Patent Publication Number: US-9835736-B2

Title: Thermally-protected scintillation detector

Description:
RELATED APPLICATIONS 
     This application is a continuation of co-pending U.S. patent application Ser. No. 12/573,194, filed Oct. 5, 2009, which claims benefit of U.S. Provisional Application No. 61/104,115, filed on Oct. 9, 2008; U.S. Provisional Application No. 61/160,734, filed on Mar. 17, 2009; and U.S. Provisional Application No. 61/160,746, filed Mar. 17, 2009. Each of the aforementioned related patent applications is herein incorporated by reference. 
    
    
     BACKGROUND 
     The present disclosure relates generally to scintillation detectors and, more particularly, to thermal protection for scintillation detectors. 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     Scintillation detectors are employed in a variety of settings to detect nuclear and electromagnetic radiation. In the presence of such radiation, a scintillator crystal may produce detectable wavelengths of light. This light may be converted to an electrical signal by a light detection device, such as a photomultiplier tube, and the electrical signal may be subsequently analyzed to determine, for example, an amount of detected radiation. By way of example, scintillation detectors may assist in the indirect determination of formation lithology by detecting gamma-ray scattering in a subterranean formation, as well as the direct determination of the formation lithology by detecting neutron-induced gamma-rays caused by neutrons emitted into the subterranean formation. 
     When scintillation detectors are employed for downhole well-logging, the scintillator crystals of such scintillation detectors may be subjected to a rapid increase or decrease in temperature due to heat from the surrounding formation. Certain scintillation detectors, such as NaI(Tl) detectors, may operate correctly at temperatures up to 200° C. without any protection. Many new scintillation materials, such as LaBr 3 :Ce and LaCl 3 :C, among others, may function at temperatures even beyond 200° C. Many of the new scintillation materials, however, while capable of operating at a very high temperature, may tend to crack or shatter if heated or cooled too rapidly. 
     SUMMARY 
     Certain aspects commensurate in scope with the originally claimed embodiments are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the embodiments might take and that these aspects are not intended to limit the scope of the presently disclosed subject matter. Indeed, the embodiments may encompass a variety of aspects that may not be set forth below. 
     Embodiments of the present disclosure relate to systems, methods, and devices for thermally protecting a scintillator crystal of a scintillation detector. In one example, a thermally-protected scintillator may include a scintillator crystal and a thermal protection element, which may partially surround the scintillator crystal. The thermal protection element may be configured to prevent the scintillator crystal from experiencing a rate of change in temperature sufficient to cause cracking and/or non-uniform light output, or a combination thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Advantages of the presently disclosed subject matter may become apparent upon reading the following detailed description and upon reference to the drawings in which: 
         FIG. 1  is a schematic block diagram of a well logging system employing a thermally protected scintillation detector, in accordance with an embodiment; 
         FIG. 2  is a schematic cross-sectional view of a thermally-protected scintillation detector employing a hygroscopic scintillator crystal, in accordance with an embodiment; 
         FIG. 3  is a schematic cross-sectional view of a thermally-protected scintillation detector employing a non-hygroscopic scintillator crystal, in accordance with an embodiment; 
         FIG. 4  is a schematic cross-sectional view of a thermally-protected scintillation detector in which a shock absorption layer and thermally conductive layer are combined, in accordance with an embodiment; 
         FIG. 5  is a schematic cross-sectional view of a thermally-protected scintillation detector having a thermal protection element extending over a photomultiplier tube, in accordance with an embodiment; 
         FIG. 6  is a schematic cross-sectional view of a thermally-protected scintillation detector having a thermal protection element partially extending over a photomultiplier tube, in accordance with an embodiment; 
         FIG. 7  is a schematic cross-sectional view of a thermally-protected scintillation detector employing a partially-open Dewar flask integrated with a housing of a scintillator crystal, in accordance with an embodiment; 
         FIG. 8  is a schematic cross-sectional view of a thermally-protected scintillation detector employing an extended partially-open Dewar flask with magnetic shielding, in accordance with an embodiment; 
         FIG. 9  is a schematic cross-sectional view of a thermally-protected scintillation detector employing an extended partially-open Dewar flask integrated with a housing of a scintillator crystal, in accordance with an embodiment; 
         FIG. 10  is a perspective view of a high temperature heater layer integrated into a thermally-protected scintillation detector, in accordance with an embodiment; 
         FIG. 11  is a schematic cross-sectional view of a scintillation detector employing the heater of  FIG. 10 , in accordance with an embodiment; 
         FIG. 12  is a flowchart describing an embodiment of a method for performing a warm-up procedure using the thermally-protected scintillation detector of  FIG. 11 ; and 
         FIG. 13  is a flow chart describing an embodiment of a method for performing a cool-down procedure using the thermally-protected scintillation detector of  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific embodiments of the present disclosure will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     Present embodiments relate generally to scintillation detectors for use in high-temperature environments (e.g., approximately 200° C. and above), which may include subterranean wells. Specifically, because entering and exiting such high-temperature environments may cause scintillator crystals capable of high-temperature operation to rapidly heat up and rapidly cool down, unprotected scintillator crystals may crack or break due to temperature stresses. Accordingly, various passive and active thermal-protection schemes for scintillator crystals are provided below. These thermal protection schemes may be simpler, smaller, and less expensive than traditional complete Dewar flasks, which may use a complex and mechanically fragile design and which typically may include a large thermal mass with a long stopper to allow extended use of the scintillation detector before exceeding its design temperature. 
     As described in the present disclosure, one or more thermal protection elements integrated into the package of the scintillator may reduce the rate of temperature change experienced by the scintillation detector, which may limit internal thermal stresses and may reduce the likelihood of scintillator damage caused by such stresses. Additionally or alternatively, embodiments of scintillator packages incorporating these thermal protection elements may provide an even temperature distribution in the scintillator crystal. This even temperature distribution may improve the spectroscopy performance of the scintillator crystal, as the light output of the scintillator crystal may be a function of the local temperature of the scintillator material. If the light output of the scintillator crystal is not uniform throughout, the energy resolution and associated spectroscopy performance may be impaired. In the following disclosure, the thermal protection elements may both reduce thermal stress and provide greater uniformity of light output by reducing thermal gradients in the scintillator crystal material. 
     In one example of a passive thermal protection element incorporated into a scintillator package, one or more thermal insulation layers may partially surround the scintillator crystal. These thermal insulation layers may reduce the rate of heat transfer to and from the scintillator crystal. Similarly, one or more thermally conductive layers may partially surround the scintillator crystal to provide for more even heating and cooling of the scintillator crystal. In another example, a partially-open Dewar flask may partially surround the scintillator crystal. These thermal protection elements may be extended beyond the scintillator component to partially surround a photomultiplier tube (PMT) component of the scintillation detector, which may further enhance the thermal protection of the scintillator crystal of the scintillation detector. In addition, certain embodiments of the partially-open Dewar flask may be modified to include magnetic shielding as well. 
     As noted above, a thermal protection element incorporated into a scintillator package may also actively prevent temperature stresses from harming the scintillator crystal. The active measures may include, for example, partially surrounding the scintillator crystal with a heating device. Prior to placing the scintillation detector into a high-temperature environment, the heating device may heat the scintillator crystal such that the rate of temperature change does not exceed a threshold amount (e.g., 2° C. per minute). Similarly, when the scintillation detector is removed from the high-temperature environment, the heating device may occasionally heat the scintillator crystal as the crystal cools to prevent the temperature from changing at a rate that would exceed the threshold amount. In this way, temperature stresses from rapid temperature change may be actively averted. These active measures may be combined with the passive measures discussed above. 
     With the foregoing in mind,  FIG. 1  represents an embodiment of a well-logging system  10  employing a thermally-protected scintillation detector. As illustrated in  FIG. 1 , the well-logging system  10  may include a downhole tool  12  and data processing circuitry  14 . By way of example, the downhole tool  12  may be a slickline or wireline tool for logging an existing well, or may be installed in a borehole assembly for logging while drilling (LWD). The data processing system  14  may be incorporated into the downhole tool  12  or may be at a remote location. The downhole tool  12  may include an external housing  16  that includes a variety of well-logging components. 
     In some embodiments, a radiation source  18  may be employed to emit radiation into a surrounding formation, which may emit such radiation as neutrons, gamma rays, and/or other particles or electromagnetic radiation. By way of example, the radiation source  18  may be an electronic neutron source, such as a Minitron™ by Schlumberger Technology Corporation by Schlumberger Technology Corporation. In other embodiments, the downhole tool  12  may not include the radiation source  18 . 
     If a radiation source  18  is employed, a shield  20  may prevent errant radiation from traveling directly to a thermally-protected scintillation detector  22 . The scintillation detector  22  may include a scintillator component  24  and a photomultiplier tube component  26 . A scintillator crystal in the scintillator component  24  may generate light in the presence of certain radiation (e.g., x-rays or gamma-rays), which may occur spontaneously in the surrounding formation or when radiation from the radiation source  18  interacts with the surrounding formation. The photomultiplier tube component  26  may generate an electrical signal from the light generated by the scintillator crystal in the scintillator component  24 . As described below, a package encompassing the scintillator component  24  may incorporate passive and/or active thermal protection elements for the scintillator crystal of the scintillator component  24  that partially surround the scintillator crystal. In some embodiments, the thermal protection element and/or the package for the scintillator crystal of the scintillator component  24  may extend over all or a portion of the photomultiplier tube component  26 . 
     As discussed below, the scintillator component  24  may include a high-temperature scintillator crystal. Such a high-temperature scintillator crystal may be hygroscopic or non-hygroscopic. Examples of hygroscopic high-temperature scintillator crystals may include scintillator crystals of LaBr 3 :Ce and/or LaCl 3 :C available from Saint-Gobain, as well as scintillator crystals of mixed La-halides available from General Electric Company. Various oxide-based scintillator crystals with excellent high temperature performance may also be used, many of which are non-hygroscopic. These may include, for example, LuAP:Ce, LuYAP:Ce, YAP:Ce, LuAG:Pr, and LPS (Lutetium Pyro-Silicate, Lu 2 Si 2 O 7 ), to name a few. 
     The high-temperature scintillator crystals mentioned above may be capable of operating at temperatures much greater than 200° C. However, these scintillator crystals may crack or break if subjected to rapid temperature changes. Thus, as described in greater detail below, the thermally-protected scintillation detectors  24  may include passive and/or active measures to prevent such rapid temperature changes. The passive measures may include thermal insulation or a partially-open Dewar flask to reduce heat transfer from the surrounding formation into the scintillator crystal. The active measures may include controlling the rate of temperature increase of the scintillator crystal by heating the scintillator crystal before and after the scintillator detector enters a high-temperature environment. 
     Signals from the thermally-protected scintillation detectors  22  may be transmitted to the data processing system  14  as data  28 . The data processing system  14  may include a general-purpose computer, such as a personal computer, configured to run a variety of software, including software implementing a technique for determining formation properties based on radiation detected by the scintillation detectors  22 . In some embodiments, the data processing system  14  may be an embedded processor in the downhole tool  12 . 
     The downhole tool  12  may transmit the data  28  to the data acquisition circuitry  30  of the data processing system  14  via, for example, a telemetry system communication downlink or a communication cable. After receiving the data  28 , the data acquisition circuitry  30  may transmit the data  28  to data processing circuitry  32 . In accordance with one or more stored routines, the data processing circuitry  32  may process the data  28  to ascertain one or more properties of a subterranean formation surrounding the downhole tool  12 , which may be indicated by a report  34 . The data processing circuitry  32  may employ any suitable technique for determining the properties of the subterranean formation. 
       FIGS. 2-9  represent embodiments of the scintillation detectors  22  incorporating passive measures to protect high-temperature scintillator crystals. Specifically,  FIGS. 2-6  represent embodiments employing thermal insulation and  FIGS. 7-9  represent embodiments employing a partially-open Dewar flask, which can also be considered thermal insulation, but due to its characteristics, may impart certain additional benefits. Turning first to  FIG. 2 , the thermally-protected scintillation detector  22  may include the scintillator component  24  and the photomultiplier component  26 . The scintillator component  24  may include a scintillator crystal  36 , a reflector layer  38 , a shock absorber layer  40 , a thermal conductor layer  42 , a thermal insulation layer  44 , and a scintillator housing  46 . In general, the various layers surrounding or partially surrounding the scintillator crystal  36  may form the “package” which encapsulates or partially encapsulates the scintillator crystal  36 . In the embodiment of  FIG. 2 , the scintillator crystal  36  may be hermetically sealed behind an optical coupling  48  and a window  50  to prevent moisture from damaging the scintillator crystal  36 . As such, the scintillator crystal  36  may be a hygroscopic or non-hygroscopic scintillator crystal capable of operating under high temperatures. Such scintillator crystals may include, for example, LaBr 3 :Ce or LaCl 3 :Ce scintillator crystals available from Saint-Gobain, scintillator crystals of mixed La-halides available from General Electric Company, and/or oxide-based scintillator crystals with excellent high temperature performance such as LuAP:Ce, LuYAP:Ce, YAP:Ce, LuAG:Pr, and LPS (Lutetium Pyro-Silicate, Lu 2 Si 2 O 7 ). 
     The reflector layer  38  may surround the scintillator crystal  36  to reflect light generated by the scintillator crystal  36  toward the photomultiplier component  26 . The reflector layer  38  may include, for example, Teflon®, Al 2 O 3  or TiO 2 , or other materials in the form of sheets, cast shapes, powders or paint. The next layer surrounding the reflective layer  38  may be the shock absorber layer  40 . The shock absorber layer  40  may be capable of contracting or expanding to accommodate differential thermal expansion and/or contraction of the scintillator crystal  36 . The shock absorber material  40  may be a solid material like a high temperature elastomer (e.g. Viton or a Silicone based Elastomer) and/or may include radial or axial springs. The elastomer may include ribs or other features to provide room for its thermal expansion or contraction while providing mechanical support to the scintillator  36 . 
     The thermal conductor layer  42  and the thermal insulation layer  44  may operate in concert to reduce the rate of temperature change in any particular location of the scintillator crystal  36 . In particular, the thermal insulation layer  44  may reduce the rate at which heat is transferred between the scintillator housing  46  and the remaining layers between the scintillator housing  46  and the thermal conductor  42 . The thermal insulation layer  44  may include, for example, various elastomers and similar materials (e.g., a viton sheet or silicone), fiberglass, an aerogel, plastics (e.g., peek), Teflon® materials such as perfluoroalkoxy polymer resin (PFA), polytetrafluoroethylene (PTFE), or fluorinated ethylene propylene (FEP), and/or a polyimide film such as Kapton®. As illustrated in  FIG. 2 , the thermal insulation layer may partially surround the scintillator crystal  36 , leaving an opening for the optical coupling  48  and window  50 . The thermal conductor layer  42  may distribute any heat transferred through the thermal insulation layer  44  evenly across the surface of the scintillator  36 . In certain embodiments, the thermal conductor layer  42  may cover greater scintillator crystal  36  surface by extending into the center of the scintillator crystal  36  via a hole drilled in the scintillator  36 . The thermal conductor layer  42  may include any thermally-conductive material, such as aluminum, copper, or stainless-steel. 
     The scintillator housing  46  may represent any standard housing for a scintillator crystal. In some embodiments, an additional thermal conductor layer may surround all or part of the scintillator housing  46  to insure heat is evenly distributed across the surface area of the scintillator housing  46 . Since the scintillator crystal  36  may be a hygroscopic scintillator crystal, the scintillator housing  46  may be constructed to seal the scintillator crystal  36  from external moisture. As such, an optical coupling  48  may join the scintillator crystal  36  to an optical window  50  attached to the scintillator housing  46 . 
     The photomultiplier component  26  may similarly include an optical coupling  48  and a window  50  to connect to the scintillator component  24 . The photomultiplier component may include a photomultiplier tube  52 , which may not necessarily include thermal protection, as many available photomultiplier tubes  52  may be capable of operating under rapidly varying temperatures. In alternative embodiments, the photomultiplier component  26  may include a micro-channel plate (MCP) in lieu of the standard multiplier structure or the photomultiplier component  26  may be an avalanche photodiode (APD). Additionally, while the photomultiplier component  26  is illustrated as optically coupled to the scintillator component  24 , which is hermetically sealed to protect the scintillator crystal  36 , it also may be possible to mount the scintillator crystal  36  directly to the photomultiplier tube  52 , if both are encased in a single hermetically sealed package. 
       FIG. 3  represents an alternative embodiment of the scintillation detector  22  illustrated in  FIG. 2 . The embodiment of the scintillation detector  22  of  FIG. 3  is substantially the same as that of  FIG. 2 , including the scintillator component  24  and the photomultiplier component  26 . The scintillator component  24  may similarly include the scintillator crystal  36 , the reflector layer  38 , the shock absorber layer  40 , the thermal conductor layer  42 , the thermal insulation layer  44 , and the scintillator housing  46 . However, the scintillation crystal  36  may be a non-hygroscopic scintillation crystal, such as an oxide-based scintillator crystal with high temperature capabilities, such as LuAP:Ce, LuYAP:Ce, YAP:Ce, LuAG:Pr, or LPS (Lutetium Pyro-Silicate, Lu 2 Si 2 O 7 ). Because the scintillator crystal  36  is non-hygroscopic, the thermal protection elements need not be sealed within in the scintillator housing  46 . As such, the optical coupling  48  and window  50  may be omitted from the scintillator component  24 . Like the embodiment discussed above, the optical coupling  48  and the optical window  50  of the photomultiplier component  26  may optically couple the scintillator crystal  36  to the photomultiplier tube  52 . 
       FIG. 4  represents another embodiment of the scintillation detector  22 , in which the shock absorbing layer  40  may be combined into the thermal insulation layer  44 . Like the embodiment of the scintillation detector  24  of  FIG. 2 , the embodiment of the scintillation detector  22  of  FIG. 4  may include the scintillator component  24  and the photomultiplier component  26 . The scintillator component  24  may similarly include the scintillator crystal  36 , the reflector layer  38 , the thermal conductor layer  42 , the thermal insulation layer  44 , and the scintillator housing  46 . Like the embodiments discussed above, the optical couplings  48  and the optical windows  50  may optically couple the scintillator crystal  36  to the photomultiplier tube  52 . However, the thermal insulation layer  44  may be designed to incorporate the shock absorbing characteristics associated with the shock absorbing layer  40  of the embodiments of  FIGS. 2 and 3  above. The material could be a high temperature elastomer (e.g. Viton or a high temperature Silicone elastomer). In some embodiments, when the reflector layer  38  employs a metallic reflector, such as silver, the reflector layer  38  may also be combined into the thermal conductor layer  42 . 
     It should be noted that, in some embodiments, the thermal conductor layer  42  may be eliminated entirely if the thermal protection provided by the thermal insulation layer  44  is sufficient. In other words, if the thermal conductivity of the scintillator crystal  36  material and the reduced rate of temperature increase provided by the thermal insulation layer  44  are sufficient to protect the scintillation crystal  36  from excessive thermal stress, the thermal conductor layer  42  may also be omitted. 
     For certain applications of the downhole tool  12 , such as traversing a zone of steam flood during well logging, the temperature of the surrounding environment may increase very rapidly. Under such extreme conditions, the thermal protection measures discussed above may not sufficiently protect all elements of the scintillation detector  22 . To protect the photomultiplier tube  52  from these high temperatures, as well as to prevent the transfer of heat through the window  50  via the photomultiplier tube  52 , the thermal protection measures described above may be adapted. In particular, these adaptations may take two forms, including mounting the entire scintillation detector  22  inside a thermally protective housing and expanding the thermal protections described above to cover all or part of the photomultiplier tube  52 . In the first case, the thermal protection measures may not be integrated into the housing of the scintillator component  24 , but the outer diameter of the scintillation detector  22  may be expanded. To maintain the same diameter, the size of the scintillator crystal  36  may be reduced. The second case, in which the thermally protective elements have been expanded to cover all or part of the photomultiplier tube  52 , is illustrated in  FIG. 5 . In general, the thermal protection may not extend over any heat-generating elements, such as resistors, diodes or active electronic components that may be mounted on the photomultiplier tube  52  to provide the correct operating voltage or to amplify the signals available at the output of the photomultiplier. In addition, extending the thermal protection beyond the photomultiplier window helps insure a more uniform temperature distribution on the photocathode and thereby a better spectroscopy performance. 
     The embodiment of the scintillation detector  22  of  FIG. 5  may include the scintillator component  24  and the photomultiplier component  26 . The scintillator component  24  may include the scintillator crystal  36 , the reflector layer  38 , the shock absorbing layer  40 , the thermal conductor layer  42 , the thermal insulation layer  44 , and the scintillator housing  46 . Like the embodiments discussed above, the optical couplings  48  and the optical windows  50  may optically couple the scintillator crystal  36  to the photomultiplier tube  52 . In the embodiment of  FIG. 5 , however, the thermal insulation layer  44  and the thermal conductor layer  42  may extend over the photomultiplier tube  52 , surrounded by an outer housing  46 . This may reduce the amount of heat that may reach the scintillator crystal  36  via the photomultiplier tube  52 . The thermal conductor layer  42  should not extend over the photomultiplier tube  52  to prevent excessive heat from reaching the scintillator crystal  36  through the thermal conductor layer  42 . 
     As shown in  FIG. 6 , the thermal insulation layer  44  may extend only as far as may optimally provide a reduction in heat transfer to the scintillation crystal  36  from an external high-temperature environment. In  FIG. 6 , the scintillator component  24  of the scintillation detector  22  may include the scintillator crystal  36 , the reflector layer  38 , the shock absorbing layer  40 , the thermal conductor layer  42 , the thermal insulation layer  44 , and the scintillator housing  46 . Like the embodiments discussed above, the optical couplings  48  and the optical windows  50  may optically couple the scintillator crystal  36  to the photomultiplier tube  52  of the photomultiplier component  26 . 
     Unlike the embodiments described above, the embodiment of the scintillation detector  22  of  FIG. 6  may include partial thermal protection over the photomultiplier tube  52 . The non-thermally protected length of the photomultiplier tube  52  is denoted as L 1 , while the thermally-protected length is denoted as L 2 . Extending the length L 2  beyond a certain distance may provide diminishing thermal protection for the scintillator crystal  36 , but may add additional manufacturing costs, weight, and size to the scintillation detector  22 . Accordingly, optimal distances L 1  and L 2  may be determined by modeling the reduction in heat transfer to the scintillator crystal  36  at various values of L 1  and/or L 2 . 
       FIGS. 7-9  represent embodiments of the thermally-protected scintillation detector  22  employing a partially-open Dewar flask to obtain thermal protection from rapid heating and cooling. In the embodiment of  FIG. 7 , the scintillation detector  22  may include the scintillator component  24  and the photomultiplier component  26 . The scintillator component  24  may include the scintillator crystal  36 , the reflector layer  38 , the shock absorber layer  40 , the thermal conductor layer  42 , and the scintillator housing  46 . Like the embodiments discussed above, the optical couplings  48  and the optical windows  50  may optically couple the scintillator crystal  36  to the photomultiplier tube  52 . The scintillator housing  46  may be constructed to form a partially-open Dewar flask that causes a vacuum  54  to separate the two housing layers  46 . Following construction of the partially-open Dewar flask, the two housing layers  46  may include a pinch-off  56  and weld  58 . It should be understood that certain details regarding the construction of the partially-open Dewar flask, such as internal supports, thermal radiation reflectors, and so forth, are not shown, as they are well known in the art. 
     Like the embodiments discussed above, certain thermally-protective elements may extend to cover all or part of the photomultiplier tube  52 , which may also serve to thermally protect the scintillator crystal  36  in a manner similar to a stopper in a traditional Dewar flask. In the embodiment illustrated in  FIG. 7 , a layer of electrical and/or thermal insulation  60  may shield a portion of the photomultiplier tube  52  beneath the outer layer of the scintillator housing  46 . The precise distance over which the layer  60  and scintillator housing  46  may extend may be determined through thermal and/or electrical modeling. The layer  60  may cover few, if any, heat-generating components of the photomultipler tube  52 , such as resistors, diodes, or active electronic components that may be mounted on the photomultiplier tube  52 . 
       FIG. 8  illustrates an alternative embodiment of the scintillation detector  22  illustrated in  FIG. 7 , in which the partially-open Dewar flask may be completely separate from the scintillator housing  46 . In the embodiment of  FIG. 8 , the scintillation detector  22  may include the scintillator component  24  and the photomultiplier component  26 . The scintillator component  24  may include the scintillator crystal  36 , the reflector layer  38 , the shock absorber layer  40 , the thermal conductor layer  42 , and the scintillator housing  46 . Like the embodiments discussed above, the optical couplings  48  and the optical windows  50  may optically couple the scintillator crystal  36  to the photomultiplier tube  52 . The partially-open Dewar flask may be formed by an inner wall  62  and an outer wall  64  joined by a weld  58 , and the space between may be evacuated to produce an insulative vacuum  54 . Like the embodiment of  FIG. 7 , a pinch-off  56  may be used in forming the partially-open Dewar flask. In some embodiments, it may not be practical or desirable to extend the partially-open Dewar flask over the photomultiplier tube  52 . Under such conditions, the inner wall  62  and outer wall  64  may be shortened to approximately the length of the partially-open Dewar flask of  FIG. 7 . The partially-open Dewar flask may cover few, if any, heat-generating components of the photomultipler tube  52 , such as resistors, diodes, or active electronic components that may be mounted on the photomultiplier tube  52 . 
     Because the partially-open Dewar flask is constructed in such a way as to overlap the front end of the photomultiplier tube  52 , thermal leakage from the photomultiplier  52  may be reduced, which may assure a more uniform scintillator crystal  36  temperature. Indeed, the photomultiplier  52  may effectively thermally protect the scintillator crystal  36  in a manner similar to a stopper in a traditional Dewar flask. Additionally, the shape of the partially-open Dewar flask may also result in a uniform temperature of a photocathode of the photomultiplier tube  52 . A non-uniform photocathode temperature may lead to a non-uniform special distribution of the quantum efficiency (QE) of the photomultiplier tube  52  and, as a consequence, may lead to poorer spectroscopy performance. 
     Using the embodiment of  FIG. 8 , the thermal protection function provided by the partially-open Dewar flask may be combined with a magnetic shielding function. Specifically, the materials of the inner wall  62  and the outer wall  64  of the partially-open Dewar flask may be chosen to magnetically shield the photomultiplier tube  52 . For example, the inner wall  62 , the outer wall  64 , or both the inner wall  62  and the outer wall  64  may be constructed of materials with high magnetic permeability. Additionally or alternatively, the inner wall  62  may have a layer of a material with a very high permeability and a relatively low saturation (e.g., Admu 80 from AD-Vance Magnetics), and the outer wall  64  may be constructed of or may have a layer of a material with a lower permeability and higher saturation (e.g., soft iron). 
       FIG. 9  illustrates another alternative embodiment of the scintillation detector  22  of  FIG. 7 , in which the partially-open Dewar flask may be formed in the scintillator housing  46 , but which may extend to cover all or part of the photomultiplier tube  52 . In the embodiment of  FIG. 9 , the scintillation detector  22  may include the scintillator component  24  and the photomultiplier component  26 . The scintillator component  24  may include the scintillator crystal  36 , the reflector layer  38 , the shock absorber layer  40 , the thermal conductor layer  42 , and the scintillator housing  46 . Like the embodiments discussed above, the optical couplings  48  and the optical windows  50  may optically couple the scintillator crystal  36  to the photomultiplier tube  52 . The two scintillator housing layers  46  may extend to cover all or part of the photomultiplier tube  52 , between which a partially-open Dewar flask may be formed. As such, the scintillation detector  22  may also include the weld  58  and the pinch-off  56 . Some embodiments may also include the layer of electrical and/or thermal insulation  60 . To effectively thermally insulate the scintillator crystal  36 , the layer  60  may cover few, if any, heat-generating components of the photomultipler tube  52 , such as resistors, diodes, or active electronic components that may be mounted on the photomultiplier tube  52 . 
     The scintillator housing  46  and optical windows  50  in the embodiments of  FIGS. 7-9  are illustrated as hermetically sealing the scintillator crystal  36 . However, if the scintillator crystal  36  is non-hygroscopic, the scintillator crystal  36  may alternatively couple directly to the window  50  of the photomultiplier tube  52 , as generally illustrated above with reference to  FIG. 3 . Moreover, even if the scintillator crystal  36  is hygroscopic, the scintillator crystal  36  may couple directly to the window  50  of the photomultiplier tube  52  if the photomultiplier tube  52  is hermetically sealed with the scintillator crystal  36 . 
       FIGS. 10-13  describe a manner of actively providing thermal protection for the scintillator crystal  36  of the thermally-protected scintillation detector  22 . Because certain scintillator crystals (e.g., LaBr 3 ) may crack or break if the rate of temperature change exceeds a threshold rate of change (e.g., 2° C. per minute),  FIG. 10  illustrates a heating device  66  that may be used to prevent the scintillator crystal  36  from heating or cooling too quickly. Specifically, the heating device  66  may preheat the scintillator crystal  36  before the scintillator crystal  36  enters a high-temperature environment to prevent excessive temperature increases, and may occasionally provide heat after the scintillator crystal  36  exits the high-temperature environment to prevent excessive temperature decreases. The heating device  66  may be, for example, a polyimide heater pad or sleeve, such as the Kapton® heater by Hi-Heat Industries. 
     As illustrated in  FIG. 10 , the heating device  66  may receive electrical power via electrical leads  68 . The electrical power may travel through a resistive path  70  to generate heat. In some embodiments, the heating device  66  may be a flexible film etched onto a metal foil, such as polyimide. Such a heating device  66  may withstand extreme temperature ranges, including high temperatures (e.g., 200° C. or greater). The heating device  66  may have rapid warm-up times and a quick response, as the resistive path  70  may run cooler. As such, the heating device  66  may thus be ideal for service in harsh environments such as subterranean formations. 
     The heating device  66  may be relatively thin, having a thickness D of approximately 0.005 inches, and may include a control circuit, as well as temperature sensors and other conventional devices for heaters. Using the temperature sensors, the control circuit may carry out such algorithms as described below with reference to  FIGS. 12 and 13  for warming and cooling a scintillator crystal  36  to prevent excessive temperature change. Additionally or alternatively, the data processing system  14  may control the heating device  66 , in which case the data processing system  14  may carry out these algorithms. It should be noted that the heating device  66  may have significant power density; in one embodiment, the heating device  66  may have a density of 5 watts per square inch, at 120V. 
     To heat the scintillator crystal  36  of the scintillation detector  22 , the heating device  66  may be mounted to a portion of the outer housing  46  or installed internally to the housing  46 . Generally, if the heating device  66  is mounted to the outer housing  46 , other thermal protection should not be employed by the scintillation detector  22 . Additionally or alternatively, the heating device  66  may be disposed internally to a scintillation detector  22  that employs passive thermal protection measures. One such configuration is illustrated in  FIG. 11 . 
       FIG. 11  represents one embodiment of the thermally-protected scintillation detector  22  that employs the heating device  66 , with certain control algorithms, to prevent rapid heating and/or cooling of the scintillator crystal  36 . Although the scintillation detector  22  of  FIG. 11  is illustrated as including passive measures (e.g., the thermal insulation layer  44  and/or the thermal conductor layer  42 ), the heating device  66  may be employed with or without such passive thermal protection. In the embodiment of  FIG. 11 , the scintillation detector  22  may include the scintillator component  24  and the photomultiplier component  26 . The scintillator component  24  may include the scintillator crystal  36 , the reflector layer  38 , the shock absorber layer  40 , the thermal conductor layer  42 , the thermal insulation layer  44 , and the scintillator housing  46 . Like the embodiments discussed above, the optical couplings  48  and the optical windows  50  may optically couple the scintillator crystal  36  to the photomultiplier tube  52 . 
     Thermally coupled to the thermal conductor layer  42 , the heating device  66  may cause heat to evenly reach the scintillator crystal  36  to prevent cracking or breaking A temperature sensor  72 , if not integrated into the heating device  66 , may measure the surface temperature of the scintillator crystal  36 . A control circuit associated with the heating device  66 , or the data processing circuitry  14 , may control when the heating device  66  is active based on temperatures detected by the temperature sensor  72 . 
       FIG. 12  is a flowchart  74  describing an embodiment of a method for performing a warm-up procedure using the heating device  66 , which may slowly heat the scintillator crystal  36  prior to its introduction to a high-temperature environment, such as a subterranean well. The method provided by flowchart  74  of  FIG. 12  may be implemented by control circuitry included in the heating device  66  or by the data processing circuitry  14 . In a first step  76 , the control circuitry may begin the warm-up procedure because the scintillation detector  22  is to be placed in a high-temperature environment. In step  78 , the heating device  66  may be activated, causing the scintillator crystal  66  to be heated. 
     As indicated by decision block  80 , if the temperature of the scintillator crystal  86  has reached a target temperature (e.g., the expected temperature of a downhole formation), the heating device  66  may be deactivated in step  82 . The scintillator detector  22  may thereafter enter the high-temperature environment without experiencing a rapid increase in the scintillator crystal  36  temperature, which may cause the scintillator crystal  36  to become damaged. If the target temperature has not been reached, the process may flow to decision block  84 . 
     In decision block  84 , the control circuitry may consider whether the temperature increase has approached and/or exceeded a threshold rate of change. The particular threshold rate of change may vary depending on the characteristics of the scintillator crystal  36 . By way of example, if the scintillator crystal  36  is formed of LaBr 3 , the designated maximum rate of temperature change may be 2° C. per minute. If the rate of temperature increase does not exceed the threshold rate of change, the heating device  66  may continue to heat the scintillator crystal  36  until the target temperature is reached, as shown in decision block  80 , or until the rate of change approaches the threshold, as shown in decision block  84 . On the other hand, if the rate of temperature change does approach the threshold rate of change, the amount of power supplied to the heating device  66  may be decreased and/or the heating device  66  may be briefly deactivated, in step  86 . When the heating device  66  becomes active again, the process may continue until the scintillator crystal  36  reaches the target temperature. 
       FIG. 13  describes an embodiment of a method for regulating a cool-down of the scintillation detector  22  using the heating device  66 , which may ensure that the scintillator crystal  36  does not too rapidly cool after exiting a high-temperature environment, such as a subterranean well. The method provided by the flowchart  88  of  FIG. 13  may be implemented by control circuitry included in the heating device  66  or by the data processing circuitry  14 . The control circuitry may continuously monitor the cool down rate in step  90  by comparing the current temperature with previous temperatures. In decision block  92 , the control circuitry may check the cool down rate to see if it exceeds a threshold rate of change. If so, the heater  66  may be briefly activated in step  94 . This activation may continue until the cool down rate falls below the threshold and the heater is deactivated in step  96 . 
     As noted above, the particular threshold rate of change may vary depending on the characteristics of the scintillator crystal  36 . By way of example, if the scintillator crystal  36  is formed of LaBr 3 , the designated maximum rate of temperature change may be 2° C. per minute for a crystal with a diameter of about 2.5 in and a length of 3 in. The maximum allowable rate of change is a function of crystal size, and may be calculated through modeling techniques. Also, the amount of heat provided by the heating device  66  may be calibrated to be sufficient to prevent the scintillator crystal  36  from cooling too rapidly, while still permitting the temperature of the scintillator crystal  36  to continue to drop. 
     While the embodiments set forth in the present disclosure may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the disclosure is not intended to be limited to the particular forms disclosed. The disclosure is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure as defined by the following appended claims.