Photomultiplier

Abstract

The present invention relates to a photomultiplier that realizes significant improvement of response time properties with a structure enabling mass production. The photomultiplier comprises a sealed container, and, in the sealed container, a photocathode, at least one dynode set, a dynode unit including a part of insulating supporting members holding the one dynode unit, and a gain control unit are housed. The gain control unit has an insulating base plate, and the insulating base plate is integrally fixed with a control dynode and a final stage dynode that belong to each dynode set together with an anode. By the insulating base plate thus being clamped by the pair of insulating supporting members, the anode, the control dynode, and the final stage dynode constitute a part of an electron multiplier section.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially broken-away view of a general configuration of an embodiment of a photomultiplier according to the present invention;

FIGS. 2A and 2B are an assembly process diagram and a cross sectional view, respectively, for explaining a structure of a sealed container in the photomultiplier according to the present invention;

FIG. 3 is an assembly process diagram for explaining a structure of an electron multiplier section in the photomultiplier according to the present invention;

FIG. 4 is a diagram for explaining a structure of a pair of insulating supporting members that constitute a portion of the electron multiplier section shown in FIG. 3;

FIG. 5A is a diagram for explaining a structure that joins a focusing electrode unit and the pair of insulating supporting members, and FIG. 5B is a diagram for explaining a structure that joins gain control units and the pair of insulating supporting members;

FIG. 6 is a perspective view for explaining a cross sectional structure of the electron multiplier section taken on line I-I shown in FIG. 1; and

FIGS. 7A and 7B are assembly process diagrams for explaining a structure of a gain control unit as a fourth structural feature of the photomultiplier according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of a photomultiplier according to the present invention will be explained in detail with reference to FIGS. 1, 2A-2B, 3-4, 5A-5B, 6, 7A, and 7B. In the explanation of the drawings, constituents identical to each other will be referred to with numerals identical to each other without repeating their overlapping descriptions.

FIG. 1 is a partially broken-away view of a general arrangement of an embodiment of a photomultiplier according to the present invention. FIGS. 2A and 2B are an assembly process diagram and a sectional view, respectively, for explaining a structure of a sealed container in the photomultiplier according to the present invention.

As shown in FIG. 1, the photomultiplier according to the present invention has a sealed container 100, with a pipe 600, which is used to depressurize the interior to a predetermined degree of vacuum (and the interior of which is filled after vacuum drawing), provided at a bottom portion, and has a photocathode 200 and an electron multiplier section 500 provided inside the sealed container 100.

As shown in FIG. 2A, the sealed container 100 is constituted by a faceplate 110, a tube body (bulb) 120 having the faceplate 110 that is fusion-joined to one end and that extends along a predetermined tube axis AX, and a stem 130 that is fusion-joined to the other end of the tube body 120 and that constitutes a bottom portion of the sealed container 100 provided with the pipe 600. FIG. 2B is a cross sectional view of the sealed container 100 taken on line I-I of FIG. 2A and shows, in particular, a portion at which the faceplate 110 is fusion-joined to the one end of the tube body 120. The faceplate 100 has a light incidence surface 110a and a light emission surface 110b that opposes the light incidence surface 110a, and the photocathode 200 is formed on the light emission surface 110b positioned at the inner side of the sealed container 100. The tube body 120 is a hollow member that is centered about the tube axis AX and extends along the tube axis AX. The faceplate 110 is fusion-joined to one end of this hollow member and the stem 130 is fusion-joined to the other end. The stem 130 is provided with a penetrating hole that extends along the tube axis AX and puts the interior of the sealed container 100 in communication with the exterior. Lead pins 700 are arranged so as to surround this penetrating hole. At the position at which the penetrating hole is provided, the pipe 600, for evacuating the air inside the sealed container 100, is attached to the stem 130.

An installation position of the electron multiplier section 500 in the tube axis AX direction inside the sealed container 100 is defined by the lead pins 700 that extend into the sealed container 100 from the stem 130. The electron multiplier section 500 also comprises a focusing electrode unit 300 for modifying trajectories of photoelectrons emitted into the sealed container 100 from the photocathode 200, and a dynode unit 400 for cascade multiplication of the photoelectrons.

In the following explanation, a multichannel photomultiplier, with which four electron multiplier channels CH1 to CH4 are constituted by two sets of electrodes (dynodes) arranged so as to sandwich the tube axis AX, shall be explained as an embodiment of the photomultiplier according to the present invention.

FIG. 3 is an assembly process diagram for explaining a structure of the electron multiplier section 500 in the photomultiplier according to the present invention. In FIG. 3, the electron multiplier section 500 has the focusing electrode unit 300 and the dynode unit 400.

The focusing electrode unit 300 is constituted by laminating a mesh electrode 310, a shield member 320, and a spring electrode 330. The mesh electrode 310 has a metal frame which is provided with an opening that allows photoelectrons from the photocathode 200 to pass through. The opening defined by the frame portion of the mesh electrode 310 is covered by a metal mesh that is provided with a plurality of openings. The shield member 320 has a metal frame provided with the opening that allows photoelectrons from the photocathode 200 to pass through. The frame portion that defines the opening of the shield member 320 is provided with shield plates 323a, 323b that extend toward the photocathode 200 and with shield plates 322a, 322b that extend toward the stem 130. The shield plates 323a, 323b respectively enable control of positions of incidence of photoelectrons onto first dynodes DY1 and function to adjust an electric field lens formed between the photocathode 200 and the focusing electrode unit 300 to improve the CTTD (that is, the TTC) response properties. The shield plates 322a, 322b are respectively positioned so as to close a space that is open at opposite ends of the first dynodes DY1. The shield plates 322a, 322b are set to a potential that is higher than that of the first dynodes DY1 (and equal to that of second dynodes DY2) and function to strengthen the electric field between the first dynodes DY1 and the second dynodes DY2. The efficiency of incidence onto the second dynodes DY2 of secondary electrons that propagate from the first dynodes DY1 to the second dynodes DY2 can thereby be improved, and the spread of transit times of secondary electrons between the first dynodes DY1 and the second dynodes DY2 is reduced. The spring electrode 330 has a metal frame provided with an opening that allows photoelectrons from the photocathode 200 to pass through. The frame portion of the spring electrode 330 is provided with metal springs 331 (electrode portions), which, by being pressed against an inner wall of the sealed container 100, maintain the entirety of the electron multiplier section 500, on which the focusing electrode unit 300 is mounted, at a predetermined position inside the sealed container 100. The frame portion of the spring electrode 330 is also provided with partitioning plates 332 that partition the second dynodes DY2, positioned immediately below, into two in a longitudinal direction of the second dynodes DY2. The partitioning plates 332 are set to the same potential as the second dynodes DY2 and function to effectively reduce the crosstalk between mutually adjacent electron multiplier channels that are formed from an electrode set of one series.

On the other hand, the dynode unit 400 has a pair of insulating supporting members (a first insulating supporting member 410a and a second insulating supporting member 410b) that hold the focusing electrode unit 300 of the above-described structure and clampingly hold at least two electrode sets that cascade-multiply the photoelectrons from the photocathode 200. Specifically, the first and second insulating supporting members 410a, 410b integrally clamp the pair of first dynodes DY1, the pair of second dynodes DY2, a pair of third dynodes DY3, a pair of fourth dynodes DY4, a pair of fifth dynodes DY5, a pair of seventh dynodes DY7, and a pair of gain control units 430a, 430b, with the dynodes or units of each pair being disposed along the tube axis AX and across the tube axis AX with respect to each other. Metal pins 441, 442 for setting the respective electrodes at predetermined potentials are mounted onto the first and second insulating supporting members 410a, 410b. The first and second insulating supporting members 410a, 410b clampingly hold, in addition to the respective electrodes, a bottom metal plate 440 that is set to a ground potential (0V).

In a state of being installed at upper portions of the first and second insulating supporting members 410a, 410b, the pair of first dynodes DY1 have metal fixing members 420a, 420b welded to both ends. Each of the pair of gain control units 430a, 430b has an insulating base plate 431 and onto this insulating base plate 431 are mounted a corresponding sixth dynode DY6, anode 432, and eighth dynode DY8. Here, each sixth dynode DY6 is constituted by two electrodes that are mounted on the insulating base plate 431 in an electrically separated state. Each anode 432 is constituted by two electrodes that are mounted on the insulating base plate 431 in an electrically separated state. Each eighth dynode DY8 is a common electrode for the two electrodes that constitute the sixth dynode DY6 and the two electrodes that constitute the anode 432.

As described above, each of the gain control units 430a, 430b belongs to one of the two electrode sets arranged so as to sandwich the tube axis AX. Thus, by these gain control units 430a, 430b being arranged together with the partitioning plates 332, the four-channel photomultiplier, with which two electron multiplier channels are formed by each electrode set, is arranged. The sixth dynode DY6 in each of the gain control units 430a, 430b is also constituted by two electrodes, and thus, for the photomultiplier as a whole, four electrodes are allocated as the sixth dynodes DY6 respectively to the electron multiplier channels. By individually adjusting the potentials of the electrodes allocated as the sixth dynodes DY6 to the respective electron multiplier channels, each electron multiplier channel can be adjusted in gain independent of the others.

FIG. 4 is a diagram for explaining a structure of the pair of insulating supporting members 410a, 410b that constitute a portion of the electron multiplier section shown in FIG. 3. Because the first insulating supporting member 410a and the second insulating supporting member 410b are identical in shape, just the first insulating supporting member 410a will be explained below and explanation of the second insulating supporting member 410b will be omitted.

The first insulating supporting member 410a comprises: a main body that holds the first electrode set of the first to fifth dynodes DY1 to DY5, the seventh dynode DY7 and the gain control unit 430a, and the second electrode set of the first to fifth dynodes DY1 to DY5, the seventh dynode DY7 and the gain control unit 430b; and protruding portions that extend from the main body toward the photocathode 200.

The main body of the first insulating supporting member 410a is provided with fixing slits 412a, 413a for fixing the first electrode set, and fixing slits 412b, 413b for fixing the second electrode set (the same fixing slits are provided in the main body of the second insulating supporting member 410b as well).

Of the first electrode set, one of fixing tabs provided at opposite ends of the second dynode DY2, one of fixing tabs provided at opposite ends of the third dynode DY3, one of fixing tabs provided at opposite ends of the fourth dynode DY4, one of fixing tabs provided at opposite ends of the fifth dynode DY5, and one of fixing tabs provided at opposite ends of the seventh dynode DY7 are inserted into the fixing slits 412a and these electrode members are thereby integrally clamped by the first and second insulating supporting members 4210a, 410b. Also, as shown in FIG. 5B, fixing tabs of one end among fixing tabs provided at opposite ends of the gain control unit 430a belonging to the electrode set of the first series are inserted in the fixing slits 413a. Of the second electrode set, one of fixing tabs provided at opposite ends of the second dynode DY2, one of fixing tabs provided at opposite ends of the third dynode DY3, one of fixing tabs provided at opposite ends of the fourth dynode DY4, one of fixing tabs provided at opposite ends of the fifth dynode DY5, and one of fixing tabs provided at opposite ends of the seventh dynode DY7 are inserted into the fixing slits 412b and these electrode members are thereby integrally clamped by the first and second insulating supporting members 4210a, 410b. Also, fixing tabs of one end among fixing tabs provided at opposite ends of the gain control unit 430b belonging to the electrode set of the second series are inserted in the fixing slits 413b.

Furthermore, notches 415 for clampingly holding a bottom metal plate 440 is provided at a bottom portion of the first insulating supporting member 410a (the same holds for the second insulating supporting member 410b). Also, pedestal portions 411, on which the first dynodes DY1 are mounted, are formed at portions sandwiched by the protruding portions of the first insulating supporting member 410a, and a notch 414 for holding the focusing electrode unit 300 is formed in each of the protruding portions (the same holds for the second insulating supporting member 410b). Specifically, as shown in FIG. 5A, notches formed in the focusing electrode unit 300 are inserted in the notches 414 respectively provided in the protruding portions of the first insulating supporting member 410a, and the focusing electrode unit 300 is thereby clampingly held integrally by the first and second insulating supporting members 410a, 410b. FIG. 5A is a diagram for explaining the structure that joins the focusing electrode unit 300 and the pair of insulating supporting members 410a, 410b, and FIG. 5B is a diagram for explaining the structure that joins the gain control units 430a, 430b and the pair of insulating supporting members 410a, 410b.

FIG. 6 is a perspective view for explaining a cross sectional structure of the electron multiplier section taken on line I-I shown in FIG. 1. As shown in FIG. 6, the electron multiplier section 500 has two electrode sets arranged so as to sandwich the tube axis AX. In each of these two electrode sets, mutually adjacent electron multiplier channels that can be adjusted in gain independently of each other are arranged by the corresponding partitioning plate 332, provided in the spring electrode 330 that constitutes a portion of the focusing electrode unit 300, and by the disposition of the corresponding gain control unit 430a or 430b. In the electron multiplier section 500 shown in FIG. 6, four electron multiplier channels are thus formed in correspondence to photoelectron emission positions of the photocathode 200.

In the one electrode set (first electrode set), among the two electrode sets arranged so as to sandwich the tube axis AX, to which the gain control unit 430a belongs, a secondary electron emitting surface is formed on each of the first dynode DY1 to the eighth dynode DY8. The set potential of each of the first dynode DY1 to the eighth dynode DY8 is increased in the order of the first dynode DY1 to the eighth dynode DY8 to guide the secondary electrons successively to the dynode of the next stage. The potential of the anode 432 is higher than the potential of the eighth dynode DY8. For example, the photocathode 200 is set to −1000V, the first dynode DY1 is set to −800V, the second dynode DY1 is set to −700V, the third dynode DY1 is set to −600V, the fourth dynode DY1 is set to −500V, the fifth dynode DY1 is set to −400V, the sixth dynode DY1 is set to −300V (made variable to enable gain adjustment), the seventh dynode DY1 is set to −200V, the seventh dynode DY1 is set to −200V, the eighth dynode DY1 is set to −100V, and the anode 432 is set to the ground potential (0V). The focusing electrode unit 300, with the partitioning plates 332, is set to the same potential as the second dynodes DY2.

The photoelectrons emitted from the photocathode 200 arrive at the first dynode DY1 after passing through the mesh openings of the focusing electrode unit 300 that is set to the same potential as the second dynode DY2. The shield plate 322b, set to the same potential as the second dynode DY2, is disposed at a space that is opened in the longitudinal direction of the first dynode DY1, and by this, the electric field between the first dynode DY1 and the second dynode DY2 is strengthened, the efficiency of incidence onto the second dynode DY2 of the secondary electrons, propagating from the first dynode DY1 to the second dynode DY2, can be improved, and the spread of transit times of the secondary electrons between the first dynode DY1 and the second dynode DY2 is reduced. The secondary electron emitting surface is formed on an electron arrival surface of the first dynode DY1, and in response to the incidence of photoelectrons, secondary electrons are emitted from the first dynode DY1. The secondary electrons emitted from the first dynode DY1 propagate toward the second dynode DY2, which is set to a higher potential than the first dynode DY1. The second dynode DY2 is separated into two electron multiplier channels by the partitioning plate 332 that extends from the focusing electrode unit 300, and a structure is realized with which, crosstalk between the adjacent electron multiplier channels is suppressed by adjustment of the trajectories of the secondary electrons from the first dynode DY1. The secondary electron emitting surface is also formed on an electron arrival surface of the second dynode DY2, and the secondary electrons emitted from the secondary electron emitting surface of the second dynode DY2 propagate toward the third dynode DY3, which is set to a higher potential than the second dynode DY2. The secondary electrons emitted from the secondary electron emitting surface of the third dynode DY3 are likewise cascade-multiplied as the electrons proceed in the order of the fourth dynode DY4, the fifth dynode DY5, and the sixth dynode DY6. The sixth dynode DY6 is constituted by the two electrodes that constitute portions of the gain control unit 430a and by suitable adjustment of the set potentials of these two electrodes, the gains of the adjacent electron multiplier channels can be adjusted independent of each other. The secondary electrons emitted from the secondary electron emitting surfaces of the respective electrodes constituting the sixth dynode DY6 arrive at the seventh dynode DY7, and secondary electrons are emitted from the secondary electron emitting surface of the seventh dynode DY7 toward the anode 432 with mesh openings. The eighth dynode DY8 is set to a lower potential than the anode 432 and functions as an inverting dynode that emits secondary electrons, which have passed through the anode 432, back to the anode 432. The other electrode set, to which the gain control unit 430b belongs, also functions in the same manner.

Next, the structural feature of the photomultiplier according to the present invention will be explained using FIGS. 7A and 7B. FIGS. 7A and 7B are assembly process diagrams for explaining a structure of a gain control unit as the fourth structural feature of the photomultiplier according to the present invention. By the structural feature, the number of parts of the photomultiplier is reduced to realize a structure suited for mass production. Specifically, the fourth structural feature is realized by the gain control unit 430a (430b) that integrates a structure, in an electrode set of one series, near the anode 432.

As shown in FIG. 7A, the gain control unit 430a, which is allocated to one electrode set, has the insulating base plate 431. By the sixth dynode DY6, the anode 432, and the eighth dynode DY8, which functions as the inverting dynode, being mounted onto the insulating base plate 431, the gain control unit 430a is obtained as shown in FIG. 7B. In the present embodiment, two electrode sets are arranged so as to sandwich the tube axis AX, and because the gain control unit 430b belonging to the other electrode set also has the same structure, explanation thereof will be omitted.

In the gain control unit 430a, the sixth dynode DY6 is constituted by two electrodes that are mounted in an electrically separated state onto the insulating base plate 431. The anode 432 is also constituted by two electrodes that are mounted in an electrically separated state onto the insulating base plate 431. The eighth dynode DY8 is a common electrode for the two electrodes that constitute the sixth dynode DY6 and the two electrodes that constitute the anode 432. In particular, the sixth dynode DY6 is constituted by two electrodes that constitute a portion of the gain control unit 430a, and by adjusting the setting potentials of these two electrodes as suited, the gains of the adjacent electron multiplier channels can be adjusted independent of each other. The anode 432 is also constituted by two electrodes and by the gain control unit 430a being applied to one electrode set, a plurality of electron multiplier channels that can be individually adjusted in gain can be realized in the one electrode set.

As shown in FIG. 2B, in the photomultiplier according to the present invention, the light emission surface 110b of the faceplate 110 is constituted by the flat region and the curved-surface processed region, positioned at the periphery of the flat region and including the edges of the light emission surface 110b. The surface shape of the peripheral region of the light emission surface 110b of the faceplate 110 is thus intentionally changed in order to adjust the angles of emission of photoelectrons from the photocathode 200 positioned at the peripheral region. The spread of transit times of photoelectrons propagating from the photocathode 200 to the first dynode DY1 is thus reduced effectively and is made not to depend on the emission positions of the photoelectrons.

Also, the first dynodes DY1, respectively belonging to two electrode sets, are arranged back-to-back with respect to each other while sandwiching the tube axis AX as shown in FIG. 6. In this case, the collection efficiency of the photoelectrons that arrive at the periphery of the first dynodes DY1 is improved significantly. For example, because an electrode for guiding photoelectrons from the photocathode 200 to the first dynodes DY1 is not required between the photocathode 200 and the first dynodes DY1, a stronger electric field strength in comparison to the conventional art can be obtained at a peripheral region of the photocathode 200 and the intervals of the equipotential lines are also made uniform. Photoelectrons emitted from a peripheral region of the photocathode 200 thus do not arrive directly at the second dynodes DY2 without arriving at the first dynodes DY1.

Furthermore, the width in the longitudinal direction of the first dynode DY1 is preferably set greater than the interval between the pair of insulating supporting members 430a, 430b. In this case, the effective surface of arrival of photoelectrons from the photocathode 200 is expanded. Also, as the shield structure at the periphery of the first dynodes DY1, the shield plates 322a, 322b are disposed at positions at which the plates close the space that is open at opposite ends of the first dynodes DY1 as shown in FIGS. 3 and 6. The shield plates 322a, 322b are set to a higher potential (equal to the potential of the second dynodes DY2) than the first dynodes DY1 and function to strengthen the electric field between the first and second dynodes DY1, DY2. The efficiency of incidence onto the second dynodes DY2 of the secondary electrons propagating from the first dynodes DY1 to the second dynodes DY2 can thereby be improved, and the spread of transit times of secondary electrons between the first and second dynodes DY1, DY2 is reduced.

Also, for effectively reducing the crosstalk between electron multiplier channels, the present photomultiplier has the partitioning plates 332, each of which partitions a second dynode DY2 in two in the longitudinal direction of the second dynode DY2. The second dynodes DY2 are set to a higher potential than the first dynodes DY1, which emit secondary electrons according to the incidence of photoelectrons from the photocathode, and are positioned at positions at which the secondary potential from the first dynodes DY1 arrives. By each partitioning plate 332 positioned inside each second dynode DY2, crosstalk between mutually adjacent electron multiplier channels that are constituted by a dynode set of one series is reduced effectively. That is, the trajectories of electrons that propagate successively along the plurality of stages of dynodes are significantly reduced in the possibility of crossing across to adjacent electron multiplier channels in this process (the crosstalk between adjacent electron multiplier channels is reduced significantly).

Preferably, each of the partitioning plates 332 is a metal tab of the focusing electrode unit 300 that is disposed between the photocathode 200 and the dynode unit 400 and is set to the same potential as the second dynodes. In this case, the metal tab of the focusing electrode unit 300 extends in the direction directed from the photocathode to the electron multiplier section.

As a structure for disposing at least a part of the metal tab of the focusing electrode unit 300 inside each second dynode DY2, each second dynode DY2 preferably has a slit that puts the front surface, on which the secondary electron emitting surface is formed, in communication with the back surface that opposes the front surface. By a tip of the metal tab of the focusing electrode unit 300 inserted into the space between the first dynode DY1 and the second dynode DY2 via the slit of the second dynode DY2, two electron multiplier channels can be formed in one dynode set.

As described above, in accordance with the photomultiplier according to the present invention, the crosstalk between electron multiplier channels formed by one electron set can be effectively reduced, and the TTS, CTTD, and other response time properties are improved significantly. In addition, by the gain control unit, with which a portion of the dynodes and the anode are integrated, the number of parts in the assembly process can be reduced and a plurality of electron multiplier channels can be arranged with a simpler structure.

From the invention thus described, it will be obvious that the embodiments of the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.

Claims

1. A photomultiplier comprising: a sealed container including a hollow body section extending along a predetermined tube axis, and a faceplate provided so as to intersect the tube axis and transmitting light with a predetermined wavelength;a photocathode provided inside sealed container so as to emit photoelectrons into said sealed container in response to incidence of the light with the predetermined wavelength;a dynode unit provided inside the sealed container so as to cascade-multiply photoelectrons emitted from said photocathode, said dynode unit including:at least one dynode set constituted by a plurality of dynodes that respectively have a secondary electron emitting surface; anda pair of insulating supporting members clampingly and integrally holding said one dynode set; anda gain control unit for controlling a gain of at least one electron multiplier channel formed by said one dynode set,wherein said gain control unit has:an insulating base plate, opposite ends of which are clamped by said pair of insulating supporting members;a control dynode, fixed to said insulating base plate, belonging to said one dynode set, said control dynode controlling the gain of the electron multiplier channel by adjustment of a setting potential;an anode, fixed to said insulating base plate, for capturing secondary electrons cascade-multiplied by said electron multiplier channel, said anode being set to a higher potential than any of the dynodes of said one dynode set; anda final stage dynode, fixed to said insulating base plate at a position where secondary electrons that have passed through said anode arrive, belonging to said one dynode set, said final dynode reversing the secondary electrons that have passed through said anode back to said anode.

2. A photomultiplier according to claim 1, wherein said anode has a mesh structure, in which a plurality of holes are arranged in parallel to a reference plane of said insulating base plate.

3. A photomultiplier according to claim 1, wherein said control dynode includes a plurality of electrically separated electrodes, and wherein said anode includes a plurality of mesh electrodes, that are respectively separated electrically and respectively correspond to each of said plurality of electrodes constituting said control dynode.

4. A photomultiplier according to claim 1, further comprising a partitioning plate comprised of a conductive material, said partitioning plate, of said one dynode set, being set to a higher potential than a first dynode that emits secondary electrons in response to incidence of photoelectrons, and partitioning a second dynode, which is arranged at a position where secondary electrons from said first dynode arrive, into two in a longitudinal direction of the second dynode.

5. A photomultiplier according to claim 4 further comprising a focusing electrode unit provided between said photocathode and said dynode unit and being set to the same potential as said second dynode, wherein said partitioning plate includes a metal tab of said focusing electrode unit that extends in a direction directed from said photocathode to said dynode unit.

6. A photomultiplier according to claim 5, wherein said second dynode has a slit that puts a front surface, on which the secondary electron emitting surface is formed, in communication with a back surface that opposes the front surface, and wherein said metal tab of said focusing electrode unit extends in the direction directed from said photocathode to said dynode unit such that a tip thereof is positioned, via said slit of said second dynode, in a space between said first dynode and said second dynode.