Electron tube

Abstract: An electron tube of the present invention has an electron multiplication unit for multiplying an incident electron flow by secondary electron emission. This electron multiplication unit is constituted by stacking a plurality of dynodes toward an incident side of the electron flow. A plurality of through holes are arranged and formed in each dynode, in which one end on the incident side of the electron flow is used as an input opening, and the other end is used as an output opening. An acceleration electrode unit projecting toward the through hole of the upper dynode is provided at an edge portion of the input opening. As described above, the acceleration electrode unit is provided at the edge portion of the input opening of the through hole formed in each dynode. For this reason, a damping electric field is pushed up by the acceleration electrode unit and deeply warped into the through hole of the upper dynode. With the action of the damping electric field, the secondary electrons are properly guided to the next dynode, thereby improving the electron collection efficiency.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an electron tube having an electron multiplication unit for multiplying an incident electron flow by secondary electron emission. Related Background Art Conventionally, a technique disclosed in Japanese Patent Laid-Open No. 5-182631 is known as a technique of such a field. Fig. 8 shows the sectional structure of the dynodes of the conventional electron tube described in this prior art. In Fig. 8, of a plurality of dynodes stacked in an electrically insulated state, continuous nth and (n + 1)th dynodes are shown. A dynode 100 has a plate 102 in which a plurality of through holes or apertures 101 are formed. The arrangement position of the plate 102 is inverted for each stage such that the inclination of the through holes 101 is inverted for each stage. As for the through holes 101, an output opening 104 has a diameter larger than that of an input opening 103. A predetermined voltage is applied to the plate 102 of each stage by a power supply 105 such that the potentials of the dynodes 100 are sequentially increased. In this example, a voltage value V 1 applied to the nth dynode 100 is 100 V. A voltage value V 2 applied to the (n + 1)th dynode 100 is 200 V. Since each through hole 101 of the plate 102 has a surface with conductivity, the upper and lower surface of the plate 102 is charged at the same potential by the voltage applied from the power supply 105. With this structure, electrons incident on each through hole 101 of the nth dynode 100 collide against an inclined portion 106, thereby emitting secondary electrons from a secondary electron emission layer formed on the inclined portion 106. The emitted secondary electrons are guided to a damping electric field formed by a potential difference between the nth dynode 100 and the (n + 1)th dynode 100, incident on the through holes 101 of the (n + 1)th dynode 100, and similarly amplified. The distribution state of the potentials between the nth dynode 100 and the (n + 1)th dynode 100 is indicated by a dotted line in Fig. 8. Equipotential lines of 120 V, 150 V, and 180 V are represented by A, B, and C, respectively. The equipotential line B is present at an intermediate position between the nth dynode 100 and the (n + 1) the dynode 100. The equipotential lines A and C are warped into the through holes 101 of the nth dynode 100 and the (n + 1)th dynode 100, respectively. As described above, each of the through holes 101 has the output opening 104 with a diameter larger than that of the input opening 103. For this reason, the equipotential line A is deeply warped into the through holes 101 as compared to the equipotential line C. As described above, when the equipotential line A is deeply warped into the through holes 101, the damping electric filed in the through holes 101 is strengthened to easily guide secondary electrons 107 emitted from the lower portion of the inclined portion 106 of the nth dynode 100 to the (n + 1)th dynode 100. Japanese Patent Laid-Open Nos. 2-291654 and 2-291655 also disclose conventional electron tubes. SUMMARY OF THE INVENTION According to one aspect of the invention there is provided an electron tube comprising: a first dynode plate, a plurality of first apertures extending through the plate, each aperture having an incident opening for receiving electrons incident on the plate and an emission opening for emitting multiplied electrons therefrom, a second dynode plate located adjacent the first dynode plate, a plurality of second apertures extending through the second dynode plate, each aperture having an incident opening for receiving electrons emitted from the first dynode plate and incident upon the second dynode plate, and an emission opening for emitting multiplied electrons therethrough, characterised in that the second dynode plate has protruding elongated acceleration electrodes each disposed on a surface facing the first dynode plate with each elongated electrode extending lengthwise along at least part of an edge of an incident opening of the second dynode plate, respectively, and arranged to protrude towards the respective emission opening of the first dynode plate. One embodiment in accordance with the present invention requires the acceleration electrodes of the second dynode plate to protrude into a corresponding one of the apertures of the first dynode plate. In another embodiment of the present invention said incident opening has a rectangular shape, each acceleration electrode has a parallelepiped shape, and an elongate edge of each incident opening matches the longitudinal direction of each acceleration electrode respectively. Conveniently, each acceleration electrode has a triangular cross-section or an inverted U-shaped cross-section. Preferably, each emission opening of each of said plurality of through holes has a cross-section larger than that of said input opening. In an alternative embodiment according to the present invention a central axis of each aperture is inclined by a predetermined angle with respect to an upper surface of each of said first and said second dynode plates, respectively. Each central axis of each aperture may be inclined at an angle of 50° in respect to the upper surface of each of the first and second dynode plates. In another alternative embodiment according to the present invention a secondary electron radiation layer is located on a first inner wall of each aperture, the first inner wall facing each incident opening, respectively. A lower portion of each first inner wall may be a recessed curved surface. The electron tube can be a photomultiplier or an image multiplier. According to another aspect of the present invention there is provided a photomultiplier comprising an electron tube in accordance with a first aspect of the present invention. In one embodiment of the multiplier in accordance with the invention the apertures in adjacent dynodes are oriented differently from each other and define a convoluted path through a stack of dynodes. As disclosed above, the acceleration electrode is located close to the incident opening of the aperture formed in the second dynode. For this reason, a damping electric field is pushed up by the acceleration electrode and deeply warped into the aperture of the first dynode. With the action of the damping electric filed, the electrons are properly guided from the first dynode to the second dynode, thereby improving electron collection efficiency. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional side elevational view showing the structure of an electron tube according to an embodiment of the present invention; Fig. 2 is a plan view showing the structure of the electron tube according to the embodiment of Figure 1; Fig. 3 is a sectional view showing two continuous dynodes of a plurality of dynodes constituting an electron multiplication unit of the invention; Fig. 4 is a partial sectional view showing the shape of an electron multiplication aperture formed in one dynode; Fig. 5 is a view showing the distribution state of the potentials of the two continuous dynodes according to the present invention. Fig. 6 is a view showing the size of each portion of the dynode; Figs. 7A and 7B are perspective views showing other shapes of the acceleration electrode unit provided to the dynode; and Fig. 8 is a sectional view showing a conventional electron multiplication unit. DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the accompanying drawings. Fig. 1 is a sectional side view showing the structure of an electron tube according to this embodiment. Fig. 2 is a plan view showing the structure of the electron tube according to this embodiment. Referring to Figs. 1 and 2, in the electron tube of this embodiment, an electron multiplication unit 20 for multiplying an incident electron flow is arranged in a column-like vacuum vessel 10. The vacuum vessel 10 is constituted by a cylindrical metal side tube 11, a circular light-receiving surface plate 12 provided at one end of the metal side tube 11, and a circular stem 13 provided at the other end of the metal side tube 11. A photocathode 21 is arranged on the lower surface of the light-receiving surface plate 12. A focusing electrode 22 is arranged between the photocathode 21 and the electron multiplication unit 20. The electron multiplication unit 20 is constituted by stacking dynodes 24 each having a large number of electron multiplication holes or apertures 23. An anode 25 and a last-stage dynode 26 are sequentially arranged below the dynodes 24. The stem 13 serving as a base portion is connected to external voltage terminals. Twelve stem pins 14 for applying a predetermined voltage to the dynodes 24 and 26 and the like extend through the stem 13. Each stem pin 14 is fixed to the stem 13 by a tapered hermetic glass 15. Each stem pin 14 has a length to reach a to-be-connected dynode 24 or 26. The distal end of the stem pin 14 is connected to the connecting terminal (not shown) of the corresponding dynode 24 or 26 by resistance welding. The materials of the above-described members are as follows. As the material of the metal side tube 11, the stem 13, and the stem pins 14, Kovar metal, SUS (stainless steel), aluminum, or iron-nickel is used. As the material of the light-receiving surface plate 12, Kovar glass, UV glass, quartz, MgF 2 , or sapphire is used. As the material of the photocathode 21, bialkali, multialkali, GaAs, or CsTe is used. As the material of the focusing electrode 22, SUS (stainless steel) or tungsten is used. As the material of the dynodes 24 and 26, and the anode 25, SUS (stainless steel), aluminum, nickel, or CuBe is used. Light 30 incident on the light-receiving surface plate 12 excites electrons in the photocathode 21 on the lower surface to emit photoelectrons in the vacuum. The photoelectrons emitted from the photocathode 21 are focused on the uppermost dynode 24 by the matrix-like focusing electrode 22 (Fig. 2), and secondary multiplication is performed. Secondary electrons emitted from the uppermost dynode 24 are applied to the lower dynodes 24 to repeat secondary electron emission. A secondary electron group emitted from the last-stage dynode 24 is extracted from the anode 25. The extracted secondary electron group is externally output through the stem pins 14 connected to the anode 25. The structure of the dynodes 24 as a characteristic feature of this embodiment will be described below with reference to the perspective view in Fig. 3. Fig. 3 shows the structure of the continuous nth and (n + 1)th dynodes 24 of the plurality of dynodes 24 stacked in an electrically insulated state. The dynode 24 has a plate 24 1 whose surface has conductivity. A plurality of electron multiplication holes 23 are regularly arranged and formed in the plate 24 1 . Rectangular input openings 24 2 each serving as one end of the electron multiplication hole 23 are formed in the upper surface of the plate 24 1 . Substantially square output openings 24 2 each serving as the other end of the electron multiplication hole 23 are formed in the lower surface. A parallelepiped acceleration electrode unit 24 4 is provided to the edge portion of the input opening 24 2 of each electrode multiplication hole 23. The electron multiplication hole 23 is inclined with respect to the incident direction of electrons which are incident through the input opening 24 2 . A secondary electron radiation layer 24 5 is formed on an inclined portion of the inner wall of each electron multiplication hole 23, where the electrons incident through the input opening 24 2 collide. The secondary electron radiation layer 24 5 is formed by vacuum-depositing an antimony (Sb) layer in the region of the secondary electron radiation layer 24 5 of the plate 24 1 , and causing this layer to react with alkali. In addition to this forming method, if CuBe is used as the material of the plate 24 1 , the region of the secondary electron radiation layer 24 5 of the plate 24 1 can be activated and formed in oxygen. The nth dynode 24 and the (n + 1)th dynode 24 are stacked while the arrangement position of the plate 24 1 is inverted such that the inclination of the electron multiplication holes 23 is inverted for each stage. In addition, the acceleration electrode units 24 4 of the (n + 1)th dynode 24 enter the electron multiplication holes 23 of the nth dynode 24. Since one long side of the acceleration electrode unit 24 4 is shorter than one side of the output opening 24 3 , the acceleration electrode unit 24 4 of the (n + 1)th dynode 24 does not contact the output opening 24 3 of the nth dynode 24. As described above, when the acceleration electrode units 24 4 enter the electron multiplication holes 23, a damping electric field for guiding the secondary electrons can be deeply warped into the electron multiplication holes 23. In this embodiment, the interval between the acceleration electrode unit 24 4 and the output opening 24 3 is 80 µm. This interval depends on the potential difference between the nth dynode 24 and the (n + 1)th dynode 24. The minimum value of the interval is 20 µm, and the maximum value is 160 µm. The acceleration electrode units 24 4 do not necessarily enter the electron multiplication holes 23 of the upper stage. When the acceleration electrode units 24 4 only slightly project upward from the upper surface of the plate 24 1 , an effect for pushing up the damping electric field can be sufficiently obtained. However, to obtain a larger effect, it is preferable that the acceleration electrode units 24 4 enter the electron multiplication holes 23 of the upper stage. The acceleration electrode units 24 4 can enter the electron multiplication holes 23 of the upper stage to the position of a lower end 24 6 of the secondary electron radiation layer 24 5 (the upper end of the vertical surface of the output opening 24 3 ) at maximum. Fig. 4 is a partial sectional view showing the shape of the electron multiplication hole 23 formed in the nth dynode 24, which sectional view is obtained upon taking along a direction perpendicular to the longitudinal direction of the acceleration electrode unit 24 4 . The electron multiplication hole 23 taken along the longitudinal direction of the acceleration electrode unit 24 4 has a rectangle section. The electron multiplication hole 23 of the (n + 1)th dynode 24 also has the same shape except that the direction is different. The electron multiplication hole 23 has a substantially tapered shape extending toward the output opening 24 3 such that the diameter of the output opening 24 3 in the sectional direction is about twice that of the input opening 24 2 in the sectional direction. The central axis of the electron multiplication hole 23 is inclined to the right side of Fig. 4 by about 50° with respect to the upper surface of the plate 24 1 . Of the inner wall of the electron multiplication hole 23, an inner wall 24 7 (a surface on which the secondary electron radiation layer 24 5 is formed) facing the input opening 24 2 is inclined to the right side of Fig. 4 by about 60° with respect to the upper surface of the plate 24 1 . An inner wall 24 8 (a surface opposing the inner wall 24 7 ) facing the output opening 24 3 is inclined to the right side of Fig. 4 by about 40° with respect to the upper surface of the plate 24 1 . The inner wall 24 7 can be divided into four portions in a direction perpendicular to the upper surface of the plate 24 1 . A portion corresponding to about 2/9 from the end portion of the input opening 24 2 is a plane perpendicular to the upper surface of the plate 24 1 . A portion corresponding to about 4/9 from that portion is a plane having an angle of about 70° with respect to the upper surface of the plate 24 1 . A portion corresponding to about 1/9 from the end portion of the output opening 24 3 is a plane perpendicular to the upper surface of the plate 24 1 . A portion corresponding to about 2/9 from that portion is a recessed curved surface having an angle of about 30° with respect to the upper surface of the plate 24 1 . Similarly, the inner wall 24 8 can be divided into four portions in a direction perpendicular to the upper surface of the plate 24 1 . A portion corresponding to about 1/7 from the end portion of the input opening 24 2 is a plane having an angle of about 30° with respect to the upper surface of the plate 24 1 . A portion corresponding to about 3/7 from that portion is a plane having an angle of about 70° with respect to the upper surface of the plate 24 1 . A portion corresponding to about 2/7 from the end portion of the output opening 24 3 is a recessed curved surface having an angle of about 35° with respect to the upper surface of the plate 24 1 . A portion corresponding to about 1/7 from that portion is a plane perpendicular to the upper surface of the plate 24 1 . Additionally, a plane parallel to the upper surface of the plate 24 1 is present on the inner wall 24 8 at a position separated from the upper end by about 1/7 the total distance. The length of the plate in the sectional direction is about 5/8 the diameter of the input opening 24 2 in the sectional direction. The input openings 24 2 are formed in the upper surface of the plate 24 1 at an equal interval. The interval between the adjacent input openings 24 2 in the sectional direction of the plane is about twice the diameter of the input opening 24 2 in the sectional direction. On the plane between the adjacent input openings 24 2 , the parallelepiped acceleration electrode unit 24 4 is formed at the end portion of the input opening 24 2 on the inner wall 24 8 side. The length of the acceleration electrode unit 24 4 in the sectional direction is about 2/7 the interval between of the adjacent input openings 24 2 in the sectional direction of the plane. Fig. 5 is a view showing the distribution state of the potentials of the nth dynode 24 and the (n + 1)th dynode 24. A voltage value V 1 applied to the nth dynode 24 is 100 V, and a voltage value V 2 applied to the (n + 1)th dynode 24 is 200 V. As in the above-described prior art (Fig. 8), equipotential lines of 120 V, 150 V, and 180 V are represented by A, B, and C, respectively. In this case, only the equipotential line C is warped into the electron multiplication holes 23 of the (n + 1)th dynode 24 through the input openings 24 2 . The equipotential lines A, B, and C are pushed up by the acceleration electrode units 24 4 of the (n + 1)th dynode 24, which project into the electron multiplication holes 23 of the nth dynode 24, so that the equipotential lines A, B, and C are warped into the electron multiplication holes 23 of the nth dynode 24 through the output openings 24 3 . Particularly, the equipotential line A is formed to be deeply warped into the electron multiplication holes 23 of the nth dynode 24. Therefore, if the area of the output opening 24 3 is not changed, the equipotential line, i.e., the damping electric field for guiding the secondary electrons can be deeply warped into the electron multiplication holes 23 as compared to the prior art (Fig. 8) which has no acceleration electrode unit 24 4 . For this reason, the damping electric field in the electron multiplication holes 23 is strengthened, so that the secondary electrons emitted from the upper stage of the secondary electron radiation layer 24 5 , which cannot be guided to the lower dynode 24 in the prior art, can be properly guided to the lower dynode 24, thereby improving the electron collection efficiency. Fig. 6 is a view showing the size of each portion of the nth dynode 24 and the (n + 1)th dynode 24. Referring to Fig. 6, the nth dynode 24 and the (n + 1)th dynode 24 are stacked at an interval d 1 of 0.09 mm. The acceleration electrode unit 24 4 has a width d 2 of 0.12 mm and a thickness d 3 of 0.12 mm. An interval d 4 between the adjacent acceleration electrode units 24 4 is 1.0 mm. The dynode 24 is constituted by three plates 24 11 to 24 13 bonded each other. The plates 24 11 to 24 13 have thicknesses d 5 of 0.18 mm, d 6 of 0.25 mm, and d 7 of 0.25 mm, respectively. As the interval d 1 between the nth dynode 24 and the (n + 1)th dynode 24, the minimum value within a range not to cause discharge between the dynodes 24 is selected, which depends on the potential difference between the dynodes 24. Therefore, if the potential between the dynodes 24 is reduced, this interval can be smaller than 0.09 µm. In the above-described embodiment, a photomultiplier has been exemplified as an electron tube having an electron multiplication unit. However, the present invention is not limited to the photomultiplier and may also be applied to an electron multiplier or image multiplier for amplifying the luminance of an input optical image as far as it is an electron tube having an electron multiplication unit for multiplying an incident electron flow by action of secondary electron emission. In addition, in this embodiment, the area of the output opening is larger than that of the input opening, and the electron multiplication hole has a prismatic shape extending toward the output opening. However, the area of the input opening may be equal to that of the output opening such that the electron multiplication hole has a prismatic shape while the opposing surfaces are parallelly arranged. The shape of the electron multiplication hole is not limited to the prismatic shape and may also be a cylindrical shape. In this case, the input opening and the output opening are circular. The input opening and the output opening may have the same diameter. Alternatively, the output opening may have a larger diameter. The input opening and the output opening may have different shapes. For example, the input opening may be circular while the output opening is square. Furthermore, in this embodiment, the parallelepiped acceleration electrode unit is used. However, the acceleration electrode unit is not limited to the parallelepiped shape. As shown in Fig. 7A, it may be a column having a triangular section. Alternatively, it may be an inverted U-shaped column, as shown in Fig. 7B. In this embodiment, the acceleration electrode units enter the electrode multiplication holes of the upper stage. However, they do not necessarily enter the electron multiplication holes. It is sufficient that the acceleration electrode units project from the upper surface of the plate toward the electron multiplication holes of the upper stage. Even when the acceleration electrode units do not enter the electron multiplication holes of the upper stage, the damping electric field can be pushed up deeply into the electron multiplication holes.

Claims: An electron tube comprising: a first dynode plate (24 1 ), a plurality of first apertures (23) extending through the plate, each aperture having an incident opening (24 2 ) for receiving electrons incident on the plate and an emission opening (24 3 ) for emitting multiplied electrons therefrom, a second dynode plate (24 1 ) located adjacent the first dynode plate, a plurality of second apertures (23) extending through the second dynode plate, each aperture having an incident opening (24 2 ) for receiving electrons emitted from the first dynode plate and incident upon the second dynode plate, and an emission opening (24 3 ) for emitting multiplied electrons therethrough, characterised in that the second dynode plate (24) has protruding elongated acceleration electrodes (24 4 ) each disposed on a surface facing the first dynode plate (24) with each elongated electrode extending lengthwise along at least part of an edge of an incident opening (24 2 ) of the second dynode plate, respectively, and arranged to protrude towards the respective emission opening (24 3 ) of the first dynode plate. A tube according to claim 1, wherein the acceleration electrodes of the second dynode plate (24) protrude into a corresponding one of the apertures (23) of said first dynode plate (24). A tube according to claim 1 or 2, wherein said incident opening (24 2 ) has a rectangular shape, each acceleration electrode (24 4 ) has a parallelepiped shape, and an elongate edge of each incident opening is the same as the longitudinal direction of each acceleration electrode, respectively. A tube according to any of claims 1 to 3, wherein each acceleration electrode has a triangular cross-section or an inverted U-shaped cross-section. A tube according to any of claims 1 to 4, wherein each emission opening (24 3 ) of each of said plurality of apertures (23) has a cross-section larger than that of said incident opening. A tube according to claim 5, wherein a central axis of each aperture (23) is inclined by a predetermined angle with respect to an upper surface of each of said first and said second dynode plates (24 1 ), respectively. A tube according to claim 6, wherein each central axis is inclined at an angle of 50° in respect to the upper surface of each of the first and second dynode plates (24 1 ). A tube according to claim 6 or 7, further comprising a secondary electron radiation layer located on a first inner wall (24 7 ) of each aperture (23), the first inner wall facing each incident opening (24 2 ), respectively. A tube according to claim 8, wherein a lower end portion of each first inner wall (24 7 ) is a recessed curved surface. A tube according to any of claims 1 to 9, wherein said electron tube is a photomultiplier for amplifying photoelectrons emitted upon reception of incident photons. A tube according to any of claims 1 to 10, wherein said electron tube is an image multiplier for multiplying a luminance of an input optical image. An electron multiplier comprising an electron tube as claimed in any of claims 1 to 11. An electron multiplier as claimed in claim 12, wherein the apertures (23) in adjacent dynodes (24 1 ) are oriented differently from each other and define a convoluted path through a stack of dynodes.

Inventors: KYUSHIMA HIROYUKI, KYUSHIMA, HIROYUKI, KYUSHIMA, HIROYUKI, C/O HAMAMATSU PHOTONICS K.K.
Applicants: HAMAMATSU PHOTONICS KK, HAMAMATSU PHOTONICS K.K.
Application year: 1995
Main classification: H01J43
IPC codes: H01J31, H01J43