Abstract:
The present invention relates to a photomultiplier having a structure for performing a high gain and achieving a higher productivity in a state keeping or improving an excellent high-speed response. In the photomultiplier, an electron-multiplying unit accommodated in a sealed container has a structure that enables an integrated assembly of a focusing electrode, an accelerating electrode, a dynode unit, and an anode. Specifically, the focusing electrode has one or more notched portions to be grasped by a part of each of the insulating support members for grasping directly the dynode unit and so on when the focusing electrode itself is rotated around the tube axis of the sealed container. With this construction, the focusing electrode is fixed to the pair of insulating support members in a state that the focusing electrode is aligned with high accuracy by using the pair of insulating support member as a reference member.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to Provisional Application filed on Mar. 31, 2005 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 that enables a cascade-multiplication of secondary electrons by emitting sequentially the secondary electrons through a plurality of stages in response to incidence of photoelectrons.  
         [0004]     2. Related Background Art  
         [0005]     In recent years, developments of TOF-PET (Time-of-Flight-PET) are earnestly proceeding as a PET (Positron-Emission Tomography) apparatus for the next generation in the field of nuclear medicine. In particular, in the TOF-PET apparatus, two gamma rays emitted from a radioactive isotope administered in a body are simultaneously measured, and therefore a photomultiplier with a large capacity having an excellent high-speed response is employed for the detectors arranged so as to surround a subject.  
         [0006]     For example, a photomultiplier shown in JP-A-5-114384 is known as the aforementioned one. In the conventional photomultiplier has a construction such that a focusing electrode and an accelerating electrode are arranged in this turn from a cathode toward a first-stage dynode. In this case, the focusing electrode is the one correcting an orbit of each photoelectron emitted from the cathode such that the photoelectrons may be focused on the first-stage dynode. In addition, the accelerating electrode is the one accelerating the photoelectrons emitted from the cathode to the first-stage dynode, and has a function to reduce variations in transit time from the cathode to the first-stage dynode caused by the emission area of the photoelectrons of the cathode.  
         [0007]     A photomultiplier with an excellent high-speed response can be obtained by the configuration arranging the focusing electrode and accelerating electrode between the cathode and the first-stage dynode, as mentioned above.  
       SUMMARY OF THE INVENTION  
       [0008]     The inventors have studied the foregoing prior art in detail, and as a result, have found problems as follows.  
         [0009]     Namely, in the conventional photomultiplier, an electron-multiplying unit housed in a sealed container and performing an excellent high-speed response is constructed by a dynode unit such that a plurality of stages of dynodes together with an anode are sandwiched between a pair of insulating fixing plates, a focusing electrode, and an accelerating electrode. In the assembly work, the accelerating electrode is fixed to the dynode unit by a specific metal member, while the focusing electrode is fixed to the accelerating electrode through a glass member. In the photomultiplier including the thus assembled electron-multiplying unit, a high positional accuracy is required for fixings of the focusing electrode and accelerating electrode to perform a high-speed response of the photomultiplier.  
         [0010]     However, the fixing of the focusing electrode to the accelerating electrode is carried out such that the two ends of the glass material are fixed by welding at the fixing area extending from the focusing electrode and the fixing area extending from the accelerating electrode, respectively. For this reason, the fixing work of the focusing electrode is a work involving a high level of difficulty such that some experience for the worker himself is required. In addition, because the number of steps for assembling the whole electron-multiplying unit may be increased, upon mass-production of the multiplier, it is difficult to shorten the producing time and reduce variations in performance thereof.  
         [0011]     The present invention is made to solve the aforementioned problem, and in order to perform a high gain and achieve a higher productivity in a state keeping or improving a high-speed response, it is an object to provide a photomultiplier having a structure which enables an integrated assembly of an electron-multiplying unit including a focusing electrode and an accelerating electrode, that is, a structure preferred to the mass-production.  
         [0012]     A photomultiplier according to the present invention comprises a sealed container of which the inside is kept in a vacuum state, and a cathode, a focusing electrode, an accelerating electrode, a dynode unit, and an anode each to be accommodated in the sealed container. In addition, the dynode unit and anode are unitedly held in a state sandwiched by a pair of insulating support members. The cathode emits photoelectrons as first electrons within the sealed container in response to incidence of light having a predetermined wavelength. The dynode unit includes a plurality of stages of dynodes for emitting secondary electrons in response to the photoelectrons reached from the photocathode to cascade-multiply sequentially the photoelectrons. The anode takes out the secondary electrons cascade-multiplied by the dynode unit as a signal. The focusing electrode functions to correct the orbit of each photoelectron emitted from the photocathode, and is arranged between the photocathode and dynode unit. Furthermore, the focusing electrode has a through hole through which the photoelectrons from the photocathode pass. The accelerating electrode functions to accelerate the photoelectrons reached from the photocathode via the focusing electrode, and is arranged between the focusing electrode and dynode unit. Also, the accelerating electrode has a through hole through which the photoelectrons reached from the photocathode via the focusing electrode pass.  
         [0013]     In particular, in the photomultiplier according to the present invention, the focusing electrode has one or more notched portions to be grasped by a pair of each of the pair of insulating support members when the focusing electrode itself rotates around the tube axis of the sealed container. Note that the notched portions are arranged along the edge portion defining the through hole in the focusing electrode.  
         [0014]     As a specific structure of the focusing electrode, for instance, it is preferable that the pair of insulating support members each have one or more protruding portions extending toward the photocathode, at least serving as a reference of the arranged position of the focusing electrode. In addition, the protruding portions each have a fixture structure for fixing the focusing electrode in a state of supporting directly the focusing electrode. In this case, each of the protruding portions is arranged at a predetermined position to be at least surrounded by the through hole of the focusing electrode in a state of grasping the dynode and anode.  
         [0015]     As described above, in the photomultiplier, when the protruding portions (attached with the fixture structure) at least serving as a reference of the arranged position of the focusing electrode is respectively provided on the pair of insulating support members for holding the dynode unit and anode, the focusing electrode may be fixed unitedly to the pair of insulating support members, together with the dynode unit and anode. In other words, due to the structure fixing the focusing electrode, provided at a part of the pair of insulating support members for grasping unitedly the dynode unit and anode, the focusing electrode constructing a part of the electron-multiplying unit can be easily aligned by using the pair of insulating support members as a reference member. As a result, on assembly of the electron-multiplying unit, alignment work with high precision between the members, specific fixing members and fixing jigs becomes unnecessary, which enables to improve drastically the productivity of the electron-multiplying unit accommodated in the sealed container. Furthermore, variations in performance between produced photomultipliers can be reduced irrespective of skilled degree of workers themselves.  
         [0016]     In addition, it is preferable that each fixture structure provided at the protruding portions includes a slit groove for pinching a part of the edge portion of the through hole of the focusing electrode. In this case, it is preferable that the notched portions each are constituted by an introducing area for accommodating the protruding portions through the through hole of the focusing electrode, and a fixing area for restricting the rotation of the focusing electrode around the tube axis of the sealed container.  
         [0017]     Besides, it is preferable that the focusing electrode further has one or more reinforcing members prepared with a slit groove for pinching one of the protruding portions. In this case, the reinforcing members are welded to the focusing electrode in a state of pinching a part of each protruding portions by the slit groove of the reinforcing member. Thus, when the part of the focusing electrode is pinched by the slit groove corresponding to the part of the focusing electrode, alignment work and fixing work of the focusing and accelerating electrodes can be carried out simultaneously.  
         [0018]     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.  
         [0019]     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 spirit and scope of the invention will be apparent to those skilled in the art from this detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a partially cutaway view illustrating a schematic structure of a photomultiplier of a first embodiment according to the present invention;  
         [0021]      FIG. 2  is an assembly process view for explaining the construction of an electron-multiplying unit applied to the photomultiplier according to the present invention;  
         [0022]      FIG. 3  is a view for explaining the structure of a pair of insulating support members constructing a part of the electron-multiplying unit;  
         [0023]      FIG. 4  is a plan view and a side view for explaining the structure of a lower electrode in an accelerating electrode;  
         [0024]      FIG. 5  is a plan view and a side view for explaining the structure of an upper electrode in the accelerating electrode;  
         [0025]      FIG. 6  is a view for explaining a mounting process of the accelerating electrode to the pair of insulating support members;  
         [0026]      FIG. 7  is an enlarged view for explaining the mounting process of  FIG. 6  in further detail;  
         [0027]      FIG. 8  is a plan view and a side view for explaining the structure of the focusing electrode;  
         [0028]      FIG. 9  is a view for explaining a mounting process of the focusing electrode to the pair of insulating support members;  
         [0029]      FIG. 10  is an enlarged view for explaining the mounting process of  FIG. 9  in further detail; and  
         [0030]      FIG. 11  is a side view illustrating an electron-multiplying unit applied to the photomultiplier according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]     In the following, embodiments of a photomultiplier according to the present invention will be explained in detail with reference to FIGS.  1  to  11 . In the explanation of the drawings, constituents identical to each other will be referred to with numerals identical to each other without repeating their overlapping descriptions.  
         [0032]      FIG. 1  is a partially cutaway view illustrating a schematic structure of a photomultiplier of an embodiment according to the present invention.  
         [0033]     As shown in  FIG. 1 , a photomultiplier  100  includes a sealed container  110  provided with a pipe  130  (solidified after evacuation) for evacuating the inside at the bottom thereof, a cathode  120  provided in the sealed container  110  and an electron-multiplying unit.  
         [0034]     The sealed container  110  is constituted by a cylindrical body having a face plate, the inside of which is formed with a cathode  120 , and a stem supporting a plurality of lead pins  140  in their penetrating state. The electron-multiplying unit is held at a predetermined position within the sealed container  110  by the lead pins  140  extending from the stem to the inside of the sealed container  110 .  
         [0035]     The electron-multiplying unit is constituted by a focusing electrode  200 , an accelerating electrode  300 , and a dynode unit  400  disposing an anode thereinside. The focusing electrode  200  is an electrode correcting an orbit of each photoelectron emitted from the cathode  120  such that the photoelectrons may be focused to the dynode unit  400 , and has a through hole which is arranged between the cathode  120  and dynode unit  400  and through which the photoelectrons from the cathode  120  pass. In addition, the accelerating electrode  300  is an electrode accelerating the photoelectrons emitted from the cathode  120  to the dynode unit  400 , and has a through hole that is arranged between the focusing electrode  200  and dynode unit  400  such that the photoelectrons passed through the through hole of the focusing electrode can be further accelerated toward the dynode unit  400 . Due to the accelerating electrode  300 , a variation in transit time of the photoelectrons reached from the cathode  120  to the dynode unit  400  can be reduced, though it is caused by the photoelectrons emitting area of the cathode  120 . Furthermore, the dynode unit  400  includes a plurality of stages of dynodes cascade-multiplying sequentially secondary electrons emitted in response to the photoelectrons reached from the cathode  120  through the focusing electrode  200  and accelerating electrode  300 , an anode taking out the secondary electrons cascade-multiplied by means of these plurality of stages of dynodes, and a pair of insulating support members grasping unitedly these plurality of stages of dynodes and the anode.  
         [0036]      FIG. 2  is an assembly process view for explaining the construction of the electron-multiplying unit applied to the photomultiplier according to the present invention.  
         [0037]     As shown in  FIG. 2 , the electron-multiplying unit is constituted by the focusing electrode  200 , accelerating electrode  300 , and dynode unit  400  including the anode. The focusing electrode  200  is provided with a through hole through which the photoelectrons from the cathode  120  pass. The accelerating electrode  300  is constituted by an upper electrode  310  and a lower electrode  320  to improve an assembling efficiency of the electron-multiplying unit. These upper electrode  310  and lower electrode  320  are integrated by welding at several spots during the assembly work of the electron-multiplying unit. The dynode unit  400  is constituted by first to seventh dynodes DY 1 -DY 7  each grasped by the first and second insulating support members  410   a,    410   b,  an anode  420 , and a reflection-type dynode DY 8  reversing the electrons passed through the anode  420  toward the anode  420  again. In addition, in each of the first to seventh dynodes DY 1 -DY 7  and the reflection-type dynode DY 8 , a reflection-type emission surface of secondary electrons is formed by receiving photoelectrons or secondary electrons to emit newly secondary electrons toward the incident direction of the electrons. In addition, fixed pieces DY 1   a,  DY 1   b  are provided to be grasped by the first and second insulating support members  410   a,    410   b  at the two ends of the first dynode DY 1 . Similarly, the second dynode DY 2  has fixed pieces DY 2   a,  DY 2   b  at its two ends; the third dynode DY 3  has fixed pieces DY 3   a,  DY 3   b  at its two ends; the fourth dynode DY 4  has fixed pieces DY 4   a,  DY 4   b  at its two ends; the fifth dynode DY 5  has fixed pieces DY 5   a,  DY 5   b  at its two ends; the sixth dynode DY 6  has fixed pieces DY 6   a,  DY 6   b  at its two ends; the seventh dynode DY 7  has fixed pieces DY 7   a,  DY 7   b  at its two ends; the anode  420  has fixed pieces  420   a - 420   d  at its two ends; and the eighth dynode DY 8  has fixed pieces DY 8   a,  DY 8   b  at its two ends.  
         [0038]     The lower electrode  320  of the accelerating electrode  300  is grasped by the first and second insulating support members  410   a,    410   b  together with the first to seventh dynodes DY 1 -DY 7 , anode  420 , and reflection-type dynode DY 8 . Thus, the upper electrode  310  is fixed by welding at the lower electrode  320  in a grasped state by the first and second insulating support members  410   a,    410   b.  On the other hand, the focusing electrode  200  is mounted at the protruding portions provided at the upper portions (cathode  120  side) of the first and second insulating support members  410   a,    410   b,  and fixed at the first and second insulating support members  410   a,    410   b  by welding of reinforcing members  250   a,    250   b.    
         [0039]     In addition, as described above, in a state that the first to seventh dynodes DY 1 -DY 7 , anode  420 , and reflection-type dynode DY 8  are unitedly grasped, the first and second insulating support member  410   a,    410   b  are further grasped by metal clips  450   a - 450   c;  thus, the aforementioned members are stably held by the first and second insulating support members  410   a,    410   b.    
         [0040]      FIG. 3  is a view for explaining the structure of the first and second insulating support members  410   a,    410   b  constituting a part of the electron-multiplying unit. In this case, since the first and second insulating support members  410   a,    410   b  have the same structure, only the second insulating support member  410   b  will now be explained for their common structure description below.  
         [0041]     The insulating support member  410   b  is provided with alignment holes D 1 -D 8  and  42  to be inserted by fixed pieces DY 1   b -DY 8   b,    420   b  of the first to seventh dynodes DY 1 -DY 7 , anode  420 , and reflection-type dynode DY 8 . Also, the insulating support member  410   b  is provided with notched portions  411   a - 411   c  hooking the metal clips  450   a - 450   c  in order to easily secure to the insulating support member  410   a  grasping the members DY 1 -DY 8 ,  420  together.  
         [0042]     In particular, protruding portions  430   a,    430   b  extending upwardly are provided at the insulating support member  410   b.  Namely, the protruding portions  430   a,    430   b  extend toward the cathode side when the electron-multiplying unit is mounted in the sealed container  110 . Then, at the protruding portion  430   a,  a slit groove  431   a  for aligning and fixing the accelerating electrode  300  as a first fixture structure, and a slit groove  432   a  for aligning and fixing the focusing electrode  200  as a fixture structure are provided. Similarly, at the protruding portion  430   b,  a slit groove  431   b  for aligning and fixing the accelerating electrode  300  as a first fixture structure, and a slit groove  432   b  for aligning and fixing the focusing electrode  200  as a fixture structure are provided.  
         [0043]     Next, the structure of the accelerating electrode  300  will be explained with reference to  FIG. 4  and  FIG. 5 .  FIG. 4  is a plan view and a side view for explaining the structure of the lower electrode  320  constituting a part of the accelerating electrode  300 . Also,  FIG. 5  is a plan view and a side view for explaining the structure of the upper electrode  310  constituting a part of the accelerating electrode  300 .  
         [0044]     The accelerating electrode  300  can be obtained by welding at several spots of the lower electrode  320  and upper electrode  310  having the structures as shown in  FIGS. 4 and 5 . The lower electrode  320  is directly inserted and fixed in the slit grooves  431   a,    431   b,  which are provided at the respective protruding portions  430   a,    430   b  of the first and second insulating support members  410   a,    410   b.    
         [0045]     Specifically, as shown in  FIG. 4 , the lower electrode  320  is provided with notched portions  320   a - 320   d  to be grasped to the first and second insulating support members  410   a,    410   b  together with the first to seventh dynodes DY 1 -DY 7 , anode  420 , and reflection-type dynode DY 8 . In addition, at the flange portion located at the outer periphery of a through hole  321  provided at the accelerating electrode  320 , the notched portions  320   a - 320   d  are arranged to surround the through hole  321 . On the other hand, as shown in  FIG. 5 , the upper electrode  310  is constituted by a body unit  312  defining a through hole  311  and a flange portion at one open end of the body unit  311 . At the outer periphery of the flange portion, slit grooves  310   a - 310   d  to sandwich the protruding portions  430   a,    430   b  provided on each of the first and second insulating support members  410   a,    410   b  are formed, and fixing section  313   a,    313   b  to be fixed by welding to the lower electrode  320  are provided.  
         [0046]     The lower electrode  320  and upper electrode  320  having the aforementioned structure, as shown in  FIG. 6 , are fixed in a welded state to the first and second insulating support members  410   a,    410   b  arranged to oppose each other.  
         [0047]     First, the lower electrode  320  is grasped by the first and second insulating support members  410   a,    410   b  with the first to seventh dynodes DY 1 -DY 7 , anode  420 , and reflection-type dynode DY 8 . At this time, the lower electrode  320  is grasped by the first and second insulating support members  410   a,    410   b  in a state that areas (parts corresponding to regions  321   a - 321   d  shown in  FIG. 4 ) provided with the notched portions  320   a - 320   d  of the flange portion are fit in the slit grooves  431   a,    431   b  formed at the protruding portions  430   a,    430   b,  respectively. As a result, the lower electrode  320  is fixed to the first and second insulating support members  410   a,    410   b  in a state that the flange portion thereof is surrounded by the protruding portions  430   a,    430   b.  Furthermore,  FIG. 7  is an enlarged view illustrating a setting situation of the notched portion  320   a  of the lower electrode  320  in particular. Note that the lower electrode  320  is aligned to only the direction designated by the arrow S 1  in  FIG. 7  when it is grasped by the first and second insulating support members  410   a,    410   b;  however, it is still slightly rotatable to the direction designated by the arrow S 2 .  
         [0048]     Subsequently, the upper electrode  310 , as shown in  FIG. 6 , is disposed on the lower electrode  320  in a state that the protruding portions  430   a,    430   b  are pinched into the slit grooves  310   a - 310   d.  At this time, the upper electrode  310 , which is different from the lower electrode  320 , is movable to the direction represented by the arrow S 1  in  FIG. 7 , but cannot be rotated to the direction represented by the arrow S 2 . For this reason, when the fixing areas  313   a,    313   b  provided at the outer periphery of the flange portion of the upper electrode  310  are welded at the lower electrode  320 , the upper electrode  310  and lower electrode  320  are unitedly fixed (aligned) to the first and second insulating support members  410   a,    410   b.    
         [0049]     Furthermore,  FIG. 8  is a plan view and a side view for explaining the structure of the focusing electrode  200 .  
         [0050]     In particular, the focusing electrode  200  is constituted by the body unit  210  shown in  FIG. 8  (substantially a main body of the focusing electrode; there are some cases that the body unit  210  herein may be simply called ‘focusing electrode’) and the reinforcing members  250   a,    250   b  controlling the rotation of the body unit  210 . The body unit  210 , as shown in  FIG. 8 , has a flange portion that has a cylindrical shape, extends from one opening end of the body unit to the inside, and defines the through hole  211 . At the flange portion, notched portions  220   a - 220   d  are formed to be grasped by slit grooves  432   a,    432   b  provided at the protruding portions  430   a,    430   b  of the first and second insulating support members  410   a,    410   b.  Note that these notched portions  220   a - 220   d  is constituted by introducing portions  221   a - 221   d  for housing the protruding portions  430   a,    430   b  via the through hole  211  in the focusing electrode  200 , and fixing portions  222   a - 222   d  for limiting the rotation of the body unit  210  around the tube axis of the sealed container  110 .  
         [0051]     The body unit  210  having the aforementioned structure is fixed to the slit grooves  432   a,    432   b  formed at the respective protruding portions  430   a,    430   b  of the first and second insulating support members  410   a,    410   b  in such a manner that the body unit  210  itself rotates around the tube axis of the sealed container  110 .  
         [0052]     Specifically, as shown in  FIG. 9 , the protruding portions  430   a,    430   b  of the first and second insulating support members  410   a,    410   b  that grasp the first to seventh dynodes DY 1 -DY 7 , anode  420 , reflection-type dynode DY 8 , and accelerating electrode  300  are inserted into the through hole  211  of the body unit  210 . The situation of this case is shown in an enlarged view of  FIG. 10 .  
         [0053]     In other words, the protruding portions  430   a,    430   b  are inserted from the introducing portions  221   a - 221   d  in the notched portions  220   a - 220   d  along the direction designated by the arrow S 4  in  FIG. 10 . Thereafter, the body unit  210  rotates in the direction designated by the arrow S 3  shown in  FIG. 10 , so that the slit grooves  432   a,    432   b  of the protruding portions  430   a,    430   b  can abut with the fixing sections  222   a - 222   d.  At this time, the slit grooves  432   a,    432   b  of the protruding portions  430   a,    430   b  may grasp the areas designated by  223   a - 223   d  of the flange portion of the body unit  210 . In this way, the body unit  210  itself is fixed to the direction designated by the arrow S 4  in  FIG. 10 . However, since the body unit  210  is not fixed to the direction designated by the arrow S 3 , the reinforcing members  250   a,    250   b  are fixed by welding to restrict the rotation along the direction designated by the arrow S 3  of the body unit  210 .  
         [0054]     The reinforcing member  250   a  is constituted by a main body plate  251   a  abutted with the flange portion of the body unit  210  and a spring portion  252   a  abutted with the side of the body unit  210 . Also, the main body plate  251   a  is provided with a slit groove  253   a  for pinching the protruding portions  430   a  of the first and second insulating members  410   a,    410   b  arranged to oppose each other. In similar, the reinforcing member  250   b  is constituted by a main body plate  251   b  abutted with the flange portion of the body unit  210  and a spring portion  252   b  abutted with the side of the body unit  210 . Also, the main body plate  251   b  is provided with a slit groove  253   b  for pinching the protruding portion  430   b  of the first and second insulating members  410   a,    410   b  arranged to oppose each other.  
         [0055]     These reinforcing members  250   a,    250   b  are inserted from the direction designated by the arrow S 5  in  FIG. 11  (the slit grooves  253   a,    253   b  pinching the protruding portions  430   a,    430   b ). As described above, the body unit  210  is fixed in the direction designated by the arrow S 4  in  FIG. 10 ; however, it is not fixed in the direction designated by the arrow S 3 . On the other hand, the reinforcing members  250   a,    250   b  pinch the protruding portions  430   a,    430   b  by the slit grooves  253   a,    253   b  to thereby be fixed in the direction designated by the arrow S 3 , while they are fixed in the direction designated by the arrow S 4 . When the above body unit  210  and each of the reinforcing members  250   a,    250   b  are fixed by welding, the focusing electrode  200  is unitedly fixed (aligned) to the first and second insulating members  410   a,    410   b.    
         [0056]     The electron-multiplying unit to be housed in the sealed container  110  through the above assembly processes.  
         [0057]     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.