Patent Publication Number: US-7906754-B2

Title: Photomultiplier tube and radiation detecting device

Description:
TECHNICAL FIELD 
     The present invention relates to a photomultiplier tube and a radiation detecting device. 
     BACKGROUND ART 
     Conventionally, in a photomultiplier tube having a reflection-type final-stage dynode, electrons emitted from a photocathode provided at one side of a vacuum vessel are multiplied by an electrode layer section including a plurality of dynodes in a layer arrangement, the multiplied electrons are further multiplied by the reflection-type final-stage dynode in a reflection direction, and the electrons are detected by an anode that is provided at the photocathode side of the reflection-type final-stage dynode. In such a photomultiplier tube, insulators are inserted between each of the dynodes and the anode, and the dynodes and the anode are stacked in a layer arrangement with predetermined spaces (for example, refer to patent document 1). In another example, each of dynodes and an anode is connected to a stem pin that supplies each of the dynodes and the anode with an electric potential (for example, refer to patent documents 2 and 3). 
     Patent document 1: Japanese Patent Application Publication No. H6-310085 (page 3, FIG. 4) 
     Patent document 2: Japanese Patent Application Publication No. H11-3677 (page 3, FIG. 1) 
     Patent document 3: Japanese Patent Application Publication No. 2003-338260 (pages 2-5, FIG. 3) 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     Because the above-described photomultiplier tubes have a layered structure where each electrode is stacked in a layer arrangement, it is desired that anti-vibration performance is improved and that noises in detected signals are reduced. 
     In view of the foregoing, it is an object of the present invention to provide a photomultiplier tube and a radiation detecting device provided with the same that can improve anti-vibration performance and that can reduce noises. 
     Method for Solving the Problems 
     In order to attain the above objects, the present invention provides a photomultiplier tube including: a vacuum vessel having a faceplate constituting one end and a stem constituting another end; a photocathode that converts incident light incident through the faceplate to electrons; an electron multiplying section that multiplies the electrons emitted from the photocathode; and an electron detecting section that transmits output signals in response to the electrons multiplied by the electron multiplying section, the photocathode, the electron multiplying section, and the electron detecting section being provided within the vacuum vessel, characterized in that the electron multiplying section includes dynodes stacked in a plurality of stages; the electron detecting section includes an anode that is arranged between a first dynode at a final stage and a second dynode at a stage before the first dynode; the stem is provided with a support means for placing the anode spaced apart from the first dynode, the support means being made of an electrically conductive material; and the anode and the second dynode are stacked with an inter-layer member made of an insulating material interposed therebetween. 
     With this configuration, the anode is placed on the support means of an insulating material, and no insulating body exists between the anode and the first dynode at the final stage. Hence, it is possible to prevent noises occurring from light emission generated by electrons colliding on an insulating body. Further, because the support means is provided, the anti-vibration performance can be improved. 
     At this time, it is preferable that a support protrusion made of an insulating material be provided on a surface of the stem confronting the photocathode, and that the first dynode be supported by the support protrusion. 
     With this configuration, because the first dynode that is the final stage dynode is supported by the support protrusion made of the insulating material, the positioning accuracy of each dynode in the electrode stacking direction can be increased. Further, because the creepage distances between the stem pin and the first dynode and between the side tube and the first dynode can be made long by the support protrusion, creeping discharge can be prevented. 
     In the above-described photomultiplier tube, it is preferable that the inter-layer member and the support means be arranged coaxially. With this configuration, the electrodes can be fixed by applying pressure in the electrode stacking direction, thereby improving the anti-vibration performance. 
     Here, it is preferable that the first dynode be formed with a fitting section that is fitted with the support protrusion. With this configuration, the positioning for arranging the first dynode is facilitated, and the positioning accuracy in the electrode surface can be improved. 
     Further, it is preferable that the first dynode be formed with a cutout, and that the support means pass through a region that is cut out by the cutout. In this way, by providing the cutout so that the support means and the first dynode do not contact, the support means and the first dynode can be separated electrically while the effective area of the first dynode is ensured. 
     A radiation detecting device having the above-described effects can be obtained by disposing, outside of the faceplate of any one of the above-described photomultiplier tubes, a scintillator that converts radiation to light and that outputs the light. 
     EFFECT OF THE INVENTION 
     According to the photomultiplier tube and the radiation detecting device of the present invention, a photomultiplier tube and a radiation detector with high anti-vibration performance and reduced noises can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a radiation detecting device  1  according to an embodiment of the present invention; 
         FIG. 2  is a partial enlarged view of a photomultiplier tube  10 ; 
         FIG. 3  is a schematic view of a stem  50  as viewed from the upper side in z-axis direction; 
         FIG. 4  is a schematic view of a dynode Dy 10  as viewed from the upper side in the z-axis direction; 
         FIG. 5  is a schematic view of an anode  25  as viewed from the upper side in the z-axis direction; 
         FIG. 6  is a plan view of the anode  25 ; 
         FIG. 7  is a schematic view of a dynode Dy 9  as viewed from the upper side in the z-axis direction; 
         FIG. 8  is a plan view of the dynode Dy 9 ; 
         FIG. 9  is a schematic cross-sectional view of a radiation detecting device  100  according to a modification of the present invention; and 
         FIG. 10  is a partial enlarged view of  FIG. 9 . 
     
    
    
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
         
           
               1 : radiation detecting device 
               3 : scintillator 
               5 : incident surface 
               7 : output surface 
               10 : photomultiplier tube 
               13 : faceplate 
               14 : photocathode 
               15 : side tube 
               18 : vacuum vessel 
               15   a ,  37   a : flange section 
               21 : support member 
               23 : inter-layer member 
               27 : anode pin 
               31 : positioning protrusion 
               32 : fitting section 
               33 : spacer 
               35 : stem pin 
               37 : ring-shaped side tube 
               50 : stem 
               50   a : annular recess 
               51 : base member 
               53 : upper holding member 
               53   a : upper surface 
               55 : lower holding member 
           
         
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, an embodiment of the present invention will be described while referring to the accompanying drawings.  FIGS. 1 through 8  are drawings showing a radiation detecting device according to a first embodiment of the present invention. In each drawing, the substantially same parts are designated by the same reference numerals to avoid duplicating description. Note that, in the following description, the terms “upper”, “lower”, and the like are used based on a state shown in each drawing, for descriptive purposes. 
       FIG. 1  is a schematic cross-sectional view of a radiation detecting device  1 .  FIG. 2  is a partial enlarged view of a photomultiplier tube  10 . As shown in  FIGS. 1 and 2 , the radiation detecting device  1  includes a scintillator  3  that converts incident radiation to light and outputs the light, and the photomultiplier tube  10  that converts incident light to electrons, multiplies the electrons, and detects the electrons. The radiation detecting device  1  is a device that detects incident radiation and outputs signals. The photomultiplier tube  10  has a tubular shape with a substantially circular cross-section. The direction of the tube axis is defined as z-axis, the horizontal axis of  FIG. 1  is defined as x-axis, and the axis perpendicular to the drawing surface of  FIG. 1  is defined as y-axis. 
     The scintillator  3  includes an incident surface  5  at one side in the z-axis direction and an output surface  7  at the other side, and has a substantially cylindrical shape. Radiation incident on the incident surface  5  is converted to light inside the scintillator  3 , and the light propagates within the scintillator  3  and is outputted from the output surface  7 . The photomultiplier tube  10  is in facial contact with the output surface  7  of the scintillator  3 . The central axis of the scintillator  3  and the tube axis of the photomultiplier tube  10  are approximately coaxial. 
     In the photomultiplier tube  10 , a vacuum vessel  18  is configured by hermetically connecting and fixing a faceplate  13  that constitutes one end section in the z-axis direction, a stem  50  that constitutes the other end section, a ring-shaped side tube  37  provided at the periphery of the stem  50 , and a side tube  15  having a tubular shape. A focus electrode  17 , an electron multiplying section provided with a plurality of dynodes Dy 1 -Dy 10 , and an electron detecting section provided with an anode  25  that detects electrons and outputs signals are arranged within the vacuum vessel  18  of the photomultiplier tube  10 . 
     The faceplate  13  is formed of glass, for example, and has a substantially circular plate shape. A photocathode  14  for converting incident light to electrons is provided at the inner side of the faceplate  13 , that is, at the lower side in the z-axis direction. The photocathode  14  is formed by reaction of alkali metal vapor to preliminarily vapor-deposited antimony, for example. The photocathode  14  is provided on an approximately entire surface of the inner side of the faceplate  13 . The photocathode  14  converts the light that is outputted from the scintillator  3  and that is incident on the faceplate  13  to electrons and emits the electrons. 
     The side tube  15  is formed of metal, for example, and has a substantially cylindrical shape. The side tube  15  constitutes a side surface of the photomultiplier tube  10 . The side tube  15  is provided with the same electric potential as the photocathode  14 . A flange section  15   a  is formed at the lower end section of the side tube  15 . The ring-shaped side tube  37  provided at the lower side of the side tube  15  is formed of metal, for example, and has a substantially cylindrical shape. The ring-shaped side tube  37  is hermetically fixed to the stem  50 , such that the ring-shaped side tube  37  surrounds the side of the stem  50 . The upper end section of the ring-shaped side tube  37  constitutes a flange section  37   a.    
     The faceplate  13  is fixed to one end section of the side tube  15 , and the flange section  15   a  of the other end section is welded to the flange section  37   a  of the ring-shaped side tube  37 , allowing the side tube  15  and the ring-shaped side tube  37  to be hermetically fixed to each other. Further, the ring-shaped side tube  37  and the stem  50  are hermetically fixed to each other to form the vacuum vessel  18 . 
     As shown in  FIG. 1 , the stem  50  has a three-layer structure including a base member  51 , an upper holding member  53  joined with the upper side of the base member  51  (the inner side of the vacuum vessel  18 ), and a lower holding member  55  joined with the lower side of the base member  51  (the outer side of the vacuum vessel  18 ). 
     The base member  51  is a circular-plate shaped member formed of insulating glass including kovar, for example, as primary component. The base member  51  takes on black color to such a degree that light from the lower side does not transmit to inside the vacuum vessel  18 . The upper holding member  53  is a circular-plate shaped member formed of insulating glass having a melting point higher than the base member  51  by adding alumina powder, for example, to kovar. The upper holding member  53  is black colored so as to absorb light emission inside the vacuum vessel  18  efficiently. Like the upper holding member  53 , the lower holding member  55  is a circular-plate shaped member formed of insulating glass having a melting point higher than the base member  51  by adding alumina powder, for example, to kovar. The lower holding member  55  takes on white color due to difference in composition of added alumina powder, and has higher physical strength than the base member  51  and the upper holding member  53 . 
       FIG. 3  is a schematic view of the stem  50  as viewed from the upper side in the z-axis direction. As shown in  FIGS. 1 and 3 , a plurality of stem pins  35  is hermetically inserted in the stem  50 , and is arranged at substantially circular positions and spaced apart from each other in the circumferential direction. Hence, each of the base member  51 , the upper holding member  53 , and the lower holding member  55  constituting the stem  50  are formed with bores at positions where the stem pins  35  are inserted. 
     In the stem  50 , the upper holding member  53  is in close contact with and joined with an upper surface of the base member  51 , and the lower holding member  55  is in close contact with and joined with a lower surface of the base member  51 . At this time, the base member  51 , the upper holding member  53 , and the lower holding member  55  are stacked and joined in a state where the axial centers of the plurality of bores formed in the base member  51 , the upper holding member  53 , and the lower holding member  55  are aligned with one another. Further, the bores of the upper holding member  53  and of the lower holding member  55  are formed to have a larger diameter than the openings of the base member  51 . Each of the stem pins  35  extends through the bores formed in each of the base member  51 , the upper holding member  53 , and the lower holding member  55 . An annular recess  50   a  is defined by the bore of the upper holding member  53  and the stem pin  35  extending through the bore. Another annular recess  50   a  is defined by the bore of the lower holding member  55  and the stem pin  35  extending through the bore. Each stem pin  35  is fusion-bonded at the annular recess  50   a  by fusing the base member  51 . 
     Each stem pin  35  is formed of an electrically conductive material. Each stem pin  35  is inserted in the stem  50  so as to be fixed with the stem  50  as described above, extends upward in z-axis, and is connected to a predetermined electrode. The stem pins  35  are formed in lengths that correspond to the positions of electrodes to which the stem pins  35  are connected. 
     At least two (three in the present embodiment) among the above-described annular recesses  50   a  formed in each of the upper holding member  53  and the lower holding member  55  constitute annular recesses  50   b  having large diameters to allow a positioning jig to insert therethrough to the base member  51  during the assembly of the stem  50 . Further, an annular recess  50   c  is defined by the bore of the upper holding member  53  and an anode pin  27  extending through the bore. The annular recess  50   c  is opened at an upper surface  53   a  of the upper holding member  53  (See  FIG. 1 ). The anode pin  27  is fusion-bonded at the annular recess  50   c  by fusing the base member  51 . 
     As shown in  FIGS. 2 and 3 , a positioning protrusion  31  is provided on the upper surface  53   a  of the upper holding member  53  inside the vacuum vessel  18 , the positioning protrusion  31  being a support protrusion for supporting a dynode Dy 10  thereon. The positioning protrusion  31  is formed of an insulating glass that is the same as the upper holding member  53 . As shown in  FIG. 2 , the positioning protrusion  31  is fitted to a fitting section  32  formed on a lower surface of the dynode  10  confronting the stem  50 . Further, a plurality of spacers  33  is provided on the upper surface  53   a , the plurality of spacers  33  being support protrusions for placing the final-stage dynode Dy 10  thereon. The spacers  33  are formed of an insulating glass that is the same as the upper holding member  53  of the stem  50 . Three spacers  33  are provided in the present embodiment. 
     Further, a plurality of upwardly protruding support members  21  is provided on the upper surface  53   a , serving as support means for placing the anode  25  thereon. In the present embodiment, the support members  21  are provided at four positions on the upper surface  53   a  that are spaced apart from each other by 90 degrees in the circumferential direction. The support member  21  is formed of an electrically conductive material, and includes a placing section  20  and a support section  22  as its cross-section is shown in  FIG. 2 , for example. The placing section  20  and the support section  22  have cylindrical shapes. The diameter of the placing section  20  is formed to be larger than the diameter of the support section  22 . The placing section  20  and the support section  22  are connected coaxially. The support section  22  is arranged on the upper surface  53   a . The placing section  20  is arranged to support the anode  25 . Thus, the support member  21  enables the anode  25  to be supported stably. 
       FIG. 4  is a schematic view of the dynode Dy 10  as viewed from the upper side in the z-axis direction. As shown in  FIG. 4 , the dynode Dy 10  is a first dynode that is provided spaced apart upward from the stem  50  in the z-axis direction and in confrontation with and in substantially parallel to the stem  50 . The dynode Dy 10  is a flat-plate shaped electrode having an electron multiplying function on its substantially entire surface. A protruding section  41  is formed at a portion of the dynode Dy 10  that corresponds to the positioning protrusion  31 , the protruding section  41  being capable of abutting the positioning protrusion  31 . As described above, the fitting section  32  is provided on the surface of the protruding section  41  confronting the upper surface of the stem  50 . The fitting section  32  is fitted with the positioning protrusion  31  and is joined with the positioning protrusion  31  by laser welding, thereby determining the position of the dynode Dy 10  in the xy plane. Protruding sections  34  are formed at portions of the dynode Dy 10  that correspond to the spacers  33 , the protruding sections  34  being capable of abutting the spacers  33 . 
     In this way, the positioning protrusion  31  is fitted with the fitting section  32  of the protruding section  41 , and the protruding sections  34  are placed on the spacers  33 , thereby supporting the entire dynode Dy 10 . Because of the configuration where the dynode Dy 10  is placed on the positioning protrusion  31  and the spacers  33 , the dynode Dy 10  is positioned with respect to all of the x-axis, y-axis, and z-axis directions in a state spaced apart from the upper surface  53   a.    
     Cutouts  29  are provided at the four corners of the dynode Dy 10  for avoiding contact with the support members  21 . A protruding section  36  is formed at a portion of the dynode Dy 10  that corresponds to the stem pin  35 . The dynode Dy 10  is connected to the stem pin  35  and is supplied with a predetermined electric potential that is higher than an electric potential supplied with a dynode Dy 9  and lower than an electric potential supplied with the anode  25 . 
       FIG. 5  is a schematic view of the anode  25  as viewed from the upper side in the z-axis direction.  FIG. 6  is a plan view of the anode  25 . As shown in  FIGS. 5 and 6 , the anode  25  is a substantially rectangular thin-plate electrode having a plurality of slits  26  extending in the y-axis direction for passing electrons therethrough. The anode  25  detects electrons emitted from the dynode Dy 10 . The anode  25  is arranged to substantially cover the dynode Dy 10 , and is placed on the placing sections  20  of the support members  21  at the four corners. Thus, the anode  25  is positioned with respect to the z-axis direction, and is arranged spaced apart upward from the dynode Dy 10  in the z-axis direction and in confrontation with and in substantially parallel with the dynode Dy 10 . A protruding section  28  is formed at a portion of the anode  25  that corresponds to the anode pin  27 , and is connected to the anode pin  27 . The anode  25  is supplied with a predetermined electric potential and outputs detected signals. 
       FIG. 7  is a schematic view of the dynode Dy 9  as viewed from the upper side in the z-axis direction.  FIG. 8  is a plan view of the dynode Dy 9 . The slits  26  of the anode  25  are not shown in  FIG. 7 . As shown in  FIGS. 7 and 8 , the dynode Dy 9  is a substantially rectangular thin-plate electrode. Electron multiplying pieces  30  extend spaced apart from each other and in parallel with each other, the electron multiplying pieces  30  having a predetermined shape (not shown) with concavities and convexities in a cross-section taken along the xz plane and an elongated shape in a cross-section taken along the yz plane, thereby forming slit-shaped electron multiplying openings  30   a  extending in the y-axis direction between the adjacent electron multiplying pieces  30 . 
     The dynode Dy 9  is a second dynode that is arranged to substantially cover the anode  25 . The four corners are placed on inter-layer members  23 , allowing the entirety of the dynode Dy 9  to be supported. The dynode Dy 9  is provided spaced apart upward from the anode  25  in the z-axis direction and in confrontation with and in substantially parallel with the anode  25 . The inter-layer members  23  are insulating members arranged coaxially with the support members  21  in the z-axis direction. The inter-layer members  23  have spherical shapes or disk shapes with convex portions at the center of the top and bottom surfaces. Here, the dynode Dy 9  may be provided with fitting sections that are dented in the z-axis direction for facilitating fixing with the inter-layer members  23 . Further, the dynode Dy 9  is formed with a protruding section  36 . The stem pin  35  is connected to the protruding section  36 , allowing the dynode Dy 9  to be supplied with a predetermined electric potential. 
     The dynodes Dy 8 -Dy 1  are thin-plate electrodes having electron multiplying pieces, in the same manner as the dynode Dy 9 . The dynodes Dy 8 -Dy 1  are arranged sequentially from the direction of the stem  50  in a layered arrangement, and arranged spaced apart from each other and in confrontation with and in substantially parallel to each other with the inter-layer members  23  arranged coaxially with the support members  21  interposed between each of the dynodes. Further, the protruding section  36  is formed at a predetermined position of each of the dynodes Dy 8 -Dy 1 . The stem pin  35  is connected to the protruding section  36 , thereby supplying a predetermined electric potential. Further, the dynodes Dy 9 -Dy 1  are supplied by the stem pins  35  to electric potentials. The electric potentials supplied with the dynodes Dy 9 -Dy 1  become sequentially higher from the photocathode  14  side toward the stem  50  side. 
     A focus electrode  17  is further arranged in confrontation with the photocathode  14 . The inter-layer member  23  is interposed between the focus electrode  17  and the dynode Dy 1 . The focus electrode  17  is connected to the stem pin  35  and is supplied with the same electric potential as is supplied with the photocathode  14 . The focus electrode  17  is a thin-plate electrode having a plurality of focus pieces that extend in the y-axis direction, where slit-shaped multiplying openings are formed between the adjacent focus pieces. The focus electrode  17  does not have an electron multiplying region. The focus electrode  17  converges electrons emitted from the photocathode  14  to be incident on the electron multiplying region of the dynode Dy 1  efficiently. 
     With the radiation detecting device  1  according to the present embodiment having the above-described configuration, when radiation is incident on the incident surface  5  of the scintillator  3 , light in response to the incident radiation is outputted to the output surface  7 . When the light outputted by the scintillator  3  is incident on the faceplate  13  of the photomultiplier tube  10 , the photocathode  14  emits electrons in response to the incident light. The focus electrode  17  provided in confrontation with the photocathode  14  converges the electrons emitted from the photocathode  14  to be incident on the dynode Dy 1 . The dynode Dy 1  multiplies the incident electrons and emits the electrons to the lower stage dynode Dy 2 . The electrons multiplied sequentially by the dynodes Dy 1 -Dy 9  in this way pass through the slits of the anode  25 , and are further multiplied by the dynode Dy 10  in the reflection direction to reach the anode  25 . The anode  25  detects the reached electrons and outputs the electrons as signals to the outside through the anode pin  27 . 
     As described above in details, according to the radiation detecting device  1  of the present embodiment, it is possible to detect radiation that is incident on the scintillator  3  and to output signals to the outside. 
     The emission of the electrons reflected and multiplied by the flat plate dynode Dy 10  spreads wider, because these electrons multiplied by dynode Dy 10  are the largest number among electrons multiplied by other dynodes. Hence, if an insulating body exists between the anode  25  and the dynode Dy 10 , there is possibility that electrons collide on the insulating body so as to emit light, and the light reaches the photocathode  14  and generates false signals (noises). In the photomultiplier tube  10  used for the above-described radiation detecting device  1 , however, the anode  25  is placed on the support members  21  which are conductive material, and no insulating body exists between the anode  25  and the final-stage dynode Dy 10 . Thus, electrons can be prevented from colliding on an insulating body and emitting light to generate noises. Further, the anode  25  is placed on the placing section  20  constituting the support members  21 , and the inter-layer members  23  are arranged coaxially in the z-axis direction with the support members  21 , thereby supporting each electrode. Hence, each electrode can be fixed by applying pressure in the electrode stacking direction. Thus, the anti-vibration performance is improved, and the positioning accuracy in the electrode stacking direction is also improved. 
     Because the dynode Dy 10  is placed on the positioning protrusion  31  as a support protrusion and the spacers  33 , the positioning accuracy in the z-axis direction can be improved. Additionally, because the fitting section  32  is formed on the lower surface of the dynode Dy 10  confronting the stem  50  to be fitted with the positioning protrusion  31 , the positioning of the dynode Dy 10  in the xy plane can be facilitated, thereby improving the positioning accuracy in the electrode surface (in the xy plane). Further, the positioning protrusion  31  ensures the creepage distances between the dynode Dy 10  and each stem pin  35  and between the dynode Dy 10  and the side tube  15  so that effects of preventing creeping discharge can be obtained. 
     Because the dynode Dy 10  is provided with the cutouts  29  so as not to contact the support members  21 , all the region of the dynode Dy 10  can be used as an electron multiplying region. The support means  21  and the dynode Dy 10  can be separated electrically, while the effective area of the dynode Dy 10  is ensured. 
     Next, a modification will be described while referring to  FIGS. 9 and 10 . In the present modification, the substantially same parts as the above-described embodiment are designated by the same reference numerals to avoid duplicating description.  FIG. 9  is a schematic cross-sectional view of a radiation detecting device  100  according to the modification.  FIG. 10  is an enlarged view of a region A in  FIG. 9 . 
     In the present modification, as shown in  FIGS. 9 and 10 , the anode  25  is placed on and supported by a support member  121 , instead of the support members  21  in the first embodiment. The support member  121  is configured by a placing section  123  and a support section  125 . Both the placing section  123  and the support section  125  have cylindrical shapes, where the diameter of the placing section  123  is configured to be larger than the diameter of the support section  125 . Further, the central axis of the support section  125  in the z-axis direction is an axis  63 . The central axis of the placing section  123  is shifted toward the center of the photomultiplier tube  10  from the axis  63  of the support section  125 . 
     Although the dynodes Dy 1 -Dy 9  and the focus electrode  17  are stacked with the inter-layer members  23  interposed therebetween, an axis  61  of the inter-layer members  23  is not coincident with the axis  63  of the support member  121 . A coaxial configuration is preferable in view of ensuring strength in the stacking direction. However, a non-coaxial configuration can be used as in the present modification, by changing the shape of the support member  121 , especially, the strength and size of the placing section  123 , and the length of the support section  125 . Because the other configuration, operation, and effects are similar to the radiation detecting device  1  according to the first embodiment, description is omitted. 
     It would be apparent that the radiation detecting device according to the present invention is not limited to the above-described embodiments, and that various changes and modifications may be made therein within the scope of the subject matter of the present invention. 
     For example, although the configuration of the stem  50  is a three-layer structure including the upper holding member  53 , the base member  51 , and the lower holding member  55 , other configurations may be used. The lower surface of the lower holding member  55  in the z-axis direction (the outer side of the vacuum vessel  18 ) protrudes downward from the lower end of the ring-shaped side tube  37 . However, the fixing position of the stem  50  relative to the ring-shaped side tube  37  is not limited to the above-described configuration. 
     The shapes of the support members  21  and  121  are not limited to the above-described shapes, and may be other shapes such as polygonal columns, provided that the anode  25  can be placed thereon. 
     The shape of each of the dynodes Dy 1 -Dy 10  is not limited to the above-described shape, and may be another shape such as a circle. 
     INDUSTRIAL APPLICABILITY 
     The radiation detecting device of the present invention is applicable to an image diagnostic apparatus for medical use.