Abstract:
The present invention relates to a photomultiplier that realizes a significant improvement of response time characteristics by a structure enabling mass production. The photomultiplier comprises a sealed container, and, in the sealed container, a photocathode, an electron multiplier section, and an anode are respectively disposed. The electron multiplier section includes multiple stages of dynode units, and each of the multiple stages of dynode units is fixed with one end of the associated dynode pin while being electrically connected thereto. In particular, the dynode pin, whose one ends are fixed to the multiple stages of dynode units, are held within an effective region of the electron multiplier section contributing to secondary electron multiplication, when the electron multiplier section is viewed from the photocathode side. By this configuration, a focusing distance from the photocathode to a first stage dynode unit can be shortened effectively and the effective region of the electron multiplier section can be enlarged to effectively reduce variations in transit time of photoelectrons propagating from the photocathode to the first stage dynode unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to Provisional Application No. 61/030364 filed on Feb. 21, 2008 by the same Applicant, which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a photomultiplier capable of successively emitting secondary electrons in multiple stages in response to incidence of photoelectrons from a photocathode and thereby performing cascade multiplication of the secondary electrons. 
         [0004]    2. Related Background Art 
         [0005]    The development of TOF-PET (time-of-flight PET) as a next-generation PET (positron emission tomography) apparatus is being pursued actively in the field of nuclear medicine in recent years. In a TOF-PET apparatus, because two gamma rays, emitted from a radioactive isotope administered into a body, are measured simultaneously, a large number of photomultipliers having excellent, high-speed response properties are used as measuring devices disposed so as to surround a subject. 
         [0006]    In particular, in order to realize high-speed response properties of higher stability, multichannel electron multipliers, in which a plurality of electron multiplier channels are prepared and electron multiplications are performed in parallel at the plurality of electron multiplier channels, are being applied to next-generation PETs such as that mentioned above in an increasing number of cases. For example, a multichannel electron multiplier described in International Publication WO2005/091332 has a structure in which a single incidence surface plate is partitioned into a plurality of light incidence regions (each being a photocathode to which a single electron multiplier channel is allocated), and a plurality of electron multiplier sections (each including a dynode unit, in turn including multiple stages of dynodes, and an anode), prepared as electron multiplier channels that are allocated to the plurality of light incidence regions, are sealed inside a single glass tube. A photomultiplier with the structure where a plurality of photomultipliers are contained inside a single glass tube is generally called a multichannel photomultiplier. 
         [0007]    A multichannel photomultiplier thus has a structure where a function of a single-channel photomultiplier, in which photoelectrons emitted from a photocathode disposed on an incidence surface plate are electron multiplied by a single electron multiplier section to obtain an anode output, is shared by the plurality of electron multiplier channels. For example, with a multichannel electron multiplier, with which four light incidence regions (photocathodes for electron multiplier channels) are arrayed in two dimensions, because for one electron multiplier channel, a photoelectron emission region (effective region of the photocathode) is made ¼ or less of the incidence surface plate, electron transit time differences among the respective electron multiplier channels can also be improved readily. Consequently, in comparison to the electron transit time differences within the entirety of a single channel photomultiplier, a significant improvement in electron transit time differences can be anticipated with the entirety of a multichannel electron multiplier. 
       SUMMARY OF THE INVENTION 
       [0008]    The present inventors have examined the above conventional multichannel photomultiplier, and as a result, have discovered the following problems. That is, in the conventional multichannel photomultiplier, because electron multiplications are performed by electron multiplier channels that are allocated in advance according to positions of discharge of photoelectrons from the photocathode, positions of respective electrodes are designed optimally to reduce electron transit time differences according to each electron multiplier channel. By such improvement of the electron transit time differences in each electron multiplier channel, improvements are also made in the electron transit time differences of the multichannel photomultiplier as a whole and consequently, the high-speed response properties of the multichannel photomultiplier as a whole are improved. 
         [0009]    However, in such a multichannel photomultiplier, no improvements have been made in regard to the spread of the average electron transit time differences among the electron multiplier channels. Also, in regard to a light exiting surface (surface positioned in the interior of the sealed container) of the incidence surface plate on which the photocathode is formed, the light exiting surface is distorted in shape in a peripheral region that surrounds a central region, which includes a tube axis of the sealed container, and especially in boundary portions (edges of the light exiting surface) at which the light exiting surface and an inner wall of a bulb intersect. Equipotential lines between the photocathode and the dynodes or between the photocathode and the focusing electrode are thereby distorted, and even within a single channel, photoelectrons that fall astray may be generated depending on the photoelectron emission position. The presence of such stray photoelectrons cannot be ignored for further improvement of high-speed response properties. 
         [0010]    Furthermore, because a large number of photomultipliers are required for the manufacture of a TOF-PET apparatus, adoption of a structure that is more suited for mass production is desired with photomultipliers that are applied to a TOF-PET apparatus, etc. 
         [0011]    The present invention has been developed to eliminate the problems described above. It is an object of the present invention to provide a photomultiplier that is significantly improved as a whole in such response time characteristics as TTS (transit time spread) and CTTD (cathode transit time difference) by realizing reduction of emission-position-dependent photoelectron transit time differences of photoelectrons emitted from a photocathode in a structure more suited for mass production. 
         [0012]    Presently, PET apparatuses having a TOF (time-of-flight) function added are being developed. In photomultipliers used in such a TOF-PET apparatus, CRT (coincidence resolving time) response characteristics are also important. Conventional photomultipliers do not meet the CRT response characteristics requirements of TOF-PET apparatuses. Because the present invention is based on a conventional PET apparatus, a bulb outer diameter is maintained in its current state, and trajectory design is carried out to enable CRT measurements that meet the requirements of a TOF-PET apparatus. Specifically, improvement of the TTS, which is correlated with the CRT response characteristics, is aimed at, and trajectory design is carried out to improve both the TTS across an entire incidence surface plate and the TTS in respective incidence regions. 
         [0013]    A photomultiplier according to the present invention comprises, together with a sealed container whose interior is depressurized to a predetermined degree of vacuum, a photocathode; an electron multiplier section including multiple stages of dynode units, and an anode that are respectively disposed inside the sealed container. The photomultiplier further comprises a plurality of lead pins (hereinafter referred to as “dynode pins”) for setting each of the multiple stages of dynode units to a predetermined potential. The photocathode emits photoelectrons into the sealed container in response to light with a predetermined wavelength. The electron multiplier section includes N (≧2) stages of dynode units to emit secondary electrons in response to the photoelectrons arriving from the photocathode and perform successive cascade multiplication of the secondary electrons. The N stages of dynode units are stacked via insulating spacers from the photocathode toward the anode. Each of the dynode units has one or more dynodes that are respectively set to a same potential. The anode is disposed inside the sealed container so as to sandwich the electron multiplier section together with the photocathode and captures the secondary electrons emitted from the electron multiplier section. One end of each of the dynode pins is fixed while being electrically connected to the associated dynode unit. 
         [0014]    In particular, the photomultiplier according to the present invention has a structure where the plurality of dynode pins are held within an effective region in the electron multiplier section defined as a minimum field region containing all dynodes constituting the multiple stages of dynode units when the electron multiplier section is viewed from the photocathode side. In the present specification, the effective region in the electron multiplier section is the field region, contributing to secondary electron multiplication, as viewed from the photocathode side and is defined as an electron incidence surface of the electron multiplier section on a plane orthogonal to a central axis of a bulb of the sealed container. More specifically, the field region is a minimum region that, when contours of all dynodes included in the electron multiplier section are projected onto the electron incidence surface of the electron multiplier section, contains all projected components of the contours. A boundary line defining the effective region of the electron multiplier section thus partially coincides with a portion of projected components of one of the dynode contours. 
         [0015]    In a conventional photomultiplier, the dynode pins are disposed along a periphery of the effective region of the electron multiplier section that avoids the effective region in which the dynodes are disposed and are specifically disposed along an outer periphery of a frame that supports the dynodes. Meanwhile, with the photomultiplier according to the present invention, because the dynode pins are disposed inside the effective region of the electron multiplier section, the effective region of the electron multiplier section can be enlarged as compared with the conventional photomultiplier. By enlargement of the effective region, trajectory modifications, especially of photoelectrons emitted from a periphery of the photocathode opposing the electron incidence surface of the electron multiplier section, are lessened in degree, and a focusing distance (transit distance of photoelectrons to arrival at the dynode unit of the first stage from the photocathode) is thus reduced significantly. 
         [0016]    In each dynode unit, the plurality of dynodes that are respectively set to the same potential are disposed so that the fixed one end of the associated dynode pin is sandwiched by at least two of the dynodes. In particular, an n-th (2≦n≦N) stage dynode unit from the photocathode toward the anode includes: the dynodes, respectively set to the same potential; a supporting frame for maintaining fixed the intervals between the dynodes; and the associated dynode pin among the plurality of dynode pins. A portion of the supporting frame has a shape positioned between at least two dynodes among the plurality of dynodes and includes a through hole for letting the dynode pin associated to an (n−1)-th stage dynode unit penetrate through without electrical contact. 
         [0017]    A portion of the insulating spacer, positioned between the n-th stage dynode unit and an (n+1)-th stage dynode unit, has a through hole holding the dynode pin associated to the (n−1)-th stage dynode unit and constitutes a part of the n-th stage dynode unit by being fixed to the n-th stage dynode unit. Here, the insulating spacer is disposed so that a center of the through hole coincides with a center of the through hole provided in the portion of the supporting frame in the n-th stage dynode unit. Furthermore, the insulating spacer, positioned between the n-th stage dynode unit and the (n+1)-th stage dynode unit has a structure for defining a position, along a direction directed from the photocathode to the anode, of the dynode pin associated to the n-th stage dynode unit. 
         [0018]    More specifically, the supporting frame of the n-th stage dynode unit preferably has an H shape formed by a pair of supports, disposed so as to sandwich all of the plurality of dynodes, and a connecting portion, having both ends fixed to the pair of supports and disposed so as to be sandwiched by at least two dynodes among the dynodes set to the same potential. Here, the connecting portion is provided with a structure to which one end of the associated dynode pin is fixed. Likewise, the insulating spacer, positioned between the n-th stage dynode unit and the (n+1)-th stage dynode unit (and constituting a part of the n-th stage dynode unit), has an H shape to secure a space for supporting the dynodes and a space for a dynode pin supporting structure. That is, the insulating spacer also has a pair of supports, associated to the pair of supports of the supporting frame in the n-th stage dynode unit, and a connecting portion, associated to the connecting portion of the supporting frame in the n-th stage dynode unit. By making the insulating spacer have the H shape, a space can be provided between dynode units even when the dynode units are respectively stacked in closely contacting states, thereby enabling evacuation to be performed readily in a manufacturing process and enabling an alkali metal vapor to be supplied adequately from the photocathode to the respective dynode units. The alkali metal vapor means as a material gas for forming the photocathode and a secondary electron emitting surface of each dynode. 
         [0019]    The through hole for letting the dynode pin associated to the (n−1)-th dynode unit penetrate through without electrical contact is thus formed in the connecting portion of the supporting frame in the n-th stage dynode unit. Likewise, the through hole for holding the dynode pin associated to the (n−1)-th stage dynode unit is formed in the connecting portion of the insulating spacer that constitutes a part of the n-th stage dynode unit, and this insulating spacer is disposed so that the center of the through hole coincides with the center of the through hole formed in the connecting portion of the supporting frame in the n-th stage dynode unit. 
         [0020]    As an example of a structure for fixing the insulating spacer to the supporting frame and a dynode pin positioning structure, for example, a step is formed inside the through hole formed in the insulating spacer positioned between the n-th stage dynode unit and the (n+1)-th stage dynode unit. Meanwhile, a flange that contacts the step formed inside the through hole of the insulating spacer is disposed on the dynode pin associated to the n-th stage dynode unit. The position, along the direction directed from the photocathode to the anode, of the dynode pin associated to the n-th stage dynode is thus defined by the step. Also, when one end of a dynode pin is fixed to the supporting frame (connecting portion) of the associated dynode unit in a state where the flange contacts the step of the insulating spacer, the insulating spacer itself is pressed against the supporting frame by the flange. By such cooperation of the step formed in the through hole of the insulating spacer and the dynode pin, the structure for fixing the insulating spacer to the supporting frame and the dynode pin positioning structure are realized. 
         [0021]    Furthermore, the insulating spacer positioned between the stacked dynode units may include a plurality of spacer elements. Specifically, the insulating spacer, positioned between the n-th stage dynode unit and the (n+1)-th stage dynode unit, includes a plurality of spacer elements, respectively having the same shape and being stacked in direct contacting states along the direction directed from the photocathode to the anode. In this case, by adjusting the number of the spacer elements, each dynode unit interval (interval between supporting frames) can be changed arbitrarily. 
         [0022]    Also, the insulating spacer, positioned between the n-th stage dynode unit and (n+1)-th stage dynode unit, may have a plurality of light shielding portions arranged so as to plaster the openings sandwiched by the dynodes in the n-th stage dynode unit. Here, each of the light shielding portions has a plurality slits each letting an alkali metal vapor pass therethrough. The light shielding portions, provided in the insulating spacers positioned between the stacked dynode units, functions to prevent that light generated in the anode side reaches the photocathode side, and the slits make an alkali metal vapor for photocathode formation pass from the anode side to the photocathode side. 
         [0023]    The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention. 
         [0024]    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 be apparent to those skilled in the art from this detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a partially broken-away view of a general configuration of an embodiment of a photomultiplier according to the present invention; 
           [0026]      FIGS. 2A and 2B  are an assembly process diagram and a sectional view for describing a structure of a sealed container in the photomultiplier according to the present invention; 
           [0027]      FIG. 3  is a diagram of a sectional structure taken on line I-I of the photomultiplier shown in  FIG. 1 ; 
           [0028]      FIG. 4  is an assembly process diagram for describing respective structures of a focusing electrode unit, an electron multiplier section, and an anode unit in the photomultiplier according to the present invention; 
           [0029]      FIG. 5  is a schematic perspective view of an internal unit (unit in which the focusing electrode unit, the electron multiplier section, and the anode unit are stacked integrally) completed via the assembly process shown in  FIG. 4 ; 
           [0030]      FIG. 6  is an assembly process diagram for describing a configuration of the focusing electrode unit; 
           [0031]      FIGS. 7A to 7D  are an assembly process diagram and sectional views for describing a first configuration of a fourth stage dynode unit that constitutes a part of the electron multiplier section; 
           [0032]      FIGS. 8A to 8C  are process diagrams for describing a method for manufacturing dynodes in each dynode unit ( FIG. 7A ); 
           [0033]      FIGS. 9A to 9D  are a perspective view and sectional views for describing a configuration of an insulating spacer positioned between dynode units; 
           [0034]      FIGS. 10A and 10B  are sectional views for describing a stacked structure of the dynode units; 
           [0035]      FIGS. 11A and 11B  are an assembly process diagram and sectional views for describing a second configuration of a fourth stage dynode unit that constitutes a part of the electron multiplier section; 
           [0036]      FIGS. 12A and 12B  are an assembly process diagram and sectional views for describing a third configuration of a fourth stage dynode unit that constitutes a part of the electron multiplier section; 
           [0037]      FIG. 13  is an assembly process diagram for describing a first configuration of the anode unit; 
           [0038]      FIGS. 14A and 14B  are assembly process diagrams for describing a second configuration of the anode unit; 
           [0039]      FIGS. 15A and 15B  are assembly process diagrams for describing a third configuration of the anode unit; 
           [0040]      FIGS. 16A and 16B  are schematic perspective views of an internal unit in which the focusing electrode unit of  FIG. 6 , the electron multiplier section of  FIGS. 12A and 12B , and the anode unit  FIGS. 14A and 14B  are stacked integrally; 
           [0041]      FIG. 17  is a diagram of a sectional structure taken on line XVIII-XVIII of the internal unit shown in  FIGS. 16A and 16B ; 
           [0042]      FIGS. 18A to 18C  are partially broken-away views for describing various dynode structures applicable to a dynode unit, and  FIG. 18D  is a conceptual diagram for describing structural features of the present invention; 
           [0043]      FIGS. 19A to 19C  are a plan view and sectional views of a dynode unit for describing a structure of the dynode unit and an effective region of an electron multiplier section; 
           [0044]      FIGS. 20A to 20C  are conceptual diagrams for describing technical effects of the photomultiplier according to the present invention by comparison with a conventional art; 
           [0045]      FIGS. 21A to 21C  are diagrams for describing trajectories of photoelectrons emitted from a photocathode for describing structural characteristics and effects of the photomultiplier according to the present invention; 
           [0046]      FIGS. 22A to 22C  are sectional views, corresponding to  FIGS. 21A to 21C , of a photomultiplier of a first comparative example prepared for describing the structural characteristics and effects of the photomultiplier according to the present invention and are diagrams for describing photoelectron trajectories in the photomultiplier according the first comparative example; 
           [0047]      FIGS. 23A to 23C  are sectional views, corresponding to  FIGS. 21A to 21C , of a photomultiplier of a second comparative example prepared for describing the structural characteristics and effects of the photomultiplier according to the present invention and are diagrams for describing photoelectron trajectories in the photomultiplier according the second comparative example; and 
           [0048]      FIGS. 24A and 24B  are an assembly process diagram and a sectional view for describing another structure of a sealed container in the photomultiplier according to the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0049]    In the following, embodiments of a photomultiplier according to the present invention will now be explained in detail with reference to  FIGS. 1 ,  2 A and  2 B,  3  to  6 ,  7 A to  12 B,  13 ,  14 A to  16 B,  17 , and  18 A to  24 B, respectively. In the description of the drawings, portions and elements that are the same shall be provided with the same symbol, and overlapping description shall be omitted. 
         [0050]      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 sectional view for describing a structure of a sealed container in the photomultiplier according to the present invention.  FIG. 3  is a diagram of a sectional structure taken on line I-I of the photomultiplier shown in  FIG. 1 . 
         [0051]    As shown in  FIG. 1 , the photomultiplier according to the present invention comprises a sealed container  100 , having a pipe  600 , used to depressurize an interior of the sealed container  100  to a predetermined degree of vacuum (and the interior of which is filled after vacuum drawing), disposed at a bottom, and has a photocathode  200 , a focusing electrode unit  300 , an electron multiplier section  400 , and an anode unit  500  disposed inside the sealed container  100 . 
         [0052]    As shown in  FIG. 2A , the sealed container  100  is constituted by an envelope portion, and a stem  130  provided with the pipe  600 , the stem  130  being joined by fusion to one end of the envelope portion and constitutes a bottom of the sealed container  100 . A top  110  of the envelope portion functions as an incidence surface plate (hereinafter, the top of the envelope portion shall be referred to as the “incidence surface plate”). The envelope portion is a hollow glass member with which the incidence surface plate  110  and a bulb  120 , extending along a predetermined tube axis AX, are formed integrally.  FIG. 2B  is a sectional view of the sealed container  100  taken on line I-I in  FIG. 2A , and particularly shows a section of a vicinity of the incidence surface plate  110  including a portion of the bulb  120 . The incidence surface plate  110  includes a light incidence surface  110   a  and a light exiting surface  110   b  opposing the light incidence surface  110   a,  and has the photocathode  200  formed on the light exiting surface  110   b  positioned at an inner side of the sealed container  100 . The bulb  120  is a hollow glass member centered about the tube axis AX and extends along the tube axis AX. The incidence surface plate  110  is positioned at one end of the hollow member and the stem  130  is joined by fusion to the other end. The stem  130  has a through hole extending along the tube axis AX and putting the interior of the sealed container  100  in communication with an exterior. Lead pins  700  for electrical communication of the interior and the exterior of the sealed container  100  are disposed so as to surround the through hole. The lead pins  700  are connected to a bleeder circuit positioned at the exterior of the sealed container  100  and an amplifying circuit that amplifies an anode signal. At the position at which the through hole is disposed, the pipe  600 , for evacuating the air inside the sealed container  100 , is attached to the stem  130 . The pipe  600  is sealed at one end at an end of manufacture of the photomultiplier to keep the interior of the sealed container  100  in an airtight, vacuum state. 
         [0053]    An installation position of the electron multiplier section  400  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 focusing electrode unit  300 , mainly including a focusing electrode and being for modifying trajectories of photoelectrons emitted into the sealed container  100  from the photocathode  200 , is disposed on an electron incidence surface of the electron multiplier section  400 . 
         [0054]    To emit secondary electrons in response to photoelectrons arriving from the photocathode  200  via the focusing electrode unit  300  and perform successive cascade multiplication of the secondary electrons, the electron multiplier section  400  includes N (≧2) stages of dynode units as shown in  FIG. 3 . In the present embodiment, eight stages of dynode units are stacked via insulating spacers from the photocathode  200  toward the anode unit  500 . In the present embodiment, the dynode unit stacked at a first stage includes a plurality of second dynodes, and the dynode unit stacked at a second stage includes a plurality of first dynodes. The first dynodes emit secondary electrons in response to the incidence of the photoelectrons from the photocathode  200 , and the second dynode emits further secondary electrons in response to the incidence of the secondary electrons from the first dynodes. The first dynodes are held by the second stage dynode unit so that secondary electron incidence surfaces of the first dynodes directly oppose the photocathode  200  and the photoelectrons from the photocathode  200  are captured more efficiently. In the present embodiment, each dynode has a line focus type (inline type) cross-sectional shape. 
         [0055]    In the description that follows, a multichannel photomultiplier, in which twelve electron multiplier channels CH 1  to CH 12  are formed by six series of electrode sets (dynode sets each forming two electron multiplier channels) disposed to sandwich the tube axis AX, shall be described as the embodiment of the photomultiplier according to the present invention. 
         [0056]    First,  FIG. 4  is an assembly process diagram for describing a structure of an internal unit (the focusing electrode unit  300 , the electron multiplier section  400 , and the anode unit  500 ) in the photomultiplier according to the present invention. 
         [0057]    The focusing electrode unit  300  includes a metal frame (focusing electrode)  310 , having a plurality of openings for letting photoelectrons pass through, insulating spacers  320   a  and  320   b,  and lead pins  330   a  and  330   b.  One ends of the lead pins  330   a  and  330   b  are fixed to the metal frame  310  via the insulating spacers  320   a  and  320   b,  and the other ends of the lead pins  330   a  and  330   b  penetrate through the electron multiplier section  400  and are electrically connected directly or via metal wires to the lead pins  700  fixed to the stem  130 . 
         [0058]    The electron multiplier section  400  includes eight stages of dynode units DY 1  to DY 8  stacked via insulating spacers. In the present specification, the first dynodes are the dynodes at which the photoelectrons from the photocathode  200  arrive first, and the other dynodes are hereinafter referred to as the second to eighth dynodes in an order of arrival of the secondary electrons. As mentioned above, in the present embodiment, the second dynodes are held by the first stage dynode unit, and the first dynodes are held by the second stage dynode unit. Thus in the description that follows, the first stage dynode unit holding the second dynodes shall be indicated as “DY 2 ,” the second stage dynode unit holding the first dynodes shall be indicated as “DY 1 ,” and subsequent dynode units shall be expressed respectively as “DY 3 ” to “DY 8 ” so that the dynodes that are held can be discerned. In the present embodiment, the dynode unit DY 8  integrally holds final stage dynodes. 
         [0059]    The dynode units DY 1  to DY 8  are respectively the same in basic structure, and for example, the fourth stage dynode unit DY 4  (holding the fourth dynodes) includes: a supporting frame  410 , supporting the plurality of fourth dynodes; an insulating spacer  420 ; and a dynode lead pin (dynode pin)  430  for setting the fourth stage dynode unit DY 4  to a predetermined potential. Each of the respective supporting frames  410  of the dynode units DY 1  to DY 8  has formed therein through holes for allowing the dynode pins  430  of the dynode units positioned at upper stages to pass through without the electrical connection. 
         [0060]    The anode unit  500  includes: a ceramic substrate  510 ; a plurality of electrodes (anode electrodes)  520 , disposed on the ceramic substrate  510  and functioning as anodes; and a plurality of lead pins  530 , one ends of which are connected to the anode electrodes  520 . The one ends of the lead pins  530  are fixed to the anode electrodes  520  via the ceramic substrate  510  and the other ends of the lead pins  530  are electrically connected directly or via metal wires to the lead pins  700  fixed to the stem  130 . 
         [0061]    The focusing electrode unit  300 , the multiple stages of dynode units DY 1  to DY 8 , and the anode unit  500  as described above are respectively stacked along a direction directed from the photocathode  200  to the anode unit  500 . The stacked state is maintained by attachment of side wall substrate members  510   a  to  510   d  (see  FIG. 6 ), which are insulation, for preventing deviation of the stacked dynodes and the respective units, to side surfaces of the stacked units. The internal unit (unit in which the focusing electrode unit, the electron multiplier section, and the anode unit are stacked integrally) completed via the above-described assembly process is schematically shown in  FIG. 5 . As shown in  FIG. 5 , the dynode pins  430  respectively associated to the dynode units DY 1  to DY 8  penetrate through the ceramic substrate  510  of the anode unit  500  in a state of being aligned in a straight line inside an effective region AR 1  of the electron multiplier section  400  to be described below. The other ends of the dynode pins  430  are electrically connected directly or via metal wires to the lead pins  700  extending from the stem  130 . 
         [0062]    Respective set potentials of the first stage dynode unit DY 2 , the second stage dynode unit DY 1 , the third stage dynode unit DY 3 , . . . , the eighth stage dynode unit DY 8  are increased in the order of the first dynodes to the eighth dynodes to guide the secondary electrons successively to the dynodes of subsequent stages. Thus, the potential of the anode electrodes  520  in the anode unit  500  is higher than the potential of the eighth dynodes. For example, the photocathode  200  is set to −1000V, the first dynodes held by the second stage dynode unit DY 1  are set to −800V, the second dynode held by the first stage dynode unit DY 2  are set to −700V, the third dynodes held by the third stage dynode unit DY 3  are set to −600V, the fourth dynodes held by the fourth stage dynode unit DY 4  are set to −500V, the fifth dynodes held by the fifth stage dynode unit DY 5  are set to −400V, the sixth dynodes held by the sixth stage dynode unit DY 6  are set to −300V, the seventh dynodes held by the seventh stage dynode unit DY 7  are set to −200V, the eighth dynodes held by the eighth stage dynode unit DY 8  are set to −100V, and the anode electrodes  520  are set to the ground potential (0V). The focusing electrode unit  300  is set to the same potential as the second dynodes held by the first stage dynode unit DY 2 . 
         [0063]    The photoelectrons emitted from the photocathode  200  arrive at the first dynodes held by the second dynode unit DY 1  after passing through the openings formed in the metal frame  310  of the focusing electrode unit  300  that is set to the same potential as the second dynodes. Secondary electron emitting surfaces are formed on electron arrival surfaces of the first dynodes, and in response to the incidence of photoelectrons, secondary electrons are emitted from the first dynodes. The secondary electrons emitted from the first dynodes propagate toward the second dynodes set to a higher potential than the first dynodes and held by the first stage dynode unit DY 2 . Secondary electron emission surfaces are also formed on electron arrival surfaces of the second dynodes, and the secondary electrons emitted from the secondary electron emitting surface of the second dynodes propagate toward the third dynodes, which are set to a higher potential than the second dynodes and held by the third stage dynode unit DY 3 . As the secondary electrons emitted from secondary electron emitting surfaces of the third dynodes propagate in a likewise manner in the order of the fourth dynodes, the fifth dynodes, the sixth dynodes, the seventh dynodes, and the eighth dynodes, respectively held by the fourth to eighth stage dynode units DY 4  to DY 8 , the secondary electrons are cascade multiplied. The secondary electrons emitted from the eighth dynodes held by the final stage (eighth stage) dynode unit DY 8  arrive at the anode electrodes  520  of the anode unit  500  and are taken out to the exterior of the sealed container  100  via the lead pins  700  electrically connected to the lead pins  530 . 
         [0064]    A specific structure of the focusing electrode unit  300  shall now be described using  FIG. 6 .  FIG. 6  is an assembly process diagram for describing a configuration of the focusing electrode unit  300 . 
         [0065]    As shown in  FIG. 6 , the focusing electrode unit  300  includes: the metal frame (focusing electrode)  310 , having the plurality of openings for letting photoelectrons pass through; the insulating spacers  320   a  and  320   b;  and the lead pins  330   a  and  330   b.    
         [0066]    Specifically, the metal frame  310  includes an outer frame, having an opening area capable of containing the entire effective region of the electron multiplier section  400 , and separating frames, each for partitioning an opening that exposes dynodes each functioning as two electron multiplier channels. The pair of insulating spacers  320   a  and  320   b  are fixed to a lower surface (surface opposing the anode unit  500 ) of the outer frame. The insulating spacers  320   a  and  320   b  function to electrically separate the electron multiplier section  400  and the focusing electrode unit  300  and maintain fixed an interval between the units  400  and  300 . Through holes for letting the lead pins  330   a  and  330   b  of the metal frame  310  pass through are formed in the insulating spacers  320   a  and  320   b.  The one ends of the lead pins  330   a  and  330   b  are fixed by welding, crimping, etc., to an upper portion of the metal frame  310 , and the other ends of the lead pins  330   a  and  330   b  are directly or indirectly connected to the lead pins  700  fixed to the stem  130 . To assemble the focusing electrode unit  300 , the lead pins ( 330   a,    330   b ) are penetrated through the respective through holes with the metal frame  310  and the insulating spacers  320   a  and  320   b  being overlapped and then the ends of the lead pins  330   a  and  330   b  are fixed to the metal frame  310  by welding or crimping. Flanges  331   a  and  331   b  are disposed on the lead pins  330   a  and  330   b,  respectively, and because the flanges  331   a  and  331   b  cannot pass through the through holes formed in the insulating spacers  320   a  and  320   b  (that is, inner diameters of the through holes of the insulating spacers  320   a  and  320   b  are smaller than outer diameters of the flanges  331   a  and  331   b ), the respective members constituting the focusing electrode unit  300  are made integral by this assembly work. Furthermore, fixing tabs  310   a  to  310   d  for attaching the side wall substrate members  510   a  to  510   d  are disposed on an outer periphery of the outer frame. Only the side wall substrate member  510   a  among the side wall substrate members  510   a  to  510   d  is shown in  FIG. 6  (illustration of the side wall substrate members  510   b  to  510   d  also is omitted). An engaging portion  511   a  is disposed at one end of the side wall substrate member  510   a.  By the fixing tabs  310   a  being joined to the engaging portion  511   a  after the focusing electrode unit  300 , the electron multiplier section  400 , and the anode unit  500  have been stacked as shown in  FIG. 4 , the side wall substrate member  510   a  functions to maintain the stacked structure. Although not illustrated, the remaining side wall substrate members  510   b  to  510   d  have the same structure and function in the same manner as the side wall substrate member  510   a.    
         [0067]    Meanwhile, the flanges that contact the insulating spacers  320   a  and  320   b  are disposed on the lead pins  330   a  and  330   b,  respectively. By the flanges thus being disposed on the lead pins  330   a  and  330   b,  respectively, the lead pins  330   a  and  330   b  are fixed to the metal frame  310  and the flanges function to press the insulating spacers  320   a  and  320   b  against the metal frame  310 , and the insulating spacers  320   a  and  320   b  are thereby respectively fixed to the metal frame  310 . The focusing electrode unit  300  may be assembled in the order of: fixing the lead pins  330   a  and  330   b  to the metal frame  310  and thereafter fixing the insulating spacers  320   a  and  320   b  to the metal frame  310  with the lead pins  330   a  and  330   b  being put in penetrating states. 
         [0068]      FIGS. 7A to 7D  are an assembly process diagram and sectional views for describing a first configuration of the fourth stage dynode unit DY 4  that constitutes a part of the electron multiplier section  400 . The dynode units DY 1  to DY 8  that constitute the electron multiplier section  400  have the same basic structures as the fourth stage dynode unit DY 4  shown in  FIGS. 7A to 7D .  FIGS. 7B to 7D  are sectional views of a connecting portion  410   b  in the supporting frame  410 , respectively. 
         [0069]    The dynodes respectively held by the fourth, sixth, and eighth stage dynode units DY 4 , DY 6 , and DY 8  are basically the same in a cross-sectional shape, and the dynodes respectively held by the fifth and seventh stage dynode units DY 5  and DY 7  are basically the same in a cross-sectional shape. The dynode units DY 1  to DY 8  of the respective stages include: the metal supporting frames  410 ; the ceramic insulating spacers  420  for electrically separating the dynode units DY 1  to DY 8  from each other and defining the intervals between the dynode units DY 1  to DY 8 ; and the metal dynode pins  430  prepared for the dynode units DY 1  to DY 8  respectively to set the dynode units DY 1  to DY 8  respectively to the predetermined potentials. 
         [0070]    For example, as shown in  FIG. 7A , in the case of the fourth stage dynode unit DY 4 , the supporting frame  410  is constituted by a pair of supports  410   a  disposed to sandwich all of the plurality of dynodes  414 , and a connecting portion  410   b  with both ends fixed to the pair of supports  410   a  and being set to the same potential as the supports  410   a.  In particular, the connecting portion  410   b  is disposed so as to be sandwiched by at least two dynodes among the dynodes  414 , and by the connecting portion  410   b  being disposed thus, the supporting frame  410  has an H shape. 
         [0071]    The connecting portion  410   b  has formed therein through holes  411  for letting the dynode pins associated to the dynode units of at least the upper stages (the first to third stage dynode units DY 1  to DY 3  in the case of the fourth stage dynode unit DY 4 ) penetrate through without electrical contact and a through hole for fixing one end of the associated dynode pin  430  by welding, crimping, etc., in a penetrated state. Here, the one end of the associated dynode pin  430  is electrically connected to the supporting frame  410 , and the other end of the dynode pin  430  is directly or indirectly connected to the lead pin  700  fixed to the stem  130  while being in a state of penetrating through the dynode units positioned in lower stages. Also formed in the connecting portion  410   b  are through holes  415  for letting the lead pins  330   a  and  330   b,  the one ends of which are fixed while being electrically connected to the focusing electrode unit  300  positioned above the electron multiplier section  400 , penetrate through to the stem  130  side. The connecting portion  410   b  furthermore has formed therein embosses  412  for positioning with respect to the insulating spacer of the upper stage dynode unit (the third stage dynode unit DY 3  in the case of the fourth stage dynode unit DY 4 ), and embosses  413  for positioning with respect to the insulating spacer  420  that is directly fixed to the supporting frame  410  itself. In particular,  FIG. 7B  shows a sectional structure of the through hole  411  in the connecting portion  410   b  taken on line III-III in  FIG. 7A ,  FIG. 7C  shows a sectional structure of the emboss  412  in the connecting portion  410   b  taken on line IV-IV in  FIG. 7A , and  FIG. 7D  shows a sectional structure of the emboss  413  in the connecting portion  410   b  taken on line V-V in  FIG. 7A . 
         [0072]    The insulating spacer  420  also has an H shape like the supporting frame  410  and has portions associated to the pair of supports  410   a  and the connecting portion  410   b  that constitute the supporting frame  410 . That is, the insulating spacer  420  also has a pair of supports and a connecting portion. In particular, through holes  423  are also formed in the connecting portion of the insulating spacer  420  at positions corresponding to the through holes  411  and  415  formed in the connecting portion  410   b  of the supporting frame  410 . The through holes  423  are disposed to coincide with the centers of the through holes  411  and  415  formed in the connecting portion  410   b  of the supporting frame  410 . 
         [0073]    Furthermore, the insulating spacer  420  not only separates the dynode units of the respective stages from each other electrically but also defines the interval between dynode units. Thus in the present embodiment, the insulating spacer  420  includes a plurality of spacer elements  420   a  and  420   b  that have the same shape. By adjusting the number of the spacer elements, the dynode unit interval (interval between supporting frames) can be changed arbitrarily. The spacer elements  420   a  and  420   b  that constitute the insulating spacer  420  are stacked in direct contacting states along the direction directed from the photocathode  200  to the anode unit  500 . For example, in the present embodiment, a single spacer element is installed respectively between the first stage dynode unit DY 2  and the second stage dynode unit DY 1 , between the second stage dynode unit DY 1  and the third stage dynode unit DY 3 , and between the third stage dynode unit DY 3  and the fourth stage dynode unit DY 4 . Two spacer elements are installed in the respective intervals between the fourth to eighth stage dynode units DY 4  to DY 8 . Eight spacer elements are installed between the eighth stage dynode unit DY 8  and the anode unit  500 . 
         [0074]    To assemble each of the dynode units DY 1  to DY 8 , the supporting frame  410  and the insulating spacer  420  is overlapped, and the dynode pin  430  is fixed to the supporting frame  410  with the dynode pin  430  penetrating through the respective through holes  411  and  423 . That is, at an upper surface side of the supporting frame  410 , the dynode pin  430  is fixed to the supporting frame  410  by welding the dynode pin  430  and the supporting frame  410  or by crimping an end of the dynode pin  430 . Here, although below the focusing electrode unit  300 , the respective dynode units are stacked in the order of: the dynode unit DY 2 , holding the second dynodes; and the dynode unit DY 1 , holding the first dynodes; the electron multiplication is performed in the order of: the first dynodes held by the second stage dynode unit DY 1 ; and the second dynodes held by the first stage dynode unit DY 2 . Such a structure is adopted to stack the dynode units compactly and efficiently and yet realize optimal electron trajectories. 
         [0075]    Here, the plurality of dynodes  414 , both ends of each of which are supported by the pair of supports  410   a,  are formed integral to the pair of supports  410   a  as shown in  FIGS. 8A to 8C  and constitute a part of the supporting frame  410 . 
         [0076]    That is, the supporting frame  410  and a plate portion that is to become dynodes are cut out integrally from a single metal plate as shown in  FIG. 8A . In the plate portion, both ends of which are connected to the supporting frame  410 , depressions that are to become the dynodes are formed additionally by pressing. Specifically, two depressions are formed adjacently as shown in  FIG. 8B , and these depressions become two mutually adjacent electron multiplier channels. The plate portion, in which the two dynodes have been formed, is then bent in a direction indicated by an arrow S 1  to obtain the dynodes  414  integrally held by the supporting frame  410  ( FIG. 8C ). 
         [0077]      FIGS. 9A to 9D  are a perspective view and sectional views for describing a configuration of the insulating spacer  420  disposed between the dynode units. In particular,  FIGS. 9A to 9D  show a structure of the spacer element  420   a  ( 420   b ) that constitutes the insulating spacer  420 , and as shown in  FIG. 9A , the spacer element  420   a  ( 420   b ) has an H shape like the supporting frame  410 . That is, the spacer element  420   a  ( 420   b ) constitutes a pair of supports  421 , associated to the pair of supports  410   a  of the supporting frame  410 , and a connecting portion  422 , associated to the connecting portion  410   b  of the supporting frame  410 . 
         [0078]    In the connecting portion  422  of the spacer element  420   a  ( 420   b ), through holes  423  and  426  are formed at positions corresponding to the through holes  411  and  415  of the connecting portion  410   b  of the supporting frame  410 . The connecting portion  422  also has formed therein embosses  424  for positioning with respect to the supporting frame  410 , and embosses  425  for positioning with respect to the supporting frame of the dynode unit positioned below. Here, when the insulating spacer  420  is formed by stacking a plurality of the spacer elements, the embosses  424  and  425  do not function.  FIG. 9B  shows a sectional structure of the through hole  423  in the connecting portion  422  taken on line VI-VI in  FIG. 9A ,  FIG. 9C  shows a sectional structure of the emboss  424  in the connecting portion  422  taken on line VII-VII in  FIG. 9A , and  FIG. 9D  shows a sectional structure of the emboss  425  in the connecting portion  422  taken on line VIII-VIII in  FIG. 9A . 
         [0079]      FIGS. 10A and 10B  are sectional views for describing a stacked structure of the dynode units. As described above, the dynode units DY 1  to DY 8  of the respective stages each include: the supporting frame  410 , holding the plurality of dynodes  414 ; the insulating spacer  420 ; and the dynode pin  430 , having one end weld-connected to the supporting frame  410  by a solder  432 . When the elements  410 ,  420 , and  430  are assembled integrally, the dynode pin of the dynode unit positioned at an upper stage is inserted into the through hole of the dynode unit positioned immediately below as shown in  FIG. 10A . By successively repeating this process, the stacked structure of the dynodes units is obtained as shown in  FIG. 10B . In  FIGS. 10A and 10B , the third stage dynode unit DY 3  is shown as the dynode unit of the upper stage, and the fourth stage dynode unit DY 4  is shown as the dynode unit immediately below. In regard to the order of assembly of the respective dynode units, the insulating spacer  420  may be fixed to the supporting frame  410  after the supporting frame  410  and the one end of the associated dynode pin  430  have been fixed. In this case, a flange  431  of the dynode pin  430  is unnecessary. 
         [0080]    Here, a step is formed in the through hole  423  of each of the spacer elements  420   a  and  420   b  that constitute the insulating spacer  420 . Meanwhile, the flange  431 , contacting the step formed in the through hole  423  of the spacer  420   b  (the spacer element of the lowermost layer in a case where a plurality of spacer elements are stacked), is disposed on the dynode pin  430  associated to the dynode unit of each stage. The position of the associated dynode pin  430  along the direction directed from the photocathode  200  to the anode unit  500  is thus defined by the step. Also, when the one end of the dynode pin  430  is fixed to the supporting frame  410  (the connecting portion) in the state where the flange  431  contacts the step of the spacer element  420   b,  the entire insulating spacer  420  is pressed against the supporting frame  410  by the flange  431 . By such cooperation of the step formed in the through hole  423  of the spacer element  420  and the dynode pin  430 , a structure for fixing the entire insulating spacer to the supporting frame  410  and a structure for positioning the dynode pin  430  are realized. 
         [0081]    A configuration of dynode unit is not limited to the above-described configurations, but can be modified in various manners. For example,  FIGS. 11A and 11B  are an assembly process diagram and sectional views for describing a second configuration of a fourth stage dynode unit that constitutes a portion of the electron multiplier section. In addition,  FIGS. 12A and 12B  are an assembly process diagram and sectional views for describing a third configuration of a fourth stage dynode unit that constitutes a portion of the electron multiplier section. In the following, as second and third configurations, the fourth stage dynode unit DY 4  will be referred. 
         [0082]    As shown in  FIG. 11A , the fourth stage dynode unit DY 4  according to the second configuration comprises a supporting frame  420 A holding a plurality of dynodes  414   a,  an insulating spacer  420 A, and a dynode pin  430 . The supporting frame  410 A is constituted by a pair of supports  410   a  disposed so as to sandwich all dynodes  414   a,  and a connection portion  410   b  with both ends fixed to the pair of supports  410   a  and being set to the same potential as the supports  410   a.  As compared with the supporting frame  410  according to the first configuration shown in  FIG. 7A , the second configuration differs from the first configuration in a dynode shape to be held. In other words, in the supporting frame  410  according to the first configuration, both two dynodes  414  are held by the pair of supports  410   a.  On the other hand, in the supporting frame  410 A, one dynode  414   a  is held by the pair of supports  410   a.    
         [0083]    The insulating spacer  420  in the second configuration, similar to the insulating spacer  420  in the first configuration, has potions  421 A and  422 A corresponding to the supports  410   a  and the connecting portion  410   b  that constitutes the supporting frame  410 A. Here, though the insulating spacer  420  in the first configuration is constituted by the spacers elements  420   a  and  420   b,  the insulating spacer  420 A is constituted by a single member. 
         [0084]    In addition, the dynode pin  430  has the same configuration as the first and second configurations. That is, in such a second configuration, the dynode pin  430  is provided with an alignment flange  431 . The fourth stage dynode unit DY 4 , as shown in  FIG. 11B , can be obtained by fixing one end of the dynode pins  430  to the supporting frame  410 A through the through hole provided in the connecting portion  422 A of the insulating spacer  420 A in the sate of overlapping the supporting frame  410 A and the insulating spacer  420 A. In this time, the supporting frame  410 A and the dynode pin  430  are electrically connected to each other. 
         [0085]    Next, a dynode unit according to the third configuration ( FIGS. 12A and 12B  show only fourth stage dynode unit DY 4 ), similar to the first and second configurations, also comprises a supporting frame  410 B holding a plurality of dynodes  414   a,  an insulating spacer  420 B, and a dynode pin  430 . The supporting frame  410 B in the third configuration has the same configuration as the supporting frame  410 A in the second configuration. Here, the insulating spacer  420 B in the third configuration, similar to the second configuration, portions  421 B corresponding to the pair of supports  410   a  in the supporting frame  410 B and a portion  422 B corresponding to the connecting portion  410   b,  but the third configuration differs from the second configuration in the point of further comprising a plurality of light shielding portions  423 B disposed so as to plaster the openings positioned between the dynodes  414   a.  Also, each of the plurality of light shielding portions  423 B is provided with a plurality of slits  450 . By this configuration, the light shielding portions  423 B function to shield light propagating from the anode side to the photocathode side, and, on the other hand, each of the slits  450  functions to pass an alkali metal vapor for photocathode formation therethrough from the anode side to the photocathode side. As described above, the dynode unit according to the second configuration ( FIGS. 7A to 7D ) and the dynode unit according to the third configuration differ in a configuration of insulating spacer. 
         [0086]    In such a third configuration, the dynode pin  430  also has the same configuration as the first and second configuration. In other words, in the third constitution, the dynode pin  430  is provided with an alignment flanges  431 . The fourth stage dynode unit DY 4 , as shown in  FIG. 12B , can be obtained by fixing one end of the dynode pins  430  to the supporting frame  410 A through the through hole provided in the connecting portion  422 A of the insulating spacer  420 A in the sate of overlapping the supporting frame  410 A and the insulating spacer  420 A. At this time, the supporting frame  410 A and the dynode pin  430  are electrically connected to each other. Also, by the light shielding portions  423 B in the insulating spacer  420 B, the openings positioned between the dynodes  414   a  are plastered. 
         [0087]      FIG. 13  is an assembly process diagram for describing a first configuration of the anode unit. 
         [0088]    As shown in  FIG. 13 , the anode unit  500  includes: the ceramic substrate  510 ; the plurality of anode electrodes  520 , disposed on the ceramic substrate  510 ; and the lead pins  530  (anode pins), the one ends of which are respectively fixed while being electrically connected to the anode electrodes  520 . In the ceramic substrate  510 , openings  511  are formed in correspondence to the positions of the anode electrodes  520 , and through holes  512  are formed for supporting and letting portions of the anode pins  530  pass through. On a rear surface of the ceramic substrate  510  are disposed auxiliary members  560   a  to  560   d  for mounting the other ends of the side wall substrate members  510   a  to  510   d  to the anode unit  500 . Furthermore, alkali source pellets  540 , for forming the secondary electron emitting surfaces of the cathode  200  and the dynodes, are mounted on the auxiliary members  560   a  and  560   b,  and a getter  550  is mounted on the auxiliary member  560   c.  To assemble the anode unit  500 , the lead pins  530 , having the flanges  531 , are penetrated through the respective through holes with the anode electrode  520 , the ceramic substrate  510 , and the auxiliary members  560   a  to  560   b  being overlapped sequentially. Here, by welding the anode electrodes  520  and the one ends of the anode pins  530  or by crimping the ends of the anode pins  530  on the upper surfaces of the anode electrodes  520 , the anode pins  530  are fixed to the anode electrodes  520  via the ceramic substrate  510  and the auxiliary members  560   a  to  560   d.  By the ends of the anode pins  530  being fixed to the anode electrodes  520 , the flanges  531  disposed on the anode pins  530  function to press the ceramic substrate  510  and the auxiliary members  560   a  to  560   d  against the anode electrodes  520 . 
         [0089]    In  FIG. 13 , only the side wall substrate member  5   10   a  among the side wall substrate members  510   a  to  510   d  is shown (illustration of the side wall substrate members  510   b  to  510   d  is omitted). A slit  511   b  is formed in the other end of the side wall substrate member  510   a.  By the slit  511   b  and a fixing tab of the auxiliary member  560   a  being joined after the focusing electrode unit  300 , the electron multiplier section  400 , and the anode unit  500  have been stacked as shown in  FIG. 4 , the side wall substrate member  510   a  functions to maintain the stacked structure. Although not illustrated, the remaining side wall substrate members  510   b  to  510   d  also have the same structure and function in the same manner as the side wall substrate member  510   a.    
         [0090]    The anode unit  500  described above can be realized by various configurations. For example,  FIGS. 14A and 14B  are assembly process diagrams for describing a second configuration of the anode unit. In addition,  FIGS. 15A and 15B  are assembly process diagrams for describing a third configuration of the anode unit. 
         [0091]    As shown in  FIG. 14A , the anode unit  500  according to the second configuration a ceramic substrate  510 A, a plurality of anode electrodes  520  to be provided on the ceramic substrate  510 A, and lead pins (anode pin)  530  fixed to the anode electrodes  520  while one end of each lead pin  530  is electrically connected to the associated one of the anode electrodes  520 . The ceramic substrate  510 A is provided with openings  511 A in according to the arrangement of the anode electrodes  520 , and through holes for respectively passing and supporting the anode pins  520 . Each of the anode pins  530  is provided with an alignment flange  531 . In addition, unlike the first configuration, on the rear surface of the ceramic substrate  510 A, spring members  570 , which functions to maintain the setting position of the internal unit including the anode unit  500  inside the sealed container  100 , are fixed. 
         [0092]    To assemble the anode unit  500 , in the state that the anode electrodes  520  and the ceramic substrate  510 A the rear surface of which the spring members  570  are attached are overlapped, let the anode pins  530  each having a flange  531  penetrate through the through holes thereof. At this time, the anode pins  530  are fixed to the anode electrodes  520  through the ceramic substrate  510 A, by welding one end of the anode pin  530  to the associated anode electrode  520  or crimping the end of the anode pin  530 , on the upper surface of the associated anode electrode  520 . The flange  531  provided on each of the anode pin  530  functions to push the ceramic substrate  510 A to the anode electrodes  520  by fixing the anode pins  530  to the associated anode electrodes  520 . The anode unit  500  according to the second configuration, as shown in  FIG. 14B , can be obtained via the above assembling process. 
         [0093]    Next, the anode unit  500  according to the third configuration, as shown in  FIG. 15A , can improve a linearity by reflecting type anode electrodes  520 B provided. 
         [0094]    In other words, the anode unit  500  according to the third configuration comprises a ceramic substrate  510 B, and a plurality of reflecting type anode electrodes  520 B provided with the ceramic substrate  50 B. On both ends of each reflecting type anode electrode  520 B, the electrode pieces  521 B for electron output. Therefore, as shown in  FIG. 15B , the anode unit  500  according to the third configuration can be obtained by inserting the electrode pieces  521 B of each reflecting type anode electrode  520 B into the slit-shaped through holes provided on the ceramic substrate  510 B. 
         [0095]    Each part constituting the internal unit housed in the sealed container  100  can be realized in the above various configurations. As an example,  FIGS. 16A and 16B  are schematic perspective views of an internal unit in which the focusing electrode unit of  FIG. 6 , the electron multiplier section of  FIGS. 12A and 12B , and the anode unit  FIGS. 14A and 14B  are stacked integrally. In other words,  FIG. 16A  is a perspective view of an internal unit according to another configuration when the internal unit is viewed from the photocathode side, and  FIG. 16B  is a perspective view of an internal unit according to another configuration when the internal unit is viewed from the stem side. 
         [0096]    In addition,  FIG. 17  is a diagram of a sectional structure taken on line XVIII-XVIII of the internal unit shown in  FIGS. 16A and 16B . Here, the dynode unit of  FIGS. 12A and 12B  comprises an insulating spacer  420 B having a plurality of light shielding portions  423 B each provided with a plurality of slits  450 . The arrow B 1  shown in  FIG. 17  indicates propagation paths of alkali metal vapor passing through each stage dynode unit from the stem side to the photocathode side. On the other hand, the arrow B 2  indicates propagation paths of light generated near the anode electrodes  520 . As shown in  FIG. 17 , in the insulating portion  420 B constituting each stage dynode unit, the light shielding portions  423 B disposed so as to plaster the openings positioned between the dynodes  414   a  shields most of light generated near the anode electrodes  520 . In addition, light passing through the slits  450  provided in each light shielding portion  423 B is also shielded by the dynodes  414   a  positioned at the upper stage. On the other hand, the alkali metal vapor directing from the stem side to the photocathode side smoothly flows by the structure in which the stage dynode units are stacked while being separated at a predetermined distance and the structure in which a plurality of slits  450  are provided in each light shielding portion  423 B. 
         [0097]    Although in the above-described embodiment, each of the dynodes held by the dynode units DY 1  to DY 8  of the respective stages has a line focus shape, the dynode shape is not restricted to the line focus shape. For example, a dynode unit DY shown in  FIG. 18A  is a metal channel plate formed by adhering together two metal plates, each having electron multiplier holes formed therein. In this case, the electron multiplier holes formed in the metal channel plates correspond to being the dynodes held by the dynode unit DY A dynode unit DY shown in  FIG. 18B  has a structure in which a mesh electrode is sandwiched by two metal frames, each having openings. With the dynode unit DY shown in  FIG. 18B , the opening portions of the metal frames function as mesh dynodes. In a dynode unit DY shown in  FIG. 18C , a metal frame and dynodes held thereby are formed integrally by etching. 
         [0098]    As described above, the electron multiplier section  400  is obtained by the stacking of the multiple stages of the dynode units DY 1  to DY 8 , in which various dynodes are held. When the dynode units DY 1  to DY 8  of the respective stages are stacked, the dynode pins associated to the dynode units DY 1  to DY 8  of the respective stages are disposed to penetrate through a space in which the dynodes  430  are disposed as shown in  FIG. 18D . The space through which the lead pins  430  penetrate as viewed from the photocathode  200  side is the effective region of the electron multiplier section  400 . 
         [0099]      FIGS. 19A to 19C  are a plan view and sectional views of the fourth stage dynode unit DY 4  for describing the structure of the fourth stage dynode unit DY 4  and the effective region of the electron multiplier section  400 . As mentioned above, the dynode units DY 1  to DY 8  of the respective stages all have the same structure, and the fourth stage dynode unit DY 4  is shown in  FIGS. 19A to 19C  as a representative unit.  FIG. 19A  is a plan view of the fourth stage dynode unit DY 4  as viewed from the photocathode  200  side,  FIG. 19B  is a sectional view of the fourth stage dynode unit DY 4  taken on line IX-IX in  FIG. 19A , and  FIG. 19C  is a sectional view of the fourth stage dynode unit DY 4  taken on line X-X in  FIG. 19A . 
         [0100]    As shown in  FIG. 19A , the fourth stage dynode unit DY 4  includes the supporting frame  410  holding the plurality of dynodes  414 , with each of which one electron multiplier channels are formed (the same applies to the other dynode units DY 1  to DY 3  and DY 5  to DY 8 ). The effective region AR 1  in the electron multiplier section  400  is the field region as viewed from the photocathode  200  side that contributes to secondary electron multiplication, and is defined as the photoelectron incidence surface of the electron multiplier section  400  on a plane orthogonal to the central axis AX of the bulb  120  in the sealed container  100 . That is, the effective region is a minimum region that, when contours of all dynodes  414  included in the electron multiplier section  400  are projected onto the photoelectron incidence surface of the electron multiplier section  400 , contains all projected components of the contours. A boundary line defining the effective region AR 1  of the electron multiplier section  400  thus partially coincides with a portion of projected components of one of the dynode contours as shown in  FIG. 19A . 
         [0101]    By the dynode pins  430  associated to the dynode units DY 1  to DY 8  of the respective stages being disposed inside the effective region AR 1  of the electron multiplier section  400  shown in  FIG. 19A , the following effects are provided.  FIGS. 20A and 20B  are conceptual diagrams for describing technical effects of the photomultiplier according to the present invention by comparison with a conventional art. 
         [0102]    Normally, a peripheral region of a light exiting surface of the incidence surface plate  110 , on which the photocathode  200  is formed, is processed to a curved surface as shown in  FIG. 20A . Thus, in comparison to photoelectrons emitted from near a center of the photocathode  200 , trajectories of photoelectrons emitted from the peripheral region are more greatly modified in a space defined by a focusing distance D. In this case, in a conventional photomultiplier, if an adequate focusing distance D cannot be secured, cascade multiplication of the photoelectrons emitted from the peripheral region of the photocathode  200  cannot be performed (the photoelectrons collide with the focusing electrode, etc., before reaching the first dynodes). 
         [0103]    With the conventional photomultiplier, a dynode pin is fixed to a fixing tab DYb disposed along a periphery of an effective region of a electron multiplier section that avoids the effective region in which the dynodes are disposed, that is, specifically, at an outer periphery of a frame DYa that supports the dynodes as shown in  FIG. 20B . The effective region AR 2  of the electron multiplier section defined at an inner side of the frame DYa is thus restricted by just the dynode pin disposing space. 
         [0104]    On the other hand, with the photomultiplier according to the present invention, because the dynode pins  430  are disposed inside an effective region AR 3  (=AR 1 ) of the electron multiplier section  400  as shown in  FIG. 20C , it becomes possible to enlarge the effective region of the electron multiplier section in comparison to the conventional photomultiplier. By enlargement of the effective region AR 3 , trajectory modifications, especially of photoelectrons emitted from the peripheral region of the photocathode  200  opposing the photoelectron incidence surface of the electron multiplier section  400 , are lessened in degree. The focusing distance D is thus reduced significantly (the photomultiplier can be made compact). 
         [0105]    Effects of the above-described structural characteristics shall now be described more specifically using  FIGS. 21A to 21C .  FIGS. 21A to 21C  are diagrams for describing trajectories of photoelectrons emitted from the photocathode  200  for describing the structural characteristics and effects of the photomultiplier according to the present invention.  FIG. 21A  is a plan view of the incidence surface plate  110  as viewed from the light incidence surface  110   a  side, and the effective region AR 1  of the electron multiplier section  400  is enlarged to a degree such that it substantially coincides with an effective cathode area (practically coincident with the light exiting surface  110   b  in the incidence surface plate  110 ) of the incidence surface plate  110 . Here as shown in  FIG. 20A , the effective region of the electron multiplier section  400  is the field region as viewed from the photocathode  200  side that contributes to secondary electron multiplication, and is defined as the photoelectron incidence surface of the electron multiplier section  400  on the plane orthogonal to the central axis AX of the bulb  120  in the sealed container  100 .  FIG. 21B  is a sectional view of the photomultiplier taken on line XI-XI shown in  FIG. 21A , and  FIG. 21C  is a sectional view of the photomultiplier taken on line XII-XII shown in  FIG. 21A . 
         [0106]      FIGS. 22A to 22C  are sectional views, corresponding to  FIGS. 22A to 22C , of a photomultiplier of a first comparative example prepared for describing the structural characteristics and effects of the photomultiplier according to the present invention and are diagrams for describing photoelectron trajectories A 2  in the photomultiplier according the first comparative example. The prepared photomultiplier according to the first comparative example is a multichannel photomultiplier (four channels) having two first dynodes DY 1  (two channels are disposed adjacently in each dynode) with back sides facing the central axis AX of the bulb. 
         [0107]      FIG. 22A  is a plan view of an incidence surface plate as viewed from a light incidence surface side of the photomultiplier according to the first comparative example and is a plan view corresponding to  FIG. 21A .  FIG. 22B  is a sectional view of the photomultiplier taken on line XIII-XIII shown in  FIG. 22A , and  FIG. 22C  is a sectional view of the photomultiplier taken on line XIV-XIV shown in  FIG. 22A . 
         [0108]    With the photomultiplier according to the first comparative example, a focusing distance D 2 , which is a photoelectron transit distance from a photocathode to the first dynodes DY 1 , is significantly long in comparison to the focusing distance D 1  ( FIGS. 21B and 21C ) of the photomultiplier according to the present invention. Distance variation of the trajectories A 2  of the photoelectrons that differ in an emission position on the photocathode is thus large (fluctuation of the photoelectron transit time is large). Also, with the photomultiplier according to the first comparative example, the trajectories A 2  of the photoelectrons emitted from a peripheral region of the photocathode must be curved greatly to avoid both a ceramic substrate, for holding the dynodes, and dynode pins (disposed in a periphery of the effective region of the electron multiplier section), for applying predetermined voltages to the respective dynodes. This is done to avoid incidence onto a focusing electron and other metal members disposed between the photocathode and the electron multiplier section and to avoid incidence of photoelectrons onto side wall portions of the first dynode DY 1  (portions at which a secondary electron emitting surface is not formed). With the photomultiplier according to the first comparative example in which trajectory modifications of such large degree are performed, a transit time difference between photoelectrons emitted from near a center of the photocathode and photoelectrons emitted from the peripheral region becomes large. 
         [0109]    Meanwhile,  FIGS. 23A to 23C  are sectional views, corresponding to  FIGS. 21A to 21C , of a photomultiplier of a second comparative example, prepared for describing the structural characteristics and effects of the photomultiplier according to the present invention and are diagrams for describing photoelectron trajectories in the photomultiplier according the second comparative example. As with the first comparative example, the photomultiplier according to the second comparative example is a multichannel photomultiplier having four electron multiplier channels.  FIG. 23A  is a plan view of an incidence surface plate as viewed from a light incidence surface side of the photomultiplier according to the second comparative example and is a plan view corresponding to  FIG. 21A .  FIG. 23B  is a sectional view of the photomultiplier taken on line XV-XV shown in  FIG. 23A , and  FIG. 23C  is a sectional view of the photomultiplier taken on line XVI-XVI shown in  FIG. 23A . 
         [0110]    Although a basic structure of the photomultiplier according to the second comparative example is the same as that of the first comparative example, a focusing distance D 3  from the photocathode to the first dynode DY 1  is forcibly designed to be shorter than the focusing distance D 2  of the photomultiplier according to the first comparative example. With the second comparative example, because a focusing distance that is adequate for curving the trajectories A 3  of the photoelectrons emitted from the periphery of the photocathode cannot be secured, the photoelectrons collide with the focusing electrode disposed between the photocathode and the electron multiplier section. 
         [0111]    On the other hand, with the photomultiplier according to the present invention ( FIGS. 21A to 21C ), because the dynode pins are disposed within the effective region AR 1  of the photomultiplier  400 , the effective region AR 1  is more enlarged than in the conventional photomultipliers according to the first and second comparative examples ( FIGS. 22A to 23C ). By enlargement of the effective region AR 1 , the trajectory modifications, especially of the photoelectrons emitted from the peripheral region of the photocathode  200  opposing the photoelectron incidence surface of the electron multiplier section  400 , are lessened in degree. The focusing distance D 1  is thus reduced significantly, and the transit distance difference between photoelectrons emitted from a central region of the photocathode  200  and photoelectrons emitted from the peripheral region becomes small (fluctuations in transit time are small). Also, by the peripheral region of the effective region AR 1  of the electron multiplier section  400  being enlarged, it becomes possible to make the photoelectrons, emitted from the peripheral region of the photocathode  200 , be incident on the first dynodes (first dynode unit DY 1 ) without greatly modifying the trajectories A 1  of the photoelectrons. 
         [0112]    In the above-described embodiment, the sealed container  100  of the photomultiplier according to the present invention includes: the envelope portion, in which the incidence surface plate and the bulb are formed integrally (with the top  110  of the envelope portion, supported by the bulb  120 , functioning as the incidence surface plate); and the stem  130 , holding the evacuating pipe  600  and the lead pins  700 . However, the sealed container applied to the photomultiplier is not restricted to the above-described structure. For example, as shown in  FIG. 24A , a sealed container  900  may include: an incidence surface plate  910 ; a bulb  920 ; and a stem  930 ; which are respectively independent glass members. The incidence surface plate  910  has a light incidence surface  910   a  and a light exiting surface  910   b  that oppose each other, and the photocathode  200  is formed on the light exiting surface  910   b  of the incidence surface plate  910  positioned at an inner side of the sealed container  900 . The bulb  920  has a shape extending along the predetermined tube axis AX and the incidence surface plate  910  is joined by fusion to one end thereof. The stem  930 , constituting a bottom of the sealed container  900 , is joined by fusion to the other end of the bulb  920 , and, an evacuating pipe  940  is disposed and lead pins  950 , electrically connecting the interior and the exterior of the sealed container  900 , are installed in respectively penetrating states in the stem  930  as well.  FIG. 24B  is a sectional view of a structure of the other sealed container taken on line XVII-XVII shown in  FIG. 24A  and particularly shows a structure near the incidence surface plate  910 , on the inner side of which is formed the photocathode  200 . Even with such a sealed container  900 , by the photocathode  200  being formed on the light exiting surface  910   b  of the incidence surface plate  910 , the effects of the above-described photomultiplier are obtained. 
         [0113]    As described above, with the photomultiplier according to the present invention, trajectory modifications of the photoelectrons emitted from the peripheral region of the photocathode can be lessened, and because a structure with a short focusing distance can consequently be realized, such response time characteristics, as TTS and CTTD, are improved significantly. 
         [0114]    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.