A photomultiplier tube enhanced in simplicity and flexibility of mounting, a photomultiplier tube unit enhanced in photomultiplier tube assembling efficiency when unitized, and a radiation detector enhanced in assembling efficiency for a plurality of photomultiplier tubes. The photomultiplier tube (1) has a hermetically sealed vessel (5) easily screw-fixed in a predetermined position due to screwing means (30) provided in the stem plate (4). As a result, the photomultiplier tube (1) can be very easily attached or detached so that even an unskilled person can mount the photomultiplier tube (1) easily and accurately in a predetermined position by screwing.

TECHNICAL FIELD

The present invention relates to a photomultiplier tube for detecting weak light incident on a faceplate by multiplying electrons emitted from the faceplate, a photomultiplier tube unit including photomultiplier tubes, and a radiation detector employing photomultiplier tubes and/or photomultiplier tube units.

BACKGROUND ART

Japanese patent application Kokai publication No. 5-100034 discloses a scintillation camera wherein photomultiplier tubes are closely arranged together on a top surface of a scintillator. Sockets of the photomultiplier tubes are used for mounting the photomultiplier tubes onto the scintillator. A spiral spring is disposed around each socket, connecting each photomultiplier tube with a pressing plate facing the scintillator. A photocathode of the photomultiplier tube is pressed to the scintillator and fixed thereon by the spiral spring. In this way, a predetermined spring force is used to fix each photomultiplier tube on the scintillator.

However, a problem arose in the conventional photomultiplier tubes described above. Since the photomultiplier tube itself does not include a specific fixing means, various fixing parts such as a spring and pressing plate are required for fixing the photomultiplier tubes to predetermined locations. As a result, the mounting procedures for fixing the photomultiplier tube to a predetermined location becomes troublesome, making the fixing structure more complex. Further, when these photomultiplier tubes are incorporated in a predetermined photodetecting device, sockets for inserting stem pins of the photomultiplier tube is used to fix the photomultiplier tube into the photodetecting device.

In view of the foregoing, it is an object of the present invention to provide a photomultiplier tube with improved simplicity and flexibility of mounting.

It is another object to provide a photomultiplier tube unit capable of improving assembly operations of modularized photomultiplier tubes.

It is further object to provide a radiation detector capable of improving the efficiency of assembling a plurality of photomultiplier tubes.

DISCLOSURE OF INVENTION

A photomultiplier tube according to the present invention includes a photocathode for emitting electrons in response to light incident on a faceplate; an electron multiplier provided in an hermetically sealed vessel for multiplying electrons emitted from the photocathode; and an anode for generating an output signal based on electrons multiplied by the electron multiplier. The photomultiplier tube is characterized by the hermetically sealed vessel including a stem plate having stem pins for fixing the electron multiplier and the anode thereon; a metal side tube enclosing the electron multiplier and the anode, the metal side tube having an open end to which the stem plate is fixed; and the faceplate fixed to another open end of the side tube. The faceplate is made from glass. Screw means is provided at a lower surface of the stem plate.

By providing the stem plate with a screwing means, the photomultiplier tube of the present invention simplifies the process of fixing the hermetically sealed vessel to a predetermined location by using screws. Generally, photomultiplier tubes are not provided with its own special fixing structure in order to enhance its flexibility. Further, due to increased sensitivity properties of photomultiplier tubes in recent years, there have been more opportunities for incorporating such photomultiplier tubes in various devices. However, the operations required for mounting and replacing individual photomultiplier tubes in such equipment requires proficiency. In order to facilitate the mounting of photomultiplier tubes or the replacement of faulty photomultiplier tubes, screw means is provided on the stem plate of the photomultiplier tube according to the present invention. By standardizing the screwing means, it is possible to standardize a method of mounting photomultiplier tubes, thereby extremely simplifying the attachment and detachment operations and improving the flexibility of the photomultiplier tube. By employing such a simple operation as the insertion of screws, even an unskilled person can easily mount the photomultiplier tube at a predetermined position with accuracy.

In the photomultiplier tube according to the present invention, the screw means includes a spacer projecting from the lower surface of the stem plate. The spacer has a female thread in the interior thereof.

When a photomultiplier tube having the above structure is mounted at a predetermined position, the stem plate can be fixed in place while still maintaining spaced away from the mounting area by the spacer, thereby encouraging heat dissipation from the stem plate and contributing to improved performance of the photomultiplier tube. Further, by forming the spacer of an electrically insulating material, it is possible to prevent electrical effects of the photomultiplier tube operating at a high voltage from being transferred externally.

The photomultiplier tube according to present invention further includes a circuit board extending parallel to the stem plate and electrically connected to the stem pins. The circuit board is secured to the stem plate by screwing a male screw into the screw means. With the above structure, the circuit board is integrally attached to the photomultiplier tube be means of the male screws. The above structure simplifies the operation for assembling the circuit board and photomultiplier tube, decreasing the time required for assembly, and decreasing production cost. When either the circuit board or the photomultiplier tube malfunctions, the circuit board and photomultiplier tube can be easily separated. Therefore, the operations for replacing parts are facilitated.

The photomultiplier tube according to present invention may further include a first circuit board detachably provided with the stem plate. The first circuit board may be secured to the stem plate by screwing a screw member into the screw means through the first circuit board. If the screwing means has female threads, male screws are used as the screw member to fix the first circuit board to the stem plate. If the screwing means has male screws, nuts having female threads are used as the screw member for fixing the first circuit board to the stem plate

The screw means may include a spacer projecting from the lower surface of the stem plate. Preferably, the spacer is integral with the stem plate by using the same material as that of the stem plate. The spacer spaces the first circuit board away from the stem plate. The screw member may be made from an electrically insulating material.

The screw means may include a spacer projecting from the lower surface of the stem plate. Preferably, the spacer is made from an electrically insulating material. The spacer spaces the first circuit board away from the stem plate. In this case, the screw member made from an electrically insulating material is preferably used.

The photomultiplier tube according to present invention may further include a second circuit board detachably provided with the stem plate and the first circuit board. The first and second circuit boards may be secured to the stem plate by screwing the screw member into the screw means through the first and second circuit boards.

A photomultiplier tube unit according to the present invention includes a plurality of photomultiplier tubes that are juxtaposed, each of the plurality of the photomultiplier tubes having a photocathode for emitting electrons in response to light incident on a faceplate; an electron multiplier provided in an hermetically sealed vessel for multiplying electrons emitted from the photocathode; and an anode for generating an output signal based on electrons multiplied by the electron multiplier. The hermetically sealed vessel includes: a stem plate having stem pins for fixing the electron multiplier and the anode thereon; a metal side tube enclosing the electron multiplier and the anode, the side tube having one open end to which the stem plate is fixed; and the faceplate fixed to another open end of the side tube. The faceplate is made from glass. Screw means is provided on a lower surface of the stem plate. The hermetically sealed vessels are secured on a single substrate by screwing male screw members into the screw means while the hermetically sealed vessels are juxtaposed on the substrate.

In the above photomultiplier tube unit, it is possible to arrange a plurality of photomultiplier tubes on a single circuit board using male screw members. This structure enables the photomultiplier tubes to be modularized with the simple operation of inserting screws. Hence, it is possible to facilitate the attaching and detaching operations of a plurality of photomultiplier tubes on a single circuit board and the replacement of individual photomultiplier tubes in the event of a malfunction. When modularizing the photomultiplier tubes, these photomultiplier tubes can be easily incorporated into a variety of equipment.

In the photomultiplier tube unit according to present invention, the substrate is a circuit board electrically connectable to the stem pins. The male screw members are electrically insulating screws. With this structure, a plurality of photomultiplier tubes is easily mounted on a single circuit board by means of the male screw members. Accordingly, the operation for assembling the circuit board and a plurality of the photomultiplier tubes is facilitated. The time required for the assembly operation is shortened. The costs of the product are reduced. When either the circuit board or the photomultiplier tube malfunctions, the circuit board and photomultiplier tube can be easily separated. The above structure facilitates such operations as replacing parts and avoiding discarding the entire unit.

A radiation detector according to the present invention includes a scintillator for emitting fluorescent light in response to radiation generated from an object; a plurality of photomultiplier tubes arranged in a manner that faceplates of the photomultiplier tubes face the scintillator. Each of the photomultiplier tubes generates an electrical charge based on the fluorescent light emitted from the scintillator. The radiation detector includes a position calculating processor for processing an output from the photomultiplier tube and generating a signal for indicating a position of radiation generated in the object. Each of the plurality of photomultiplier tubes has a photocathode for emitting electrons in response to light incident on a faceplate; an electron multiplier provided in an hermetically sealed vessel for multiplying electrons emitted from the photocathode; and an anode for producing an output signal based on electrons multiplied by the electron multiplier. The hermetically sealed vessel includes: a stem plate having stem pins for securing the electron multiplier and the anode; a metal side tube for enclosing the electron multiplier and the anode, the side tube having one open end to which the stem plate is fixed; and the faceplate fixed to another open end of the side tube, the faceplate being made from glass. Screw means is provided at a lower surface of the stem plate. The hermetically sealed vessels are arranged to be secured to a single substrate by screwing male screws into the screw mean members while the hermetically sealed vessels are juxtaposed on the substrate.

Since the radiation detector employs units including a plurality of photomultiplier tubes arranged on a single circuit board and fixed by male screw members, a complex process is not required when replacing individual photomultiplier tubes in radiation detectors (such as a gamma camera) in which a plurality of photomultiplier tubes are incorporated. Replacement operations can be performed on individual units. Therefore, the time required for the replacement operation is reduced. Moreover, by employing a structure using screws, it is possible to facilitate the operation for attaching and detaching each photomultiplier tube in relation to the circuit board and for replacing individual photomultiplier tubes in the detached units.

BEST MODE FOR CARRYING OUT THE INVENTION

The following description will be made for explaining preferred embodiments of a photomultiplier tube, a photomultiplier tube unit, and a radiation detector according to the present invention in details, referring to the accompanying drawings.

FIG. 1is a perspective view showing a photomultiplier tube according to the present invention.FIG. 2is a cross-sectional view of the photomultiplier tube in FIG.1. The photomultiplier tube1includes a side tube2having a substantially rectangular section and formed from a metal material (such as Kovar metal and stainless steel). A glass faceplate3is fused to one open end A of the side tube2. A photocathode3afor converting light to an electron is formed on an inner surface of the faceplate3. The photocathode3ais formed by reacting alkali metal vapor with antimony pre-deposited on the faceplate3. A stem plate4made from a metal material (such as Kovar metal and stainless steel) is welded to the other open end B of the side tube2. The assembly of the side tube2, faceplate3, and stem plate4forms a hermetically sealed vessel5. The vessel5has a low height of approximately 10 mm.

A metal evacuating tube6is provided in the center of the stem plate4. The evacuating tube6is used to evacuate the vessel5by a vacuum pump (not shown) after the assembly of the photomultiplier tube1is over. The evacuating tube6is also used for introducing alkali metal vapor into the vessel5during the production of the photocathode3a.

A stacked electron multiplier7in a block shape is disposed inside the vessel5. The electron multiplier7has an electron multiplying section9in which ten stages of flat dynodes8are stacked. Stem pins10formed from Kovar metal penetrate the stem plate4and support the electron multiplier7in the vessel5. The tip of each stem pin10is electrically connected to each dynode8. Pinholes4aare formed in the stem plate4, enabling the stem pins10to penetrate the stem plate4. Each of the pinholes4ais filled with a tablet11formed from Kovar glass, which forms a hermetic seal between the stem pins10and the stem plate4. Each stem pin10is fixed to the stem plate4by the tablet11. The stem pins10are classified into two groups: one group for dynode pins10A connected individually to each dynode8, and the other group for anode pins10B connected individually to each of anodes12described later.

The anodes12are positioned below the electron multiplying section9in the electron multiplier7. The anodes12are fixed to the top ends of the anode pins10B. A flat focusing electrode13is disposed between the photocathode3aand the electron multiplying section9above the top stage of the electron multiplier7. A plurality of slit-shaped openings13ais formed in the focusing electrode plate13. The openings13aare arranged parallel to each other with respect to one direction. Slit-shaped electron multiplying holes8aare formed in the dynode8. The number of electron multiplying holes8ais the same as that of the openings13a. The electron multiplying holes8aare arranged parallel to each other in one direction. The electron multiplying holes8aextend in a direction substantially orthogonal to the surface of the dynodes8.

Electron multiplying paths L are formed by arranging the electron multiplying holes8ain each dynode8along the direction of the stack. A plurality of channels are formed in the electron multiplier7by associating the path L with the corresponding opening13ain the focusing electrode plate13. The anodes12are configured in an 8×8 arrangement, so that each anode12corresponds to a predetermined number of channels. Since the anode12is connected to the corresponding anode pin10B, output signals can be extracted through each anode pin10B.

Hence, the electron multiplier7has a plurality of linear channels. A predetermined voltage is applied across the electron multiplying section9and anodes12by the stem pin10connected to a bleeder circuit (not shown). The photocathode3aand the focusing electrode plate13are maintained at the same potential. The potential of each dynode is decreasing from the top of the dynode toward the anodes12. Accordingly, incident light on the faceplate3is converted to electrons at the photocathode3a. The electrons are guided into a certain channel by the electron lens effect generated by the focusing electrode plate13and the first stage of the dynode8on the top of the electron multiplier7. The electrons guided into the channel are multiplied through each stage of the dynodes8while passing through the electron multiplying paths L. The electrons are collected by the anodes12to be outputted as an output signal.

As shown inFIG. 3, the stem plate4is brought into contact with the open end B of the side tube2such that a side face4bof the stem plate4contacts an inner surface2cin the vicinity of a lower end2aof the side tube2. When welding the side tube2and the stem plate4, made of metal, together to form a hermetic seal, a lower end face2dof the side tube2is approximately flush with a lower face4cof the stem plate4in order that the lower end surface2ddoes not project below the stem plate4. In other words, the above structure extends the lower end2aof the side tube2in the substantial axial direction of the tube2, and eliminates lateral projection like a flange at the lower end of the photomultiplier tube1. In this embodiment, a junction F between the side tube2and stem plate4is laser-welded by irradiating a laser beam on the junction F from a point directly below and external to the junction F or in a direction toward the junction F.

By eliminating the flange-like overhang at the lower end of the photomultiplier tube1, it is possible to reduce the external dimensions of the photomultiplier tube1, though the above structure of the photomultiplier tube1and the side tube2may be improper for resistance-welding. Further, when several photomultiplier tubes1are arranged, it is possible to minimize dead space between neighboring photomultiplier tubes1as much as possible by placing the neighboring side tube2of the photomultiplier tubes1close together. Laser welding is employed to bond the stem plate4and side tube2together in order to achieve a thin structure of the photomultiplier tube1and to enable high-density arrangements of the photomultiplier tube1.

The above laser welding is one example for fusing the stem plate4and side tube2. When the side tube2and the stem plate4are welded together using the laser welding, it is unnecessary to apply pressure across the junction F between the side tube2and stem plate4in contrast to resistance welding. Hence, no residual stress is induced at the junction F, avoiding cracks from occurring at this junction during the usage. The usage of the laser welding greatly improves the durability and sealability of the photomultiplier tube1. Laser welding and electron beam welding prevent generation of heat at the junction F, compared to the resistance welding. Hence, when the photomultiplier tube1is assembled, there is very little effect of heat on the components in the vessel5.

The side tube2is formed by pressing a flat plate made from metal such as Kovar and stainless steel into an approximately rectangular cylindrical shape having a thickness of approximately 0.25 mm and a height of approximately 7 mm. The glass faceplate3is fixed to the open end A of the side tube2by fusion. As shown inFIG. 4, an edge portion20is formed on an upper end of the side tube2which the glass faceplate3faces. The edge portion20is provided around the whole upper end of the side tube2. The edge portion20curves outwardly with a curved part20aformed on an inner surface2cside of the side tube2. A tip20bof the edge portion20is formed like a knife-edge. Hence the top of the side tube2can easily pierce the glass faceplate3, thereby facilitating the assembly process and improving reliability when the side tube2and glass faceplate3are fused together.

When fixing the side tube2with an edge portion20having the above shape to the glass faceplate3, the metal side tube2is placed on a rotating platform (not shown) with the bottom surface of the glass faceplate3contacting the tip20bof the edge portion20. Next, the side tube2is heated by a high-frequency heating device while the glass faceplate3is pressed downwardly by a pressure jig. At this time, the heated edge portion20gradually melts the glass faceplate3, and penetrates therein. As a result, the edge portion20is brought into embedded in the glass faceplate3, ensuring a tight seal at the juncture between the glass faceplate3and side tube2.

The edge portion20extends upwardly from the side tube2rather than extends laterally from the side tube2like a flange. When embedding the edge portion20into the glass faceplate3as close to a side surface3cas possible, it is possible to increase the effective surface area of the glass faceplate3to nearly 100% and to minimize the dead area of the glass faceplate3to nearly 0%.

When the side surface3cof the glass faceplate3is extended by a predetermined length external from the outer surface2bof the side tube2, an overhanging part3A having a predetermined length of extension is formed in the glass faceplate3, expanding the effective surface area of a photocathode3aformed on the glass faceplate3. When the glass faceplate3is fused to the metal side tube2, the above fusing method for fusing glass and metal is employed due to the combination of metal and glass. The overhanging part3A of the glass faceplate3functions extremely effectively to ensure a fusing area necessary to fuse the glass faceplate3and side tube2. The increase of the amount of overhang in the overhanging part3A avoids the side surface3cfrom deformating during the fusion process, allowing the side surface3cto retain its form throughout the process.

As shown inFIGS. 2 and 5, the photomultiplier tube1has four threaded portions30. Each threaded portion30is provided in each corner of the stem plate4. The threaded portion30includes a cylindrical spacer31projecting from the lower surface4cof the stem plate4. The cylindrical spacer31has a female threaded hole31atherein. The cylindrical spacer31is made from the same material as that of the stem plate4, and integrally molded with the stem plate4. The cylindrical spacer31may be made from electrically insulating material such as resin separately from the stem plate4.

By forming the threaded portions30with the stem plate4, the vessel5can be easily mounted at a predetermined position. Additionally, standardization of the threaded portions30may contribute to standardizing a method for fixing the photomultiplier tube1. For example, when a photomultiplier tube1in a photodetector malfunctions, a photomultiplier tube1having the same standard specifications can be easily installed at the same position in a correct manner in the photodetector. When the photomultiplier tube1is mounted at a predetermined position of a substrate, the stem plate4is spaced away from the substrate by the cylindrical spacer31. The above structure ensures heat dissipation from the stem plate4, and contributes to the enhanced performance of the photomultiplier tube1. When the cylindrical spacer31is formed from an electrically insulating material, it is possible to prevent the electrical effects of the photomultiplier tube1operating at a high voltage from being transferred externally.

Next, another embodiment of the photomultiplier tube1having the threaded portions30will be described. Referring toFIGS. 2,6, and7, a voltage dividing circuit (bleeder circuit) which is connectable to the dynode pins10A, or a first circuit board33which is connectable to the anode pins10B and has circuit patterns for anode output may be fixed to the photomultiplier tube1. The first circuit board33has metal socket pins34corresponding to the anode pins10B and metal socket pins35corresponding to the dynode pins10A. An evacuating tube insertion hole33afor inserting the evacuating tube6is formed in the center of the first circuit board33. Screw insertion holes36are formed in the first circuit board33at positions corresponding to the spacers31. After the anode pins10B and dynode pins10A are inserting into the socket pins34and socket pins35, and the screw insertion holes36is aligned with the female thread31aof the spacers31, electrical insulated screws32(male screws) are screwed into the female threaded holes31afrom below, thereby fixing the first circuit board33integrally to the photomultiplier tube1and parallel to the stem plate4.

As described above, the first circuit board33is integrally fixed to the photomultiplier tube1by using the screws32. This structure facilitates the operation for assembling the first circuit board33and the photomultiplier tube1. As a result, the time required for assembly can be shortened and the cost of the product reduced. In case when either the first circuit board33or the photomultiplier tube1malfunction, the photomultiplier tube1can easily be separated from the first circuit board33. Therefore, the operation for replacing parts can be facilitated.

As shown inFIGS. 8 and 9, a second circuit board37may be fixed parallel to the first circuit board33under the bottom side of the first circuit board33. The second circuit board37is electrically connected to the first circuit board33through connecting pins (not shown), and has a function for calculating a position such as an AD converter. Spacers38that are electrically isolated and cylindrical in shape protrude from the top surface of the second circuit board37at positions corresponding to the screw insertion holes36formed in the first circuit board33. The spacers38maintain the first circuit board33and second circuit board37at a predetermined distance from each other. A screw insertion hole38ais formed in each spacer38. Electrically insulating screws32A (male screws) are screwed into the female threads31athrough the screw insertion holes38ato fix the first circuit board33and second circuit board37integrally to the photomultiplier tube1. Since the first circuit board33and second circuit board37are fixed to the photomultiplier tube1by screws, these three parts can easily be separated by unscrewing the screws. Further, a scintillator M may be integrally fixed to the faceplate3of the photomultiplier tube1.

Next, a preferred embodiment of a photomultiplier tube unit and a radiation detector according to the present invention will be described.

As shown inFIG. 10, a radiation detector40is a gamma camera as one example. The radiation detector40has been developed as a diagnostic device used in nuclear medicine. The gamma camera40has a detecting unit43supported by an arm42of a support frame39. The detecting unit43is positioned directly above a bed41on which a patient P serving as the object of examination reclines.

As shown inFIG. 11, a casing44of the detecting unit43accommodates a scintillator46which is positioned opposite to the patient. The scintillator46is fixed directly to a group of photomultiplier tubes G without an interposing glass light guide. The group of photomultiplier tubes G includes a plurality of photomultiplier tubes1arranged densely in a matrix configuration. The faceplate3of each photomultiplier tubes1is orientated downwardly to the scintillator46in order to directly receive fluorescent light emitted from the scintillator46. A conventional light guide is no longer needed, because the thickness of the faceplate3is increased to compensate for the thickness of the light guide.

A position calculating processor49is provided in the casing44for performing calculations based on electrical charges from each photomultiplier tube1. The group of photomultiplier tubes G is fixed to the position calculating processor49by screw means. The position calculating processor49electrically connected to the group of photomultiplier tubes G generates an X signal, a Y signal, and a Z signal to form a three-dimensional image on a display (not shown). Gamma rays emitted from the affected part of the patient P are converted to predetermined fluorescent light by the scintillator46. Each of the photomultiplier tubes1converts the energy of this fluorescent light into electrical charges. The position calculating processor49generates positions signals based on the electrical charges. In this way, it is possible to monitor the distribution of radiation energy from the object on the display for use in diagnoses.

While the above description has been given for the gamma camera40as one example of a radiation detector, another radiation detector used in nuclear medicine diagnoses is a Positron CT (commonly designated as PET). This apparatus also includes many the photomultiplier tubes1.

Further, the group of photomultiplier tubes G has the photomultiplier tubes1arranged in a matrix, as described above. As shown inFIG. 12, the group of photomultiplier tubes G includes a photomultiplier tube unit S having four 2×2 of the photomultiplier tubes1. The arrangement of the photomultiplier tubes1in the unit S is one example.

Next, the matrix-shaped photomultiplier tube unit S will be described in detail, wherein components having the same structure as those of the components shown inFIG. 8are represented by the same numerals.

As shown inFIGS. 12 and 13, when configuring a photomultiplier tube unit S using the photomultiplier tubes1described above, the photomultiplier tubes1having the same structure are arranged in a 2×2 matrix. The neighboring side surfaces3cof the four faceplates3are in close contact, while neighboring side tubes2are separated from one another. Neighboring faceplates3can be easily and reliably fixed together by adhesive.

The stem plate4of each photomultiplier tube1in the 2×2 photomultiplier tube unit S has a cylindrical spacer31, as one example of the threaded portions30. The photomultiplier tubes1are arranged on an upper surface of a single first circuit board50. The first circuit board50may include a voltage dividing circuit (bleeder circuit) which is connectable to each of the dynode pins10A, or a circuit pattern which is connectable to each of the anode pins10B for extracting anode output. The first circuit board50is also provided with metal socket pins51corresponding to anode pins10B and metal socket pins52corresponding to dynode pins10A.

A single second circuit board55is provided under the first circuit board50and parallel thereto. The second circuit board55is electrically connected to the first circuit board50through connecting pins (not shown). The second circuit board55has a function for calculating a position, such as an AD converter. Spacers56that are electrically isolated and cylindrical in shape protrude from the top surface of the second circuit board55at positions corresponding to screw insertion holes53formed in the first circuit board50. The spacers56maintain a predetermined interval between the first circuit board50and the second circuit board55. Screw insertion holes56aare formed in the spacers56. Electrically insulating screws32B (male screws) are screwed into the31athrough the female threads56ato fix the first circuit board50and second circuit board55integrally to the four photomultiplier tubes1. Each photomultiplier tube1can be easily separated from the first circuit board50and second circuit board55by unscrewing the screws. The scintillator46may also be integrally fixed to the faceplate3of each photomultiplier tube1.

With this structure, a plurality of the photomultiplier tubes1are integrally mounted onto the first circuit board50and second circuit board55using the male screws32B. Accordingly, this structure simplifies the assembly of the first circuit board50, the second circuit board55, and the photomultiplier tube1, thereby reducing the assembly time and reducing the cost of the product. In case that any one of the first circuit board50, the second circuit board55, and the photomultiplier tube1malfunctions, the first circuit board50, the second circuit board55, and the photomultiplier tube1can be easily separated, thereby facilitating the operation of replacing parts. Additionally, discarding the entire unit may be avoided.

The present invention is not limited to the preferred embodiment described above. For example,FIG. 15shows another type of photomultiplier tube1A having a stem plate4A with screw portions60. The screw portions60have annular spacers65in order to improve mountability. Female threads65aare formed at predetermined positions on the bottom surface of the spacers65. The photomultiplier tube1A is fixed to a base61of a common photodetector. Hence, screw insertion holes62are formed in the base61at positions corresponding to the screw portions60. Socket openings64are formed in the base61for inserting a socket63into the stem pins10. Screws32C (male screws) are screwed into the female threads65aof the screw portions60through the screw insertion holes62, thereby fixing the photomultiplier tube1A to the base61.

The circuit boards33,37,50, and55are configured to have components required for the photomultiplier tube1. The components may be changed appropriately depending on the application thereof. Further, the circuit boards33and50described above can also be formed of plastic or ceramics in a flat shape on which no circuit is mounted.

INDUSTRIAL APPLICABILITY

A photomultiplier tube, a photomultiplier tube unit, and a radiation detector according to the present invention have a lot of different applications in imaging devices for a low luminescent object, such as gamma cameras.