LIGHT PIPE SCINTILLATION CRYSTAL ASSEMBLY OF A RADIATION DETECTOR

Abstract: A light pipe (18) for scintillation crystal (12) assembly of a radiation detector, which comprises a body which has been formed of an adhesive such as a silicone rubber com­ pound which after curing is elastic and light transparent. In a preferred embodiment the adhesive comprises embedded masking elements for modifying the distribution of scintilla­ tion light.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention This invention relates to a light pipe for a scintillation crystal assembly of a radiation detector and a method for producing the same. A preferred field of application of the invention is in a scintillation gamma camera as raoiation detector, which comprises a plurality of photomultiplier tubes ccnnected with the light pipe as part of a sclntiliation crystal assembly. 2. Description of the Prior Art Conventional light pipes, such as described for example in the U.'. Patent 3,723,735 (Spelha et al.) or U.S. Patent 4,029,964 (Asne) R consist of an acrylic plastic, such as plexiglass . The liyht pipe is sculpturea by means of circular grooves into pads and interstices. The grooves have to be cut into the body of the light pipe. Tne photomultiplier tubes are coupled to the pfads by means of a semi-fluia grease-like silicone compounc. To hola the photomultiplier tubes in their positicn requires them to be spring biased. Also the radiation detector of the b.5. Patent 3,784,819 (Martone et al.) makes use of a licht pipe of an acrylic plastic. However, the crystal side of the light transmitter is coated with a reflective material, forming a plurality of masking elements. The masking elements modify the distribution of scintillation light in the light pipe. Thus they provioe for a better light distribution, which results in an improved image display. The process of fabricating a conventional light pipe out of a piece of acrylic plastic by means of cutting the sculpture is relatively complicated and expensive. Also the method of positioning of masking elements is not optimal. SUMMAnY OF THE INVENTION 1. Otiects It is an object of this invention to provide an improved light pipe for a scintillation crystal assembly of a radiation oetector. It is also an object of this invention to provide a method for pro3ucing such an improved light pipe. Furthermore, it is an object of this invention to provide a detector head, which comprises an improved light pipe in a very simple, however, efficient connection with other components of the detector heaG. 2. Summary According to this invention a light pipe is provided for a scintillation crystal assembly of a radiation detector which comprises a body which has been formea of an adhesive which after curing is elastic and light transparent. Also, according tc this invention a method has been provided for prooucing a light pipe for a scintillation crystal assembly of a radiation detector, comprising the steps of: a) providing a casting-mold having the negative shape of the light pipe; b) filling the casting-mold with an adhesive, which after the curing is elastic and light transparent; c) curing the adhesive; and d) removing the cast body out of the casting-molo, this cost body being the light pipe. Finally, according to this invention a radiation detector has been provided, which comprises: a) a scintillation crystal assembly having a light pipe; and b) a photomultiplier tube bonded by means of an adhesive to the light pipe, which adhesive after the curing is elastic and light transparent; wherein the light pipe is formed of the same aahesive as the bond between light pipe and photomultiplier tube. According to this invention a light pipe has been created which can be very easily fabricated in its final shape by casting. Cutting of gr.oves is no lcnger necessary. Also the light pipe is of tne same material as the bond between light pipe and photomultiplier tubes. Thus the light pipe according to this invention provices for an excellent optical matching between scintillation crystal and photomultiplier tu"es. Furthermore, tne irnproved light pipe provides for an easier way of placing of masking elements. According to a preferred embodiment of this invention the body of the light pipe comprises embedded masking elements for modifying the distribution of light. A preferred method for embeoding those masking elements comprises the steps of: a) filling the casting-mold to a prescribed aepth with the adhesive and allowing it to cure; b) applying the appropriate pattern of mask material, by silk screen inking of tne surface of the cured adhesive for example; and c) repeating these steps until the full pattern of masking elements has been applied and the light pipe is formea with its final depth. The foregoing and other objects, features anc advantages of the invention will be apparent from the following more particular description of preferred embooiments of the invention, as illustrated in the accompanying drawings. BRIEF DESCRIPTION OF ThE DRAWINGS In the drawings: Fig. 1 shows a top view of a scintillation crystal assembly in a detector head, such as a scintillation gamma camera, comprising a light pipe according to this invention; Fig. 2 shows the scintillation crystal assemoly of Fig. 1 in a cross section; and Fig. 3 shows in a cross section a casting-mold for the light pipe. DFTAILEG DESCRIPTION OF THE PREFERRED EMBODIMENT In Figs. 1 and 2 the scintillation crystal assembly 10, which has hexagonal shape, comprises a scintillation crystal 12 (e.g. NaI(Tl), an aluminum cover 14, a glass disc 16 and a light pipe 18. The body of the light pipe 18 is sculpturea by means of circular grooves 20 into (e.g. 37) circular pads 22 and interstices 24. On each pad 22 a photomultiplier tube PMT1 to PMTN has to be placed. Thus a detector head (e.g. gamma camera head) will be created, which comprises for example a total of N = 37 photomultiplier tubes. Merely three (of e.g. N= 37) photomultiplier tubes are shown in Figs. 1 and 2 for illustrating purpose. Each photomultiplier tube PMT1 to PMTN has a bottom 26 and is bonded with its bottom 26 to a corresponding pad 22 by means of an elastic light transparent silicone rubber compound. The silicone rubber compound has a transmission of greater than G.8 through 1 cm at 400nm. wavelengths and a moderate refractive index from about 1.4 to 1.6, at 400 n; ;. Such a silicone ruboer adhesive is available for example from General Electric Company, Silicone Products Division, RTV Products Department, Waterford, New York 12188 unter the name RTV 615 or RTV 655 (see General Electric's brochure CDS-1870A 1RTV Silicone Rubber Product Data RTV 615, RTV 655 and RTV 670, High Streng Transparent Silicone Rubber Compound"). Other usable silicone rubber adhesives are for example the Sylgard R Silicone Elastomers 182 or 184 available from the Dow Corning Corporation, Midland, Michigan 48640 (see page 22 of Dow Corning's brochure "Materials for High Technology Applications"). In the present case RTV 615 of General Electric is used as adhesive. The silicone rubber bond is indicated with the numeral 28. According to this invention also the lignt pipe 18 has been formea of the same elastic light transparent silicone rubber compound, for example RTV 615 of General Electric, as has been used for bonding the bottoms 26 of the photomultiplier tubes PMT1 to PMTN to the light pipe 18. Preferably the light pipe 18 as shaped according to Figs. 1 and 2 has been produced in one piece by casting. This is for exampîe indicated in Fig. 3. Fig. 3 shows in a cross section a casting-molo 30, which is the sculpture negative of the light pipe 18 of Figs. 1 and 2. As described in the brochure Product Data CDS-1870A of General Electric ten parts of the A compound and one part of the B compound have previously been mixed and stirred in a container such as to produce a homogeneous mixture free of air bubbles. The mixture is then cast into the interior volume 32 of the castingmo1o 3.. After the curing the light pipe 18 can be remcveo from the casting-melo 30 and bonded to the glass disc 16 of tne scintillation crystai assembly 10 preferably also by means of the silicone rubber aohesive RTV 615. After having bonoed the photomultiplier tubes PMT1 to PMTN to the pads 22, the light pipe 18 and both the silicone rubber bond 28 between photomultiplier tubes and light pipe and the silicone rubber bond 34 between light pipe and glass disc 16 form an optically uniform unit. This unit proviaes an excellent optimal aatching with respect to the scintillation flashes travelling from the scintillation crystal 12 via glass disc 16 to the photoultiplier tubes PMT1 to PTN. According to a preferred emtodiment of this invention masking elements, such as dots or rings of reflective or absorptive material, are embedded in the body of the light pipe 18, as indicated in Figs. 2 and 3 by tne reference numeral 36. The masking elements modify the distribution of the scintillation light so that an improvement in camera performance is achieved. The masking elements 36 have been applied to the silicone rubber compounö mixture, during the casting of the light pipe 18. Thereby the castingmolo has been filled to a prescribed depth with the silicone rubber copound. After the curing of this layer of silicone rubber compound the appropriate pattern of mask material, by silk-screen inking of the surface of the curea silicone rubber compound for exarple, has been applied. These steps have been repeated until the full pattern of masking elements has been applied and the light pipe is formed with its final depth. Having thus described the invention with particular reference to the preferred fcrms thereof, it will be obvious to those skilled in the art to shich the invention pertains, after unoerstanding the invention, that various changes and modifications may be made therein without departing from the spirit and scope of the invention as defined by the claims appenoed hereto.

Claims: WHAT IS CLAIMED IS: 1. A light pipe for a scintillation crystal assembly of a radiation detector, comprising a body which has been formed of an adhesive which after curing is elastic and light transparent. 2. A light pipe according to claim 1, wherein the adhesive is a silicone rubber compound. 3. A light pipe according to claim 1, wherein the body formed of the adhesive comprises embedoed masking elements for modifying the distribution of scintillation light. 4. A method for producing a light pipe for a scintillation crystal assembly of a radiation detector, comprising the steps of: a) providing a castin-mold having the negative shape of the light pipe; b) filling the casting-mold with an adhesive, which after the curing is elastic and light transparent; c) curing the adhesivE; and d) removing the cast body out of the casting-mold, this cast body being the light pipe. 5. A method according to claim 4, further comprising the step of making use of an silicone rubber componno as adhesive. 6. A method according to claim 4, further comprising the steps of: a) filling the casting-mold to a prescribed depth with the adhesive and allowing it to cure; b) applying an appropriate pattern of mask material, for forming masking elements for modifying the distribution of scintillation light, on the surface of the cured adhesive; and c) repeating these steps until the full pattern of masking elements has been applied ano the light pipe is formed with its final depth. 7. A method according to claim 6, further comprising the step of applying the appropriate patterns of mask material by silk-screen inkins on the surface of the cured adhesive. 8. A radiaticn detector comprising: a) a scintillation crystal assembly having a light pipe; and b) a photcmultiplier tube bonded by means of an adhesive on the light pipe1 which adhesive after the curing is elastic ano light transparent; wherein the light pipe is formed of the same adhesive as the bom between light pipe and photomultiplier tube. 9. A radiation detector according to claim 8, wherein the scintillation crystal assembly further comprises a glass disc covering the scintillation crystal of the scintillation crystal assembly, wherein the light pipe is bonded with the glass disc by the same adhesive of which the light pipe has been formed.

Inventors: HAAS, WERNER J., DR., STOUB, EVERETT W., DR.
Applicants: SIEMENS AKTIENGESELLSCHAFT BERLIN UND MUNCHEN
Application year: 1984
Main classification: G01T1
IPC codes: G01T1