Radiation detector and its manufacturing method

Abstract: A radiation detector comprises a plate-like monocrystal (2) converting a radiation ray radiated from an object to be examined to a visible ray, a glass plate (5), provided on the monocrystal (2), transmitting the visible ray generated in the monocrystal (2), and a plurality of photomultipliers, arranged on the glass plate (5), converting the visible ray inputted through the glass plate (5) from the monocrystal (2) to an electrical signal.

Description: The present invention relates to improvements of a radiation detector to be used in a scintillation camera. If a radioactive medicine, which is labeled by a radioisotope (hereinafter called "RI"), is added to a body, the radioactive medicine is absorbed in a specific organ or tissue, and concentrated thereon. By use of this property, a scintillation camera detects a γ-ray, which is emitted from the radioactive medicine at random, for a fixed period of time, and obtains an RI distribution (scintigram). This is useful for the diagnosis of the shape and function of the organ, an existence of a focus, and a metabolic function. DE-A-29 34 665 is directed to a γ camera which comprises a scintillator. Light photons generated by the scintillator are guided through a light guiding plate serving as a light collecting lens, to a plurality of photomultiplier tubes. The scintillator can also be formed as part of the front collimator. A further conventional radiation detector of Anger type used in the scintillation camera is structured as shown in Fig. 1. More specifically, a scintillator 30 comprises an aluminum plate 32 and a glass plate 34, and a flange 35 of an iron frame. The aluminum plate 32 is attached to a front surface (γ-ray incident surface) of a monocrystal 31. The monocrystal 31 is formed of sodium iodide (NaI) plate, and receives the γ ray, and emits a flash. The aluminum plate 32 protects the monocrystal 31 from deliquescence. The glass plate 34 is adhered to the back surface (output surface of scintillation light) of the monocrystal 31 by a silicon adhesive 33. The glass plate 34 protects the monocrystal 31 from deliquescence, and scintillation light is passed therethrough. The scintillator 30 is annularly formed by a flange 35 of an iron frame not to be deformed. The conventional radiation detector is structured as follows. A plurality of studs 44 are formed in the flange 35, so that a frame is formed. A light guide 37 of an acrylic plate is adhered to the glass plate 34 of the scintillator 30 by hard two-part liquid silicon 36. A plurality of photomultipliers 39 having a photoelectric surface 40 on the bottom of the light guide 37 are tightly arranged on the upper surface of the light guide 37 by a coupling agent 38 such as optical grease. The photomultipliers 39 are pressed to the light guide 37 and fixed thereto by a case 43 through a spring 42, which is inserted into a socket 41. The scintillation light, which is generated in the monocrystal 31, is passed through the adhesive 33, glass plate 34, two-part liquid silicon 36, light guide 37, coupling agent 38, and the outer tube of the photomultiplier 39 in this order, and detected at the photoelectric surface 40. In an electric circuit of the scintillation camera, an output of each photomultiplier 39 of the radiation detector is received. An energy signal in which the output is added, that is, a signal, which is proportional to the whole amount of emitted scintillation light, is selected by a window, which is set in accordance with energy of the incident γ ray, so that only effective data is collected. Also, the output of the photomultipliers 39 is multiplied by a coefficient in accordance with the position of the arrangement, and added, so that the incident position of the γ ray is calculated and the RI distribution is outputted. The conventional radiation detector has a structure in which a plurality of members such as the glass plate 34, the light guide 37, and the like are layered. Due to this, when scintillation light exceeds a critical angle and enters the portion between the members having a different refractive index, for example, a boundary surface between the glass plate 34 and silicon 36 or a boundary surface between the silicon 36 and the light guide 37, scintillation light is totally reflected. Therefore, the components of the total reflection is increased as the scintillation position is away from the center of sensibility of the photomultiplier 39. Due to this, the change of sensibility, which is called a positional response of the photomultiplier, to the position of scintillation of the photomultiplier 39 is decreased as the γ ray enters a position, which is away from the center of sensibility. As a result, the electric circuit of the scintillation camera calculates the incident position of the γ ray in a state that the incident position is biased to the center of sensibility, and shows a tendency for a luminescent spot to be concentrated on the center of sensibility. Therefore, there is a disadvantage in that uniformity, which is one of the important basic performance of the scintillation camera, is decreased. In the conventional scintillation camera, the output of the photomultiplier is nonlinearly compensated in the electric circuit based on the positional response in which a point source is collected as being moved on the surface of the scintillator, thereby decrease in uniformity is controlled. However, the compensation circuit is excessively loaded by such a compensation, and other basic performances of the scintillation camera such as intrinsic resolving power and energy resolving power must be given up to some extent. In consideration of the above problem, the present invention has been made, and a first object is to provide a radiation detector wherein a positional response is improved by optical means so that load to be applied to a nonlinear circuit can be reduced, and intrinsic resolving power and energy resolving power can be improved as improving uniformity. A second object is to simplify the structure of the radiation detector, thereby miniaturizing the detector, lightening the weight of the detector, and reducing the number of assembling steps. A radiation detector of the present invention comprises the features mentioned in claim 1. Therefore, according to the radiation detector of the present invention, since scintillation light reaches the photomultiplier through only the glass plate member, unlike the conventional structure, no component of total reflection is generated at the boundary surface between the glass plate and the light guide, and the positional response can be improved. As a result, uniformity can be improved and intrinsic resolving power and energy resolving power can be improved at the same time. Some further features of particular embodiments of the invention are mentioned in the dependent claims. Moreover, according to the present invention, there is provided a method for manufacturing a radiation detector in accordance with claim 8. According to the manufacturing method of the radiation detector of the present invention, since it is unnecessary to take time to provide a spring or further mount the light guide on the glass plate provided on the monocrystal, the number of assembling steps can be reduced. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a cross sectional view showing a conventional radiation detector; Fig. 2 is a cross sectional view showing a radiation detector according to a first embodiment of the present invention; Fig. 3 is a plane view showing the arrangement of photomultipliers; Fig. 4 is a view showing a characteristic of a positional response; Fig. 5 is a cross sectional view showing a radiation detector according to a second embodiment of the present invention; Fig. 6 is a plane view of a glass plate according to a third embodiment of the present invention; Fig. 7 is a plane view of a glass plate according to a fourth embodiment of the present invention; Fig. 8 is a cross sectional view showing a radiation detector according to a fifth embodiment of the present invention; and Fig. 9A to 9E are views showing steps of detaching a photomultiplier from a glass plate according to a sixth embodiment of the present invention. Embodiments of the present invention will be explained with reference to the drawings. Fig. 2 shows the first embodiment of the present invention. More specifically, a scintillator 1 is structured as follows. An aluminum plate 3 is attached to a front surface (γ-ray incident surface) of a monocrystal 2. The monocrystal 2 is formed of sodium iodide (NaI) plate, and receives a γ ray, and emits a flash. The aluminum plate 3 protects the monocrystal 2 from deliquescence. A glass plate 5 having a thickness of 9 - 16mm is formed on the back surface (output surface of scintillation light) of the monocrystal 2 in accordance with the total thickness of the glass plate and the light guide of the conventional scintilator. Then, the portion between the monocrystal 2 and the glass plate 5 is filled with an acrylic adhesive 4 having substantially the same refractive index as that of the glass plate 5 (for example, 1.51). Since the glass plate 5 has the above-mentioned thickness, the glass plate 5 not only protects the monocrystal 2 from deliquescence but also functions as a light guide. The scintillator 1 is annularly formed by a flange 6 of an iron frame not to be deformed. According to the radiation detector of the present invention, a plurality of studs 7 are formed in the flange 6, so that the frame is formed. As shown in Fig. 3, a plurality of photomultipliers 9 having a photoelectric surface 10 on the bottom are tightly arranged on the glass plate 5 of the scintilator 1, and the photomultipliers 9, which are provided at the most end portions, are arranged on the glass plate 5 in an overhanging state. The portions between the glass plate 5 and the photomultipliers 9 are filled with an acrylic adhesive 8 having substantially the same refractive index as that of at least one of the glass plate 5 and the outer tube (borosilicate glass or potassium free glass is generally used.) of the photomultipler 9, and adhered. Then, the scintilator 1 and the photomultipliers 9 are contained in a case 12 constituting a dark box and a magnetic screen box. According to the above-structured radiation detector of the present invention, the following technical advantages can be realized. First, scintillation light, which is generated in the monocrystal 2, is passed through the adhesive 4 having substantially the same refractive index as that of the glass plate 5, the glass plate having a predetermined thickness, the adhesive 8 having substantially the same refractive index as that of at least one of the glass plate 5 and the outer tube of the photomultipler 9, and the outer tube of the photomultiplier 9 in order and, reaches to the photoelectric surface 10 of the photomultiplier 9. Therefore, optical loss is extremely small. In other words, since the total reflection of scintillation light, which is generated in the portion between the members having the different refractive index, the portion between the monocrystal and the adhesive, the portion between the adhesive and the glass plate, the portion between the glass plate and a silicon adhesive, the portion between the silicon adhesive and the light guide, the portion between the light guide and a coupling agent, and the portion between the coupling agent and the photomultiplier, is decreased, a characteristic of sensibility to the scintillation position of the photomultipler, that is, a characteristic of a positional response is improved. Fig. 4 is a view showing a characteristic of a positional response wherein a vertical axis is an output level of the photomultiplier, a horizontal axis is a distance from the center d0 of sensibility of the photomultiplier, ▴ shows a prior art, and X shows the present embodiment but adopting a silicon adhesive between the monocrystal 2 and the glass plate 5 instead of the acrylic adhesive 4. As a result, load to be applied to a nonlinear circuit can be reduced, and intrinsic resolving power and energy resolving power can be improved as improving uniformity. The specific measured value showing improvement of the positional resolving power and energy resolving power are FWHM 0.2 mm or more of positional resolving power and 0.8% or more of energy resolving power. According to this embodiment, since the characteristic of the positional response is good, the positional resolving power and energy resolving power can be maintained to the same extent as the conventional case even if the spacing of the next photomultipliers is extended. Therefore, in this case, it is possible to provide a radiation detector whose maintenance is simply performed at low cost. Second, the photomultipliers 9, which are arranged at the most end portions, are held on the glass plate 5 in an overhanging state. Thereby, the photomultipliers 9 can be arranged in a wide range, which is more than the incident area of the monocrystal 2, and distortion of an image of the peripheral portion of the monocrystal 2 can be reduced. Third, the photomultipliers 9, which are arranged at the most end portions, are held on the glass plate 5 in an overhanging state. Thereby, there is no need of changing the size of the area of the glass plate 5, of filling a light guide on the glass plate 5. As a result, the conventional flange 6 is used as it is, and the detector can be prevented from being enlarged. Fourth, since the detector of the present invention has the simple structure in which the conventional acrylic plate and the conventional spring are removed, the detector can be thinned and lighten, and the number of assembling steps can be reduced. A second embodiment of the present invention will be explained. Fig. 5 is a cross sectional view showing the structure of the second embodiment. It is noted that the same reference numerals are added to the same portions as the first embodiment of Fig. 2, and the specific explanation is omitted. In the first embodiment, the photoelectric surface 10 is formed on only the bottom surface of the outer tube of the photomultipler 9. However, in the second embodiment, a photoelectric surface 13 is formed the portion, which is from the bottom surface of the outer tube of the photomultipler 9 to the side surface thereof, which has a height of 3 - 10 mm. Also, the acrylic adhesive 8 is packed up to the side surface of the photoelectric surface 13. Therefore, as shown in Fig. 5, the photomultiplier 9 can detect scintillation light with a light receiving angle 1, which is wider than the conventional light receiving angle 0, so that the positional response, which is better than the first embodiment, can be realized as shown in Fig. 4. It is noted that is Fig. 4 ▴ shows the prior art and ▪ shows this embodiment but adopting a silicon adhesive between the monocrystal 2 and the glass plate 5 instead of the acrylic adhesive 4. Additionally, according to this embodiment, even if the acrylic adhesive 8 is packed in only the bottom surface of the photomultiplier 9 similar to the first embodiment without packing the adhesive 8 up to the side surface of the photoelectric surface 13, the positional response, which is better than the first embodiment, can be realized as shown in  of Fig. 4. A third embodiment of the present invention will be explained. Fig. 6 is a plane view showing the glass plate according to the third embodiment of the present invention. It is noted that the entire structure of the radiation detector of this embodiment is the same as that of the first embodiment of Fig. 1. This embodiment is characterized in that a ring-shaped marking 14, which shows the position where the photomultipler 9 is arranged on the upper surface of the glass plate 5 of the scintilator 1. Therefore, since the marking 14 may be used when the photomultiplier 9 is arranged on the glass plate 5, the assembling work can be correctly and efficiently performed. A fourth embodiment of the present invention will be explained. Fig. 7 is a plane view showing the glass plate according to the fourth embodiment of the present invention. It is noted that the entire structure of the radiation detector of this embodiment is the same as that of the first embodiment of Fig. 1. This embodiment is characterized in that coating material 15 containing a reflection agent is applied to a portion (slanted lines) where no photomultipler 9 is arranged on the upper surface of the glass plate 5 by silk printing. Therefore, according to this embodiment, the photomultiplier 9 can detect scintillation light well, so that the positional response can be improved. A fifth embodiment of the present invention will be explained. Fig. 8 is a cross sectional view showing the structure of the fifth embodiment. It is noted that the same reference numerals are added to the same portions as the first embodiment of Fig. 2, and the specific explanation is omitted. This embodiment is characterized in that the portion where no photomultiplier 9 is arranged on the the upper surface of the glass plate 5 is cut in a section-v form, and a seating 16, which receives the photomultiplier 9, is formed on the upper surface of the glass plate 5. The surface of the cut is coated by a reflection agent 15 1 . Therefore, according to the fifth embodiment, the seating 16 may be used when the photomultiplier 9 is arranged on the upper surface of the glass plate 5. Also, the use of the seating 16 is useful for thinning the thickness of the glass plate 5 and reducing the number of photomultipliers 9. A sixth embodiment of the present invention will be explained. This embodiment relates to a jig, which is used when the photomultiplier 9 is detached from the glass plate 5, and its detaching method. Figs. 9A to 9E are views showing steps of detaching the photomultiplier from the glass plate by the jig. According to this embodiment, a socket 11, which is screwed to a screw thread 17 of the photomultiplier 9, is detached as shown in Figs. 9A and 9B. Then, as shown in Figs. 9C and 9D, a jig 18 for detachment in place of the socket 11 is screwed to the screw thread 17. The jig 19 is a cylinder in which a screw thread 19 is screwed to the inner wall of one side, and a handle 20 is fixed to the other side. Then, a jig 18 is held and the photomultiplier 9 is rotated or inclined on the glass plate 5. As a result, the acrylic adhesive 8 is peeled from the photomultiplier 9 and the glass plate 5, so that the photomultiplier 9 can be easily detached from the glass plate 5. After detaching the photomultiplier 9 from the glass plate 5, the surface of the glass 5 is cleaned by alcohol, and another photomultiplier 9 is adhered to the glass 5 by the acrylic adhesive 8. According to this embodiment, since the photomultiplier 9 can be easily detached from the glass plate 5, the exchange of the photomultiplier 9 due to the trouble can be easily performed. It is noted that rubber having high coefficient of friction is formed in the inner wall of the cylinder without forming the screw thread 19, and the jig may be attached to the photomultiplier 9 by frictional force of rubber. The present invention is not limited to the above-mentioned embodiments, and various modifications may be worked.

Claims: A radiation detector comprising: a scintillator (1) having a plate-like monocrystal (2) converting radiation rays radiated from an object to be examined to visible rays, and a glass plate (5) provided on said monocrystal, said glass plate transmitting said visible rays generated in said monocrystal (2), and protecting said monocrystal (2) from deliquescence, a plurality of photomultipliers (9), arranged on said glass plate (5) of said scintillator to be opposite to said monocrystal (2), and converting the visible rays to electrical signals, at least one photomultiplier (9) being arranged at the edge of said glass plate (5) so as to extend partially beyond the edge of said glass plate; and an adhesive (8) joining said photomultipliers (9) on said glass plate (5) of said scintillator (1), said adhesive (8) having substantially the same refractive index as that of at least one of an input surface of said photomultipliers (9) and of said glass plate (5). The radiation detector according to claim 1, characterized in that said glass plate (5) has a thickness of 9 mm or greater. A radiation detector according to claim 1 or 2, characterized by further comprising an adhesive (4) which adheres said glass plate (5) and said monocrystal (2), and has substantially the same refractive index as that of said glass plate (5). A radiation detector according to anyone of claims 1 to 3, characterized in that said photomultipliers (9) form a photoelectric surface (40) from the front surface to the side surface. The radiation detector according to claim 4, wherein said photoelectric surfaces (40) of said photomultipliers are buried in an acrylic adhesive. The radiation detector according to anyone claims 1 to 5, wherein the adhesive (8) is an acrylic adhesive. The radiation detector according to anyone of claims 1 to 6, wherein said adhesive (8) is joining said photomultipliers on said glass plate in such a manner that each photomultiplier (9) can be detached from the glass plate (5). A method for manufacturing a radiation detector comprising the steps of: adhering a glass plate (5) transmitting visible rays to a plate-like monocrystal (2) converting radiation rays radiated from an object to be examined to said visible rays by an adhesive (4) having substantially the same refractive index as said glass plate (5); arranging a plurality of photomultipliers (9), converting said visible rays to electrical signals, on the glass plate (5), at least one of the photomultipliers (9) being arranged at the edge of said glass plate (5) so as to extend partially beyond the edge of said glass plate; and adhering said photomultipliers (9) to said glass plate (5) by an adhesive (8) having substantially the same refractive index as that of at least one of said photomultipliers (9) and said glass plate (5).

Inventors: SHUTO KEISEI, TAKAYAMA TAKUZOU, YAMAKAWA TSUTOMU, SHUTO, KEISEI, TAKAYAMA, TAKUZOU, YAMAKAWA, TSUTOMU, Shuto, Keisei, c/o Intellectual Property Div., Takayama, Takuzou, c/o Intellectual Property Div., Yamakawa, Tsutomu, c/o Intellectual Property Div.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1992
Main classification: G01T1
IPC codes: G01T1