Abstract:
Systems and devices are provided that relate to a gas-filled radiation detector with an internal optical fiber. The internal optical fiber may detect photons emitted during ionization avalanche events triggered by incident radiation. Such a radiation detector may include a housing, a fill gas within the housing, and an optical fiber within the housing. The fill gas may interact with radiation through an ionization avalanche that produces light. The optical fiber within the housing may capture the light and transmit the light out of the housing.

Description:
BACKGROUND 
     This disclosure relates generally to downhole radiation detectors and, more particularly, to gas-filled tube radiation detectors that collect a light signal using an internal optical fiber. 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, 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. 
     Many different downhole tools are used to determine the properties of a geological formation surrounding a well. Some of these downhole tools detect radiation from the formation—either naturally occurring or emitted from a radiation source in the downhole tool—to ascertain the properties of the formation. The radiation detectors of these tools generally take one of two forms: a scintillation detector or a gas-filled tube radiation detector. 
     A scintillation detector detects radiation by converting the energy of the radiation into light. Specifically, scintillating crystals in the scintillation detector may generate light when incident radiation strikes the crystals. A photomultiplier tube (PMT) may amplify the light into an electrical signal that can be interpreted by electronic components. Although scintillation detectors are effective, they may be expensive to manufacturer. 
     Gas-filled tube radiation detectors, also commonly referred to as Geiger-Müller tubes, may present a lower-cost option. A gas-filled tube radiation detector may detect radiation using a tube filled with an ionizing gas in an electric field. When incident radiation enters the gas-filled tube, an ionization avalanche may occur that causes electrons to rapidly move toward an anode in the gas-filled tube. These electrons produce an electrical signal on the anode that can be detected by electronics. Although gas-filled tube radiation detectors may be less costly, these detectors may also be less sensitive than scintillation detectors. Moreover, both gas-filled tube radiation detectors and scintillation detectors may use electronics located near the detectors to collect the electrical signals. Electronics that are used near the detectors may be subject to harsh downhole environmental conditions. 
     SUMMARY 
     A summary of certain embodiments disclosed herein is set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of aspects that may not be set forth below. 
     Systems and devices are provided relating to a gas-filled radiation detector that uses an optical fiber to detect a light signal. In one example, a radiation detector includes a housing, a fill gas within the housing, and an optical fiber within the housing. The fill gas may interact with radiation through an ionization avalanche that produces light. The optical fiber within the housing may capture the light and transmit the light out of the housing where the light may be detected. 
     In another example, a downhole radiation detection system that detects radiation in a borehole of a geological formation may include a gas tube radiation detector and a signal detection component. The gas tube radiation detector may generate photons when struck by incident radiation. The photons may be captured and transmitted out of the gas tube radiation detector as an optical signal by an optical fiber inside the gas tube radiation detector. The signal detection component may detect the optical signal from the optical fiber. 
     In another example, a drilling system includes a section of drill string with a mandrel and an annular gas tube radiation detector. The annular gas tube radiation detector may be disposed inside or outside of the mandrel and may include an optical fiber doped with a conductive material to serve as an anode. The optical fiber may also capture light produced by ionization avalanches in the gas tube radiation detector. 
     Various refinements of the features noted above may exist in relation to various aspects of the present disclosure. Further features may also be incorporated in these various aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to one or more of the illustrated embodiments may be incorporated into any of the above-described aspects of the present disclosure alone or in any combination. The brief summary presented above is intended only to familiarize the reader with certain aspects and contexts of embodiments of the present disclosure without limitation to the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which: 
         FIG. 1  is a block diagram of a drilling system with a downhole tool employing a radiation detector with an optical fiber to collect a light signal, in accordance with an embodiment; 
         FIG. 2  is a block diagram of such a gas-filled tube radiation detector with an optical fiber to collect a light signal, in accordance with an embodiment; 
         FIG. 3  is a cross-sectional view of the radiation detector of  FIG. 2  at cut lines  3 - 3 ; 
         FIG. 4  is a cross-sectional view of an example of the optical fiber used to collect the light signal, in accordance with an embodiment; 
         FIG. 5  is a cross-sectional view of the optical fiber of  FIG. 4  at cut lines  5 - 5 ; 
         FIG. 6  is a schematic diagram of a radiation detector with an axially woven optical fiber to collect a light signal, in accordance with an embodiment; 
         FIG. 7  is a cross-sectional view of the radiation detector of  FIG. 6  at cut lines  7 - 7 ; 
         FIG. 8  is a schematic diagram of a gas-filled tube radiation detector using a first segment of an optical fiber that is axially woven to collect light and a second segment of the optical fiber to serve as an anode, in accordance with an embodiment; 
         FIG. 9  is a cross-sectional view of the radiation detector of  FIG. 8  at cut lines  9 - 9 ; 
         FIG. 10  is a schematic diagram of a section of drill string with an annular gas-filled radiation detector disposed outside of a mandrel of the drill string, in accordance with an embodiment; 
         FIG. 11  is a cross-sectional view of the section of drill string of  FIG. 10  at cut lines  11 - 11 ; 
         FIG. 12  is a schematic diagram of a section of drill string with an annular gas-filled radiation detector disposed within an inner mandrel of the drill string, in accordance with an embodiment; 
         FIG. 13  is a cross-sectional view of the section of the drill string shown in  FIG. 12  at cut lines  13 - 13 ; and 
         FIG. 14  is a schematic diagram of a gas-filled radiation detector with pressure compensation, in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific embodiments of the present disclosure will be described below. These described embodiments are only examples of the presently disclosed techniques. Additionally, in an effort to provide a concise description of these embodiments, all features of an actual implementation may not be 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. 
     When introducing elements of various embodiments of the present disclosure, the articles “a,” “an,” and “the” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Additionally, it should be understood that references to “one embodiment” or “an embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
     This disclosure relates to a gas-filled radiation detector that can be deployed in a downhole tool. As mentioned above, a gas-filled radiation detector is filled with a gas that generates ionization avalanches when struck by incident radiation. These ionization avalanches produce not only electrons detectable as an electrical signal, but also photons. The photons from the ionization avalanches may represent light—whether in the visible spectrum or outside the visible spectrum—that may be collected by an optical fiber or a bundle of optical fibers within the gas-filled tube and output as an optical signal. The optical fiber may carry the signal to a signal detection component to convert the optical signal into an electrical and/or digital signal. The efficiency of the optical fiber may permit a vast separation, if desired, between the gas-filled tube radiation detector and the signal detection component. 
     The optical fiber or bundle of fibers may be doped to exhibit a variety of properties. For instance, the optical fiber or fibers may be doped with a conductive material, thereby allowing the optical fiber or fibers to replace a separate anode wire in the gas-filled tube radiation detector. Additionally or alternatively, the optical fiber or fibers may be doped with a material that permits optically simulated luminescence (OSL). The OSL-enabling material may cause the optical fiber or fibers to store light or radiation energy until light of a first wavelength pumped into the optical fiber or fibers releases the energy in the form of light of a second wavelength. Additionally or alternatively, the optical fiber or fibers may be doped with an amplifying material that may amplify the optical signal detected by the optical fiber or fibers. 
     The gas-filled radiation detector of this disclosure may appear in downhole tools conveyed in any suitable manner. For example, the gas-filled radiation detector may be used in downhole tools conveyed by wireline or coiled tubing. In other examples, the gas-filled radiation detector may appear in logging while drilling (LWD) or measurement while drilling (MWD) tools in a drill string used to drill a well. For instance, a drilling system  10  may convey a downhole tool that uses the gas-filled tube radiation detector, as shown in  FIG. 1 . The drilling system  10  of  FIG. 1  includes a drill string  12  used to drill a borehole  14  into a rock formation  16 . A drill collar  18  of the drill string  12  encloses the various components of the drill string  12 . Drilling fluid  20  from a reservoir  22  at the surface  24  may be driven into the drill string  12  by a pump  26 . The hydraulic power of the drilling fluid  20  causes a drill bit  28  to rotate, cutting into the rock formation  16 . The cuttings from the rock formation  16  and the returning drilling fluid  20  exit the drill string  12  through a space  30 . The drilling fluid  20  thereafter may be recycled and pumped, once again, into the drill string  12 . 
     A variety of information relating to the rock formation  16  and/or the state of drilling of the borehole  14  may be gathered while the drill string  12  drills the borehole  14 . For instance, a measurement-while-drilling (MWD) tool  32  may measure certain drilling parameters, such as the temperature, pressure, orientation of the drilling tool, and so forth. Likewise, a logging-while-drilling (LWD) tool  34  may measure the physical properties of the rock formation  16 , such as density, porosity, resistivity, and so forth. These tools and others may rely on electrical power for their operation. As such, a turbine generator  36  may generate electrical power from the hydraulic power of the drilling fluid  20 . 
     The MWD tool  32  and/or the LWD tool  34  may employ a radiation detection system  40  as shown in  FIG. 2 . In other embodiments, however, the radiation detection system  40  of  FIG. 2  and other drawings discussed below may be deployed in a downhole tool conveyed by wireline, coiled tubing, or any other suitable means of conveyance. The radiation detection system  40  of  FIG. 2  includes a gas-filled tube radiation detector  42  and a signal detection component  44 . The radiation detector  42  may be very close to the signal detection component  44  (e.g., a few centimeters) or relatively remote (e.g., a few kilometers). Indeed, in some embodiments, the signal detection component  44  may be located at the surface rather than downhole, sparing the signal detection component  44  from the harsh downhole environment. In  FIG. 2  and others that follow, the radiation detector  42  may be described in relation to a longitudinal z-axis, a radial r-axis, and a circumferential c-axis. The coordinate system shown in  FIG. 2  will be used throughout the various drawings discussed below to represent the spatial relationship between various system components. 
     A cylindrical housing  46  of the radiation detector  42  may enclose an ionizing fill gas  48 . As will be discussed below, the fill gas  48  may be selected to detect ionizing radiation (e.g., gamma-rays and x-rays) or non-ionizing radiation (e.g., neutrons). The housing  46  may be formed using stainless steel. In some examples, the thickness of the housing  46  may be approximately 2 to 3 millimeters, depending on the diameter of the radiation detector  42 . When the fill gas  48  is maintained at a high enough pressure to withstand the pressure of the environment, the housing  46  of the radiation detector  42  may include a beryllium window that may be permeably to low-energy gamma-rays. 
     An optical fiber  50  may be centered in the cylindrical radiation detector  42  and positioned generally axially along the z-axis. The optical fiber  50  may connect through an optical coupling  52  to a high voltage (HV) source  54 . In the example of  FIG. 2 , the optical fiber  50  may be doped with a conductive material, allowing the optical fiber  50  to carry a charge. The HV source  54  may supply a voltage potential of between approximately 500V to 2000V, allowing the optical fiber  50  to serve as an anode in the radiation detector  42  when the housing  46  is grounded. This causes an electric field to form between the optical fiber  50  and the housing  46 . 
     The resulting electric field may cause the fill gas  48  to react to incident radiation. When radiation  56  (e.g., a gamma-ray) passes through the housing  46  and into the fill gas  48 , an ionization avalanche event  58  may occur. The ionization avalanche event  58  may begin when the radiation  56  strikes a molecule of the fill gas  48 , causing the molecule to eject an electron and become an ion. The electron accelerates toward the positively charged optical fiber  50 , while the ion approaches the grounded housing  46 . When the accelerated electron strikes another molecule of the fill gas  48 , another electron may be released and another ion may be generated, and this process may continue as more and more of the fill gas  48  becomes involved. The ionization avalanche event  58  may not only release electrons, however, but also may release photons  59 . The photons  59  represent discrete components of light, and may be in the visible spectrum or outside the visible spectrum. Indeed, the wavelength of the photons  59  may depend on the makeup of the fill gas  48 , but the photons  59  may have sufficient energy in some embodiments to cause further ionization. That is, in some cases, at least some of the photons  59  released during the ionization avalanche event  58  may also produce more ions, continuing the ionization avalanche event  58  until a substantial amount of the fill gas  48  has become ionized. After one ionization avalanche event  58  occurs, there may be a brief period of time while the ions regain electrons, becoming non-ionized, before another ionization avalanche event  58  can happen. 
     The photons  59  may do more than just sustain the ionization avalanche event  58 . Indeed, the photons  59  may be used to detect when the ionization avalanche events  58  occur in lieu of a conventional electrical signal due to electrons on an anode. In some embodiments, the photons  59  may enter the optical fiber  50  and pass through an optical coupling  60  to output an optical signal  62 . In other embodiments, the optical fiber  50  may be doped with a material that allows optically stimulated luminescence (OSL) and the energy of the photons  59  may be absorbed by the optical fiber  50  and released at a later time. It may be noted that the optical signal  62  may be obtained more quickly than an electrical signal that would be produced directly by the electrons in the ionization avalanche events  58 . That is, the photons  59  from the ionization avalanche events  58  may travel much more quickly to the optical fiber  50  than the electrons and, therefore, the optical signal  62  may appear more quickly. 
     To ensure the photons  59  are accumulated by the optical fiber  50 , the radiation detector  42  may include focal mirrors  64  to focus light reflecting off of the back of the cylindrical radiation detector  42  toward the optical fiber  50 . The radiation detector  42  also may include a reflective material on the interior of the housing  46 , and may be, in some examples, a series of parabolic reflectors  66 . Turning briefly to  FIG. 3 , which represents a cross-sectional view of the radiation detector  42  along cut lines  3 - 3 , the parabolic reflectors  66  can be more easily seen. In the example of  FIG. 3 , the housing  46  of the radiation detector  42  includes a corrugated surface that forms the parabolic reflectors  66 . These parabolic reflectors  66  may be micro-machined or sputtered and may include any suitable reflective material, such as Al 2 O 3 . In general, the corrugated parabolic reflectors  66  may be shaped to have a parabolic focal point on the central optical fiber  50 . In examples for which the optical fiber  50  or optical fibers  50  appear elsewhere in the radiation detector  42  (e.g., in the configurations of  FIGS. 6-9 ), the corrugated parabolic reflectors  66  may be formed so as to concentrate the light on individual strokes of different optical fibers  50  or bundles of optical fibers  50  inside the radiation detector  42 . 
     Returning to the discussion of  FIG. 2 , the optical signal  62  collected by the optical fiber  50  subsequently may be detected and/or interpreted by the signal detection component  44 . As mentioned above, the signal detection component  44  may be located relatively close to the radiation detector  42  (e.g., a few centimeters) or may be located relatively far from the radiation detector  42  (e.g., a few kilometers). The distance between the radiation detector  42  and the signal detection component  44  may vary dramatically in different embodiments because the optical fiber  50  may very efficiently carry the optical signal  62  great distances. The optical signal  62  may be converted, for example, by an avalanche photodiode (PD)  68  into an electrical signal  70 . In other examples, a photomultiplier tube (PMT) may convert the optical signal  62  into the electrical signal  70 . An electrical amplifier  72  may amplify the electrical signal  70  to enable the signal to be detected by electrical signal detection circuitry  74 . The electrical signal detection circuitry  74  may include any suitable circuitry, including analog-to-digital conversion (ADC) circuitry, processing circuitry, and so forth. The information from the detected electrical signal  70  may be stored and/or used to determine a property of the formation  16 . 
     In some embodiments, the signal detection component  44  may receive the optical signal  62  as the ionization avalanche events  58  occur. Additionally or alternatively, the signal detection component  44  may receive the optical signal  62  on demand via optically stimulated luminescence (OSL) when the optical fiber  50  is doped with an OSL-enabling material. Under such conditions, the signal detection component  44  may cause the optical fiber  50  to release the optical signal  62  in the form of light of a first wavelength when an optically stimulated luminescence (OSL) laser  76  emits light of a certain second wavelength into the optical fiber  50  (e.g., through an optical coupling  78 ). As will be discussed below, doping the optical fiber  50  with the OSL-enabling material allows the optical fiber  50  to store energy when the optical fiber  50  is struck by photons  59  having a sufficiently high energy and/or the radiation  56 . 
     Specifically, when the optical fiber  50  has been doped with an OSL-enabling material and is struck by the photons  59  or other radiation  56 , electron-hole pairs may form. The electrons may become entrapped in trapping sites, which are imperfections of the lattice of the optical fiber  50 . The electrons that have moved into these trapping sites are generally stable. When the optical fiber  50  is submitted to a stimulating wavelength by the OSL laser  76 , however, the entrapped electrons free themselves, releasing optically stimulated luminescence photons of a second wavelength. This second wavelength of light may be distinguished from the first wavelength of light emitted by the OSL laser  76 . As such, the signal detection component  44  may distinguish the optical signal  62  from light emitted by the OSL laser  76 . 
     When the radiation detection system  40  uses optically stimulated luminescence (OSL), the OSL laser  76  may be operated in a continuous or pulsed mode. When operated in a continuous mode, the simulation light from the OSL laser  76  and the luminescence light emitted by the optical fiber  50  may occur substantially simultaneously. In a pulsed mode, however, the simulation light from the optical laser  76  may be emitted in pulses (e.g., having pulse widths on the order of 100 μs and in a frequency on the order of kilohertz). As a result, the luminescence light emitted by the optical fiber  50  may occur in a corresponding pulsed fashion, having pulse widths on the order of 10 or so microseconds. The simulation delay for annealing the emission of the luminescence light may be on the order of approximately 100 seconds. Thus, the pulsed mode may enable the elimination of the bias introduced by slow phosphors that may be induced in the optical fiber  50 . The pulsed mode may also reduce the temperature dependence of the luminescence of the optical fiber  50 . As mentioned above, some embodiments of the gas-filled tube radiation detector  42  may employ optically stimulated luminescence (OSL) while others may not. When the radiation detector  42  does not employ optically stimulated luminescence (OSL) to collect the photons  59 , the optical fiber  50  may not be doped with an OSL-enabling material and the signal detection component  44  may not include the OSL laser  76  and/or the optical coupling  78 . 
     Considering now composition of the fill gas  48 , any suitable gas or mixture of gases that responds to incident radiation  56  by producing ionization avalanche events  58  and photons  59  may be used. The wavelength of the photons  59  may vary depending on the composition of the fill gas  48 . In some examples, the fill gas  48  may include xenon (Xe), argon (Ar), methane (CH 4 ), carbon dioxide (CO 2 ), carbon tetrafluoride (CF 4 ), or a combination of these. For instance, the fill gas  48  may be a mixture of 90% Ar and 10% CH 4 , a mixture of 90% Xe and 10% CH 4 , or 95% Xe and 5% CO 2 , to name just a few suitable gas mixtures. In some examples, the pressure of the fill gas  48  may be maintained at a pressure of between approximately 1 and 20 atm, and may be around 10 atm. It may be noted that the higher the pressure, the greater the likelihood of a higher detection rate. 
     Additionally or alternatively, the fill gas  48  may include gas molecules to detect non-ionizing radiation, such as neutrons, which effectively become ionizing when they interact with certain matter. Thus, the fill gas  48  may additionally include helium-3 ( 3 He) and/or boron trifluoride (BF 3 ), which may interact with neutrons to by emitting an alpha particle. The release of the alpha particle may produce an ionization avalanche event  58  within the radiation detector  42 . Including gases such as these in the fill gas  48  may thereby enable a dual detection of both gamma- or X-rays and neutrons. 
     The optical fiber  50  may be bare—that is, not coated in an opaque sheath—to more easily capture the photons  59 , as shown in  FIGS. 4 and 5 . In  FIG. 4 , the optical fiber  50  is pictured in a cross-sectional schematic manner along a z-axis through the center of the optical fiber  50 .  FIG. 5  illustrates a cross-sectional view of the optical fiber  50  at cut lines  5 - 5 . The bare optical fiber  50  includes a substantially transparent cladding  90  that surrounds a substantially transparent core  92 . The cladding  90  and the core  92  may be any suitable materials of different refractive indices to cause light propagating through the optical fiber  50  to be transmitted along the optical fiber  50  through total internal reflection. To collect the light generated by the ionization avalanche events  58 , the optical fiber  50  does not include a sheathing material that would insulate the optical fiber  50 , but would also prevent light from entering the optical fiber  50  from within the radiation detector  42 . 
     The cladding  90  or the core  92  also may be seeded and/or doped with impurities to impart specific properties to the optical fiber  50  to enhance its operation as a light collector and/or radiation detector within the radiation detector  42 . Indeed, in some examples, the cladding  90  or the core  92  may be doped with a material to enable optically stimulated luminescence (OSL), a light-amplifying material, and/or an electrically conductive material. The various impurities doped into the optical fiber  50  may be seeded into the cladding  90  or formed in the core  92 . In some examples, these impurities may represent finely ground powder (e.g., on the order of a few nanometers to a few microns). The finely ground powder may be seeded onto the cladding  90  of the optical fiber  50  through electrostatic sputtering, phase vapor deposition (PVD), or thermally sprayed onto the outer surface of the cladding  90  of the optical fiber  50 , to name a few examples. Additionally or alternatively, such impurities may be embedded in the core  92  of the optical fiber  50 . During the manufacturing process, the core  90  of the optical fiber  50  may be filled in with the finely ground powder, and extruded and drawn to a suitable dimension. 
     Optically Stimulated Luminescence (OSL) Dopants 
     As mentioned above, in some embodiments of the radiation detector  42 , the optical fiber  50  or bundles of optical fibers  50  may not only collect the photons  59  but also to store the energy of the photons  59  until released by the OSL laser  76 . In addition, in some embodiments, the optical fiber  50  or bundles of optical fibers  50  may also detect incident ionizing or non-ionizing radiation through optically simulated luminescence (OSL). Many varieties of OSL-enabling materials may be seeded or formed into the cladding  90  and/or the core  92  of the optical fiber  50  or bundles of optical fibers  50  to enable this functionality. 
     To detect ionizing radiation (e.g., the photons  59  when the photons  59  have a sufficiently high energy, gamma-rays, or x-rays), any suitable material that retains optical luminescence in harsh environments that commonly occur downhole may be used (e.g., temperatures higher than 100° C.). Such OSL-enabling materials may include MgS doped with a rare earth, BaS doped with a rare earth, SrS doped with a rare earth, SrSe doped with a rare earth, αAl 2 O 3 , Al 2 O 3 :C and quartz. The rare earth doping impurities may be, for example, Sm, Eu, or Ce. OSL-enabling materials sensitive to non-ionizing radiation such as neutrons that may be seeded or formed in the optical fiber  50  may be phosphors, BeO, CaF 2 :Mn, and CaSO 4 , and may, in some embodiments, include a converting layer of fluoride oxide (V 2 O 3 ) or lithium drifted glass. 
     Optically Amplifying Dopants 
     Even without the use of optically stimulated luminescence (OSL), the optical fiber  50  or bundles of optical fibers  50  may enhance the optical signal  62  by amplifying the light of the photons  59 . Doing so may enhance the resolution of the radiation detector  42 . As such, the optical fiber  50  may be doped with laser-active rare earth ions that apply a gain to the optical signal  62  in the optical fiber  50 . The laser-active rare earth ions absorb pump light from a pump laser, which may be used in addition to or as an alternative to the OSL laser  76 . By absorbing the pump light, the laser-active rare earth ions become excited into metastable levels, allowing light amplification via simulated emission. Doping the optical fiber  50  with these materials may provide a high gain efficiency of the photons  59  traveling through the optical fiber  50  from the ionization avalanche events  58 . 
     In Table 1 below, various laser-active ions that may be doped into the optical fiber  50 , as well as associated host materials from which the optical fiber  50  may be based, are shown in table format: 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Optically 
                   
                   
               
               
                 amplifying 
                   
                 Emission 
               
               
                 dopant 
                 Host glasses 
                 wavelengths 
               
               
                   
               
             
             
               
                 neodymium 
                 silicate and phosphate 
                 1.03-1.1 μm, 0.9-0.95 μm, 
               
               
                 (Nd 3+ ) 
                 glasses 
                 1.32-1.35 μm 
               
               
                 ytterbium 
                 silicate glass 
                 1.0-1.1 μm 
               
               
                 (Yb 3+ ) 
               
               
                 erbium (Er 3+ ) 
                 silicate and phosphate 
                 1.5-1.6 μm, 2.7 μm, 0.55 μm 
               
               
                   
                 glasses, fluoride glasses 
               
               
                 thulium 
                 silicate and germanate 
                 1.7-2.1 μm, 1.45-1.53 μm, 
               
               
                 (Tm 3+ ) 
                 glasses, fluoride glasses 
                 0.48 μm, 0.8 μm 
               
               
                 praseodymium 
                 silicate and fluoride 
                 1.3 μm, 0.635 μm, 0.6 μm, 
               
               
                 (Pr 3+ ) 
                 glasses 
                 0.52 μm, 0.49 μm 
               
               
                 holmium 
                 silicate glasses, 
                 2.1 μm, 2.9 μm 
               
               
                 (Ho 3+ ) 
                 fluorozirconate glasses 
               
               
                   
               
             
          
         
       
     
     Thus, in one example, the optical fiber  50  may be doped with erbium to amplify the optical signal  62  if desired. In other examples, the optical fiber  50  may be doped with any other suitable materials to amplify the optical signal  62 , including those listed above, either alone or in various combinations with one another. 
     Electrically Conductive Dopants 
     Other materials that may be used in the cladding  90  or the core  92  of the optical fiber  50  may include electrically conductive materials. When the optical fiber  50  has been doped with electrically conductive materials, the optical fiber  50  may operate as the anode of the radiation detector  42 , as generally discussed above with reference to  FIGS. 2 and 3 . Conductive dopants such as arsenic or gallium may provide conductivity to enable the optical fiber  50  to serve as the anode of the radiation detector  42 . 
     The various dopant materials discussed above may be used in suitable combination in the optical fiber  50  to impart the respective properties allowed by each. Still, it should be understood that the optical fiber  50  may not necessarily be doped with any of the above materials in some embodiments. For instance, the radiation detector  42  may employ a separate anode wire and the optical fiber  50  instead may represent a bundle of optical fibers  50  around the separate anode wire. 
     In fact, many different configurations of the radiation detector  42  are possible. For example, as shown in  FIG. 6 , the radiation detector  42  may include a wound optical fiber  50  or bundles of optical fibers  50  wrapped within the housing  46  of the radiation detector  42 . As illustrated in  FIG. 7 , which provides a cross-sectional view of the radiation detector  42  at cut lines  7 - 7 , the optical fiber  50  or bundles of optical fibers  50  appear outside of the central area of the radiation detector  42  where the fill gas  48  is located. The optical fiber  50  or bundles of optical fibers  50  may not be charged. Instead, a metallic anode  100  may be centered in the radiation detector  42 . By way of example, the metallic anode  100  may be tungsten or gold in some embodiments. In one example, the optical fiber  50  or bundles of optical fibers  50  may be doped with an optically amplifying material to enhance the optical signal  62 . In other examples, the optical fiber  50  or bundles of optical fibers  50  may be doped with a material that enables optically stimulated luminescence (OSL). In still other examples, the optical fiber  50  or bundles of optical fibers  50  may not be doped with either of these materials. 
     Before continuing, it should be appreciated that the cross-sectional view of  FIG. 7  illustrates the optical fiber  50  or bundles of optical fibers  50  as appearing in a few discrete locations longitudinally at certain radial distances along the radial axis r of the radiation detector  42 . It should be appreciated, however, that in an actual implementation, the optical fiber  50  or bundles of optical fibers  50  may be disposed much more closely to one another and/or may wrap many more times axially through the inner circumference of the radiation detector  42 . Indeed, an actual implementation may employ many more turns to increase the amount photons  59  that can be captured by the optical fiber  50  or bundles of optical fibers  50 . 
     Since the optical fiber  50  or bundles of optical fibers  50  may not serve as the anode to the radiation detector  42 , the high voltage power source  54  may supply a high voltage potential to the anode wire  100  through a pressure coupling  102 . The pressure coupling  102  and a corresponding pressure coupling  104  may insulate the anode wire  100  from the housing  46  of the radiation detector  42 . As in the configuration of  FIGS. 2 and 3 , above, the housing  46  is grounded, thereby creating an electric field between the anode wire  100  and the housing  46  that enables the fill gas  48  to detect radiation. 
     A support structure  106  may allow the optical fiber  50  or bundles of optical fibers  50  wind around the outer circumference of the inner chamber of the radiation detector  42  where the fill gas  48  is located. The support structure  106  may be any suitable non-conductive material, including plastic or comparable materials. Winding the optical fiber  50  or bundles of optical fibers  50  through the radiation detector  42  in this manner may increase the likelihood that the photons  59  will be detected and emitted as the optical signal  62 . The optical signal  62  may exit the radiation detector  42  through an optical coupling  108  disposed in a radially central location or, as shown in  FIG. 6 , through a radially non-central location in the radiation detector  42 . 
     Other configurations of the radiation detector  42  also may wind the optical fiber  50  or bundles of optical fibers  50  in a manner similar to that shown in  FIGS. 6 and 7 .  FIGS. 8 and 9 , for example, illustrate a configuration in which a conductively doped optical fiber  50 A or bundle of optical fibers  50 A are optically joined to a non-conductive optical fiber  50  or bundles of optical fibers  50 B thorough optical couplings  110  and  52 . With this configuration, the central conductive optical fiber  50 A or bundles of optical fibers  50 A may be doped with an electrically conductive material and may serve as the anode in the radiation detector  42 . By serving as the anode, an electrical field may form between the housing  46 , which is grounded, and the conductive optical fiber  50 A, which is supplied by a high voltage (HV) power source  54 . The electric field between the conductive optical fiber  50 A and the housing  46  allows the fill gas  48  to produce ionization avalanche events  58  (e.g., in the manner of  FIG. 2 ) when radiation enters the radiation detector  42 . The optical coupling  110  may join the non-conductive optical fiber  50 B or bundles of fibers  50 B to the conductive optical fiber  50 A, through which the optical signal  62  is transmitted out of the radiation detector  42 . In some embodiments, the optical fiber  50 B or bundles of fiber  50 B may be doped with an optically amplifying material, such as erbium, to amplify the light captured by the optical fiber  50 B or bundles of optical fibers  50 B when supplied with pump light. As in the configuration of  FIGS. 6 and 7 , the non-conductive optical fiber  50 B or bundles of optical fibers  50 B are woven back and forth across the radiation detector  42  through a support structure  106  of any suitable material. 
     A cross-sectional view of the configuration of  FIG. 8  along cut lines  9 - 9  appears in  FIG. 9 . As illustrated in  FIG. 9 , the conductively doped optical fiber  50 A or bundles of optical fibers  50 A may be central to the cylindrical radiation detector  42 . At a radial distance from the center, the non-conductive optical fibers  50 B or bundles of optical fibers  50 B wind back and forth along the axial or longitudinal (z-axis) direction. Although  FIG. 9  illustrates that the non-conductive optical fiber  50 B or bundles of optical fibers  50 B wind back and forth only a relatively limited number of times, an actual implementation may employ many more turns. This may allow the non-conductive optical fiber  50 B and/or bundles of optical fibers  50 B to capture a greater number of the photons  59 . In some examples, the housing  46  of the radiation detector  42  may include the parabolic reflectors  66  shown in  FIG. 3 . The parabolic reflectors  66  may be angled to direct light toward the non-conductive optical fiber  50 B or bundles of optical fibers  50 B and/or toward the optical fiber  50 A or bundles of optical fibers  50 A. 
     Some configurations of the radiation detector  42  may be used in a drill string (e.g., the drill string  12  of  FIG. 1 ). In one example, shown in  FIGS. 10 and 11 , a section of drill string  120  includes a mandrel  122  around which the radiation detector  42  is disposed. The interior of the mandrel  122  may include a housing  124 , an inner support material  126 , and a fluid channel  128 . The housing  124  may include any suitable material, such as stainless steel. The fluid channel  128  may allow the drilling fluid  20  to pass through the mandrel  122 . Support electronics  130  may be disposed within the inner mandrel  122  and may receive the optical signal  62  via an optical coupling  132  also may supply a high voltage signal to the optical fiber  50  or bundles of optical fibers  50 , which may be doped with conductive materials. A support structure  106  may wind the optical fiber  50  or bundles of optical fibers  50  back and forth around the annular shape of the radiation detector  42 . The outer housing  46  of the radiation detector  42  of  FIG. 10  may be grounded, thereby generating an electric field between the housing  46  and the conductively doped optical fiber  50  or bundles of optical fibers  50 . 
       FIG. 11  provides a cross-sectional view of the configuration of  FIG. 10  at cut lines  11 - 11 . As seen in  FIG. 11 , the optical fiber  50  or bundles of optical fibers  50  may be wound around through the support structure  106  of the radiation detector  42 . As can also be seen in  FIG. 11 , the radiation detector  42  has an annular form that surrounds the cylindrical shape of the mandrel  122 . Although  FIG. 11  schematically illustrates the optical fiber  50  or bundles of optical fibers  50  as having a relatively limited number of windings, an actual implementation may include many more. The relatively close spacing of the optical fiber  50  or bundles of optical fibers  50  may allow the electrical field formed between the housing and the optical fiber  50  or bundles of optical fibers  50  to remain relatively uniform throughout the annular radiation detector  42  of  FIGS. 10 and 11 . 
     In another configuration, shown in  FIGS. 12 and 13 , the radiation detector  42  may be disposed within the mandrel  122  in an inner mandrel  140 . As in the configuration of the radiation detector  42  shown in  FIGS. 10 and 11 , the radiation detector  42  of  FIGS. 12 and 13  may include support electronics  130 . The support electronics  130  may receive the optical signal  62  from the optical fiber  50  or bundles of optical fibers  50  via an optical coupling  132 . In addition, the support electronics  130  may supply a high voltage signal to the optical fiber  50  or bundles of optical fibers  50 , which may serve as the anode of the radiation detector  42 . The housing  46  of the radiation detector  42  may be grounded, thereby creating an electric field between the optical fiber  50  or bundles of optical fibers  50 . A sealing wall  142  may seal the radiation detector  42  and the electronics  130  within the inner mandrel  140 . In some examples, the supporting electronics  130  may be hermetically sealed within a sealing structure  144 . 
     Because the radiation detector  42  is disposed in the inner mandrel  140  of the mandrel  122 , the material used in the housing  124  (e.g., stainless steel) may prevent some radiation from entering the radiation detector  42 . As such, the housing of the mandrel  122  near the radiation detector  42  may include beryllium windows  146 . A cross-sectional view of the configuration of  FIG. 12  at cut lines  13 - 13  appears in  FIG. 13 . As seen in  FIG. 13 , the radiation detector  42  forms an annular shape within the inner mandrel  140  of the mandrel  122 . The optical fiber  50  or bundles of optical fibers  50  may be woven axially (e.g., along the z-axis) through the radiation detector  42  as shown. In an actual implementation, the optical fiber  50  or bundles of optical fibers  50  may be located more closely to one another and wrapped with tighter turns through the support structure  106 . The beryllium windows  146  may not fully surround the radiation detector  42 , but rather may form windows within the housing  124  of the mandrel  122 . In other embodiments, the beryllium windows  146  may be smaller or larger in relation to the radiation detector  42 . 
     Finally, it may be appreciated that any of the configurations of the radiation detector  42  discussed above may benefit from maintaining the fill gas  48  at a constant pressure even as the radiation detector  42  is lowered into a borehole  14 . In an example shown in  FIG. 14 , an optical fiber  50  or bundles of optical fibers  50  may appear along an anode wire  100  of tungsten or gold. A ceramic electrical insulator  150  may electrically insulate the anode wire  100 . In some embodiments, the ceramic electrical insulator may also serve as an optical coupling to enable the optical fiber  50  or bundles of optical fibers  50  to output a signal from within the radiation detector  42 , or separate optical couplings may do so. A torsion spring  152  may account for movement by the radiation detector  42  under the harsh conditions that may occur when conveyed into the borehole  14 , preventing the optical fiber(s)  50  and the anode wire  100  from breaking from excessive tension. 
     A pressure compensation chamber  154  may allow the fill gas  48  to pass through an aperture  156  into the radiation detector  42 . The fill gas  48  may be forced through the aperture  156  by a moving piston  158  supplied with energy by a spring or an actuator. In some embodiments, the moving piston  158  may be controlled based on the temperature of the environment into which the radiation detector  42  has been deployed. Thus, a temperature sensor  160  may detect the temperature near the radiation detector  42 , and the moving piston  158  may increase or decrease the amount of force being applied to the fill gas  48  in the pressure compensation chamber  154  based on this temperature measurement. Although the pressure compensation chamber  154  is shown to be adjacent to the radiation detector  42 , the pressure compensation chamber  154  may be quite remote. For example, in some embodiments, the pressure compensation chamber  154  may be located in a high-pressure vessel farther away. 
     The specific embodiments described above have been shown by way of example, and it should be understood that these embodiments may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the particular forms disclosed, but rather to cover all modifications, equivalents, and alternatives falling within the spirit and scope of this disclosure.