced, and because of the small thermal load encountered in cooling only the photocathode, normal air convection around the tube envelope is sufficient to remove any heat generated. The cooler element of the present invention permits rapid and accurate temperature control of the photocathode and by the use of suitable feedback systems, compensation for changes in temperature conditions, caused, for example, by absorption of radiant heat, can be obtained. Since the internal cooling technique of the present invention can be applied to all existing designs of photomultiplier tubes with resultant reduction in thermal noise and enhancement of tube sensitivity, the present invention provides a significant advance in the art. It will, of course, be recognized by those skilled in the art that numerous variations of the embodiments illustrated herein can be constructed without departing from the true spirit and scope of the invention as it is defined in the following claims.