Photomultiplier

Abstract: A photomultiplier of the present invention has an electron multiplier of a structure which can be manufactured easily. This electron multiplier is constituted by dynode plates that are stacked through insulators so as to be separated from each other at a predetermined interval. Each dynode plate comprises upper- and lower-electrode plates that are electrically connected to each other. The upper- and lower-electrode plates grip at least one of the insulators such that the gripped insulator is partly exposed.

Description: The invention relates to an electron multiplier. The electron multiplying portion of an electron multiplier is constituted by stacking a plurality of dynodes in a plurality of stages at a predetermined gap. U.S. Patent No. 3,229,143 discloses an arrangement in which an electron multiplying portion is constituted by stacking a plurality of metal plates supporting dynodes in a plurality of stages through insulating balls. The invention aims to obtain the structure of an electron multiplier which decreases variations in gain at different portions on an incident surface on which an electron flow, ions, or the like is incident. It is well-known that the electron multiplier can be applied to a photomultiplier or the like. This electron multiplier has at least a dynode unit and an anode. The dynode unit for cascade-multiplying the electron flow or the like is constituted by stacking metal plates each having a plurality of dynodes in the incident direction of the electron flow or the like. Spacers made of an insulating material are arranged among these plates. The interval between the plates adjacent to each other in the stacking direction is maintained at a predetermined value by the corresponding spacer. Part of each spacer is fitted in the opening of a through hole formed in the corresponding plate, so that the respective plates are in a direction perpendicular to the stacking direction. The spacers and the plates supporting the dynodes are made of different materials and are independent members. Thus, sometimes the spacers may be shifted, among the plates, from positions where the openings of the corresponding through holes should be present, or the spacers may not be present among the plates. The spacers serve to maintain the gap among the dynode plates at a predetermined value. Hence, if the interval between the metal plates adjacent to each other in the stacking direction is not uniformly maintained at the respective portions of the metal plates, the gain differs depending on portions where the electron flow, ions, or the like is incident. According to the invention there is provided an electron multiplier comprising: an anode plate; and a dynode unit comprising a plurality of dynode plates stacked in a direction perpendicular to a major surface of said anode plate, each of said dynode plates comprising at least one dynode, an upper-electrode plate with a first through hole, a lower-electrode plate with a second through hole, the upper- and lower-electrode plates being in tight contact with and electrically connected to each other, and an insulator for separating the dynode plate from an adjacent dynode plate in the dynode unit, characterised in that the first through hole and the second through hole together form an aperture for retaining the insulator partially therewithin. A photomultiplier according to an embodiment of the present invention comprises, as shown in Fig. 1, a photocathode (21), an anode plate (25), and a dynode unit (20) provided between the photocathode (21) and the anode plate (25). The dynode unit (20) has a plurality of dynode plates (24) which are stacked in a direction perpendicular to a major surface (251) of the anode plate (25) at a predetermined gap through insulators (spacers) (27) (see Fig. 16). The direction perpendicular to the major surface (251) of the anode plate (25) coincides with a stacking direction (31) of the dynode plates (24) (see Figs. 2, 3, 4, 7, 16 and 17) and the incident direction of the electron flow or the like (see Fig. 11). The respective dynode plates (24) are electrically connected to metal pins (14) extending through the bottom portion of a vacuum vessel (10) into its interior where the dynode unit (20) is provided. Predetermined voltages are independently applied to the dynode plates (24) from an external power supply (200) through the metal pins (14). Then, the dynode plates (24) are set at appropriate potentials, respectively. Particularly, as shown in Fig. 1, each dynode plate (24) supports at least one dynode (240) and has an upper-electrode plate (24a) and, as shown in Figs. 2 and 17, a lower-electrode plate (24b) which is in tight contact with and electrically connected to the upper-electrode plate (24a) and grips the insulators (27) together with the upper-electrode plate (24a). Fig. 2 shows a case wherein each insulator (27) is spherical. Fig. 17 shows a case wherein each insulator (27) is a circular cylinder. The upper-electrode plate (24a) has a first through hole (241) formed with a tapered portion (243) at its first upper-electrode surface (24a 1 ) that is of the upper-electrode plate and is in tight contact with the lower-electrode plate (24b). The first through hole (241) holds the corresponding insulator (27) and couples a second upper-electrode surface (24a 2 ) of the upper-electrode plate (24a) on the opposite side to the first upper-electrode surface (24a 1 ) with the tapered portion (243). The lower-electrode plate (24b) has a second through hole (242). The second through hole (242) couples a first lower-electrode surface (24b 1 ) that is of the lower-electrode plate and is in tight contact with the upper-electrode plate (24a) with a second lower-electrode surface (24b 2 ) on the opposite side to the first lower-electrode surface (24b 1 ), and holds the corresponding insulator (27). An area (S2 in Fig. 2 or S5 in Fig. 17) of an opening (242a) of the second through hole (242) located on the first lower-electrode surface side of the second through hole (242) is larger than an area (S3 in Fig. 2 or S6 in Fig. 17) of an opening (242b) of the second through hole (242) located on the second lower-electrode surface side of the second through hole (242). In particular, the area (S2 or S5) of the opening (242a) of the second through hole (242) on the first lower-electrode surface side is larger than a maximum area (S1 in Fig. 2 or S4 in Fig. 17) of the section of the insulator (27), the section being in parallel to the major surface (251) of the anode plate (25). On the other hand, the area (S3 or S6) of the opening (242b) of the second through hole (242) located on the second lower-electrode surface side is smaller than the maximum area (S1 or S4) of the section of the insulator (27), the section being in parallel to the major surface (251) of the anode plate (25). When each insulator (27) is spherical, as shown in Fig. 2, its section having the maximum area is a section including its center (270). When each insulator (27) is a circular cylinder, as shown in Fig. 17, its section having the maximum area is a section including its central axis (32). The central axis (32) of the circularly insulator (27) is in parallel to the major surface (251) of the anode plate (25). Thus, an inner wall (242c) of the second through hole (242) is brought into contact with the corresponding insulator (27), and part of this insulator (27) projects from the opening (242b) of the second through hole (242) located on the second lower-electrode surface (24b 2 ). When part of this insulator (27) is gripped by the upper- and lower-electrode plates (24a and 24b), its center (270) (or the central axis (32) if the insulator (27) is a circular cylinder) is located between the first and second lower-electrode surfaces (24b 1 and 24b 2 ) of the lower-electrode plate (24b), i.e., in a gap (T) shown in Fig. 4. The gap (T) corresponds to the thickness of the lower-electrode plate (24b). The dynode unit (20) is obtained by stacking the dynode plates (24) each constituted by the upper- and lower-electrode plates (24a and 24b) gripping the corresponding insulator (27) in the above manner. When the dynode plates (24) are stacked, the insulators (27) adjacent to each other in the direction perpendicular to the major surface (251) of the anode plate (25) are brought into direct contact with each other. The adjacent insulators (27) are also in contact with the edges of the first through holes (241) formed in the upper-electrode plates (24a) (see Fig. 4). From the above arrangement, the first through holes (241) formed in each upper-electrode plate (24a) realize alignment of the dynodes (240) supported by a dynode plate (24) located above this dynode plate (on the photocathode side). Since the insulators (27) adjacent to each other are in contact with respect to the major surface (251) of the anode plate (25), the respective insulators (27) transmit a structural pressure applied from the upper side (photocathode side) directly to the lower side (anode plate side). In other words, with this structure, the physical load applied to the dynode plates (24) gripping the insulators (27), e.g., the structural pressure that might cause fracture of the dynode plates (24), is decreased. Furthermore, when the upper- and lower-electrode plates (24a and 24b) are brought into tight contact with each other, each insulator (27) is accommodated in a space defined by a side wall (243a) of the tapered portion (243) and the inner wall (242c) of the second through hole (242) as it partly projects from the opening (242b) of the second through hole (242). Therefore, each of the insulators (27) gripped by the upper- and lower-electrode plates (24a and 24b) serves as the spacer that separates the respective dynode plates (24) from each other by a predetermined interval. Since the spherical bodies are fixed by sandwiching them with the upper- and lower-electrode plates of the dynode plates, in the electron multiplier manufacturing process of stacking the plurality of dynode plates, the spherical bodies will not be shifted from the corresponding through holes. Thus, the gaps among the dynodes of the respective plates can be maintained at the predetermined value, so that variations in gain caused by the non-uniformity in the gap of the dynodes of the respective stage are decreased. In particular, when the plurality of dynode plates are stacked as in the conventional case, since the spherical bodies need not be arranged at predetermined portions on the dynode plates, the manufacture can be facilitated. As part of each spherical body projecting from the corresponding through hole is abutted against the corresponding through hole of the adjacent dynode plate, the centers of the spherical bodies and the centers of the corresponding through holes, both continuous in the vertical direction, coincide with each other, thereby performing alignment of the dynode plates of the respective stages in the horizontal direction. When the spherical bodies held by the adjacent dynode plates are abutted against each other, most of the power applied in the stacking direction (the direction perpendicular to the major surface of the anode plate) of the dynode plates acts on the series of spherical bodies, and an extra stress will not be applied to the respective plates. The insulators described above can be insulating circular cylinders in place of spherical bodies. An operation which is substantially the same as that of the spherical bodies can be obtained with the circular cylinders. The present invention will become more fully understood from the detailed description of embcdiments given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention. Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description. IN THE DRAWINGS Fig. 1 is a perspective view showing the internal structure of a photomultiplier according to an embodiment of the invention; Fig. 2 is a perspective view showing the holding structure of an insulator of a dynode plate; Figs. 3 and 4 are sectional views showing the holding structure of the insulator of the dynode plate and taken along lines A-A and B-B in Figs. 2 and 17. Figs. 5 and 6 are views for explaining the method of bringing upper- and lower-electrode plates constituting each dynode plate into tight contact with each other; Fig. 7 is a sectional view showing the structure of an electron multiplier obtained by stacking the dynode plates shown in Figs. 5 and 6; Fig. 8 is a perspective view for explaining the first engaging structure of a dynode plate and a voltage supply pin; Fig. 9 is a perspective view showing the second engaging structure of a dynode plate and a voltage supply pin; Fig. 10 is a view showing the section of an anode plate; Fig. 11 is a side view showing the structure of an electron multiplier shown in Fig. 1; Fig. 12 is a plan view of the photomultiplier according to an embodiment of the invention; Fig. 13 is a side sectional view of the photomultiplier according to an embodiment of the invention; Fig. 14 is a partially cutaway view showing the holding structure of an insulating ball of the dynode plate; Fig. 15 is a partially cutaway view showing a portion in the dynode plate where the insulating ball is disposed; Fig. 16 is a sectional view showing the structure of the electron multiplier shown in Fig. 1; and Fig. 17 is a perspective view showing the holding structure of a circularly cylindrical insulator of a dynode plate. DESCRIPTION OF THE EMBODIMENTS The structure of a photomultiplier, and in particular an electron multiplier, according to embodiments of the invention will be described with reference to Figs. 1 to 17. In order to achieve the above object, as shown in Fig. 1 a photomultiplier embodying the invention has, in a vacuum vessel 10, a photocathode 21, an anode plate 25 formed with a plurality of openings or through holes 250, and a dynode unit 20 provided between the photocathode 21 and the anode plate 25. The vacuum vessel 10 is hermetically formed with a metal side tube 11, a glass plate 12 mounted to one end of the metal side tube 11, and a circular stem 13 mounted to the other end of the metal side tube 11. Furthermore, in the vacuum vessel 10, a focusing electrode plate 22 for correcting the orbit of photoelectrons emitted from the photocathode 21 is provided between the photocathode 21 and the dynode unit 20. A plurality of openings (focusing electrodes) 23 for passing the photoelectrons therethrough are formed in the focusing electrode plate 22, and are set at a higher potential than that of the photocathode 21. In this photomultiplier, a last-stage dynode plate 26 is provided between the anode plate 25 and a major surface 13a of the stem 13. The potential of the dynode plate 26 is set to be lower than or equal to that of the anode plate 25. The dynode plate 26 serves to emit secondary electrons toward the anode by receiving the secondary electrons emitted from the dynode unit 20 and passing through the openings 250 of the anode plate 25. Thus, the secondary electrons multiplied by the dynode unit 20 are captured efficiently by the anode. The dynode unit 20 has a plurality of dynode plates 24 stacked in a direction perpendicular to the major surface 251 of the anode plate 25 such that they are spaced apart from each other by a predetermined gap through insulators (spacers) 27 (see Fig. 16). The direction perpendicular to the major surface 251 of the anode plate 25 coincides with a stacking direction 31 of the dynode plates 24 (see Figs. 2, 3, 4, 7, 16, and 17) and the incident direction of the electron flow or the like (see Fig. 11). The respective dynode plates 24 are electrically connected to connection pins 14 extending through the bottom portion (stem 13) of the vacuum vessel 10 into its interior where the dynode unit 20 is provided. The connection pins 14 are inserted in the vacuum vessel 10 perpendicularly to the major surface 13a of the stem 13, and are in direct electrical contact with the associated dynode plates 24. Predetermined voltages are independently applied from an external power supply 200 to the dynode plates 24 or the like through the connection pins 14. Thus, the plates 22, 24, 25, 26 are set at appropriate potentials, respectively. The basic structure, operating principle, and material of the above photomultiplier are described in, e.g., U.S.P. No. 4,023,063, Japanese Patent Laid-Open Nos. 60-39752, 60-182642, 62-41378 and 3-147240, and RCA Photomultiplier Handbook. As shown in Fig. 2, each dynode plate 24 is constituted by an upper-electrode plate 24a and a lower-electrode plate 24b which is in tight contact with and electrically connected to the upper-electrode plate 24a and grips the insulators 27 together with the upper-electrode plate 24a. Fig. 2 shows a case wherein the insulators 27 are spherical. Fig. 17 shows a case wherein the insulators 27 are circular cylinders. The upper-electrode plate 24a has a first through hole 241 formed with a tapered portion 243 at its first upper-electrode surface 24a 1 which is in tight contact with the lower-electrode plate 24b. The first through hole 241 aligns the insulator 27 gripped by the another dynode plate (the photocathode side of the hole 241) and couples a second upper-electrode surface 24a 2 of the upper-electrode plate 24a on the opposite side to the first upper-electrode surface 24a 1 with the tapered portion 243. The lower-electrode plate 24b has a second through hole 242. The second through hole 242 couples a first lower-electrode surface 24b 1 which is in tight contact with the first upper-electrode surface of the upper-electrode plate 24a with a second lower-electrode surface 24b 2 on the opposite side to the first lower-electrode surface 24b 1 , and grips the insulator 27. An area S2 of an opening 242a of the second through hole 242 located on the first lower-electrode surface side of the second through hole 242 is larger than an area S3 of an opening 242b of the second through hole 242 located on the second lower-electrode surface side of the second through hole 242. In particular, the area S2 of the opening 242a of the second through hole 242 on the first lower-electrode surface side is larger than a maximum area S1 of the section of the insulator 27, the section being in parallel to the major surface 251 of the anode plate 25. On the other hand, the area S3 of the opening 242b of the second through hole 242 located on the second lower-electrode surface side is smaller than the maximum area S1 of the section of the insulator 27, the section being in parallel to the major surface 251 of the anode plate 25. When each insulator 27 is spherical, as shown in Fig. 2, its section having the maximum area is a section including its center 270. When each insulator 27 is a circular cylinder, as shown in Fig. 17, its section having the maximum area is a section including its central axis 32. The central axis 32 of the circularly cylindrical insulator 27 is perpendicular to a direction (the stacking direction 31 of the dynode plates 24) perpendicular to the major surface 251 of the anode plate 25. In other words, the center axis 32 is in parallel to the major surface 251 of the anode plate 25. Thus, an inner wall 242c of the second through hole 242 is brought into contact with the corresponding insulator 27, and part of this insulator 27 projects from the opening 242b of the second through hole located on the second lower-electrode surface 24b 2 . When this insulator 27 is held by the upper- and lower-electrode plates 24a and 24b, its center 270 (or the central axis 32 if the insulator 27 is a circular cylinder) is located between the first and second lower-electrode surfaces 24b 1 and 24b 2 of the lower-electrode plate 24b, i.e., in a gap T shown in Fig. 4. The gap T corresponds to the thickness of the lower-electrode plate 24b. The dynode unit 20 is obtained by stacking the dynode plates 24 each constituted by the upper- and lower-electrode plates 24a and 24b gripping the associated insulator 27 and tightly fixed with each other in the above manner. When the dynode plates 24 are stacked, the insulators 27 that are adjacent to each other in the direction perpendicular to the major surface 251 of the anode plate 25 are brought into direct contact with each other at points C1. The adjacent insulators 27 are also in contact with the edges (indicated by reference symbols C2 and C3 in Fig. 4) of the first through holes 241 formed in the upper-electrode plates 24a (see Fig. 4). From the above arrangement, the first through holes 241 formed in each upper-electrode plate 24a realize alignment of the dynodes 241 supported by a dynode plate 24 located above this dynode plate (on the incident side of the electron flow). Since the insulators 27 adjacent in the direction perpendicular to the major surface 251 of the anode plate 25 are in contact with each other, the respective insulators 27 transmit a structural pressure applied from the upper side (photocathode side) directly to the lower side anode (plate side). In other words, with this structure, the physical load applied to the dynode plates 24 holding the insulators 27, e.g., the structural pressure that might cause fracture of the dynode plates 24, is decreased. Furthermore, when the upper- and lower-electrode plates 24a and 24b are brought into tight contact with each other, each insulator 27 is accommodated in a space defined by a side wall 243a of the tapered portion 243 and the inner wall 242c of the second through hole 242 as it partly projects from the opening 242b of the second through hole 242. Therefore, the insulators 27 held by the upper- and lower-electrode plates 24a and 24b serve as the spacers that separate the respective dynode plates 24 from each other by a predetermined gap. This arrangement will be described in more detail with reference to Figs. 3 and 4. The sectional views of Figs. 3 and 4 show the holding structure of the insulator shown in Fig. 2. These sections coincide with the section of the circularly cylindrical insulator of Fig. 17 that shows the holding structure of the circularly cylindrical insulator, which is perpendicular to the central axis 32 of the circular cylinder. As shown in Fig. 3, the tapered portion 243 having an opening with a diameter D5 is formed in the first upper-electrode surface 24a 1 of the upper-electrode plate 24a. This tapered portion 243 is communicated with the second upper-electrode surface 24a 2 by the through hole 241 having a diameter D4. The through hole 242 for coupling the first and second lower-electrode surfaces 24b 1 and 24b 2 is formed in the lower-electrode plate 24b. Upper and lower diameters D2 and D3 of the opening 242 and a diameter D1 of the insulators 27 satisfy the following relationship: D3 < D1 < D2 where D1: the diameter of the spherical insulators 27 D2: the diameter of the opening 242a of the through hole 242 on the first lower-electrode surface side D3: the diameter of the opening 242b of the through hole 242 on the second lower-electrode surface side With the above arrangement, when the upper- and lower-electrode plates 24a and 24b are brought into tight contact with each other, the insulators 27 are held by the side wall 243a of the tapered portion 243 and the inner wall 242c of the through hole 242, as shown in Fig. 4. In Fig. 4, reference symbol C1 denotes a contact point where the insulator held by a given dynode plate 24 and another insulator adjacent to this insulator in the stacking direction 31 are in contact with each other, and C2 and C3 denote contact points where this another insulator and the edge of the through hole 241 formed in the upper-electrode plate 24a are in contact with each other. When the insulator held by the dynode plate 24 located on the upper side (on the photocathode side) is fixed by the through hole (the through hole 241 formed in the upper-electrode plate) of an adjacent lower side dynode plate (on the anode plate side), the dynodes of the respective stages are aligned. The structure for bringing the upper- and lower-electrode plates 24a and 24b constituting each dynode plate 24 into tight contact with each other will be described with reference to Figs. 5 to 7. Fig. 5 shows an nth-stage dynode plate. This metal plate is provided with adhering portions 100 which are predetermined portions around the metal plate and which project in a direction perpendicular to the stacking direction. The upper- and lower-electrode plates 24a and 24b are brought into tight contact with each other and are resistance-welded at the adhering portions 100. In Fig. 5, reference symbols W denote welding portions. Fig. 5 also shows an engaging portion 900 which electrically connects a connection pin 14 (to be described later) to this metal plate. Fig. 6 is a view showing an (n+1)th-stage dynode plate. Adhering portions 100 and an engaging portion 900 are shown in Fig. 6 in the same manner as in Fig. 5. The positions where the adhering portions 100 and the engaging portion 900 are provided differ from one dynode plate to another. In other words, the positions of the adhering portions 100 and the engaging portion 900 are shifted so that the gap among the plates at the welding portions W will not become smaller than the gap at other portions when the dynode plates are stacked as shown in Fig. 7. The tight contact structure of the upper- and lower-electrode plates 24a and 24b constituting each dynode plate is disclosed in, e.g., U.S.Patent Application Serial No. 08/234,152. The arrangement for connecting the connection pins 14 and the dynode plates 24 will be described with reference to Figs. 8 and 9. Fig. 8 shows the first structure in which the engaging portion 900 for the connection pin 14 is constituted by two projections 900a and 900b. As the second structure, as shown in Fig. 9, a U-shaped projection 990 may serve as the engaging portion, so that the distal end of the connection pin 14 is engaged with the associated dynode plate 24. The structure for engaging the dynode plate with the distal end of the connection pin 14 is disclosed in, e.g., U.S. Patent Application Serial No. 08/234,142. Fig. 10 shows the section of the anode plate 25. The anode plate 25 is provided between the dynode unit 20 and the last-stage dynode plate 26 and is formed with the plurality of through holes 250. An opening gap S of the openings 250 on the dynode unit 20 side is smaller than an opening gap Q of the openings 250 on the dynode plate 26 side. A surface 251 in parallel to the dynode plate 26 is formed on the inner wall of each opening 250. Since the secondary electrons emitted from the last-stage dynode plate 26 are positively captured by the surface 252, the anode plate 25 can efficiently capture the secondary electrons. The structure of the anode plate 25 is disclosed in, e.g., U.S. Patent Application Serial No. 08/234,153. The structure of the photomultiplier embodying the invention, and in particular, of the electron multiplier, will be described in further detail with reference to Figs. 11 to 17. Referring to Fig. 11, in the electron multiplier of this embodiment, the dynode unit 20 for multiplying the incident electron flow is disposed in the circularly cylindrical vacuum vessel 10. The vacuum vessel 10 is constituted by the cylindrical metal side tube 11, the circular light-receiving surface plate 12 provided at one end of the metal side tube 11, and the circular stem 13 provided at the other end of the metal side tube 11 and constituting the base portion. The photocathode 21 is provided on the lower surface of the light-receiving surface plate 12, and the focusing electrode plate 22 is disposed between the photocathode 21 and the dynode unit 20. The dynode unit 20 is constituted by stacking the dynode plates 24 having a large number of electron multiplication holes (dynodes) 240 in a plurality of stages. The anode plate 25 and the last-stage dynode plate 26 are disposed under the stacked dynode plates 24 in this order. The insulator balls 27 made of a ceramic are incorporated into the dynode plates 24 and the anode plate 25. The stem 13 serving as the base portion is connected to the voltage terminals of the external power supply 200, and a total of 12 stem pins 14 for applying predetermined voltages to the respective dynode plates 24 and 26, and the like extend through the stem 13. Each stem pin 14 is fixed to the stem 13 with a tapered hermetic glass member 15. Each stem pin 14 has a length to reach the dynode plate to be connected, and its distal end is resistance-welded with the connection terminal 900 or 990 of the corresponding dynode plate 24 or 26. Fig. 11 is a side view of the electron multiplier according to this embodiment, and Fig. 12 is a plan view of the electron multiplier of this embodiment. Light 30 incident on the light-receiving surface plate 12 excites electrons in the photocathode 21 on the lower surface of the light-receiving surface plate 12 to emit photoelectrons in a vacuum. The photoelectrons emitted from the photocathode 21 are focused on the uppermost-stage dynode plate 24 by the grid-type focusing electrode plate 22 (see Fig. 12) to perform secondary multiplication. The secondary electrons emitted from the dynodes 240 of the uppermost-stage dynode plate 24 are supplied to the dynodes of the lower-stage dynode plates 24 to repeat secondary electron emission. The secondary electron group emitted from the last-stage dynode plate 26 is obtained by the anode plate 25. The obtained secondary electron group is output to the outside through the stem pin 14 connected to the anode plate 25. Fig. 13 is a side sectional view of the electron multiplier according to the invention. Referring to Fig. 13, the dynode unit 20 of the electron multiplier is stacked on the anode plate 25 arranged on the last-stage dynode plate 26, and the focusing electrode plate 22 is stacked on the dynode unit 20. The insulator balls 27 are incorporated into the disposing portions at the outer edges of the dynode plates 24 and anode plate 25 (to be referred to as metal plates hereinafter). The predetermined gap among the upper- and lower-stage metal plates is maintained by the projecting portions of the insulator balls 27. A plurality of insulator balls 27 are arranged along the outer edge of each dynode plate 24 and the like, and the insulator balls 27 of the upper and lower stages are in contact with each other. Thus, most of the power applied in the stacking direction 31 of the metal plates is absorbed by the series of insulator balls 27. Even if a power is applied in the stacking direction, no extra force acts on the respective metal plates. Each metal plate is constituted by bonding the upper- and lower-electrode plates 24a and 24b, and the dynodes 240 having arcuated inner side surfaces are formed in the electrode plates 24a and 24b. A technique for constituting the dynode plate from two metal plates is disclosed in, e.g., Japanese Patent Laid-Open No. 60-182642. Electron multiplication holes are formed in each metal plate. The secondary electron emission surfaces 240a are formed on the inner walls of these holes, thereby constituting the dynodes 240. The material and the like for forming the secondary electron emission film is disclosed in, e.g., "Surface Analysis of Cu-Be Dynode" (J. Vac. Sci. Technology 17(5), Sept./Oct. 1980, pp. 1221 - 1224). The last-stage dynode stage 26 and the anode plate 25 have openings 250 and 260 for passing through gas forming the secondary electron emission surface, respectively. Figs. 14 to 16 are partially cutaway views showing the structure for gripping the insulator balls 27 in the dynode plates 24, which is the characteristic portion of this embodiment. As shown in Fig. 14, the through holes 241 are formed in the upper-electrode plate 24a of each dynode plate 24, and the through holes 242 are formed at positions of the lower-electrode plate 24b corresponding to the through holes 241. The side wall of the through hole 241 is formed in the tapered manner such that its lower half is widened downward in Fig. 14. The diameter of the upper opening of the through hole 241 is smaller than the diameter of the insulator ball 27, and the diameter of the lower opening of the through hole 241 is larger than the diameter of the insulator ball 27. The side wall of the through hole 242 is formed in the tapered manner such that it is entirely widened upward in Fig. 14. The diameter of the upper opening of the through hole 242 is larger than the diameter of the insulator ball 27, and the diameter of the lower opening of the through hole 242 is smaller than the diameter of the corresponding insulator ball 27. Thus, when the insulator ball 27 is placed in the through hole 242 in the lower-electrode plate 24b, the insulator ball 27 is caught by the tapered side wall of the through hole 241, and is held such that about its lower 1/3 projects from the lower opening of the through hole 242. Subsequently, as shown in Fig. 15, the through holes 241 and 242 are mated and the upper- and lower-electrode plates 24a and 24b are bonded. Then, the insulator ball 27 is depressed from obliquely upward by the tapered side wall of the through hole 241, and is completely fixed by the upper- and lower-electrode plates 24a and 24b. Since the side walls of the through holes 241 and 242 are formed in the tapered manner, automatic alignment is performed along the tapered side walls by only placing the insulator balls 27 in the space surrounded by the through holes 241 and 242, thereby precisely setting the upper- and lower-electrode plates 24a and 24b to coincide with each other. When the insulator balls 27 are fixed in the through holes 241 and 242, the upper- and lower-electrode plates 24a and 24b around the insulator balls 27 are resistance-welded, thereby obtaining the dynode plate 24 provided the insulator balls 27 therein. As the dynode plates 24 has this structure, the dynode plates 24 and the insulator balls 27 are integrated with each other. Thus, the insulator balls 27 will not be removed from the dynode plate 24 or drop. As shown in Fig. 16, the dynode plates 24 into which the insulator balls 27 are incorporated are stacked in a plurality of stages. The stacking operation is adjusted such that part of each insulator ball 27 projecting from the through hole 242 of the dynode plate on the upper stage is partially accommodated in the associated through hole 241 of the dynode plate on the lower stage. When the stacking operation is adjusted in this manner, the centers of the series of insulator balls 27 are located on one straight line 31. The diameters, the sizes of the openings, and the inclined angles of the tapered side walls, respectively, of the through holes 241 and 242 in each of the dynode plate 24 are the same, and thus the sizes (diameters) of the opposing insulator balls 27 are the same. Accordingly, the central axis of the through holes 241 and 242 always coincides with the centers 270 of the insulator balls 27. As a result, the dynode plates 24 and 26 will not be shifted in the horizontal direction, and the stacking gap among them becomes constant. In this embodiment, the insulator balls 27 having a diameter of 0.66 mm are used, and the gap between the dynode plates adjacent in the vertical direction is 0.25 mm. With this arrangement, the dynode plates 24 and 26, the anode plate 25, and the focusing electrode plate 22 can be stacked easily and accurately. In this embodiment, the insulator balls 27 are used as the insulating spacers. However, the insulating spacers are not limited to the spherical bodies, but can be formed as the insulating circular cylinders 40 as shown in Fig. 17. Even if the insulating spacers are formed as the insulating circular cylinders 40, the same operation and effect as those of the above embodiment can be obtained. In this case, through holes 241 and 242 of opposing dynode plates 24 and the like may be formed at appropriate positions to have a shape (a V-groove shape) so that they extend along the side surfaces of the circular cylinder 40. In the embodiment shown in Fig. 17, the section of the circular cylinder 40 perpendicular to its central axis 32 coincides with the section shown in Figs. 3 and 4. This embodiment is exemplified by a photomultiplier having the photocathode 21. However, a focusing electrode plate 22, an anode plate 25, and a dynode unit 20 may be disposed in an electron multiplier having no photocathode 21, as a matter of course. In this embodiment, through holes are formed in the dynode plates. However, through holes may be formed in support plates supporting the dynodes, e.g., the support plates shown in U.S. Patent No. 3,229,143. As has been described above in detail, according to the electron multiplier embodying the present invention, since the spherical or circularly cylindrical spacers are fixed by sandwiching them with the upper- and lower-electrode plates of the dynode plates, when stacking the plurality of dynodes plates, the spacers will not be shifted. Thus, the gap among the respective stages can be maintained at the predetermined value, so that variations in gain caused by the non-uniformity in the gap of the dynodes are decreased. In particular, when the plurality of dynode plates are stacked as in the conventional case, since the spacers need not be arranged on the dynode plates, the manufacture can be facilitated. When the dynode plates are stacked, the centers of the respective spherical bodies or the central axes of the respective circular cylinders coincide with the centers of the through holes in the stacking direction. Thus, positional shifts of the dynode plates in the horizontal direction can be prevented, thereby decreasing variations in gain. Furthermore, when the spacers adjacent to each other in the stacking direction are abutted against each other, even if a power in the stacking direction is applied to the spacers, most of the power acts on the series of spacers, so that the dynode plates will not be deformed. Accordingly, the gap among the dynode plates of the respective stages can be maintained at the predetermined value. From the invention thus described, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention as claimed, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims: An electron multiplier comprising: an anode plate (25) ; and a dynode unit (20) comprising a plurality of dynode plates (24) stacked in a direction perpendicular to a major surface of said anode plate, each of said dynode plates (24) comprising at least one dynode, an upper-electrode plate (24a) with a first through hole (241), a lower-electrode plate (24b) with a second through hole (242), the upper- and lower-electrode plates being in tight contact with and electrically connected to each other, and an insulator (27,40) for separating the dynode plate from an adjacent dynode plate in the dynode unit (20), characterised in that the first through hole (241) and the second through hole (242) together form an aperture for retaining the insulator (27,40) partially therewithin. A multiplier according to claim 1, wherein the insulator (27,40) is in direct contact with the insulator in an adjacent dynode plate in the dynode unit. A multiplier according to claim 1 or 2, wherein the upper-electrode plate (24a) has a first surface (24a1) and a second surface (24a2), the first through hole (241) comprising a tapered portion (243) at the first surface (24a1) which portion is wider at the first surface (24a1), the lower-electrode plate (24b) has a first surface (24b1) and a second surface (24b2), the second through hole comprising a tapered portion (242) which portion is wider at the first surface (24b1) of the lower-electrode plate (24b), the first surface of the upper-electrode plate (24a) and the first surface of the lower-electrode plate (24b) contacting each other, and the tapered portion (243) of the first through hole (241) and the tapered portion (242) of the second through hole together defining a space for partially accommodating the insulator (27,40). A multiplier as claimed in claim 3, wherein the first through hole (241) comprises a uniform portion at the second surface (24a2). A multiplier according to claim 3 or 4, wherein the area of the second through hole (242) on the first surface (24b1) at the lower-electrode (24b) is larger than the area of a maximum section through the insulator (27,40). A multiplier according to claim 3, 4 or 5, wherein the area of the second through hole (242) on the second surface (24b2) of the lower-electrode is smaller than the area of a maximum section through the insulator (27,40). A multiplier according to any preceding claim, wherein the second through hole (242) is formed so that the insulator (27,40) extends beyond the lower-electrode plate (24b). A multiplier according to claim 7, wherein the insulator (27,40) is in contact with a peripheral portion (C2,C3) of the first through hole (241) in the adjacent dynode plate in the dynode unit (20). A multiplier according to any preceding claim, wherein the insulator comprises either a spherical body (27) or a cylindrical body (40). A multiplier according to any of claims 3 to 6, wherein the center of the insulator (27,40) is located between the first (24b1) and second (24b2) surfaces of the lower-electrode plate (24b). A photomultiplier comprising an electron multiplier as claimed in any preceding claim, and a photocathode (21).

Inventors: ATSUMI AKIRA, HASEGAWA YUTAKA, KYUSHIMA HIROYUKI, NAGURA KOJI, ATSUMI, AKIRA, HASEGAWA, YUTAKA, KYUSHIMA, HIROYUKI, NAGURA, KOJI, ATSUMI, AKIRA, C/O HAMAMATSU PHOTONICS K.K., HASEGAWA, YUTAKA, C/O HAMAMATSU PHOTONICS K.K., KYUSHIMA, HIROYUKI, C/O HAMAMATSU PHOTONICS K.K., NAGURA, KOJI, C/O HAMAMATSU PHOTONICS K.K.
Applicants: HAMAMATSU PHOTONICS KK, HAMAMATSU PHOTONICS K.K.
Application year: 1995
Main classification: H01J43
IPC codes: H01J9, H01J43