Abstract:
A scintillation detector includes a scintillation crystal directly coupled to a photomultiplier tube (PMT). The crystal/PMT subassembly is attached to a voltage divider and the entire device is hermetically sealed in a stainless steel outer case. Conductors are passed through the hermetic package from the voltage divider via a high temperature metal to ceramic pass-through. The crystal and PMT are longitudinally loaded within the outer case by springs in order to minimize vibrations in the crystal and PMT. A thermoplastic support sleeve circumscribes the crystal and the PMT to protect the crystal and PMT from excessive longitudinal loading. Preferably, the support sleeve and the crystal have similar coefficients of thermal expansion so that the crystal and the support sleeve experience similar dimensional changes due to temperature fluctuations, allowing the support sleeve to best maintain its stress-limiting function as temperature within the detector changes. The support sleeve also protects the crystal/PMT subassembly from bending loads.

Description:
FIELD OF THE INVENTION 
     The invention herein described relates generally to a scintillation detector and to a method of manufacturing a scintillation detector. The scintillation detector and method are particularly useful for borehole logging applications, but may, however, have use in other applications. 
     BACKGROUND OF THE INVENTION 
     Scintillation detectors have been employed in the oil and gas industry for well logging. These detectors have used thallium-activated sodium iodide crystals that are effective in detecting gamma rays. The crystals are enclosed in tubes or casings to form a crystal package. The crystal package has an optical window at one end of the casing which permits radiation-induced scintillation light to pass out of the crystal package for measurement by a light sensing device such as a photomultiplier tube coupled to the crystal package. The photomultiplier tube converts the light photons emitted from the crystal into electrical pulses that are shaped and digitized by associated electronics. Pulses that exceed a threshold level are registered as counts that may be transmitted “uphole” to analyzing equipment or stored locally. 
     The ability to detect gamma rays makes it possible to analyze rock strata surrounding the bore holes, as by measuring the gamma rays coming from naturally occurring radioisotopes in down-hole shales which bound hydrocarbon reservoirs. Today, a common practice is to make measurements while drilling (MWD). For MWD applications, the detector must be capable of withstanding high temperatures and also must have high shock resistance. At the same time, there is a need to maintain performance specifications. 
     As new MWD tools are developed, the need for smaller detectors that meet or exceed larger detector performance is paramount. Current geophysical detectors that use hygroscopic crystals, such as thallium-activated sodium iodide crystals, require that the crystal be hermetically sealed in a stainless steel container. In order to maintain that seal under operating conditions, typically a soda lime glass window is hermetically sealed to the stainless steel housing by means of a glass to metal seal. The window is required to transmit the scintillated light produced in the crystal to a light sensing device such as a photomultiplier tube. This window assembly, along with the multiple optical interfaces needed, degrades the light transmitted to the photomultiplier. It follows, if the window and the associated interface can be removed, a gain in optical performance can be realized. This translates into a smaller crystal that has increased system nuclear performance of a larger crystal having an interface/window assembly. Therefore, it is desirable to have the photomultiplier tube directly coupled to the crystal and hermetically sealed in the housing. 
     However, there are many problems that must be addressed in the construction of such a windowless detector. These problems include the hermeticicity of the electrical pass-throughs, the off-gassing of volatile components that may degrade the hygroscopic crystal, and the survivability of the device under extreme environmental conditions. 
     Accordingly, it will be understood from the above that it would be desirable to have a scintillation detector without an optical window which overcomes the above problems. 
     SUMMARY OF THE INVENTION 
     The present invention provides a scintillation detector wherein a scintillation crystal is directly coupled to a photomultiplier tube (PMT). The crystal/PMT subassembly is attached to a voltage divider and the entire device is hermetically sealed in a stainless steel outer case. Conductors are passed through the hermetic package from the voltage divider via a high temperature metal to ceramic pass-through. The crystal and PMT are longitudinally loaded within the outer case by springs in order to minimize vibrations in the crystal and PMT and to accommodate thermal expansion and contraction of the crystal/PMT subassembly. A thermoplastic support sleeve circumscribes the crystal and the PMT to protect the crystal and PMT from excessive longitudinal and bending loads. The support sleeve and the crystal have similar coefficients of thermal expansion so that the crystal and the support sleeve experience similar dimensional changes due to temperature fluctuations, allowing the support sleeve to best maintain its stress-limiting function and avoiding damage to the crystal/PMT, solid reflector or optical interface as temperature within the detector changes. The support sleeve is radially compressible and expandable, preferably by means of a longitudinal slot in it. 
     According to an aspect of the invention, a scintillation detector includes a sleeve supporting a light sensing device against longitudinal and/or bending is loads. 
     More particularly, according to another aspect of the invention, a scintillation detector includes a hygroscopic scintillation crystal; a light sensing device, such as a PMT, optically coupled to the crystal; a resilient biasing device which loads the crystal and the light sensing device longitudinally; and a support sleeve circumscribing the crystal and the light sensing device which limits the longitudinal load on the light sensing device and/or associated electronics. 
     According to yet another aspect of the invention, a method of manufacturing a scintillation detector includes the steps of optically coupling a hygroscopic scintillation crystal to a photomuitiplier tube; forming an equipment assembly by inserting the crystal and the photomultiplier tube in a support sleeve which limits the longitudinal loading on the photomultiplier tube; inserting the equipment assembly in a housing; longitudinally loading the equipment assembly; and sealing the housing while maintaining a longitudinal load on the equipment assembly. 
     According to a further aspect of the invention, a scintillation detector includes means for optically coupling a hygroscopic scintillation crystal and a light sensing device; means for longitudinally loading the crystal and the light sensing device; and means for limiting the longitudinal load on the light sensing device. 
     To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG  1  is a longitudinal sectional view of a scintillation detector according to the invention; 
     FIG. 2 is a perspective view of a support sleeve according to the present invention; 
     FIG. 3 is a perspective view of a boot sleeve which may be used in the exemplary embodiment; and 
     FIG. 4 is an exploded perspective diagram illustrating assembly of the components of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now in detail to the drawings, FIG. 1 illustrates an exemplary and preferred scintillation detector  10  according to the present invention. The detector  10  comprises a housing  12  encapsulating an equipment assembly  14 . The equipment assembly  14  includes a scintillation crystal  16  and a light sensing device such as a photomultiplier tube (PMT) 18 , which are bonded together to form a crystal/PMT subassembly  20 . The equipment assembly  14  also includes a voltage divider  24  electrically connected to the PMT  18 . The crystal/PMT subassembly  20  and the voltage divider  24  are circumscribed by a support sleeve  26 . The voltage divider  24  and the support sleeve  26  are engaged by and preferably attached to an end cap  28  which is also part of the equipment assembly  14 . Details of the equipment assembly  14  are given below. 
     The illustrated scintillation crystal  16  has a cylindrical surface  30  and flat end faces  32  and  34 , the surface finish of which may be sanded, polished, ground, etc., as desired. The crystal  16  may be, for example, thallium-activated sodium iodide crystal as in the illustrated embodiment. Alternatively, other crystal materials, such as a cesium iodide crystal, may be employed. The crystal  16  is surrounded at its radially outer surface  30  and axial end surface  32  by a layer  36  of reflecting material provided between the crystal and the support sleeve  26 . The layer  36  of reflecting material preferably is formed by a white thin porous unscintered polytetrafluoroethylene (PTFE) sold under the trademark TETRATEC, PTFE sold under the trademark TEFLON, or the like. Being porous, air or gas can escape from between the layer  36  and the cylindrical surface  30  and the end  32  of the crystal  16  to avoid pockets of trapped air or gas. Such pockets are usually undesirable since trapped air or gas could have a negative impact on reflectivity at the layer-crystal interface. The porous PTFE film  36  is tightly wrapped around the crystal  16  and is generally self-adhering to the cylindrical surface  30  and the circular end  32  of the crystal  16 . 
     It is desirable that the PMT  18  be a bare glass envelope phototube, in order to avoid introduction into the housing  12  of foreign materials generally associated with phototube assembly packaging that could off-gas during thermal cycling and thereby cause degradation in the crystal  16 . An exemplary PMT  18  is phototube model number R1288-01, made by Hamamatsu Photonics K.K., of Hamamatsu City, Japan, suitably modified to remove foreign materials associated with phototube assembly packaging, as noted above. The PMT  18  has a layer  40  of 0.010 inch thick mu-metal foil wrapped around it to shield the PMT  18  from magnetic fields. 
     The crystal/PMT subassembly  20  is formed by connecting the crystal  16  and the PMT  18  using bonded optical interface  42 . The end  34  of the crystal  16  and proximate end  44  of the PMT  18  are mechanically and optically bonded together by the interface  42 . It is desirable for the material of the interface  42  to not degrade or out-gas during thermal cycling, and to have good optical properties, in that it allows scintillation light to pass-through unhindered. An exemplary material exhibiting such properties is SYLGARD® 186 encapsulating resin, a high strength room-temperature-curing silicone elastomer manufactured and sold by Dow Corning Corporation. This material maintains its effectiveness and minimizes out-gassing at temperatures up to 200° C. It also transmits the scintillation light well without excessive attenuation. 
     A further desirable property of the material of the interface  42  is that it allow some flexibility in the connection of the crystal  16  and the PMT  18 , while still maintaining the crystal  16  and the PMT  18  mechanically coupled. Too much rigidity in the interface may make the PMT  18  and/or the interface  42  more vulnerable to breakage as the equipment assembly  14  is subjected to vibration and shock. Again, SYLGARD® 186 is a material that has this desirable property, although it is expected that other silicone materials and materials of other types will also possess acceptable properties. 
     It will be appreciated that the bonded optical interface may alternatively be a silicone gel pad which is cast or otherwise formed prior to adhering it to the crystal and the PMT by means of a liquid silicone, an optical grease, or the like. 
     The voltage divider  24  has a sleeve  50 , preferably made of Teflon, which circumscribes an electronics package  52  located between a socket  54  and a disk  56 . The electronics package  52  comprises electronics  58  and a potting material  59 . The potting material, like all the materials of the voltage divider  24 , is chosen to avoid out-gassing of materials that may degrade the crystal  16 . A suitable potting material is SYLGARD  170  silicone-based potting compound. 
     The electronics of the electronics package  52  are connected at one end to socket  54 , which is electrically connected with pins  60  at distal end  62  of the PMT  18 . The socket  54  is designed to mate with the pins  60 . The socket  54  may be an off-the-shelf item designed to mate with the PMT  18 , for example a socket manufactured and sold by Hamamatsu Photonics to mate with its R1288-01 phototube. 
     At the opposite end of the electronics package  52  wires  64  pass-through the disk  56  and then out of the equipment assembly  14  through hole  66  in end cap  28 . The wires are knotted between the electronics package  52  and the disk  56  for strain-relief purposes. The disk  56 , which is preferably made of Teflon, is attached to the wires  64  by applying room temperature vulcanizing silicone adhesive between the disk  56  and the knot in the wires  64 . Any strain on the wires  64  will not be transmitted to the connection between the wires  64  and the electronics in the electronics package  52 . 
     The disk  56  may be close to surface  68  of the potting material  59 . In an exemplary embodiment there is 0.125″ between the end cap  28  and the surface  68 . 
     The sleeve  50  has an inner circumferential surface  70  which mates with outer circumferential surface  72  of the socket  54 . The socket  54  is attached to the sleeve  50  by applying a room temperature vulcanizing silicone adhesive on one or both of the circumferential surfaces  70  and  72 , and pressing the socket  54  into the sleeve  50 . 
     The end cap  28  has an annular recess  76  formed therein for receiving respective ends  78  and  80  of the sleeve  50  and the support sleeve  26 . The ends  78  and  80  are adhered to the end cap  28  by use of a room temperature vulcanizing silicone adhesive. 
     The wires  64  are connected to leads  82  which pass out of the housing  12  through a hermetic conductor interface (pass-through)  84 . The leads  82  are connected to power supplies and devices for recording and/or outputting a signal. 
     Although the detector has been described above with the voltage divider inside the housing, it will be appreciated that alternatively the voltage divide may be located external to the hermetically sealed housing, with wires or other electrical connections passing through the housing, such as by use of a multiple metal to ceramic pass-throughs. 
     The support sleeve  26  provides support for the crystal/PMT subassembly  20  when the equipment assembly  14  is longitudinally loaded. Without the presence of the support sleeve  26  or some means of limiting the longitudinal loading to the crystal/PMT subassembly  20 , typical loading of the crystal/PMT subassembly  20  may cause damage to the PMT  18  under typical geophysical operating conditions. The support sleeve  26  allows adequate loading of the subassembly  20  in a longitudinal manner, while not directly pressuring outer cylindrical surface  88  of the PMT  18 . 
     A potential additional problem is “hammering” of the PMT  18  by the crystal  16 . With longitudinal shock loading, the crystal  16 , which is relatively heavy, may act as a hammer as it is pushed against the PMT  18 , which is relatively fragile. This “hammering” can cause breakage of the PMT  18 . The support sleeve  26 , and the boot  178  and the springs  140  and  152  described below, attenuate the loads on the crystal  16  and PMT  18  due to shock and/or vibration. 
     In order to optimally perform its function in limiting the longitudinal load on the crystal/PMT subassembly  20 , it is desirable that the support sleeve  26  have a coefficient of thermal expansion similar to that of the crystal/PMT subassembly  20 , particularly a coefficient of expansion substantially equal to that of the crystal  16 . It is desirable that the difference in the coefficient of thermal expansion between the support sleeve  26  and the crystal  16  be no greater than 100×10 −6  inch/inch-° C., more preferably no greater than 50×10 −6  inch/inch-° C. and still more preferably no greater than 20×10 −6  inch/inch-° C. A suitable material for the support sleeve  26  is polyetheretherketone (PEEK). PEEK has a coefficient of thermal expansion of approximately 47×10 −6  inch/inch-° C., which is close to the coefficient of thermal expansion of a thallium-activated sodium iodide crystal, which has a coefficient of thermal expansion of 60×10 −6  inch/inch-° C. 
     Because a bare glass bulb PMT is relatively fragile, the crystal  16  of the present invention is subjected to less axial load than is typical of conventional MWD devices. An exemplary embodiment of the present invention utilizing a 1″ diameter crystal is subjected to about a 100 lb longitudinal load. This is less than the 250 lb load which would be typical for prior detectors having such a crystal longitudinally loaded against an optical glass window. The use of a bonded optical interface between the crystal  16  and the PMT  18  allows the optical interface to be maintained even under this reduced axial load. 
     The support sleeve  26  also protects the bonded optical interface  42  by providing stiffness to the equipment assembly  14  against bending loads. 
     It is desirable for the support sleeve  26  to be radially compressible and expandable in order to insure a tight fit against the cylindrical surface  30  of the crystal  16 , while maintaining ease of installation of the support sleeve  26 . Having a tight fit of the support sleeve  26  against the crystal  16  keeps the reflecting layer  36  pressed against the crystal  16 , which assures good performance of the reflecting layer  36 . It is also desirable for the support sleeve  26  to be radially expandable and contractible to accommodate expansions and contractions of the crystal due to temperature changes. In a preferred embodiment, the support sleeve  26  is slotted along its longitudinal length, thereby providing a longitudinally extending gap  90 . In a exemplary embodiment, for a 1″ outer diameter crystal, the support sleeve is 8″ long, has a 1.1″ outside diameter, is 0.020″ thick, and has a 0.060″ wide gap. The tolerance for the thickness in the exemplary embodiment is ±0.003″. The thickness is selected to provide sufficient compression strength against the longitudinal loads to which the support sleeve is subjected during use, while also affording sufficient resistance to bending loads that might cause separation of the optical interface or damage to the crystal or PMT. A visual example of the support sleeve  26  with the gap  90  is illustrated in FIG.  2 . 
     It will be appreciated that other materials, for example polyamide resins sold by Dupont under the trademark VESPEL, or other thermoplastic materials, may be substituted PEEK. 
     The housing  12  includes a tubular metal casing  122  which preferably is cylindrical like the crystal  16  as in the present case. Casing  122  is closed at its rear end by a back cap  124  and at its front end by a shield cap  126  and the conductor interface  84 . The casing  122  and the back cap  124  preferably are made of stainless steel, as is conventional. The back cap  124  is joined to the rear end of the casing  122  by a vacuum type peripheral weld, such as a tungsten inert gas weld. As seen at the left in FIG. 1, cylindrical wall  128  of the casing  122  is interiorly recessed to form a welding flange  130  which defines a close fitting pocket for receipt of the back cap  124 . The back cap  124  has, opening to its outer side, an annular groove  134  spaced slightly inwardly from its circumferential edge to form a thin annular welding flange  136  and a reduced narrow thickness connecting web  138 . Welding is effected at the outer ends of the juxtaposed thin welding flanges  130  and  136 , and the reduced thickness of the connecting web  138  further reduces welding heat conduction away from the welding flanges  130  and  136  to permit formation of a desired hermetic weld. 
     The back cap  124  and the equipment assembly  14  have sandwiched therebetween, going from left to right in FIG. 1, a resilient biasing device such as a spring  140 , a thrust plate  142 , and a cushion pad  144 . The spring  140 , or other resilient biasing device, functions to axially (longitudinally) load the equipment assembly  14  and hold it in place. The spring  140  may be a stack of wave springs disposed crest to crest, or may alternatively include resilient biasing devices such as coil springs, resilient pads, and the like. 
     The thrust plate  142  functions to spread the spring force across the transverse area of the cushion pad  144  for substantially uniform application of pressure and axial loading to the equipment assembly  14 . The cushion pad  144  is made of a resilient material and preferably a silicone rubber (elastomer) to which a reflecting material such as aluminum oxide powder may be added. 
     The equipment assembly  14  is also axially loaded from the opposite end of the housing  12 . Referring to the right hand side of FIG. 1, the shield cap  126  is attached to the casing  122  by welding, in a manner similar to the welding of the back cap  124  to the casing  122 . Between the shield cap  126  and the end cap  28  are, from right to left, a tubular spring mount  150 , a resilient biasing device such as a spring  152 , a thrust plate  154 , and a cushion pad  156 . 
     The spring mount  150  is preferably made of stainless steel or aluminum, and may include a stepped mounting flange  160  upon which the spring  152  is mounted. The spring  152  may be wave springs, or may alternatively include other types of resilient biasing devices, for example a coil spring, or other resilient devices/materials. 
     The thrust plate  154  functions to spread the spring force of the spring  152  across the transverse area of the cushion pad  156  for substantially uniform application of pressure and axial loading to the equipment assembly  14 . The cushion pad  156  may be made of a similar resilient material to that of the cushion pad  144 . The spacer plate  154  has a hole  162  therein to allow passage of the wires  64  therethrough. 
     The conductor interface  84  includes a ceramic insulator  170  through which brazed leads  82  pass, and a metal ring  172 , preferably made of stainless steel, which is brazed to the ceramic insulator  170 . The metal ring  172  has a welding flange  174  which mates with welding flange  176  of the shield cap  126 . When so mated the welding flanges  174  and  176  are welded together, hermetically sealing the conductor interface  84  to the shield cap  126 . 
     The equipment assembly  14  is surrounded by a shock absorbing boot  178  which also functions to accommodate radial expansion at the crystal  16  and support sleeve  26 . The boot  178  preferably extends the length of the support sleeve and preferably grips the support sleeve  26  to aid in holding the support sleeve  26  tightly against the crystal/PMT subassembly  20 . As shown, the boot  178  is preferably cylindrical and concentric with both the crystal/PMT assembly  20  and the casing  122 . The boot  178  is made of resiliently compressible material and preferably is a silicone rubber, elastomer, or silicone elastomer, the latter preferably being a fast setting silicone elastomer. Preferably, the silicone elastomer does not include any fillers such as Al 2 O 3  powder that may degrade performance. Alternatively, the shock absorbing boot  178  may comprise any member that provides a shock absorbing function about the circumference and length of the equipment assembly  14 . The boot  178  may have a smooth inner surface  180  and outer surface  182 , or may have ribs extending axially or circumferentially on either the inner surface  180  or the outer surface  182 . In other alternative embodiments, the shock absorbing member  178  may have dimples or geometrically shaped protrusions on either the inner surface  180 , the outer surface  182 , or both. 
     As is preferred, the casing  122  and the boot  178  have interposed therebetween a boot sleeve  198  which extends longitudinally from the back cap  124  to the cushion pad  156 . The sleeve  198 , when circumscribing the boot  178  and the equipment assembly  14  in a substantially uncompressed state, has an outside diameter that exceeds the inside diameter of the tubular metal casing  122 . Therefore, to insert the sleeve  198  into the casing  122 , the sleeve  198  must be compressed, thereby causing the boot  178 , made of resilient material, to radially compress the equipment assembly  14 , which in turn radially loads the equipment assembly  14 . Preferably the sleeve  198  is metal, for example, stainless steel. Alternatively, however, the sleeve  198  may be composed of any material which has a lower coefficient of friction with the casing  122  than does the boot  178  with the casing  122 . 
     The boot sleeve  198  should be radially compressible to effectuate substantial radial compression of the boot  178  against the equipment assembly  14 . In a preferred embodiment, the sleeve  198  is slotted along its longitudinal length, thereby providing a longitudinally extending gap  199 . The longitudinally extending gap  199  may vary between a substantial width, when the boot  178  resides within the sleeve  198  without any externally applied compression, and almost no appreciable width, when the sleeve  198  and the boot  178  are under a substantial radial compressive force when inserting the sleeve  198  and boot  178  into the casing  122 . Under such compressive forces the longitudinal edges of the slotted sleeve  198  approach and may come into physical contact with one another causing the outside diameter of the sleeve  198  to be reduced. A visual example of the slotted sleeve  198  and the gap  199  is illustrated in FIG.  3 . 
     The boot sleeve  198  provides for uniform and controlled radial loading of the equipment assembly  14 , and especially of the crystal  16 . The thickness of the boot sleeve  198  along its axial length may be controlled with tight tolerances, thereby providing for uniform radial loading along the crystal&#39;s entire length. To increase or decrease the amount of radial loading, the thickness of the boot sleeve  198  may be varied, wherein a thicker sleeve increases the radial loading on the equipment assembly  14  and vice-versa. Since the thickness of the boot sleeve  198  may be tightly controlled, so too can the radial loading on the equipment assembly  14 , and thus the stiffness of the crystal  16  which forms a part of the equipment assembly  14 . 
     The boot sleeve  198  also facilitates assembly of a subassembly including the equipment assembly  14  and the boot  178 , into the casing  122 . During insertion of the subassembly into the casing  122 , the boot sleeve  198  provides a coefficient of friction between the boot sleeve  198  and the metal casing  122  which is substantially less than the coefficient of friction between the boot  178  and the casing  122 . 
     FIG. 4 is an exploded perspective view illustrating a manner in which the detector  10  may be assembled. Initially the parts are cleaned, cycled thermally, and dried. After appropriately wrapping the crystal  16  with the reflecting layer  36 , and wrapping the PMT  18  with the foil  40 , the crystal/PMT subassembly  20  is formed by bonding the scintillation crystal  16  and the PMT  18  by means of the bonded optical interface  42 , as described above. The voltage divider  24  is then coupled to the PMT  18 , and the crystal/PMT subassembly  20  and the voltage divider  24  are inserted into the support sleeve  26 , the support sleeve being radially expanded to facilitate insertion. The voltage divider  24  and the support sleeve  26  are attached to the end cap  28  with the wires  64  protruding out the hole  66 . The support sleeve  26  is then installed over the crystal/PMT subassembly  20 , and the voltage divider  24 , with both the support sleeve  26  and the sleeve  50  of the voltage divider being attached to the end cap  28 . 
     The equipment assembly is then inserted into the boot  178 . Oxide powder may be used to dust the inside of the boot  178  to facilitate insertion of the equipment assembly. The boot  178  is then inserted in the sleeve  198  to form a equipment-boot-sleeve subassembly. The casing  122  is prepared for insertion of the subassembly by welding the back cap on to the cylindrical wall, and inserting the spring  140 , the thrust plate  142 , and the cushion pad  144  into the housing  12 . At this point, the outside diameter of the boot sleeve  198 , with the boot  178  in an uncompressed state, will be greater than the inside diameter of the metal casing  122 . Therefore, to insert the boot sleeve  198  into the casing  122 , a radial compression force is applied to the boot sleeve  198  at an end first to be inserted into the casing  122  to compress the boot sleeve  198  sufficiently to enable insertion of the subassembly into the casing  122 . 
     After the equipment assembly  14 , boot  178 , and boot sleeve  198  are inserted into the metal casing  122 , the silicone pad  156 , the thrust plate  154 , and the spring  152 , are inserted into the metal casing  122 . Thereafter the spring mount  150  and the shield cap  126  are inserted against spring pressure and the shield cap  126  is welded to the metal casing  122 , with the wires  64  protruding from the housing  12  through the space where the conductor interface  84  will be inserted. The wires  64  are then connected to the inner ends of the leads  82 , and the conductor interface  84  is mated with the shield cap  126 , the shield cap  126  then being welded to the body  172  of the conductor interface  84  to complete the assembly process. 
     Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described integers (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such integers are intended to correspond, unless otherwise indicated, to any integer which performs the specified function of the described integer (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.