Abstract:
The techniques and device provided herein relate to regulating a source generator in an X-ray based equipment. In particular, an X-ray system is provided that comprises an X-ray generator and a reference detector system that regulates the output of the X-ray generator. The reference detector system comprises a direct channel that allows at least a portion of the photons to directly reach the detector crystal and a plurality of fluorescent channels, such that photon flux entering the reference detector from the fluorescent channels is negligibly impacted by variations of beam spots, shapes and/or positions.

Description:
[0001]    This application claims priority to International Patent Application No. PCT/US2015/065001 filed Dec. 10, 2016, the entirety of which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The disclosure relates to an apparatus and method of electronic X-ray generator output regulation. More specifically, this disclosure relates to stabilization of the regulation process using a design where the regulation is independent of electron beam spot position on the target material where the photon emission occurs. 
         [0003]    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. 
         [0004]    Numerous well logging tools are used to identify characteristics of geological formations where wells are drilled. For example, properties such as a density and/or photoelectric factors of the formation may be measured by downhole well-logging tools. 
         [0005]    Traditionally, radioisotopic sources, such as radiocaesium (also referred to as Caesium-137 (Cs-137) have been used for density measurement in well-logging tools. Indeed, radioisotopic sources may provide a stable flux output and relatively high energy of source photons that are suitable for consistent density measurement. Unfortunately, however, the use of chemical sources in a well-logging application may be undesirable, as strict operational standards and procedures may regulate such practices. These regulations may add operational burdens to downhole development. Accordingly, new non-chemical methods that provide reliable density measurements may be desirable. 
       SUMMARY 
       [0006]    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. 
         [0007]    Embodiments of the disclosure relate to using X-ray measurements to determine density and/or photoelectric factors of a well formation, by downhole well-logging tools. More particularly, the current embodiments relate to stabilization of an electronic photon generator used in X-ray measurements by regulating the source strength based upon signals detected by a reference detector of the X-ray measurement system. 
         [0008]    Some embodiments relate to a downhole tool comprising an X-ray system and a reference detector system. The X-ray system comprises a generator that produces a source stream of electrons and accelerates them to a beam spot on a target to generate photons. The reference detector system comprises a photomultiplier tube (PMT) configured to detect and provide an amount and energy of the source stream of photons that reach it, a detector crystal configured to interact with photons and produce scintillation light before they reach the PMT, a direct channel configured to allow at least a portion of the stream of photons to directly reach the detector crystal, a filter configured to reduce a low energy part of a resultant spectrum of the PMT, and a plurality of fluorescent channels positioned substantially symmetrically, such that photon flux entering the reference detector from the fluorescent channels is negligibly impacted by variations of the beam spot. 
         [0009]    Some embodiments relate to a downhole tool comprising an X-ray system and a reference detector system. The X-ray system comprises a generator that produces a source stream of electrons and accelerates the electrons to a beam spot on a target, where they generate photons. The reference detector system comprises a photomultiplier tube (PMT) configured to detect and provide an amount and energy of the source stream of photons that reach it, a detector crystal configured to interact with photons and produce scintillation light before they reach the PMT, a direct channel configured to allow at least a portion of the stream of photons to directly reach the detector crystal, a filter configured to reduce a low energy part of a resultant spectrum of the PMT, and a fluorescence material structure positioned and angled to generate fluorescence that reaches the detector crystal via at least one fluorescent channel or aperture, such that a photon flux entering the reference detector via the fluorescent channel or aperture is negligibly impacted by variations in the position or shape of the beam spot. 
         [0010]    Some embodiments relate to a method of constructing a downhole tool with X-ray output regulation. The method comprises constructing two or more pieces of an internal shield of the downhole tool, creating a fluorescence channel in at least one of the pieces via a milling operation, and after creating the fluorescence channels, assembling the two or more pieces to form a complete internal shield. 
         [0011]    Various refinements of the features noted above may be undertaken 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 
         [0012]    Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which: 
           [0013]      FIG. 1  is a schematic diagram of a well-logging system that uses a source-regulated X-ray equipped well-logging tool, in accordance with an embodiment; 
           [0014]      FIG. 2  is a schematic diagram of a well-logging tool that may obtain logging measurements using an X-ray system that is regulated based upon filtered spectrum data, in accordance with an embodiment; 
           [0015]      FIG. 3  is a chart illustrating an example of spectrum data obtained via the well-logging tool of  FIG. 1 , in accordance with an embodiment; 
           [0016]      FIG. 4  is a chart illustrating an example of multiple sets of spectrum data for different accelerator voltages, in accordance with an embodiment; 
           [0017]      FIG. 5  is a schematic cross section diagram of a well-logging tool that may obtain logging measurements using an X-ray system that is regulated using symmetric fluorescent channels, in accordance with an embodiment; 
           [0018]      FIG. 6  is a schematic cross section diagram of a well-logging tool that may obtain logging measurements using an X-ray system that is regulated using symmetric gaps as fluorescent channels, in accordance with an embodiment; 
           [0019]      FIGS. 7A and 7B  are schematic cross section diagrams of a section of a well-logging tool that may obtain logging measurements using an X-ray system that is regulated using symmetric gaps as fluorescent channels, in accordance with an embodiment; 
           [0020]      FIG. 8  is a schematic cross section diagram of a well-logging tool that may obtain logging measurements using an X-ray system that is regulated using symmetric gaps as fluorescent channels, where the reference detector crystal is shifted from the electron beam axis, in accordance with an embodiment; 
           [0021]      FIG. 9  is a schematic cross section diagram of a well-logging tool that may obtain logging measurements using an X-ray system that is regulated using a non-symmetric design, in accordance with an embodiment; 
           [0022]      FIG. 10  is a schematic diagram illustrating angular choice resulting in a common solid angle regardless of beam spot location, in accordance with an embodiment; 
           [0023]      FIG. 11  is a schematic cross section diagram of a well-logging tool where channels are drilled into the tool, in accordance with an embodiment; 
           [0024]      FIG. 12  is a schematic cross section diagram illustrating a well-logging tool manufactured using a split part approach, in accordance with an embodiment; 
           [0025]      FIG. 13  is a schematic cross section diagram illustrating a well-logging tool where a step on the filter is used to shield leakage photons around the filter, in accordance with an embodiment; 
           [0026]      FIG. 14  is a schematic cross section diagram illustrating two halves of a reference detector shielding block of a well-logging tool, in accordance with an embodiment; 
           [0027]      FIG. 15  is schematic cross section diagram of a well-logging tool where symmetric gaps are created by inserting a centralized inner shield, in accordance with an embodiment; and 
           [0028]      FIG. 16  is a schematic cross section diagram illustrating a well-logging tool having an annular fluorescent channel, in accordance with an embodiment 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    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. 
         [0030]    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. 
         [0031]    Many downhole tools used for logging wells drilled for hydrocarbon production, CO 2  sequestration or other purposes obtain measurements of the formation surrounding the wells. As mentioned above, traditional density measurement systems for downhole tools may use highly-regulated radioisotopes for obtaining formation density measurements that may provide increased operational burdens. Accordingly, this disclosure describes systems and methods that may stabilize source X-ray source energies and output flux, such that X-ray technology may facilitate downhole density and/or photoelectric factor measurements. 
         [0032]    With this in mind,  FIG. 1  is a schematic diagram illustrating a well-logging system  00  that may obtain logging measurements, using an X-ray system  14  of  FIG. 2 , where the X-ray system  14  is regulated based upon filtered spectrum data, as described in more detail below. The well-logging system  00  may be conveyed through a geological formation  02  via a wellbore  03 . The downhole tool  01  is conveyed on a cable  04  via a logging winch system  05 . Although the logging winch system  05  is schematically shown in  FIG. 1  as a mobile logging winch system carried by a truck, the logging winch system  05  may be substantially fixed (e.g., a long-term installation that is substantially permanent or modular). Any suitable cable  04  for well logging may be used. The cable  04  may be spooled and unspooled on a drum  06  and an auxiliary power source  07  may provide energy to the logging winch system  05  and/or the downhole tool  01 . 
         [0033]    Although the downhole tool  01  is described as a wireline downhole tool, it should be appreciated that any suitable conveyance may be used. For example, the downhole tool  01  may instead be conveyed as a logging-while-drilling (LWD) tool as part of a bottom hole assembly (BHA) of a drill string, conveyed on a slickline or via coiled tubing, and so forth. For the purposes of this disclosure, the downhole tool  01  may be any suitable measurement tool that obtains multidimensional measurements through depths of the wellbore  03 . 
         [0034]    Many types of downhole tools may obtain measurements in the wellbore  03 . For each depth of the wellbore  03  that is measured, the downhole tool  01  may generate density and/or photoelectric factor measurements. 
         [0035]    The downhole tool  01  may provide such measurements  08  to a data processing system  09  via any suitable telemetry (e.g., via electrical signals pulsed through the geological formation  02  or via mud pulse telemetry). The data processing system  09  may process the measurements  08  to identify patterns in the measurements  08 . The patterns in the multidimensional measurements  08  may indicate certain properties of the wellbore  03  (e.g., porosity, permeability, relative proportions of water and hydrocarbons, and so forth) that would be otherwise indiscernible by a human operator. 
         [0036]    To this end, the data processing system  09  thus may be any electronic data processing system that can be used to carry out the systems and methods of this disclosure. For example, the data processing system  09  may include a processor  10 , which may execute instructions stored in memory  11  and/or storage  12 . As such, the memory  11  and/or the storage  12  of the data processing system  09  may be any suitable article of manufacture that can store the instructions. The memory  11  and/or the storage  12  may be ROM memory, random-access memory (RAM), flash memory, an optical storage medium, or a hard disk drive, to name a few examples. A display  13 , which may be any suitable electronic display, may provide a visualization, a well log, or other indication of properties of the wellbore  03  based on the multidimensional measurements  08 . 
         [0037]    Turning now to a detailed discussion of the well-logging tool  01 ,  FIG. 2  is a schematic diagram of a well-logging tool  01  that may obtain logging measurements. As mentioned above, measurement systems not using chemical (or radioisotopic) sources of radiation may provide certain benefits in downhole operations. Accordingly, the current well-logging tool  01  is equipped with an X-ray system  14  that operates a generator  16  (e.g., an X-ray generator) at high voltage (e.g., more than 250 keV). To avoid instability or drift of the generator in the X-ray system  14 , the X-ray system is regulated based upon filtered spectrum data, in accordance with an embodiment. 
         [0038]    As mentioned above, the X-ray generator can be used in downhole applications to obtain measurements for the formation  02 , including, but not limited to, density and photoelectric factors. To measure downhole density and/or photoelectric factors, the downhole well-logging tool  01  uses the source  16  to provide x-rays and a measurement detector to detect x-rays. Emitted photons from the generator  16  undergo interaction with formation  02  elements, causing the photons to scatter or be absorbed by the formation  02 . Some of the scattered photons return back to the detector (or multiple detectors) (not shown) mounted in the logging tool  01 . The density and photoelectric factors are derived from the photon counts and energies that are observed at the detectors. 
         [0039]    The principle of an electronic X-ray generator is based on the so-called bremsstrahlung effect. The high energy electrons traveling in the electromagnetic field emit photons when they strike a target  20 . The emitted photon spectrum is rather broad, and the energy is less than the incident electron energy. The target  20  thickness is large enough to stop the majority of electrons in the incident electron beam. 
         [0040]    Unfortunately, unlike radioisotopic sources, the output of generator source  16  may not be stable. Fluctuation of high voltage and electron beam current can change the energy spectra of emitted photons. In other words, both the energy and the intensity of source  16  photons can vary. If the source strength is not regulated, additional uncertainty is introduced in the count rates of measured photons, as the number and energy of source photons may vary over time. Indeed, the amount of photons reaching the detectors after passage through the formation  02  depends on the energy of photons emitted by source  16  into the formation as well as the flux intensity. Accordingly, as may be appreciated, the accuracy of the formation  02  density measurement can suffer significantly as the energy and intensity of the photon source varies. Therefore, the accuracy of the density measurements may be vastly improved by regulating the output of the source  16 . The required accuracy of the regulation depends on the specification of the measurement. 
         [0041]    One mechanism for regulating the X-ray generator (e.g., source  16 ) high voltage and beam current may utilize a portion of the photons that travel through the logging tool  01 . This portion of photons may be detected by a dedicated reference detector  18 . If the reference detector  18  and the path of source photons from the radiation target  20  to this detector  18  are properly shielded, the spectrum of detected photons is independent from environmental variables and can be used for generator source  16  regulation. 
         [0042]    As illustrated in  FIG. 2 , the generator source  16 , generator target  20 , and reference detector  18  are contained inside the logging tool  01 . The reference detector  18  is shielded, such that the scattered photons produced from the borehole or formation  02  cannot reach it. In the current embodiment, a special filter  22  is positioned between the target  20  and the reference detector  18 . In some embodiments, the filter  22  may be manufactured from a high density and high Z (e.g., high atomic number) material, like tungsten, lead, or any heavy material. The high density and high z material may act to attenuate photons effectively, resulting in only part of the source radiation reaching the reference detector  18 . For example, the filter  22  may attenuate the low energy part of the source photon spectrum and passes only the high energy tail to the detector  18  (e.g., through a direct channel  23 ). 
         [0043]    In addition, in the current embodiment, the logging tool  01  includes an indirect path (e.g., channel  24 ) from the target  20  to a detector crystal  26  of the reference detector  18 . The source photons hitting the wall of the channel  24  create a fluorescent emission. The fluorescent peak has relatively low energy. The detector crystal  26  may reduce sensitivity to background radiation, while allowing a photomultiplier tube (PMT)  28  to detect and provide an amount of photons that reach it. 
         [0044]    For example,  FIG. 3  is a chart illustrating an example of spectrum data  40  obtained via the PMT  28  of the well-logging tool  01  of  FIG. 1 , in accordance with an embodiment. As illustrated, the final spectrum of detected photons looks like a two peak distribution. The low energy peak  42  is created by the fluorescent photons produced via the channel  24  of  FIG. 1 . The high energy peak  44  is created by the high energy tail of the initial spectra with the low energy part removed by the filter  22 . In some embodiments, the counts provided by the spectrum data may be summed in two bins to provide two numbers W LE  and W HE , which correspond to the integration of respectively the low and high energy parts of the spectrum. 
         [0045]    The total counts of the reference detector  18  are proportional to the electron beam current if the high voltage of the generator source  16  is fixed. Accordingly, the ratio of the count rates in the low energy and high energy windows (W LE /W HE ) is sensitive to the voltage of the generator source  16  of  FIG. 2 . Thus, fixing the counts of the low energy and high energy windows of the reference detector  18  may act to stabilize the electron beam current and end-point energy of the generator source  16 . For example, in  FIG. 4 , the high energy components (W HE )  44  of the spectrum are very sensitive to the high voltage. The low energy components (W LE )  42  are proportional to the beam current, but also depend to some extent on the high voltage. In  FIG. 4 , W LE  and W HE  would be the total count rate between 0 and 120 keV and 200 and 400 keV, respectively. 
         [0046]    This scheme works well when the electron beam spot on the target  20  (i.e. the photon emission point) does not move. Unfortunately, beam spot movement may occur during operation of the downhole tool  01  and/or as a consequence of tool-to-tool variation. When the beam spot moves, the effective geometry of the shielding setup changes, which may distort the reference detector spectra. For example, the apparent high voltage and beam current derived from the reference detector  18  window counts may be different from the nominal values of high voltage and current used to stabilize the generator output. 
         [0047]    The systems and methods described herein arrange the geometry of the fluorescent channels  24  and heavy material filter  22 , such that the reference detector  18  spectrum becomes independent of beam spot position/movement. Specifically, because the count rates of low and high energy windows of the spectrum define the apparent high voltage of the generator and electron beam current, if the spectrum does not depend on the beam spot movement, the generator parameters are also stabilized to one set of nominal values. 
         [0048]    As mentioned above, if the electron beam spot position or shape changes on the radiator target  20 , the spectrum in the reference detector  18  may be affected. The beam spot defines the effective photon source geometry. Moving the photon source changes the flux of the photons entering the detector crystal  26  through the filter  22  (e.g., the high energy window counts) if it is not positioned symmetrically. Additionally, the flux of photons entering the fluorescent channel  24  (e.g., the low energy window counts) depends on the solid angle from the photon source to the channel opening. When the counts in the low and high energy windows change, the apparent values of generator parameters may vary. 
         [0049]    Accordingly, in one embodiment, a symmetric design of the reference detector block may be used to make the detector spectrum independent of beam spot movement.  FIG. 5  illustrates a downhole tool  50  that includes a reference detector  18  with a symmetric design, in accordance with an embodiment. In the embodiment of  FIG. 5 , the crystal  26  is located along the axis of the electron beam (e.g., the Z axis  52 ). Accordingly, there are two ways for photons to reach the crystal  26 . Specifically, the photons may reach the crystal  26  via the direct channel  23  and/or the fluorescent channels  24 . Photons going through the direct channel  23  pass through the filter  22 , which may be made from high density materials such as tungsten, lead, etc. The photons passing through the direct channel  23  register as a high energy peak (high energy tail) in the detector spectrum. 
         [0050]    In the current embodiment, the target  20 , the direct channel  23 , the filter  22 , and crystal  26  are aligned substantially symmetrically, such that beam spot movement on the target  20  has a negligible effect on the high energy window counts. Further, the width of the direct channel  23  can be increased to reduce the variation of high energy window counts within desired limits. For example, limits on the variation of window counts may be defined by accuracy specifications of the density measurement. These limits may be met by varying the width of the direct channel  23 . 
         [0051]    In the current embodiment, a heavy metal inner shielding  54  surrounds the detector  18  crystal  26 . This shielding  54  may prevent the photons from the wellbore  03  and/or or formation  02  from entering the reference detector  18  crystal  26 . 
         [0052]    In the current embodiment, the symmetric design includes symmetric fluorescent channels  24  as well. These symmetric fluorescent channels  24  are positioned inside the inner shielding  54 . Instead of one path to the detector  18  crystal  26 , the current embodiment illustrates two symmetric fluorescent channels  24 . The two channels  24  are substantially symmetrically located in the XZ plane (e.g., defined by the X-Axis  56  and the Y-Axis  58 ) from the electron beam axis (e.g., the Z-axis  52 ). 
         [0053]    The symmetric design of the channels  24  may minimize the variation of spectra and keep it within the predefined limits. For example, because the two channels  24  are substantially symmetrical, if the beam spot moves in the X-direction (e.g., along the X-Axis  56 ), the flux deficit in one channel is compensated by a flux increase in another channel. 
         [0054]    The shape of the channels  24  may vary and the cross section may be circular or rectangular, for example. If the size of the channel  24  cross section is large enough, the beam spot movement in the Y-direction (e.g., along the X-Axis  58 ) may not strongly affect the low energy window counts. Further, the low energy counts may be stabilized to a desirable factor by adjusting the size of the channels  24 . Accordingly, the symmetric design of the two channels  24  may help to stabilize the reference detector  18  spectrum, even when the beam spot on the target  20  moves in the XY plane. 
         [0055]    While the current embodiment illustrates two symmetric fluorescent channels  24 , other embodiments are not limited to this number of fluorescent channels. If desirable and the geometry of the downhole tool  50  allows, additional channels  24  may be added with azimuthal symmetry. Further, the cross section of each of the channels  24  may be reduced if their number is increased, maintaining the low energy count rates within predefined limits. 
         [0056]    In some embodiments, the inner surface of the fluorescent channels  24  may be coated with a different material to change the energy of the fluorescent peak. Further, in some embodiments, tubes, plates, or coatings made by different materials may be placed inside the channels to change the energy of the fluorescent peak. 
         [0057]    Turning now to an alternative embodiment,  FIG. 6  illustrates a downhole tool  70  where a narrow gap  72  is formed in the direct channel  23  around the heavy filter  22  material, instead of using the symmetric fluorescent channels  24  of  FIG. 5 . In the current embodiment, the photons entering into the gap  72  do not have direct path to the detector  18 . Accordingly, fluorescent emission occurs on the walls of the direct channel around the filter  22 . As illustrated in the cross section  80  of  FIG. 7A , the filter  22  may be mounted to the inner shielding  54 , leaving empty passages  82  for the fluorescent and scattered photons. 
         [0058]    As illustrated in  FIG. 7A , the filter  22  may be connected to the inner shielding  54  by one or more centralizing devices  82 . Alternatively, in some embodiments, the inner shielding  54  and filter  22  may be made as one piece leaving an empty area for the fluorescent gap  82  around the filter  22 . 
         [0059]    Similar to the symmetric channels of the embodiment illustrated in  FIG. 5 , in operation, when the beam is shifted to one direction perpendicular to the generator  16  axis, the fluorescent flux passing through the gap  82  does not change, because a deficit of photons on one side of the gap is compensated for by an excess of photons on the diametrically opposite side. 
         [0060]    Further, as illustrated in the embodiment of  FIG. 7B , to obtain a desired amount of fluorescent photons of a desired energy, the walls of the direct channel  52  may be coated with different materials  86  other than the materials of inner shield and the filter  22 . Alternatively, sleeves of different materials may be inserted around the inner diameter of the inner shield  54 , the outer diameter of the filter  22 , or both. 
         [0061]    In some embodiments, such as the embodiment depicted in  FIG. 8 , the gap  72  around the filter  22  may also work when the detector  18  is no longer coaxial with the electron beam axis. In the current embodiment, when the beam spot movement is small enough in comparison to diameter of the direct channel, the variation of low and high energy window counts may remain small enough to keep the window count within desirable tolerance limits. The geometry of the direct channel  23 , filter  22  dimensions, and the gap  72  size may be adjusted to keep the uncertainty of detector window counts within the required limits. Further, specific positioning of the crystal  26  may also be adjusted to maintain stable counts. 
       Angle Based Stabilization 
       [0062]    Though the above-described embodiments may help to stabilize output regardless of beam movement, these embodiments may involve complex mechanical designs and associated tight tolerances. Further, the fluorescence material may be the same or substantially the same as the one used for the high energy filter  22  (e.g., tungsten), which may result in the interference from thermal noise, because these material may have a low fluorescent peak. For example, Tungsten has a relatively low fluorescence peak of around 59 keV. In some embodiments, it may be desirable to use a material with a higher fluorescence peak to stay above the thermal noise of the reference detector (especially at high temperature). 
         [0063]      FIG. 9  illustrates an embodiment of a downhole tool  100  having a single fluorescence channel  24 . Specifically, an angle between the target  20  and a fluorescence material structure (e.g., disc  102 ) may be used to reduce the number of fluorescence channels  24  to a single fluorescence channel  24 , while maintaining spectrum stability. 
         [0064]    In the current embodiment, the fluorescence material structure (e.g., disc  102 ) may be constructed of a material with the proper fluorescence energy (e.g., Bismuth or Gold). The fluorescence disc may be mounted on a support structure  104  of the detector  18 . In some embodiments, the support structure  104  may not have any particular fluorescent line in the measurement range of energy. Accordingly, using this technique may not require tight tolerances of the fluorescence channel  24 . Instead, the mechanical tolerance may be mainly limited to the size of the fluorescent material structure, which, in one embodiment, may be a disc  102 . 
         [0065]    For example, due to the angle of the fluorescence material structure  102 , photons sourced from the target  20  that interact with the fluorescence material structure  102  may be directed through the single fluorescence pathway  24  in an optimized angle  106 . By adjusting the fluorescence according to this optimized angle  106 , the emitted fluorescence photons may reach the crystal  26 , with relatively little dependence on beam spot positioning, as will be discussed in more detail below. 
         [0066]    If desired, in some embodiments, the high-energy filter  22  (e.g., a Tungsten filter) may be surrounded with a material that will not generate fluorescence x-rays but still attenuate fluorescence coming from the filter  22 . 
         [0067]      FIG. 10  is a diagram illustrating an effect of the angular positioning of the fluorescence disc  102  and/or support structure  104 . Specifically, the angle between the disc  102  and the target  20  may be optimized in such a way that the solid angle is the same for any beam position on the target  20 . For example, beams  108  from a first beam spot  110  may result in fluorescence that follow the same optimized angle  106  as beams  112  from a second beam spot  114 . Accordingly, the flux of x-rays generated by the disc  102  fluorescence may not depend on the beam spot location on the target  20 . 
         [0068]    When the target  20  is not aligned with the crystal  26 , a proper tilt angle on the high energy filter  22  can compensate for the effect of a beam spot movement on the low-energy part of the spectrum. Accordingly, in the current embodiment, there is no need for multiple channels. Further, the disc  102  mounted on the support structure  104  may be in the plane of the measurement detectors, which may preserve room in the downhole tool  100 . The tolerance can be well controlled and inspected before installing the disc  102  with its support structure  104 . For example, the disc  102  with its support structure  104  may be exchanged if a different fluorescence energy is desired. 
       Channel Manufacturing 
       [0069]    In some embodiments, in order to correct for beam spot movement, the size (e.g., cross section) and position of the channels  24  may require a high dimensional accuracy. Channels  24  with this type of tolerance are traditionally drilled and reamed into a high density material to the correct cross-sectional size. Unfortunately, these traditional methods have resulted in the positional variations of the drilled hole, for reasons such as:
       Position error in the initial drilling location.   Angle error in the drilled hole.   Position and angle error in the additional holes.   Drill bit movement while starting to drill an angled hole.   Drift of the drilled hole (especially for small diameter holes of long length).       
 
         [0075]      FIG. 11  illustrates an embodiment of a tool  01  with traditionally drilled channels. Variations caused by traditional channel drilling may create difficulty in ensuring a positional symmetry with high accuracy. Further, the exact intersection point  114  of the two holes and the exact geometry at the intersection  114  is generally difficult to achieve when the drilling starts in two different locations. Further, it is even more difficult to repeat the same geometry from tool-to-tool. Since the fluorescence occurs at the intersection of the two drilled holes, it is important that this geometry be well-defined and not vary. Additional problems may be introduced by the fact that traditionally drilled channels are hard to inspect after they are made. 
         [0076]    As illustrated in  FIG. 11 , positional uncertainty  116  may be introduced by an angle error in a drilled hole. For example, a hole that is a standard dimension of a fluorescent channel (e.g. 2″ long) with a standard angular tolerance (e.g., an angular tolerance of +/−0.5°) may have a lateral drift of as much as +/−0.017 inch (or 0.034 inch total). This is nearly an order of magnitude larger than the positional tolerance useful to correct for beam spot movement. The effect of such channel position due to angular and position tolerance errors can be seen (not to scale) in  FIG. 11 . 
         [0077]    Another problem with drilling four different holes to create these fluorescent channels  24  is that each hole must be drilled along a different axis. In other words, a different setup of the workpiece on the drilling machine may be necessary, which may introduce additional complexity and positional errors. After the holes are drilled from the outside of the part, some of the holes may need to be plugged near the intersection point, in order to give a consistent surface for fluorescent photon emission and to prevent photon leakage to the environment outside of the tool. For example, one set of possible plug locations  120  is shown in  FIG. 11 . This plugging operation introduces an additional source of error in the manufacturing stage because the plug could be installed to the wrong depth. 
         [0078]    As noted above, drilling a series of holes in a part can present manufacturability issues. Further, inspection of all holes as well as the intersection points may be difficult because the position cannot be inspected directly and the drift of the holes can be difficult to measure. 
         [0079]    Given the high positional accuracy required for stable X-ray regulation that is independent of beam spot movement, the scrap rate of a part made with drilled fluorescent channels may be high. 
         [0080]    The techniques described herein reduce the positional error of the channels  24  by using a different method to manufacture channels along the midplane of a shielded part. As will be appreciated, the channels  24  in this method may be rectangular instead of the circular channels that are produced by traditional drilling. 
         [0081]      FIG. 12  illustrates an embodiment of a shield  140  produced as two or more separate pieces (e.g., a top piece  142  and a bottom piece  144 ), in accordance with an embodiment. The two pieces  142  and  144  are split along a plane  146  at a distance of half the cross-sectional channel height  148  from the electron beam axis  150 . The fluorescent channels  152  are milled into the larger part of the shield (in the depicted embodiment of  FIG. 12 , the bottom piece  144 ) to a depth of the full cross-sectional channel height. 
         [0082]    Using this technique, the entire fluorescent channel  152  geometry may be machined in one milling setup with a typical numerical control (NC) machining position, having tolerances on the order of one one-thousandth of an inch. Accordingly, additional compounding positional errors are not encountered with this technique. Further, since the channels  152  are machined from the top, the drift of the channel  152  along its length will be small, and it is defined by the position tolerance of the milling machine. 
         [0083]    Additionally, manufacturing the channels  152  using this technique results in well-defined intersection points of the channels  152  and the positions and sizes of all channels  152  and intersection points may be inspected easily. Channels  152  of this type may not be deep, so the channel cross-section may be machined (e.g., milled) to tight tolerances without bending a bit, which may allow for greater control of the cross-sectional size of the channel  152 . Further, since the channels  152  are only located in one of the two pieces of the part, a channel  152  that is incorrectly machined may only result in discarding one of the two pieces instead of the entire part. 
         [0084]    Further, since the channels  152  are easily accessible from the top, the channels  152  may be easily filled or coated with an alternate material to change the energy of fluorescence (as discussed above). 
         [0085]    Features in this design prevent direct leakage of photons from the source to the reference detector  18  crystal  26 , which may degrade the ability to regulate the X-ray generator  16  effectively.  FIG. 13  illustrates an embodiment of a downhole tool  170  that provides such features. First, the high energy filter  22  has a larger diameter than the reference detector  18  crystal  26 . Accordingly, photons that leak around the filter  22  or between the two pieces of the shielding do not have a direct path to the crystal  26 . Secondly, a step  172  is added to prevent scattered photons from reaching the reference crystal  26 . 
         [0086]    The embodiment shown in  FIG. 5  prevents the leakage of photons to the crystal  26  through the gap between the two pieces of the shielding. However, leakage is still possible to the fluorescent channels  24  if the surfaces of the two mating shield pieces of this part are not flat or have machining defects. In this case, fluorescence can occur not only close to the channel  24  intersection spot but also by this leakage of photons into the channels  24 , which could make a path of fluorescent photons to the crystal  26 . Such a contribution to the low energy counts/fluorescent peak is not necessarily symmetrical with this type of leakage. However, the leakage may be completely blocked when an additional step is incorporated with the two pieces. For example,  FIG. 14  illustrates a first cross sectional view  180  and second cross sectional view  182  of two halves (first half  184  and second half  186 ) of a reference detector shielding block  188 . The initial channels  24 A that go from the target  20  may be machined in one piece (or one step) (e.g., the first half  184 ) and secondary channels  24 B may be machined in the other piece (e.g., the second half  186 ). The step  190  prevents the photons from leaking through the gap between the two halves  184  and  186  of the block  188 . 
         [0087]      FIG. 15  illustrates a schematic cross-section of a tool  200  embodiment having a fluorescent gap  202 , as described above. This design may be produced by drilling the hole in the inner shield  204 , and inserting a precision machined filter  22  that is centralized in the inner shield (e.g., via mounting screws or other centralizing devices  206 ). A cross-section as viewed from the top of the tool  200  is shown in  FIG. 16 . The inner diameter of the inner shield  54  and/or the outer diameter of the filter  22  can be coated with an alternate material to change the fluorescent energy. 
         [0088]    As illustrated in  FIGS. 15 and 16 , the form factor of a tool  20  with an annular gap  202  around the filter  22  may reduce sensitivity to diameters of the parts in the assembly, which may make the part easier to manufacture and reduce tool-to-tool variability. 
         [0089]    A larger fluorescent gap that is less sensitive to geometric tolerances on the fluorescence annulus may be made, however this may increase the number of low-energy fluorescent photons that reach the reference detector  18  crystal  26 . The number of fluorescent photons may be limited by increasing the length  201  of the channel, which may also mean increasing the length  203  of the high energy filter  22 . Such a change may decrease the number of high energy photons reaching the reference detector  18  crystal  26 . Accordingly, in the current embodiment, a cup-shaped filter  22  is used. This embodiment decouples the length  203  of the high energy filter  22  from the length  201  of the fluorescence annular channel  202 , while allowing for greater variance on the tolerance of the inner diameter of the inner shield  54 , the outer diameter of the filter  22 , as well as the concentric position tolerance of the filter  22  within the inner shield  54 . 
         [0090]    Each part shown in  FIG. 16  is cylindrical and co-axial, however other shapes may be used. The filter  22  and the annular fluorescent channels  24  may be symmetric about an imaginary line between the electron beam spot position at the target  20  (source of photons) and the center of the reference crystal  26 . However, it is not necessary for the target  20  and the crystal  26  to be coaxial. This annular design is more compact and is useful for smaller tool  01  diameters than the symmetric channel design mentioned previously. 
         [0091]    By regulating X-ray generation in logging tools during a downhole application, more accurate density and/or photoelectric factor measurements may be obtained. Accordingly, controlling a generator  16  of an X-ray system  14  of the logging tool  01  based upon the normalized difference between a plurality of windows of a high energy peak of spectrum data obtained by a reference detector  18 , may result in stabilization of the source  16 . As a result, increased accuracy in density and/or photoelectric factor measurements may be obtained, enabling decision makers to manage the wellbore  03  to more effectively produce hydrocarbons, complete the well, or perform any other suitable wellbore  03  management. 
         [0092]    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.