Patent Publication Number: US-8115386-B2

Title: Photomultiplier tube

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a photomultiplier tube for detecting incident light from outside. 
     2. Related Background Art 
     Conventionally, compact photomultiplier tubes by utilization of fine processing technology have been developed. For example, a flat surface-type photomultiplier tube which is arranged with a photocathode, dynodes and an anode on a translucent insulating substrate is known (refer to Patent Document 1 given below). The above-described structure makes it possible to detect weak light at a high degree of reliability and also downsize a device.
     Patent Document 1: U.S. Pat. No. 5,264,693   

     SUMMARY OF THE INVENTION 
     However, in the above-described conventional photomultiplier tubes, individual stages of dynodes are arranged on an insulating substrate of a casing constituted with the insulating substrate and cap members, thereby giving such a structure that multiplied electrons, the orbit of which is widened as the electrons pass between secondary electron surfaces of these individual stages of dynodes, are easily made incident onto the insulating substrate of the casing. This tendency becomes apparent when the casing is downsized for refinement. Therefore, there has been a case where the casing is electrically charged to result in a decrease in withstand voltage. 
     Under these circumstances, the present invention has been made in view of the above problem, an object of which is to provide a photomultiplier tube capable of preventing electrons from being made incident onto an insulation part of a casing between dynodes to improve a withstand voltage. 
     In order to solve the above problem, the photomultiplier tube of the present invention is a photomultiplier tube which is provided with a first substrate and a second substrate which are arranged so as to oppose each other, with the respective opposing surfaces made with an insulating material, a side wall part which constitutes a casing together with the first and the second substrates, a plurality of stages of electron multiplying parts which are arrayed on the opposing surface of the first substrate so as to be spaced away sequentially from a first end side to a second end side and each of which has a secondary electron surface extending in a direction intersecting with the opposing surface, a photocathode which is installed on the first end side so as to be spaced away from the electron multiplying part, converting incident light from outside to photoelectrons to emit the photoelectrons, and an anode part which is installed on the second end side so as to be spaced away from the electron multiplying part to take out electrons multiplied by the electron multiplying parts as a signal, in which the opposing surface of the second substrate is formed so as to cover a plurality of electron multiplying parts, and a plurality of conductive members which are electrically independent from each other and set equal in potential to the individually opposing electron multiplying parts are installed along the opposing surface at sites opposing individually the plurality of electron multiplying parts on the opposing surface. 
     According to the above-described photomultiplier tube, incident light is made incident on the photocathode, by which the light is converted to photoelectrons, these photoelectrons are made incident sequentially into a plurality of stages of electron multiplying parts on the opposing surface of the first substrate and multiplied accordingly, and the thus multiplied electrons are taken out from the anode part as an electric signal. In this instance, a plurality of conductive members equal in potential to each of the opposing electron multiplying parts are installed so as to be electrically independent from each other at sites opposing each of the plurality of stages of electron multiplying parts on the opposing surface of the second substrate opposing the first substrate. Therefore, electrons passing between the plurality of stages of electron multiplying parts are prevented from being made incident onto the opposing surface of the second substrate. It is, thereby, possible to prevent a decrease in withstand voltage due to electric charge of the surface of the substrate. 
     It is preferable that the plurality of conductive members are formed in such a manner that each of the end parts thereof on the second end side projects to the second end side more than each of the end parts of the opposing electron multiplying parts on the second end side. In this instance, electrons passing between the stages of electron multiplying parts can be reliably prevented from being made incident onto the opposing surface of the second substrate. 
     It is also preferable that the plurality of conductive members are formed in such a manner that each of the end parts thereof on the first end side is positioned to the second end side more than each of the end parts of the opposing electron multiplying parts on the first end side. According to the above constitution, a distance between adjacent conductive members is secured, thus making it possible to suppress leakage current between the conductive members and also increase a withstand voltage. 
     Further, it is also preferable that the plurality of conductive members are connected to a plurality of power feeding parts installed on the second substrate and the plurality of electron multiplying parts are electrically connected to the individually opposing conductive members and powered from the plurality of power feeding parts. In this instance, the electron multiplying parts are powered via conductive members, thus simplifying a structure in which the conductive members are set equal in potential to the electron multiplying parts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a photomultiplier tube which is related to one preferred embodiment of the present invention. 
         FIG. 2  is an exploded perspective view of the photomultiplier tube given in  FIG. 1 . 
         FIG. 3  is a partially broken perspective view showing an internal structure of the photomultiplier tube shown in  FIG. 1 , when viewed from an upper frame. 
         FIG. 4  is a partially broken perspective view showing an internal structure of the photomultiplier tube shown in  FIG. 1 , when viewed from a lower frame. 
         FIG. 5  is a partially enlarged sectional view along the line V to V in a state that the upper frame is attached to electron multiplying parts and the lower frame shown in  FIG. 3 . 
         FIG. 6  is a perspective diagram showing focusing electrodes and electron multiplying parts in  FIG. 3 , when viewed from the upper frame. 
         FIG. 7  is a partially enlarged sectional view showing a modified example of the conductive layers shown in  FIG. 5 . 
         FIG. 8  is a partially enlarged sectional view showing a modified example of the conductive layers shown in  FIG. 5 . 
         FIG. 9  is a partially enlarged sectional view showing a comparative example of the electron multiplying parts, the lower frame and the upper frame shown in  FIG. 5 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, a detailed description will be given for preferred embodiments of the photomultiplier tube related to the present invention by referring to drawings. In addition, in describing the drawings, the same or corresponding parts will be given the same numeral references to omit overlapping description. 
       FIG. 1  is a perspective view of a photomultiplier tube  1  related to one preferred embodiment of the present invention.  FIG. 2  is an exploded perspective view of the photomultiplier tube  1  shown in  FIG. 1 . 
     The photomultiplier tube  1  shown in  FIG. 1  is a photomultiplier tube having a transmission-type photocathode and provided with a casing constituted with an upper frame  2  (a second substrate), a side-wall frame  3  (a side wall part), and a lower frame  4  (a first substrate) which is constituted so as to oppose the upper frame  2 , with the side-wall frame  3  kept therebetween. The photomultiplier tube  1  is an electron tube such that a light incident direction onto the photocathode intersects with a direction at which electrons are multiplied at the electron multiplying part. Specifically, when light is made incident from a direction indicated by the arrow A in  FIG. 1 , photoelectrons emitted from the photocathode are made incident onto the electron multiplying part, thereby secondary electrons are subject to cascade amplification in a direction indicated by the arrow B to take out a signal from the anode part. 
     In addition, in the following description, the upstream side of an electron multiplying channel (the side of the photocathode) along a direction at which electrons are multiplied is given as “a first end side,” while the downstream side (the side of the anode part) is given as “a second end side.” Further, a detailed description will be given for individual constituents of the photomultiplier tube  1 . 
     As shown in  FIG. 2 , the upper frame  2  is constituted with a wiring substrate  20  made mainly with rectangular flat-plate like insulating ceramics as a base material. As the above-described wiring substrate, there is used a multilayer wiring substrate such as LTCC (low temperature co-fired ceramics) in which microscopic wiring can be designed and also wiring patterns on front-back both sides can be freely designed. The wiring substrate  20  is provided on a main surface  20   b  thereof with a plurality of conductive terminals (power feeding parts)  201  electrically connected to a photocathode  22  to be described later, focusing electrodes  37 , an electron multiplying part  31 , and the anode part  32 , to supply power from outside and take out a signal. These conductive terminals  201  are mutually connected to conductive terminals (not illustrated) on an insulating opposing surface  20   a  which opposes the main surface  20   b  inside the wiring substrate  20 , by which these conductive terminals are connected to the photocathode  22 , the focusing electrodes  37 , the electron multiplying part  31  and the anode part  32 . In addition, in  FIG. 1  and  FIG. 2 , the conductive terminals  201  are described by omitting some of them for simplifying the drawings. Further, the upper frame  2  is not limited to a multilayer wiring substrate having the conductive terminals  201  but may include a plate-like member made with an insulating material such as a glass substrate on which conductive terminals for supplying power from outside and taking out a signal are installed so as to penetrate. In addition, where the photocathode  22  is equal in potential to the focusing electrodes  37 , there may be used common conductive terminals. 
     The side-wall frame  3  is constituted with a rectangular flat-plate like silicon substrate  30  as a base material. A penetration part  301  enclosed by a frame-like side wall part  302  is formed from a main surface  30   a  of the silicon substrate  30  toward an opposing surface  30   b  thereto. The penetration part  301  is provided with a rectangular opening and an outer periphery of which is formed so as to run along the outer periphery of the silicon substrate  30 . 
     Inside the penetration part  301 , the focusing electrodes  37 , the electron multiplying part  31  and the anode part  32  are formed from the first end side to the second end side. These focusing electrodes  37 , the electron multiplying part  31  and the anode part  32  are formed by processing the silicon substrate  30  according to RIE (reactive ion etching) or others and made mainly with silicon. The focusing electrodes  37  are electrodes for guiding photoelectrons emitted from the photocathode  22  to be described later into the electron multiplying part  31  and installed between the photocathode  22  and the electron multiplying part  31 . The electron multiplying part  31  is constituted with N stages (N denotes an integer of two or more) of dynodes (electron multiplying parts) set different in potential along a direction at which electrons are multiplied from the photocathode  22  to the anode part  32  and provided with a plurality of electron multiplying channels (channels) at each stage. The anode part  32  is arranged at a position holding the electron multiplying part  31  together with the photocathode  22 . The focusing electrodes  37 , the electron multiplying part  31  and the anode part  32  are respectively connected to the lower frame  4  by anode joining, diffusion joining and joining using a sealing material such as a low-melting-point metal (for example, indium), by which they are arranged on the lower frame  4  two-dimensionally (the details will be described later). In addition, inside the penetration part  301 , columnar parts (not illustrated) which electrically connect the photocathode  22  with conductive terminals  201  for the photocathode  22  are also formed. Further, the electron multiplying part  31 , the focusing electrodes  37  and the anode part  32  are individually connected to the corresponding conductive terminals  201  inside the penetration part  301  (the details will be described later) and set in a predetermined potential via the conductive terminals  201 . For example, where dynodes are constituted at ten stages, a voltage of 100 to 1000V is applied in incremental steps at every 100V intervals to the photocathode  22  at ten stages of dynodes, and a voltage of 1100V is applied to the photocathode  22  at the anode part  32 . 
     The lower frame  4  is constituted with a rectangular flat-plate like glass substrate  40  as a base material. The glass substrate  40  forms an opposing surface  40   a  which opposes the opposing surface  20   a  of the wiring substrate  20  by glass which is an insulating material. The photocathode  22  which is a transmission-type photocathode is formed at a site opposing the penetration part  301  of the side-wall frame  3  on the opposing surface  40   a  (a site other than a region joining with the side wall part  302 ) and at the end part opposite to the side of the anode part  32 . 
     Next, the internal structure of the photomultiplier tube  1  will be described in more detail by referring to  FIG. 3  and  FIG. 4 .  FIG. 3  is a partially broken perspective view showing the internal structure of the photomultiplier tube  1 , when viewed from the upper frame  2 .  FIG. 4  is a partially broken perspective view showing the internal structure, of the photomultiplier tube  1 , when viewed from the lower frame  4 . 
     As shown in  FIG. 3 , the electron multiplying part  31  is constituted with a plurality of stages of dynodes arrayed so as to be spaced away sequentially from the first end side on the opposing surface  40   a  to the second end side (in a direction indicated by the arrow B, that is, a direction at which electrons are multiplied). The number of stages of dynodes is not limited to a specific number of stages, however,  FIG. 3  shows a case where the electron multiplying part is constituted with a 1 st  stage dynode to a 10 th  stage dynode  31   a  to  31   j . Each of the plurality of stages of dynodes  31   a  to  31   j  is provided with a secondary electron surface  33  extending in a direction approximately orthogonal to the opposing surface  40   a.    
     The photocathode  22  is installed so as to be spaced away from the 1 st  stage dynode  31   a  to the first end side on the opposing surface  40   a  behind the focusing electrode  37 , and the photocathode  22  is formed on the opposing surface  40   a  of the glass substrate  40  as a transmission-type photocathode. When incident light transmitted from outside through the glass substrate  40 , which is the lower frame  4 , arrives at the photocathode  22 , photoelectrons corresponding to the incident light are emitted, and the photoelectrons are guided into the electron multiplying part  31  by the focusing electrodes  37 . 
     The anode part  32  is installed so as to be spaced away from the tenth dynode  31   j  to the second end side on the opposing surface  40   a , and the anode part  32  is an electrode for taking out electrons which are multiplied by the electron multiplying part  31  in a direction indicated by the arrow B as an electric signal. 
     Further, as shown in  FIG. 4 , the wiring substrate  20  is arranged so as to cover the focusing electrodes  37 , the electron multiplying part  31  and the leading end of the anode part  32  by the opposing surface  20   a  thereof. A plurality of conductive layers (conductive members)  21   a  to  21   l  which are electrically independent from each other are formed at a range on the opposing surface  20   a  enclosed by the side wall part  302 . The conductive layers  21   a  to  21   l  are individually formed in a band shape along a direction substantially perpendicular to a direction at which the dynodes are arrayed (a direction along the arrow B shown in  FIG. 4 ) so as to run along a direction at which the dynodes  31   a  to  31   j  extend at sites opposing leading ends of a plurality of stages of the dynodes  31   a  to  31   j . Still further, the conductive layers  21   j ,  21   k  are individually formed in a band shape along a direction substantially perpendicular to a direction at which the dynodes are arrayed so as to run along a direction at which the anode part  32  extends and a direction at which the focusing electrodes  37  are arrayed at sites opposing the leading end of the anode part  32  and the leading end of focusing electrodes  37 . 
       FIG. 5  is a partially enlarged sectional view along the line V to V in a state that the upper frame is attached to the electron multiplying part and the lower frame shown in  FIG. 3  and a section view of the glass substrate  40  in the thickness direction and in a direction at which electrons are multiplied. The conductive layer  21   a  formed on the opposing surface  20   a  of the wiring substrate  20  is arranged in such a manner that an end part  23   a  on the second end side in a direction at which electrons are multiplied projects to the second end side more than an end part  34   a  of the dynode  31   a  on the second end side, that is, projecting to the dynode  31   b  which is a subsequent stage, and an end part  24   a  on the first end side is positioned to the second end side more than an end part  35   a  of the dynode  31   a  on the first end side, that is, being included in a range opposing the leading end of the dynode  31   a . In other words, the conductive layer  21   a  deviates from the range opposing the leading end of the dynode  31   a  to a direction at which electrons are multiplied and is formed so as to be astride the range opposing the leading end of the dynode  31   a  and a range opposing a space  39  between the dynode  31   a  and the dynode  31   b  which is a subsequent stage. Similarly, the conductive layers  21   b  to  21   j  are formed so as to deviate from a range opposing the leading ends of the dynodes  31   b  to  31   j  to a direction at which electrons are multiplied. 
     For example, where the thickness of the upper frame  2 , that of the side-wall frame  3  and that of the lower frame  4  along a direction at which light is made incident are respectively 0.5 mm, 1.0 mm and 0.5 mm, the thickness of a sealing part for sealing the upper frame  2  and the side-wall frame  3  under vacuum in a direction at which light is made incident is 0.05 to 0.1 mm and the width of dynodes  31   a  to  31   j  which constitute the electron multiplying part  31  along a direction at which electrons are multiplied is about 0.2 mm, the conductive layers  21   a  to  21   j  are set so as to be about 0.2 mm in width along a direction at which electrons are multiplied and about 0.02 mm in membrane thickness, and deviations from the end parts of the dynode  31   a  to  31   j  on the first and second end side are both set to be 0.05 mm. In this instance, the width of each of the dynodes  31   a  to  31   j  along a direction at which electrons are multiplied can be adjusted in a range from about 0.2 to about 0.5 mm, and the width of each of the conductive layers  21   a  to  21   j  along a direction at which electrons are multiplied can also be adjusted accordingly. 
       FIG. 6  is a perspective diagram which shows the focusing electrode  37  and the electron multiplying part  31 , when viewed from the upper frame. As shown in the drawing, the 1 st  stage to the 3 rd  stage dynodes  31   a ,  31   b ,  31   c  are respectively provided with square-column like conductor parts  38   a ,  38   b ,  38   c  which extend along a direction at which the dynodes  31   a ,  31   b ,  31   c  extend from base parts  36   a ,  36   b ,  36   c , which are plate-like parts at which column-like electrode parts having secondary electron surfaces  33  are erected, and which act as fixing parts to the glass substrate  40  and are also electrically integrated with column-like electrode parts. These conductor parts  38   a ,  38   b ,  38   c  are electrically connected respectively to the conductive layers  21   a ,  21   b ,  21   c , thereby the dynodes  31   a ,  31   b ,  31   c  are set equal in potential respectively to the conductive layers  21   a ,  21   b ,  21   c . More specifically, conductive raised parts  25   a ,  25   b ,  25   c  which project to the leading ends of the conductor parts  38   a ,  38   b ,  38   c  are installed respectively at sites opposing the conductor parts  38   a ,  38   b ,  38   c  at the conductive layers  21   a ,  21   b ,  21   c  on the opposing surface  20   a . And, the conductor parts  38   a ,  38   b ,  38   c  are respectively in contact with the raised parts  25   a ,  25   b ,  25   c , by which the dynodes  31   a ,  31   b ,  31   c  are electrically connected to the conductive layers  21   a ,  21   b ,  21   c . Further, these conductive layers  21   a ,  21   b ,  21   c  are electrically connected to the conductive terminals  201  by wiring inside the wiring substrate  20  (refer to  FIG. 2 ), and the dynodes  31   a ,  31   b ,  31   c  are respectively powered from the conductive terminals  201  via the raised parts  25   a ,  25   b ,  25   c  and the conductive layers  21   a ,  21   b ,  21   c . Still further, the 4 th  stage to the 10 th  stage dynodes  31   d  to  31   j , the focusing electrode  37  and the anode part  32  are also similar in connection constitution and respectively powered from the conductive terminals  201  via the conductive layers  21   d  to  21   l  and set equal in potential to the conductive layers  21   d  to  21   l.    
     According to the above described photomultiplier tube  1 , incident light is made incident onto the photocathode  22 , thereby converted to photoelectrons, and the photoelectrons are multiplied by being made incident into a plurality of stages of electron multiplying parts  31  on the glass substrate  40 , and the thus multiplied electrons are taken out as an electric signal from the anode part  32 . In this instance, on the opposing surface  20   a  of the upper frame  2  which opposes the lower frame  4 , the plurality of conductive layers  21   a  to  21   j  equal in potential respectively to the dynodes  31   a  to  31   j  are installed at sites opposing the respective leading ends of a plurality of stages of dynodes  31   a  to  31   j . It is, therefore, possible to prevent electrons passing between secondary electron surfaces  33  of the plurality of stages of dynodes  31   a  to  31   j  from being made incident onto the opposing surface  20   a  of the upper frame  2 . Thereby, it is possible to prevent a decrease in withstand voltage due to electric charge of the surface of the substrate. For example, where no conductive layer is installed on the opposing surface  20   a  of the wiring substrate  20  ( FIG. 9 ), when the orbit of electrons passing between dynodes deviates to the opposing surface  20   a  from a direction at which electrons are multiplied, the thus multiplied electrons are made incident onto an insulated surface to cause electric charge, which can be responsible for poor withstand voltage and noise defect resulting from emission. On the other hand, where a conductive layer is installed ( FIG. 5 ), when the orbit of electrons deviates to the opposing surface  20   a  from a direction at which electrons are multiplied, electrons are pushed back to the opposing surface  40   a  of the glass substrate  40 , and there is a decrease in area at which the multiplied electrons are made incident onto the insulated surface. Thus, the above problem is not found. Further, the multiplied electrons are prevented from being made incident, thus making it possible to suppress loss of the multiplied electrons and improve electron multiplying efficiency. 
     Further, in the conductive layers  21   a  to  21   j  installed on the wiring substrate  20 , each of the end parts thereof on the second end side projects to subsequent stages of the dynodes  31   a  to  31   j  (or the side of the anode part  32 ) and deviates to the second end side. It is, thereby, possible to more reliably prevent electrons passing between the stages of dynode  31   a  to  31   j  from being made incident onto the opposing surface  20   a  of the upper frame  2 . 
     Still further, in the conductive layers  21   a  to  21   j , each of the end parts thereof on the first end side deviates to the second end side with respect to the dynodes  31   a  to  31   j  and is included in a range opposing the leading ends of the dynodes. It is, thereby, possible to secure each distance between adjacent conductive layers  21   a  to  21   j , to suppress leakage current between the conductive layers and also to increase a withstand voltage to a greater extent. 
     In addition, the plurality of conductive layers  21   a  to  21   j  are set equal in potential to the opposing dynodes  31   a  to  31   j . If the conductive layers are set lower in potential than the opposing dynodes, a force which pushes back electrons will be increased but multiplication efficiency of electrons by secondary electron surfaces will be decreased. On the other hand, if they are set equal in potential, it is possible to prevent electrons from being made incident onto the substrate surface and also keep the multiplication efficiency of electrons. Further, the dynodes  31   a  to  31   j  are allowed to be powered via the conductive layers  21   a  to  21   j , by which a structure can be made simple where the conductive layers  21   a  to  21   j  are set equal in potential to the dynodes. 
     In addition, the present invention shall not be limited to the above described embodiments. For example, the width of a conductive layer formed on the wiring substrate  20  along a direction at which electrons are multiplied may be modified in the following manner. 
     For example, as shown in  FIG. 7 , the conductive layer  121   a  may be formed in such a manner that the end part  124   a  on the first end side in a direction at which electrons are multiplied is in alignment with a position of the end part  35   a  of the dynode  31   a  on the first end side. Further, as shown in  FIG. 8 , the conductive layer  221   a  may be formed in such a manner that the end part  124   a  on the first end side in a direction at which electrons are multiplied spreads to the first end side more than the end part  35   a  of the dynode  31   a  on the first end side. Accordingly, the conductive layer is reliably increased in area to prevent electric charge more efficiently. However, in view of keeping the withstand voltage between the conductive layers and preventing electric charge of the substrate at the same time, a configuration of conductive layers in  FIG. 7  is preferable to that in  FIG. 8 . A configuration in which both end parts of the conductive layer deviate to a direction at which electrons are multiplied is more preferable to the configuration of the conductive layers in  FIG. 7 . 
     In addition, in the present embodiment, the photocathode  22  is a transmission-type photocathode but may be a reflection-type photocathode. Further, the anode  32  may be arranged between the dynode  31   i  and the dynode  31   j.