Photo-multiplier tube having a metal side wall

Abstract: A photo-multiplier tube which is much faster to manufacture, prevents generation of gas within its envelope (1A), prevents deterioration of its electron multiplier assembly (dynodes) (8) and greatly reduces noise includes an all metal cylindrical side wall (2A). At one end of the side wall (2A) is an annular, flange shaped, metal sealing area (2b). A stem (4) of the photomultiplier tube has another annular flange shaped, metal sealing area (11). These two sealing areas (2b, 11) are welded together. Also a metal exhaust tube (7A) may be connected to the stem (4) by resistance welding. The metal exhaust tube (7A) is then severed using pinch-off seal at the final stage of the photo-multiplier tube production.

Description: The present invention relates to a photomultiplier tube, and more particularly to a photomultiplier tube wherein the sidewall of the photomultiplier tube envelope is made of metal. There has been proposed a box-shaped photomultiplier tube as shown in Fig. 1 comprising an evacuated envelope 1 (made entirely of glass) having a generally cylindrical, disc-shaped, transparent faceplate 3, a generally cylindrical sidewall 2, and a generally cylindrical, disc-shaped stem 4. The faceplate 3 is hermetically attached to one opening of the cylindrical sidewall 2. A photocathode 5 is formed on the interior surface of the transparent faceplate 3 using alkali metal vaporization techniques. The photocathode 5 provides photoelectrons in response to radiation incident thereon. The stem 4 is vacuum sealed to the lower opening of the cylindrical sidewall 2 e.g., by welding or heat-melt-bonding. Inside the envelope 1 is provided an electron multiplier assembly 8 comprising a plurality of dynodes. Each dynode is provided with a secondary electron emissive surface for multiplying the photoelectrons incident thereon. As shown in Fig. 1, the stem 4 is formed from a generally cylindrical glass disc 4A. A plurality of stem leads 6 (only some of which are shown) extend through the glass disc 4A into the envelope 1 for supplying voltages to the dynodes and the photocathode 5. In the center of the glass disc 4a is a heat sealed glass exhaust tube 7 protruding vertically downward. During manufacture of the photomultiplier tube and before being heat sealed, the glass exhaust tube 7 provides communication between the interior of the photomultiplier and an exhaust system (not shown). The exhaust system evacuates the envelope 1 via the glass exhaust tube 7, and then alkali metal vapor is introduced into the envelope 1 through the glass exhaust tube 7 for forming the photocathode 5. The glass exhaust tube 7 is unnecessary after production of the photomultiplier tube is complete, and so is severed at the final stage of photomultiplier tube manufacture by using a gas burner so as to be maximally shortened. The cylindrical sidewall 2 of conventional photomultiplier tubes is heated to melting at a sealing portion 2a and vacuum sealed to the cylindrical disc-shaped stem 4 thereat. After the glass exhaust tube 7 is connected to the exhaust system, the envelope 1 is evacuated and then alkali metal vapor is introduced into the envelope 1 to form the photocathode 5 and the secondary electron emissive surface of the dynodes. Afterward, the glass exhaust tube 7 is severed from the exhaust system using a gas burner and maximally shortened. Refer to Japanese Laid-open Patent Publications 60-112224, 58-54539, and 60-211758 for more detailed information on photomultiplier tube technology. In view of the fact that the cylindrical sidewall 2 and the stem 4 of the photomultiplier tube are formed entirely from glass, various problems have been known with conventional photomultiplier tubes. Firstly, light emanates from the glass caused by radioactive materials such as K 40 contained within the glass and causes production of noise. Secondly, floating electrons or ions generated during production of the photomultiplier assembly 8 strike the glass of the cylindrical sidewall or the stem and cause the glass to emit light which also produces unwanted noise. Thirdly, the photo-multiplier assembly 8 is liable to deteriorate because of a high temperature applied thereto when the stem 4 is melted to secure to the opening of the cylindrical sidewall 2 and when the glass exhaust tube 7 is severed by a gas burner. Fourthly, melting and severing of the glass exhaust tube 7 cause generation and pooling of gas at the interior section of the photo-multiplier tube, which in turn prevents forming a good vacuum. Further, severing of the glass exhaust tube 7 requires more than one step which prolongs the manufacturing time. Finally, changes in heat, especially at the stem 4, brought about when glass is heated for severing the glass exhaust tube 7, generates cracks in the glass, shifts in the alkali metal film, and other undesirable phenomena, which complicate severing and shortening operations of the glass exhaust tube 7. US-A-4,554,481 discloses a photo-multiplier tube formed from a cathode sub-assembly and a stem sub-assembly having tubular metal side walls which avoids many of the above-mentioned problems associated with a glass tube. According to this invention, a photo-multiplier tube comprises a tubular metal side wall having first and second ends in a longitudinal direction; a transparent face plate hermetically sealed to the first end of said metal side wall, said face plate having a surface; a stem hermetically sealed to the second end of said metal side wall, wherein said metal side wall, said face plate and said stem form an airtight chamber with the surface of said face plate being directed inwardly of said airtight chamber; a photocathode formed on the surface of said faceplate for producing electrons in response to incident radiation thereon; an electron multiplier assembly provided within the airtight chamber for multiplying the electrons relayed from said photocathode; and, an anode for receiving multiplied electrons from said electron multiplier assembly and producing an output signal representative of the radiation incident on said photocathode, and is characterised in that the second end of said metal side wall includes a flange shaped sealing portion and said stem includes a metal flange shaped, airtight sealing section, said sealing portion of said metal side wall being hermetically sealed to said sealing section of said stem. Particular examples of photo-multiplier tubes in accordance with this invention will now be described with reference to the accompanying drawings, in which:- Fig. 1 is a cross sectional diagram schematically showing a conventional photo-multiplier tube; Figs. 2(a), 2(b) and 2(c) are top plan view, cross sectional side view and a bottom plan view, respectively, showing a photo-multiplier tube according to a first embodiment of the present invention; Figs. 3(a), 3(b) and 3(c) are top plan view, cross sectional side view and a bottom plan view, respectively, showing a photomultiplier tube according to a second embodiment of the present invention; Figs. 4(a), 4(b) and 4(c) are top plan view, cross-sectional side view and a bottom plan view, respectively, showing a photomultiplier tube according to a third embodiment of the present invention; Figs. 5(a), 5(b) and 5(c) are top plan view, cross-sectional side view and a bottom plan view, respectively, showing a photomultiplier tube according to a fourth embodiment of the present invention; Figs. 6(a), 6(b) and 6(c) are top plan view, cross-sectional side view and a bottom plan view, respectively, showing a photomultiplier tube according to a fifth embodiment of the present invention; Figs. 7(a), 7(b) and 7(c) are top plan view, cross-sectional side view and a bottom plan view, respectively, showing a photomultiplier tube according to a sixth embodiment of the present invention; Figs. 8(a), 8(b) and 8(c) are top plan view, cross-sectional side view and a bottom plan view, respectively, showing a photomultiplier tube according to a seventh embodiment of the present invention; Figs. 9(a), 9(b) and 9(c) are top plan view, cross-sectional side view and a bottom plan view, respectively, showing a photomultiplier tube according to an eighth embodiment of the present invention; Figs. 10(a), 10(b) and 10(c) are top plan view, cross-sectional side view and a bottom plan view, respectively, showing a photomultiplier tube according to a ninth embodiment of the present invention; and Figs. 11(a), 11(b) and 11(c) are top plan view, cross-sectional side view and a bottom plan view, respectively, showing a photomultiplier tube according to tenth embodiment of the present invention. Referring to the accompanying drawings, preferred embodiments of the invention will be described wherein like parts and components are designated by the same reference numerals to avoid duplicating description. A first embodiment of the photomultipler is shown in Figs. 2(a) through 2(c). As shown therein, a photocathode 5 is provided in the inner upper surface of an envelope 1A for producing photoelectrons in response to radiation incident thereon. Inside the envelope 1A is provided an electron multiplier assembly 8 for multiplying the photoelectrons relayed from the photocathode 5. The multiplier assembly 8 includes a plurality of dynodes arranged vertically in a number of stages. Each stage includes a set of dynodes arranged in a two-dimensional matrix form or in one-dimensional array. The multiplier assembly 8 is disclosed in detail in the co-pending European Application No. claiming priority from Japanese Patent Application No. 3-344895 to which reference should be made. As shown in Fig. 2(b), the envelope 1A includes a generally cylindrical, disc-shaped, transparent faceplate 3 with a photocathode 5 deposited on its under surface, a generally-cylindrical sidewall 2A made entirely of metal, an outwardly-protruding, flange-shaped, annular sealing area 2b, and a generally cylindrical, disc-shaped stem 4. Preferably, the metal used for the sidewall 2A is of high magnetic permeability to impose external electric and magnetic shielding capability thereon. At one opening of the cylindrical sidewall 2A is a radially inwardly protruding, annular rim to the underside of which the faceplate 3 is annularly attached to form a hermetic seal. The sealing area 2b at the other opening of the sidewall 2A and is hermetically sealed to the stem 4 as described below. As can be seen from Fig. 2(b), a plurality of stem leads 6 for supplying voltages to the photocathode 5, dynodes and anode 10 extend through tapered hermetic glasses 9 and are vacuum sealed thereto. As can be seen from Fig. 2(c), the stem leads 6 are distributed substantially in a rectangular pattern. The photocathode 5 and the sidewall 2A are held at the same voltage. As can be seen from Fig. 2(b), the final-stage dynode's electrode 14 is horizontally held immediately below an anode 10 and immediately above the upper portions of the stem leads 6 protruding into the envelope 1A. Two of the stem leads 6 are connected to the electrode 14. The hermetic glass 9 is voltage proof but is tapered to reserve a longer distance between adjacent two hermetic glasses so that a leak current does not flow. When the operating voltage is low, the hermetic glass 9 need not be tapered but be cylindrical. Regardless of the level of the operating voltage, increasing the diameter of the envelope can prevent the leak current from flowing. As can be seen from Fig. 2(b), in the center of the stem 4 is a flared, downward-protruding metal exhaust tube 7A. Although Fig. 2(b) shows the metal exhaust tube 7A after being sealed using resistance welding techniques, before being sealed, the metal exhaust tube 7A connects the photomultiplier tube with an exhaust system made from, for example, a vacuum pump (not shown). Because the metal exhaust tube 7A is unnecessary after the photomultiplier tube has been produced, it can be severed using cold welding techniques at the final stage of producing the photomultiplier tube. As is also shown in Figs. 2(a) through 2(c), the stem 4 includes a metal, radially outwardly protruding, flange-like, annular portion 11. After the annular portion 11 is aligned with the sealing area 2b, the two are welded together using helium arc or resistance welding techniques. On the inner surface of the dynodes in the multiplier assembly 8 is formed a secondary electron emitting surface (not shown). A photomultiplier tube made according to the present invention has the sealing area 2b aligned with the flange-like annular portion 11. Once aligned, the two are welded together to form a vacuum seal using helium arc or resistance welding techniques. When this process is completed, the metal exhaust tube 7A is connected to the exhaust system which evacuates the envelope 1A. While the exhaust system evacuates the envelope 1A via the metal exhaust tube 7, alkali metal vapor is introduced through the metal exhaust tube 7 for forming and activating the photocathode 5 and the secondary emissive surface of the photomultiplier portion 8. Afterward the metal exhaust tube 7A is severed from the exhaust system using pinch-off seal and maximally shortened. Because the sidewall 2A is made entirely from metal, radioactive materials contained within glass such as K 40 are not present so noise caused by such materials is prevented. Also even if floating electrons or ions generated in the electron multiplying process strike the sidewall 2A, the sidewall 2A does not emit light and thus noise is greatly reduced. Additionally, the metal side wall 2A serves to shield the photomultiplier tube from external electric and magnetic fields. The sealing area 2b is aligned with the flange-like annular portion 11, then once aligned the two are welded together to form a vacuum seal using helium arc or resistance welding techniques. Because this method reduces production time and amount of heat involved with production, deterioration of the multiplier assembly 8 caused by heat can be avoided. Because the flared metal exhaust tube 7A is welded using resistance welding techniques and severed using pinch-off seal, the length of the flared metal exhaust tube 7A can be maximally reduced without generation or pooling of gas in the photomultiplier tube. Operation time can also be expected to reduce greatly. The envelope in the first embodiment is generally cylindrical, but can of course be angled. Further advantages exist in the present invention in that commonly used metal caps used for making up electrical devices, such as capacitors, diodes, can be used for the metal envelope, whereby a mass-production of the photomultiplier tubes can be accomplished with reduced cost. Figs. 3(a) through 3(c) show a second preferred embodiment of the present invention. In this preferred embodiment, welding is performed under a vacuum so the metal exhaust tube 7A can be omitted. After formation of the photocathode 5 and the secondary electron emissive surfaces of the dynodes, indium seal or resistance welding is performed using a transfer unit to weld the sealing area 2b and the annular portion 11 together. Because the interior of the photomultiplier tube is a vacuum before the sealing area 2b and the annular portion 11 are welded together, if the seals are airtight, the interior of the photomultiplier tube will remain a vacuum even after the photomultiplier tube is moved to a standard atmosphere. Therefore there is no need to evacuate the interior of the photomultiplier tube and the flare-shaped metal exhaust tube 7A is unnecessary. All advantages obtained with the first preferred embodiment can also be obtained in the second preferred embodiment. Additionally the second preferred embodiment allows omitting the metal exhaust tube 7A and a subsequent reduction in the number of required parts. A third preferred embodiment of the present invention is shown in Figs. 4(a) through 4(c). As can be seen from the figures, the plate-like anode electrode 10 of the first and second embodiments is replaced with a multianode 12 comprising rectangular shaped hermetic glass 120 for supporting the multianode 12 and a plurality of downwardly extending anode leads 121 which penetrate through the hermetic glass 120. In this embodiment, the multianode 12 is rectangular with the downwardly extending anode leads 121 formed in equidistant rows through the hermetic glass 120. The multianode 12 is fitted into a rectangular hole formed in the stem 4. In the figures, the anodes are arranged two-dimensionally but they may be arranged one-dimensionally. The advantages obtained with the first and second preferred embodiments can also be obtained with the third preferred embodiment. Additionally the present invention according to the third preferred embodiment can be used to determine the position where light was incident upon the photomultiplier tube, e.g., by determining which anode leads 121 produce the greatest current. Because the current from the anode leads 121 varies depending upon the amount of incident light, the anode leads 121 which output the greatest current will be those directly beneath the position where light was incident upon the photomultiplier tube. Figs. 5(a) through 5(c) show a fourth embodiment of the present invention. In the fourth embodiment, the end of the sidewall 2A to which the faceplate 3 is attached has no inwardly radially protruding annular rim. Instead of the faceplate 3 being annularly airtight welded to the underside of the inwardly radially protruding annular rim, the faceplate 3 is airtight welded to the open end of the sidewall 2A. The fourth embodiment obtains all the advantages of the embodiments described previously. Additionally, the fourth embodiment eliminates the annular rim of the sidewall 2A, thereby increasing the effective surface area of the photocathode 3. Also, because the pressure difference between the atmosphere and the evacuated interior of the photomultiplier tube urges the faceplate 3 towards the interior of the photomultiplier tube, and therefore naturally presses the faceplate 3 against the sidewall 2A, less surface area is required for airtight welding the faceplate 3 to the sidewall 2A than when the faceplate 3 is welded to the underside of the radially inwardly protruding annular rim. This also greatly increases reliability of the airtight seal of the envelope 1A. Figs. 6(a) through 6(c) show a fifth embodiment of the present invention. In the fifth embodiment, the faceplate 3 is airtight welded to the underside of the radially inwardly protruding annular rim of the sidewall 2A as in the first through third embodiments, the difference being that the faceplate 3 includes a generally hemispherical portion 13 that protrudes away from the interior of the photomultiplier tube. The fifth embodiment obtains all the advantages of the first through third embodiments. Additionally, the hemispherical portion 13 allows light angularly incident on the faceplate 3 to enter the photomultiplier tube instead of reflecting thereof. Figs. 7(a) through 7(c) shows the present invention according to a sixth embodiment. In the sixth embodiment, the photomultiplier portion 8 is thinner and the vertical height of the envelope 1A reduced to conform to the vertical height of the thinner photomultiplier assembly 8. The photomultiplier assembly 8 can be made from multi-layered dynodes as in the previous embodiments or from microchannel plates or semiconductor elements. Because sealing the envelope 1A by applying resistance welding techniques leaves the photomultiplier portion 8 almost unaffected by heat, such a vertically thin photomultiplier tube is possible. The sixth embodiment is particularly advantageous in that it reduces the amount of space taken up by the photomultiplier tube. Figs. 8(a) through 8(c) show a seventh embodiment of the present invention. In the seventh embodiment a generally circular hole is opened in the stem 4. Into the hole is fitted a large, generally circular, tapered hermetic glass 9A which meets the circular size of the hole. Positioned following the perimeter of the hermetic glass 9A are a plurality of leads which penetrate through the hermetic glass so one end of each lead is exposed to the interior of the photomultiplier tube and the other end is exposed to the exterior of the photomultiplier tube. In the center of the stem 4 is a metal exhaust tube 7A. The seventh embodiment is particularly advantageous in that manufacturing cost can be reduced by reducing the number of parts. Figs. 9(a) through 9(c) show an eighth embodiment of the present invention. The eighth embodiment is the same as the seventh embodiment except that in the eighth embodiment the metal exhaust tube 7A is omitted. In the eighth embodiment, as in the second embodiment, after formation of the photocathode 5 and the secondary electron emissive surface of the dynodes, indium seal or resistance welding is performed to weld the sealing area 2b and the flange-like annular portion 11 together. Because the interior of the photomultiplier tube is a vacuum before and after the sealing area 2b and the flange-like annular portion 11 are welded together, there is no need to evacuate the interior of the photomultiplier tube. Therefore the flare-shaped metal exhaust tube is unnecessary. The eighth embodiment is particularly advantageous in that the manufacturing cost can be reduced by reducing the number of parts. Also because the metal exhaust tube 7A is omitted, the leads 6 can be more easily inserted into their appropriate sockets. Because the sidewall 2A is made entirely from metal, noise caused by such radioactive materials contained within glass, such as K 40 , is prevented. Also even if floating electrons or ions strike the metal sidewall 2A, light does not emanate from the side wall 2A, providing great reductions in noise. The sealing area 2b is aligned with the flange-like annular portion 11, then once aligned the two are welded together using helium arc or resistance welding techniques to form a vacuum seal. Because this method reduces production time and amount of heat involved with production, quality problems related to heat can be avoided. Because the flared metal exhaust tube 7A is welded using resistance welding techniques and severed using pinch-off seal, the length of the flared metal exhaust tube 7A can be maximally reduced without generation or pooling of gas in the photomultiplier tube. Operation time can also be expected to reduce greatly. Figs. 10(a) through 10(c) and Figs. 11(a) through 11(c) show ninth and tenth embodiments of the present invention, respectively. The ninth embodiment is similar to the first embodiment shown in Figs. 2(a) through 2(c) except the circular cross-section of the envelope 1A in the first embodiment is square in the ninth embodiment. The cross-section of the envelope 1A may be rectangular. The tenth embodiment is also similar to the first embodiment shown in Figs. 2(a) through 2(c) except the circular cross-section of the envelope 1A in the first embodiment is hexagon in the tenth embodiment. The ninth and tenth embodiments are advantageous in that a plurality of photomultiplier tubes can be arranged without gaps forming therebetween as with circular cross-section photomultiplier tubes. Consequently less light passes between the photomultiplier tubes when tightly arranged one- or two-dimensionally and less light is lost.

Claims: A photo-multiplier tube comprising: a tubular metal side wall (2) having first and second ends in a longitudinal direction; a transparent face plate (3) hermetically sealed to the first end of said metal side wall (2), said face plate (3) having a surface; a stem (4) hermetically sealed to the second end of said metal side wall (2), wherein said metal side wall (2), said face plate (3) and said stem (4) form an airtight chamber (1A) with the surface of said face plate (3) being directed inwardly of said airtight chamber (1A); a photocathode (5) formed on the surface of said face plate (3) for producing electrons in response to incident radiation thereon; an electron multiplier assembly (8) provided within the airtight chamber (1A) for multiplying the electrons relayed from said photocathode (5); and, an anode (10) for receiving multiplied electrons from said electron multiplier assembly (8) and producing an output signal representative of the radiation incident on said photocathode (5), characterised in that the second end of said metal side wall (2) includes a flange shaped sealing portion (2b) and said stem (4) includes a metal flange shaped, airtight sealing section (11), said sealing portion (2b) of said metal side wall being hermetically sealed to said sealing section (11) of said stem (4). A photo-multiplier tube according to claim 1, wherein said sealing portion (2b) of said metal side wall (2) and said sealing section (11) of said stem are sealed together by resistance welding. A photo-multiplier tube according to claim 1 or 2, further comprising a metal exhaust tube formed in said stem, said metal exhaust tube (7A) being sealed and maximally shortened after gaseous matter within said airtight chamber (1A) has been evacuated. A photo-multiplier tube according to claim 3, wherein said metal exhaust tube (7A) is sealed by pinch-off sealing. A photo-multiplier tube according to any one of the preceding claims, wherein said electron multiplier assembly comprises a plurality of dynodes arranged in a predetermined number of stages in the longitudinal direction of said metal side wall, each stage including a predetermined number of dynodes being in one dimensional array or being arranged two dimensionally in a matrix form. A photo-multiplier tube according to any one of the preceding claims, wherein said anode (10) includes a plurality of plate shaped anode elements for receiving the electrons from said electron multiplier assembly, and a plurality of electrically isolated leads (16) connected in one-to-one correspondence to said plurality of plate shape anode elements, said plurality of leads being sealed through said stem (4). A photo-multiplier tube according to claim 6, wherein the plurality of leads (6) are electrically isolated and sealed through the stem (4) by glass (9). A photo-multiplier tube according to any one of the preceding claims, wherein the first end of said metal side wall (2) is provided with an annular, radially inwardly protruding portion with a surface confronting the airtight chamber (1A), said face plate (3) being hermetically sealed to the surface. A photo-multiplier tube according to any one of the preceding claims, wherein said face plate (3) is generally hemispheric (13) for allowing angular incident light to pass through. A photo-multiplier tube according to any one of the preceding claims, wherein said electron multiplier assembly (8) includes a micro-channel plate or a semiconductor device. A photo-multiplier tube according to any one of the preceding claims, wherein said metal side wall has a circular cross section, a square cross section, a rectangular cross section or a hexagonal cross section.

Inventors: HASEGAWA YUTAKA, ITO MASUO, KYUSHIMA HIROYUKI, OBA KOICHIRO, TAKEUCHI JUNICHI, HASEGAWA, YUTAKA, ITO, MASUO, KYUSHIMA, HIROYUKI, OBA, KOICHIRO, TAKEUCHI, JUNICHI, HASEGAWA, YUTAKA, C/O HAMAMATSU PHOTONICS K.K., Ito, Masuo, c/o Hamamatsu Photonics K.K., KYUSHIMA, HIROYUKI, C/O HAMAMATSU PHOTONICS K.K., Oba, Koichiro, c/o Hamamatsu Photonics K.K., TAKEUCHI, JUNICHI, C/O HAMAMATSU PHOTONICS K.K.
Applicants: HAMAMATSU PHOTONICS KK, HAMAMATSU PHOTONICS K.K.
Application year: 1993
Main classification: H01J43
IPC codes: H01J43