Abstract:
A rugged scintillation detector uses a hermetically sealed window assembly welded or otherwise bonded to the detector housing. A method and apparatus useful in forming a reproducible hermetic seal between the glass and an annular metal retaining ring is described. The glass and ring are chosen so that the ring diameter is slightly larger than that of the glass at room temperature. A thin layer of frit at the interface ensures a very tight mechanical seal, and a surface layer of chromium oxide on the ring ensures a secure chemical bond. The assembly device uses pressure and counterpressure to uniformly and reproducibly assemble the glass window into the retainer ring at elevated temperatures.

Description:
BACKGROUND OF THE INVENTION 
     Prior efforts at producing a rugged scintillation detector for well hole logging have met with marginal success, particularly when using a scintillation crystal that forms hydrates and becomes unusable or unreliable when exposed to moisture. The typical construction of a scintillation detector using a hygroscopic crystal such as NaI(Tl) involves the use of a hermetically sealed housing to hold the crystal, said housing having a window at one end. The window in turn is optically coupled to a photomultiplier tube capable of recording the occurrence of scintillation flashes or events occurring within the crystal. Maintaining a hermetic seal between the window and the housing has been a recurring problem, particularly where the detector is subjected to a harsh environment. Even though various remedies have been proposed in the past, none of them have been entirely successful for detectors useful in severe applications such as well hole logging. 
     U.S. Pat. Nos. 4,004,151 and 4,360,733 rely on an epoxy resin and a tapered window inside a retaining ring or housing to securely hold the window in place. For the most part, epoxy resins cannot withstand repeated thermal cycling and the assemblies are susceptible to failure within a relatively short period of time. 
     Another approach to this problem has been to use a Kovar metal ring which is bonded to glass to form a tight seal. However, the choice of glass is very limited to one specific formulation which has expansion characteristics that precisely match those of the Kovar metal. This severely restricts the selection and optimization of glass having optical properties that may be considered to be desirable for a given application. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention covers a method and apparatus for achieving a reproducible hermetic seal between a glass window and a metal retainer ring. The ring is adapted to be welded to the detector housing for a scintillation crystal. The method involves selecting a metal ring having a coefficient of thermal expansion which is between 0.5 and 2.5 ppm (parts per million)/°C. greater than the coefficient of thermal expansion of the glass window in the range of 20° to 500° C., machining the ring so that the inner diameter is between about 0.005&#34; to 0.015&#34; larger than the diameter of the glass at ambient temperatures, uniformly and completely applying a glass frit to the mating edges of the annular ring and the glass, firing the frit, heating the window and assembly to a temperature at which the frit melts to permit the glass to be assembled into the ring, and maintaining stabilization of the window during assembly by applying pressure of between 1 and 3 psi on the window and a counter pressure of between 0.2 and 0.8 psi. 
     This procedure preferably is achieved by the use of a sealing fixture which includes means for positioning the window above and in axial alignment with the metal ring, means to heat the ring sufficiently to melt the sealing frit and means to bias the window into position within the ring, the biasing means of including means to exert a downward pressure on the window, and means to apply a smaller pressure to the window in the upward direction to prevent mis-alignment of the window as it is pressed into the ring. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The FIGURE represents a cross-sectional elevated view, partially in cross section of the sealing fixture of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The method of forming an effective glass-to-metal assembly according to the present invention comprises the steps of prewetting the glass and the metal ring with a sealing frit, precisely aligning and assembling the glass into the ring while the frit is molten, cooling the assembly, and lapping and polishing both faces of the ring. The polished assembly is then welded to one end of the detector housing. 
     In more detail, a stainless steel ring is machined to have an outer diameter corresponding to that of the detector housing, an inner diameter between 0.005&#34; to 0.015&#34; larger than the diameter of the glass window and a width at least equal to the width of the glass window. The ring is machined from 446 stainless steel, a non-hardenable chromium steel having very high corrosion and heat resistance. It contains 23 to 27% chromium, 1% silicon, a maximum of 1.5% Mn with 0.25% or less of nitrogen, carbon, phosphorus and sulfur. The ring is degreased and is then heated in an air atmosphere to 500° C. to oxidize the surface, forming the mixed oxides of chromium and iron, which serve to enhance the bond with the glass. 
     The material which is selected for use as an optical window should be highly transparent in the optical range in which the crystal scintillates. A suitable glass for use with a NaI(Tl) crystal is referred to as UBK-7 and is manufactured and sold by Thermal American Fused Quartz Company. This glass is characterized by transmittance of at least 0.97 in the range of 365 nm to about 1750 nm and a refractive index of at least 1.5 in the same range. It softens at 716° C. The glass is formed into the shape of a flat circular disc having a uniform thickness between 0.20&#34; to 0.40&#34;. 
     The diameter of the glass disc is approximately 0.005&#34; to 0.015&#34; smaller than the inner diameter of the ring at ambient temperatures. The glass disc is cleaned well in acetone and is then degreased with trichloroethylene. A thick slurry of frit is prepared by thoroughly mixing 5-6 parts of a glass frit with 1 part of a vehicle such as distilled water. A suitable frit is Corning 7570 having a particle size distribution of 100 to 325 mesh, and containing a high percentage of lead oxide. The softening point of the frit is 440° C. and its coefficient of thermal expansion is 8.4 ppm/°C. in the range of 25°-300° C. The slurry of frit is applied uniformly to the inner surface of the metal ring following which the ring, while laying flat, is thermally cycled between room temperature and a temperature of about 500° C. to ensure wetting of the entire inner surface of the ring. The fired frit results in tapering of the ring ID by about 2.5 mils. 
     In like manner the frit is applied to the edge of the glass disc followed by thermal cycling, while laying flat, to ensure uniform and complete wetting of the edge of the glass. The fired frit results in the effective tapering of the glass (window) diameter by about 2-5 mils. The frit is preferably applied by a medicine dropper over a lathe machine although dipping, spraying, silk screening and other methods of application may be used. Because of the added thickness of the solidified frit layers, the effective diameter of the glass disc is greater than the inner diameter of the ring at room temperature, therby preventing assembly of the disc into the ring. 
     The frit preferably is applied to the ring by placing the ring in a chuck and slowly rotating the chuck at a speed of 20-40 rpm. With the ring rotating, a few drops of slurry are poured onto the inner surface of the ring following which the slurry is dried. The ring is then removed from the chuck and is heated in an oven as aforesaid. In like manner, the window is positioned between a pair of glass holding studs and the edge is coated with slurry followed by drying and heating. 
     The sealing cycle for the window and the ring involves heating them at the rate of 8° C./min to 500° C. in air and holding at that temperature for 30-60 minutes to melt the frit and assemble the window into the ring and cooling the assembly at the rate of 1° C./minute. The glass and ring are carefully assembled as the sealing frit is molten by very precisely pushing the window into the ring, while maintaining stabilization of the window and ring at all times. This ensures uniform and even bonding of the ring to the window. A sealing fixture capable of maintaining such stabilization during assembly is shown in the drawing. 
     The thermal expansion coefficients of the glass, the retainer ring and the frit are selected to be between about 1.0 and 2.5 ppm/°C. of one another in the temperature range of 20° to 500° C. with the coefficient of the metal retainer ring being slightly greater than that of the glass to ensure a compression fit when the assembly is cooled. 
     The sealing fixture utilizes a weight and counterweight to permit the glass window to be slowly and evenly inserted into the ring as the frit is melted. The entire fixture is adapted to be placed in a furnace (not shown) which is heated to a temperature at which the frit becomes molten. Typically, this temperature is at least 470° C. 
     Referring specifically to the drawing, the sealing fixture 2 is mounted on a levelled platform 4 and comprises a ring support 6 having a plurality of vertically extending concentric circular ribs 8a, 8b, 8c and 8d. A retainer ring 24 rests on the ribs and is restrained against shifting by a positioning ring 10. At least one of the ribs 8d has a diameter which is less than the inner diameter 25 of the ring 24. As previously mentioned, the ring I.D. and the glass (window) diameter have a taper of about 2-5 mils due to the fired frit coatings. The ring is placed on the ribs so that the larger inner diameter is at the top. The inner surface of the ring is coated with a thin layer of frit 28. A glass window 22 in the shape of a cylindrical disc rest on a piston 18 in axial alignment with the retainer ring. The circumferential edge 29 of the window is coated with a layer of frit 26. 
     The piston 18 slides vertically within the ring support 6 between the pre-assembly postion as shown, ad a lower position, limited by the shoulder 30 of stop 32. The piston stop and ring support are mounted in a frame 34 which rests on and preferably is secured to platform 4. A thermal cap 20 slip fits over the sealing fixture and is adapted by means of stud 36 and weight 12 to exert a downward pressure of between about 1/2 and 3 pounds per square inch on the glass window. The downward pressure is opposed by a counterweight 14 which works through pivot 38 to exert an upward force on the piston 18 of about 0.1 to 1 pound. The cap serves to retard heat transfer during thermal cycling, thereby serving to reduce thermal shock to the window. 
     While the glass window and ring are at room temperature, the glass will rest on the ring, but, because of the solid layers of frit 25 and 26 on the ring and window respectively, the effective diameter of the window is larger than the effective inner diameter of the ring and the window is not forced into the ring. When the entire assembly is placed in an oven which is heated to a temperature at which the frit melts, the glass is gently pressed into the ring as the downward presure of the weight 12 pushes the piston down. The counter pressure offers enough resistance to physically stabilize the glass and to keep the glass in contact with the top planar surface of the piston, thereby preventing the glass from getting tilted or mis-aligned while entering the retainer ring. The grooves 9 between the circular ribs 8a-d provide more than sufficient space to prevent the possibility of excess molten frit sticking to the sealing fixture during assembly. The glass window slides into the ring until one or more of the ribs 8a-d of the ring support prevent further downward movement of the window. 
     By using a plurality of ribs and different positioning rings of varying diameters, the sealing fixture is capable of handling windows of different diameters for use on a variety of detector assemblies. 
     The window assembly is slowly cooled at a rate(˜1° C./min) that reduces the buildup of thermal stresses within the window. As previously mentioned, the coefficient of thermal expansion of the metal ring preferably is 1.0 to 2.5 ppm/°C. greater than that of the glass, thereby causing the metal to contract more than the glass during cooling to form a compression seal with the glass. In addition, a strong chemical bond is formed between the glass and the oxide layer (believed to be due to the presence of Cr 2  O 3  and iron oxide in the oxide layer). 
     The glass-sealed ring after being welded to the appropriate pipe is leak checked by a helium leak checker to a sensitivity of 10 -10  atm-cc/sec. 
     Following the formation of the hermetic seal between the glass and the metal, the surfaces of the window and ring are ground to remove excess frit. They are then lapped so that the surfaces of the metal and glass are flush with one another and the glass is polished for maximum optical clarity. 
     The window assembly is joined to the detector housing by appropriate means such as welding. The preferred method of welding is electron beam welding although other methods of welding may be used provided they do not produce excessive heat build-up which can cause thermal stresses within the glass, possibly leading to its fracture. Electron-beam welding can be achieved using an operating power as low as 0.1 watt. The retaining ring can also be designed with one or more thermal barriers such as grooves or can be held in a heat dissipating chuck or holder during welding to minimize the amount of generated heat that reaches the window. The housing typically is composed of 304 stainless steel. 
     Appropriate means can be used to check the integrity of the glass to metal seal after assembly. For example, the ring is welded to the housing after which it is checked for leaks using a helium leak checker. In addition, after the detector is fully assembled, the adequacy of the seal can be demonstrated by immersing fully assembled detector in silicon oil in a Bell jar and evacuating the jar to remove all traces of air. Any lack of integrity of the glass-to-metal seal or housing welds will manifest itself by visual detection of a steady stream of bubbles coming from the assembly at the location of the leak. 
     A number of variations can be made in the design and construction of this device without departing from the inventive features described herein. For example the ring support and positioning ring may be replaced with an adjustable chuck. The design and shape of the piston support for the glass window may be varied, without departing from the basic assembly technique which utilizes a pressure-counterpressure means of preserving alignment of the window and the retainer ring while the window is being fitted into the ring. Another alternative is to apply the pressure/counterpressure against the ring to move it into alignment with the window restraining the window against movement. 
     Although the invention has been described in connection with the assembly of hermetically sealed housings for radiation detectors, the use of an air-tight seal between glass and metal in many other applications is manifest.