Abstract:
A handheld X-ray diffractometer comprises a miniaturized X-ray source and multiple area detectors to allow the diffractometer to obtain two-dimensional X-ray diffraction images in a large diffraction space without rotating the sample. The source and detectors are located inside of a radio opaque enclosure that protects the operator during use. The handheld diffractometer also comprises a sample monitoring and alignment system that allows an operator to observe the measuring area and to align the diffractometer to the sample from outside of the housing. A specially designed mouthpiece, which mates the diffractometer to the sample area, prevents x-ray leakage and triggers off the data collection. The detectors can be positioned to perform measurements necessary to calculate a mechanical stress in the sample. Linear detectors may also be used in place of the area detectors.

Description:
BACKGROUND 
   This invention relates to X-ray diffraction systems. X-ray diffraction is a non-destructive technique for the qualitative and quantitative analysis of crystalline material samples, which are generally provided in the form of powders or solids. In accordance with this technique, an X-ray beam is generated by an X-ray tube with a stationary anode, by a conventional rotating anode X-ray source or by a synchrotron source and directed toward the material sample under investigation. When the X-rays strike the sample, they are diffracted according to the atomic structure of the sample. 
   X-ray diffraction data can be collected using one-dimensional diffraction (1D) profiles and two-dimensional (2D) profiles. One dimensional profiles are measured by rotating the sample and detecting diffracted X-rays with scanning point detectors or linear position-sensitive detectors. Two-dimensional profiles are acquired with two-dimensional, or area, detectors and the resulting data is then processed using two-dimensional image processing and two-dimensional diffraction pattern manipulation and interpretation. A typical two-dimensional laboratory diffractometer system  100  normally consists of five components as shown in  FIG. 1 . The components include an X-ray source  102  that produces a primary X-ray beam  104  with the required radiation energy, focal spot size and intensity. X-ray optics  106  are provided to condition the primary X-ray beam  104  to a conditioned, or incident, beam  108  with the required wavelength, beam focus size, beam profile and divergence. A goniometer and stage  110  are used to establish and manipulate geometric relationships between the incident X-ray beam  108 , the sample  112  and the X-ray detector  114 . The incident X-ray beam  108  strikes the sample  112  and produces scattered X-rays  116  which are recorded in the detector  114 . A sample alignment and monitor assembly comprises a sample illuminator  118 , typically a laser, that illuminates the sample  112  and a sample monitor  120 , typically a video camera, which generates a video image of the sample to assist users in positioning the sample in the instrument center and monitoring the sample state and position. 
   The two-dimensional detector  114  intercepts and records the scattered x-rays  116  from the sample  112 , and saves and displays the diffraction pattern in a two-dimensional image frame. 
   In the laboratory, X-ray diffractometers can be used to determine crystal structure and identify compounds. During laboratory data collection, the sample and instrument components are typically moved. For example, the gonimeter is used to provide a data scan and to set a tilt angle between the incident X-ray beam and the sample. However, there are many applications that require a diffractometer to be used outside of the laboratory. For example, with in-situ stress measurements, the diffractometer must be brought to the location of the stressed member. Consequently, a portable or handheld X-ray diffractometer would be desirable. Such a handheld instrument must be light in weight, small in size and energy efficient. Aligning the instrument accurately to the sample spot to be measured is also critical to obtain accurate measurement results. However, the conventional laboratory instrument is not suitable for handheld use because the setup is bulky and large and, as set forth above, requires that the instrument components be moved during data collection. Further, it would be difficult to properly align the instrument to the sample spot. 
   SUMMARY 
   In accordance with the principles of the invention, a handheld X-ray diffractometer comprises a miniaturized X-ray source and multiple area detectors to allow the diffractometer to obtain two-dimensional X-ray diffraction images in a large diffraction space without rotating the sample. 
   In one embodiment, the source and detectors are inside of a radio opaque enclosure that protects the operator during use. The handheld diffractometer also comprises a sample monitoring and alignment system that allows an operator to observe the measuring area and to align the diffractometer to the sample from outside of the housing. A specially designed mouthpiece, which mates the diffractometer to the sample area, prevents x-ray leakage and triggers off the data collection. 
   In another embodiment, the a sample monitoring and alignment system is located within the safety enclosure and the video display is mounted outside the enclosure so that the operator can observe measuring sample surface, save images of measuring area, and align the system to the measuring area without exposure to X-rays. 
   In still another embodiment, the detectors are positioned to perform measurements necessary to calculate a mechanical stress in the sample. In particular, the detectors are positioned for measuring diffraction rings at predetermined tilt angles with respect to the sample and the incident X-ray beam. In addition, one detector is positioned to measure the diffraction from crystal planes nearly parallel to the sample surface. 
   In yet another embodiment, linear detectors are used in place of the area detectors. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective schematic view of a conventional laboratory X-ray diffraction system. 
       FIG. 2  is a partial cutaway schematic of the interior of a handheld X-ray diffractometer constructed in accordance with the principles of the invention. 
       FIG. 3  is a schematic diagram showing diffraction cones illustrating the paths of diffracted X-rays produced when an X-ray beam impinges on a crystalline sample. 
       FIG. 4  is a schematic diagram showing distortion of diffraction cones caused by mechanical stress in the sample. 
       FIG. 5  is a three-dimensional cutaway view of the inventive handheld diffractometer showing a detector arrangement suitable for measuring stress in a sample. 
       FIG. 6  is a screen shot of a video display of a view through a video microscope showing a magnified image of a metal sample surface with a laser spot on the center. 
       FIG. 7  is a partial side view of the inventive handheld diffractometer illustrating angles at which the X-ray detectors may be positioned for stress measurements. 
       FIG. 8  is a partial schematic view of an inventive handheld X-ray diffractometer that uses linear X-ray detectors in place of area X-ray detectors. 
       FIG. 9  is a schematic diagram illustrating details of a nosepiece with an interlock mechanism. 
   

   DETAILED DESCRIPTION 
     FIG. 2  shows a partial cutaway schematic side view of a two-dimensional X-ray diffractometer  200  constructed in accordance with the principles of the invention. The diffractometer  200  comprises a miniaturized X-ray source  202  that produces X-rays with the required radiation energy, focal spot size and intensity. The construction of such an X-ray source is well-known and examples can be found in U.S. Pat. Nos. 5,621,780; 5,854,822 and 7,127,033, the disclosure of which is hereby incorporated by reference. The target material in the X-ray tube can be chromium, cobalt, iron, copper, molybdenum or other metals. The most critical requirement for the tube is energy efficiency because the entire system must be powered by a battery in order to make it portable. However, it is also necessary to delivery a high dosage of X-rays in a short period of time since the operator can only hold the system steady in a given position for a short time. One effective way to achieve both high energy efficiency and high x-ray dosage is run the x-ray source  202  at substantial power for a short time. Such operation is analogous to a flashing light in a camera. 
   X-ray optics  204  condition the primary x-ray beam (not shown in  FIG. 2 ) to the required wavelength, beam focus size, beam profile and divergence. The X-ray optics  204  also direct the incident beam  205  on to the sample surface  206  at a predetermined incident angle. The X-ray beam generated from a typical X-ray tube has a broad spectrum distribution including K-alpha and K-beta lines. Typically K-alpha radiation is used for X-ray diffraction so that it is necessary to monochromatize the X-ray beam using a K-beta filter or monochromator. The K-beta filter can be inserted in any location within the X-ray beam path, including the primary and incident beams. In one embodiment, the window through which the primary X-ray beam emerges from the X-ray source is fabricated from K-beta filter material. In another embodiment the detector window is fabricated from K-beta filter material. If the K-beta filter material is used for X-ray source window, the X-ray beam will contain mainly K-alpha radiation. The X-ray beam can be collimated to a proper beam size and divergence by a pinhole collimator, a monocapillary or a polycapillary lens in a conventional fashion. Multiple two-dimensional, or area, X-ray detectors  208 ,  210  and  212  read the resulting diffraction pattern in three-dimensional space and a video microscope  214 , video camera  216  and a video display  218  allow the operator to monitor the measured sample area and align the system precisely to the intended measurement spot without exposing the operator to X-ray radiation. A laser  220  generates a laser beam that provides a reference point on the sample surface  206  for precise sample alignment. 
   Several important safety elements are included. A radiation shield  222  forming a safety enclosure covers all the space and components exposed to the direct beam  204  of the X-ray source and scattered X-rays from sample surface  206  and other components. A handle or support  224  allows the operator to position the system and an operation trigger  226  can be actuated to start data collection. A nosepiece  228  provides a safety interlock. The nosepiece  228  has an interlock mechanism (not shown in  FIG. 2 ) to ensure that X-rays can be released from the X-ray source  202  only if the sample surface  206  is securely covered by the nosepiece  228 . The nosepiece  228  can be configured to establish different angles at which the incident X-ray beam  205  is incident on the sample surface  206  and also formed to fit different sample surface shapes. The safety interlock switch may also trigger off x-ray diffraction data collection in conjunction with the trigger  226 . The nosepiece  228  may include several exchangeable parts, each of which matches a particular surface shape, such as a flat surface, a cylindrical surface or a corner. The nosepiece can also be designed to control the x-ray incident angle for different metal types to optimize the measurement condition. An oscillation mechanism may be integrated to the nosepiece to improve the measurement sampling when dealing with samples of large grain structure. A magnetic nosepiece may be used to enhance the stability of the measurement position when measuring a sample of ferrous materials. 
   The multiple X-ray area detectors  208 ,  210  and  212  eliminate the necessity to move sample and instrument components during data collection and, consequently, the inventive apparatus does not need to provide data scan motion or to change tilt angles during data collection. Therefore, the gonistat used by laboratory systems and many existing portable systems to provide the data scan and to set tilt angles can be eliminated, reducing the weight and size of the apparatus. 
   One embodiment of the inventive handheld diffractometer is particularly useful in on-site stress measurements.  FIG. 3  shows a typical pattern  300  of diffracted x-rays from a polycrystalline (powder) sample  304 . The diffraction pattern  300  from such a sample  304  forms a series of diffraction cones  306 - 314  if a large number of crystals are oriented randomly in the space covered by the incident X-ray beam  302 . Each diffraction cone, such as cone  306 , corresponds to paths followed by diffracted X-rays that are diffracted from the same family of crystalline planes in all the participating crystals. Polycrystalline materials can be single-phase or multi-phase solids. 
   Stress measurement with two dimensional X-ray detectors is based on a fundamental relationship between the stress tensor and diffraction cone distortion as shown in  FIG. 4 . This figure shows two diffraction cones  404  and  406  that are produced when an X-ray beam  402  strikes a sample  400 . Stresses in sample  400  distort the diffraction cone shape so that the Bragg angle 2θ becomes a function of the angle γ (that is, 2θ=2θ(γ)) where the particular function is uniquely determined by the stress tensor and the sample orientation. In particular, cones  404  and  406  are distorted to form cones  408  and  410 , respectively. The fundamental equation for stress measurement using two-dimensional X-ray detectors is given as: 
                 p   11     ⁢     σ   11       +       p   12     ⁢     σ   12       +       p   13     ⁢     σ   13       +       p   22     ⁢     σ   22       +       p   23     ⁢     σ   23       +       p   33     ⁢     σ   33         =     ln   ⁡     (       sin   ⁢           ⁢     θ   0         sin   ⁢           ⁢   θ       )                     where   ⁢           ⁢     p   ij       =     {               (     1   ⁢     /     ⁢   E     )     ⁡     [         (     1   +   v     )     ⁢     f   ij       -   v     ]             =         1   2     ⁢     S   2     ⁢     f   ij       +     S   1                 if   ⁢           ⁢   i     =   j                 (     1   ⁢     /     ⁢   E     )     ⁢     (     1   +   v     )     ⁢     f   ij             =       1   2     ⁢     S   2     ⁢     f   ij                 if   ⁢           ⁢   i     ≠   j           ⁢           ⁢   and   ⁢           ⁢             f   11     =     h   1   2               f   12     =     2   ⁢     h   1     ⁢     h   2                 f   22     =     h   2   2                   f   13     =     2   ⁢     h   1     ⁢     h   3                 f   23     =     2   ⁢     h   2     ⁢     h   3                 f   33     =     h   3   2                       
and
         h 1 =sin θ(sin φ sin ψ sin ω+cos φ cos ω)+cos θ cos γ sin φ cos ψ−cos θ sin γ(sin φ sin ψ cos ω−cos φ sin ω)   h 2 =−sin θ(cos φ sin ψ sin ω−sin φ cos ω)−cos θ cos γ cos φ cos ψ+cos θ sin γ(cos φ sin ψ cos ω+sin φ sin ω)   h 3 =sin θ cos ψ sin ω−cos θ sin γ cos ψ cos ω−cos θ cos γ sin ψ       
   The f ij &#39;s are the strain coefficients determined by {h 1 , h 2 , h 3 } which are components of the unit vector of the diffraction vector expressed in the sample coordinates. The term ln(sin θ 0 /sin θ) represents the diffraction cone distortion at a particular (γ, 2θ) position. S 1  and S 2  are the macroscopic elastic constants. The ω, ψ and φ angles refer to a sample orientation in an Eulerian gonistat. 
   Since the stress measurement is based on a variation of the interplanar spacing of the crystals due to stresses, knowledge of the lattice parameters without the influence of stresses is critical. Conventionally, this information is obtained by the extrapolation of data measured at other tilt angles. This extrapolation is acceptable if a linear relation is maintained between the strain and sin 2  ψ where ψ is the tilt angle between the normal of the measured crystal planes and the normal of the sample surface. However, this linear relation does not hold true if there are preferred orientation, large crystal grains or stress gradients in the sample. Therefore, prior art stress measurement systems cannot be used with accuracy in many applications. 
   In accordance with the principles of the invention, information regarding lattice parameters without the influence of stresses is obtained by measurement directly on the sample surface. Based on the mechanics, the stress component normal to the sample surface is zero or negligible. The inventive system measures a part of a diffraction ring from a crystal plane, the normal of which is very close to the normal of the sample surface. 
     FIG. 5  shows an interior configuration  500  of an embodiment of the inventive handheld diffractometer suitable for stress measurement in a three-dimensional view. The primary x-ray beam  205  hits the sample  206  at a predetermined tilt angle. The diffraction ring produced by a predetermined crystal plane is sensed by the two-dimensional X-ray detectors  208  and  210 . The diffraction ring distortion is analyzed by the computer to output the residual stress result. The two-dimensional detector  212  measures a part of a diffraction ring from another crystal plane, the normal of which is very close to the normal of the sample surface  206  so a stress-free crystal lattice parameter can be measured directly from the sample surface  206 . In another words, detector  212  detects X-rays diffracted from crystal planes which are nearly parallel to the sample surface  206 . The stress-free crystal lattice parameter is used in the stress calculation and this additional data improves the accuracy and reliability of the stress results. 
   Another important consideration for measuring x-ray diffraction data with a handheld system is that the measurement surface must be covered by the system during the data collection for safety reasons. However, it is critical to be able to observe the measurement surface and align the system accurately to the sample surface. The inventive system uses a laser beam to provide a reference on the sample surface, and a video microscope with a display located outside of the radiation enclosure to allow the operator to observe the system alignment and to keep the system aligned to the sample surface. For example, in  FIG. 5 , laser beam  515  generated by laser  220  provides a reference to the position of the sample  206 . The position of the laser spot on the sample  206  can be observed by a video system including the microscope  214 , camera  216  and video display  218 . The laser spot will appear in the center of a crosshair if the sample  206  is at the correct position to make a measurement. When the x-ray beam  205  is aimed at the sample  206  and diffraction data is being collected, the sample measurement area  206  is covered by the system enclosure to prevent leakage of X-rays for the safety of operator. Therefore, a video system must be provided to align the system accurately to the sample surface  206 . The video microscope  216  also allows an image of the measurement area to be linked to the x-ray diffraction data.  FIG. 6  is such an image of magnified metal sample surface  600  with a laser spot  602  on the center. 
     FIG. 7  shows a partial side view of  FIG. 5 . In one embodiment, detectors  208  and  210  measure X-rays diffracted from a set of crystal planes with 2θ angles in the range 120°-170° where the angle depends on the incident X-ray beam  205  wavelength and the metal type of the sample  206 . The detector  212  measures diffraction from a different set of crystal planes with a 2θ N  angle in the range 60°-90°. Both sets of the crystal planes represent different crystal orientations. Therefore, the diffraction data from both sets of crystal planes also contains information on anisotropic features of the crystals. The following table gives possible diffraction angle selections for four pure metal samples if Co-Kα incident X-ray radiation is used. One skilled in the art can also determine other possible angles if different radiation is used and different metals are measured. 
   
     
       
             
           
             
             
             
             
           
             
             
             
             
           
         
             
                 
             
             
               Co-Kα radiation 
             
           
        
         
             
                 
               Metal-Alloy 
               2θ 
               2θ N   
             
             
                 
                 
             
           
        
         
             
                 
               α-Fe 
               161.4 
               77.2 
             
             
                 
               Al 
               162.0 
               77.4 
             
             
                 
               α-Ti 
               154.2 
               74.7 
             
             
                 
               Cu 
               163.5 
               88.8 
             
             
                 
                 
             
           
        
       
     
   
     FIG. 8  shows a configuration  800  that is an alternative to the configuration  500  in  FIG. 5 . In this alternative configuration, eight linear X-ray detectors ( 802 - 816 ) are used. The detectors are arranged in a radial pattern around a center opening  818  through which the incident X-ray beam  205  passes. Adjacent linear detectors are separated by a 45° angle. The incident X-ray beam  205  hits the sample  206  at a predetermined tilt angle to produce diffracted X-rays. The eight detectors  802 - 816  measure eight points on a diffraction ring produced by diffraction from a predetermined crystal plane. The diffraction ring distortion is analyzed by a computer to generate the residual stress result. One skilled in the art would also understand the possibility and motivation to use less or more linear detectors in order to balance cost and sampling statistics. The other features described in the previous configuration also apply to this configuration. 
     FIG. 9  is a schematic cross-sectional diagram of an illustrative nosepiece structure  228 . The nosepiece  228  has an x-ray labyrinth consisting of the interleaved labyrinth rings  230  and  232  and a spring ring  234 . Labyrinth ring  232  is sealed to the radiation shield  222 . Labyrinth ring  230  is connected to labyrinth ring  232  by means of a spring ring  234  so that labyrinth ring  230  can slide relative to labyrinth ring  232 . In operation when the unit is brought into contact with the sample  206 , the bottom plane  236  of labyrinth ring  230  is brought into close contact with the sample  206  to prevent x-ray leakage when x-ray beam  206  passing through X-ray optics  204  hits the sample  206 . 
   An interlock switch  904  is provided which is connected by support  906  to labyrinth ring  232  and by support  908  to labyrinth ring  230 . When labyrinth ring  230  is tightly pressed against the sample  206 , the force compresses the spring ring  234  and closes the interlock switch  904 . The interlock switch  904  is connected by conductors  910  and  912  to trigger switch  914  and further connected to the x-ray source control (not shown in  FIG. 9 ) via terminals  916 . Trigger switch  914  is actuated when trigger  226  is depressed by an operator. Only when both the interlock switch  904  and the trigger switch  914  are closed, can the x-ray source operate. A radiation sensor  902  may be installed in the outside of the radiation shield  222  and next to the contacting surface  236  between the sample  206  and the labyrinth ring  230  so that any x-ray leakage can be detected and the unit switched off. 
   Such a nosepiece structure can cover the surface of the measuring area to prevent the leakage of x-rays during the measurement so that no x-ray leakage is beyond a safe level. It is also possible to integrate a radiation sensor directly into the nosepiece  228  so that that the dosage of any leaked x-rays is monitored. One skilled in the art could anticipate that the nosepiece may be designed to fulfill several other functions. For example, labyrinth ring  230  may comprise several exchangeable parts of which each matches a particular sample surface shape, such as flat, cylinder or corners. Labyrinth ring  230  may be made of magnetic material to enhance the stability of the measurement position when measuring a sample composed of ferrous materials. 
   Further, nosepiece  228  can also be designed to control the x-ray incident angle θ N  for different metal types to optimize the measurement condition. An oscillation mechanism (not shown in  FIG. 9 ) may be integrated to the nosepiece  228  to improve the measurement sampling when dealing with samples of large grain structure. 
   While the invention has been shown and described with reference to a number of embodiments thereof, it will be recognized by those skilled in the art that various changes in form and detail may be made herein without departing from the spirit and scope of the invention as defined by the appended claims. For example, in other embodiments, the system may contain a level sensor to help orient the handheld system relative to the sample or structure. The system may further contain a Global Position System receiver to record the geographic location of each measurement. It may also be beneficial to have several handheld units linked each other or to a station through wireless technologies. One skilled in the art can also see the possibility and motivation to modify the system so that it can be mounted permanently on critical locations of structures so that the residual stress and surface condition are remotely and constantly or frequently monitored.