Patent Publication Number: US-2023162942-A1

Title: Vacuum compatible x-ray shield

Description:
FIELD 
     The disclosure pertains to X-ray shields. 
     BACKGROUND 
     X-ray shields are required in a wide range of applications including electron microscopes and other systems where high energy particle beams are incident on matter, resulting in X-ray generation. For reasons of geometry, an X-ray shield positioned close to an X-ray generation site requires less shielding material than a shield placed farther away. However, systems generating X-rays often operate under ultra-high vacuum (UHV), while some common shielding materials are incompatible with an ultra-high vacuum. Accordingly, there remains a need for improved technologies enabling compact X-ray shields to be utilized within an ultra-high vacuum. 
     SUMMARY 
     In brief, examples of the disclosed technologies provide X-ray shielding material inside a stand-alone vacuum-tight enclosure which can be deployed within an ultra-high vacuum environment. 
     In a first aspect, the disclosed technologies can be implemented as a method of manufacturing an X-ray shield. A shell defining a chamber, and having one or more ports, is fabricated. The shell is tested to verify that the shell is free of leaks. The tested shell is filled with an X-ray shielding material. The one or more ports of the filled shell are sealed. 
     In some examples, the shell can be fabricated by an additive manufacturing process, which can incorporate one or more of: direct metal laser sintering (DMLS), selective laser melting (SLM), electron beam melting (EBM), or binder jetting (BJ). In varying examples, the shell can be rigid; can incorporate stainless steel; and/or can have a median wall thickness in a range 0.1 to 1.0 mm. The rigid shell can incorporate a material having a first average atomic number Z1, and the method can include cladding the rigid shell with another material having an average atomic number Z2 less than Z1. 
     In additional examples, the X-ray shielding material can incorporate a metal and the filling operation can include introducing the metal in a molten state into the chamber. The X-ray shielding material can incorporate a resin loaded with metal particles. The one or more ports can include a first port, used to introduce the X-ray shielding material into the chamber during the filling operation, and a second port used to release displaced fluid from the chamber during the filling operation. 
     In further examples, the sealing can include welding a respective cap onto each of the one or more ports. For a duration encompassing the testing, the shell can be temporarily isolated from an environment surrounding the shell. The testing can have a leak rate threshold less than or equal to 10-7 mbar•l/s. 
     In further examples, the disclosed technologies can be implemented as a method of reducing X-ray emission from an electron microscope housed in a vacuum enclosure. An X-ray shield can be manufactured by any of the above methods or variations. The X-ray shield can be secured within an interior volume of the vacuum enclosure. The X-ray shield can be secured within a pump coupler of the electron microscope, and can be oriented so as to block at least 80% of X-rays emitted, parallel to a longitudinal axis of the pump coupler, through an intake aperture of the pump coupler. 
     In a second aspect, the disclosed technologies can be implemented as an apparatus having a vacuum enclosure and an X-ray shield positioned within the vacuum enclosure. The X-ray shield includes an inverse vacuum bottle containing an X-ray shielding material. 
     In some examples, the apparatus can be an electron microscope having a column axis and can further include a pump coupler. The X-ray shield can be positioned within the pump coupler and oriented so as to block at least 80% of X-rays that are emitted from an X-ray generation site within the vacuum enclosure, through an intake aperture of the pump coupler. A vacuum conductance of the pump coupler is reduced due to the X-ray shield, compared to without the X-ray shield, by not more than 20%. 
     The inverse vacuum bottle can incorporate stainless steel. The stainless steel can be clad with a material having an average atomic number less than or equal to 14. The X-ray shield can be formed to include a twisted elongate member. The X-ray shielding material can incorporate at least 50% by weight of lead. A pressure within the vacuum vessel can be held below 10-9 mbar. 
     In another aspect, the disclosed technologies can be implemented as a method in which an inverse vacuum bottle containing an X-ray shielding material is placed inside a vacuum enclosure of an electron microscope, and the vacuum enclosure is pumped to a pressure below 10-9 mbar. 
     In some examples, the vacuum vessel can incorporate a pump coupler, and the placing can include securing the inverse vacuum bottle within the pump coupler. 
     The foregoing and other objects, features, and advantages of the invention will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 A- 1 B  are section views of a portion of an electron microscope suitable for deployment of disclosed technologies. 
         FIG.  2    is a section view of an apparatus with X-ray shielding conventionally deployed external to a vacuum enclosure. 
         FIG.  3    is a section view of an apparatus with an X-ray shield according to a first example of the disclosed technologies, deployed inside a vacuum enclosure. 
         FIG.  4    is a section view of an apparatus with an X-ray shield according to a second example of the disclosed technologies, deployed inside a vacuum enclosure. 
         FIG.  5    is a flowchart of a first example method according to the disclosed technologies. 
         FIGS.  6 A- 6 D  are views of a first example X-ray shield according to the disclosed technologies. 
         FIG.  7    is a flowchart of a second example method according to the disclosed technologies. 
         FIG.  8    is a block diagram of an apparatus incorporating an X-ray shield according to the disclosed technologies. 
         FIG.  9    is a flowchart of a third example method according to the disclosed technologies. 
         FIGS.  10 A- 10 B  are views of a second example X-ray shield according to the disclosed technologies. 
         FIG.  11    is a view of a third example X-ray shield according to the disclosed technologies. 
         FIGS.  12 A- 12 C  are views of a fourth example X-ray shield according to the disclosed technologies. 
         FIG.  13    is a view of a fifth example X-ray shield according to the disclosed technologies. 
         FIGS.  14 A- 14 D  are views of a sixth example X-ray shield according to the disclosed technologies. 
         FIG.  15    illustrates a generalized example of a suitable computing environment in which described embodiments, techniques, and technologies pertaining to a disclosed nonlinear optical device can be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     Introduction 
     X-ray shielding is often used around X-ray generating equipment for reasons of personnel safety. X-rays can be generated within electron microscopes (e.g. TEM and SEM), focused ion beam machines, other analytic equipment (e.g., performing electron scattering, X-ray diffraction, or similar techniques), or other equipment where high energy particle beams impinge on materials. 
     In an electron microscope, X-ray generation due to stray electrons striking an aperture plate or wall of the microscope column can be controlled or minimized through careful design. However, incidence of the electron beam on a sample is a fundamental aspect of electron microscope operation and cannot be eliminated. Moreover, electron microscopes are commonly used with beam voltages in excess of 60 keV, and the resultant X-ray production can be significant. Similar considerations apply to other particle beam equipment. 
     Accordingly, the center of a sample chamber, where a sample is normally placed for imaging by an electron microscope, represents a significant X-ray generation site. The sample chamber of an electron microscope can be congested, particularly in the case of a transmission electron microscope (TEM). In some directions, a location outside the sample chamber may be as close as it is practical to place an X-ray shield, and such a shield can be deployed without consideration for vacuum compatibility. However, the sample chamber (and the entire electron beam channel) can be maintained under ultra-high vacuum, for which purpose a pipe section dubbed a coupler can connect the sample chamber to a vacuum pump. To maximize vacuum conductance, the coupler can have a wide cross-section and can be generally free of obstructions to gas flow. Thus, the coupler can also present a clear egress path for X-rays. Because of the dimensions of the vacuum pump and the coupler, an X-ray shield provided external to the coupler can be bulky and heavy, and it can be desirable to provide an X-ray shield inside the coupler or between the coupler and the X-ray generation site at the center of the sample chamber. 
     Such internal X-ray shield positioning could be straightforward in the case of a stainless steel shield (subject to considerations of degraded vacuum conductance). However, stainless steel has a relatively low stopping power for X-rays. Under typical electron beam conditions and typical requirements for X-ray attenuation by a shield, the thickness of stainless steel required for shielding could be about 20 cm. Aluminum, with lower atomic number, could require even more - about 40 cm thickness. 
     Lead, on the other hand, has superior stopping power, and can provide sufficient shielding with merely 1.5 cm thickness. But lead is prone to outgassing, and exposed lead surfaces can be undesirable within a UHV system. Still further, a vacuum-compatible coating (generally, a metal coating), applied to a lead structure, can be susceptible to defects. That is, it can be difficult to achieve a defect-free sealed coating over the lead. Still further, it can be difficult to test whether the coated structure is in fact leak-free. 
     The disclosed technologies solve these problems by leak-testing a hollow shell and then filling the with an X-ray shielding material. The resulting X-ray shield can be deployed anywhere suitable within an ultra-high vacuum system. Only the shell material (which can be stainless steel or a low-Z vacuum compatible material such as aluminum) is exposed to the vacuum, and leak-tightness of the shield can be assured. Thus, the respective advantages of stainless steel and lead can be advantageously combined and a compact X-ray shield can be deployed within a pump coupler or at any other available location within a UHV enclosure. 
     For reasons of maintaining vacuum conductance through the pump coupler, complex shapes such as elongated twist structures can be used. Shells of such shapes can be manufactured conveniently using additive machining, although this is not a requirement. For example, sections of a shell can be manufactured by forming sheet metal, extrusion, or casting, in addition or alternatively to additive manufacturing, and welding the sections together. 
     The following section provides a brief description of a representative TEM which can benefit from the disclosed technologies. 
     Example Electron Microscope 
       FIGS.  1 A- 1 B  are section views  101 - 102  of a portion of an electron microscope, and provide context for examples of the disclosed technologies described herein. For purpose of illustration  FIGS.  1 A- 1 B  depict a common TEM configuration, however the disclosed technologies can be applied similarly to other equipment. 
       FIG.  1 A  is a vertical section through column axis  105 . Sample chamber  110  can be situated between condensor electron optics  122  and objective electron optics  128 . Optics  122 ,  128  and sample chamber  110  define a portion of an electron column of the electron microscope, within which an electron beam can be guided through channel  125 . 
     Pole pieces  124 ,  126  shape magnetic fields between them. In normal operation a sample can be held midway between pole pieces  124 ,  126  along column axis  105 . Incidence of the electron beam on the sample can lead to generation of X-rays. Thus, oval  115  denotes an X-ray generation site. 
     The interior of the electron column, including sample chamber  110  can be maintained at ultra-high vacuum during operation. Accordingly, vacuum pump  140  can be joined to sample chamber  110  by pump coupler  130  having intake aperture  132  and outlet aperture  134 . A vacuum enclosure of the electron microscope comprises all or parts of the walls of sample chamber  110 , coupler  130 , a portion of pump  140 , and additional components above or below sample chamber  110  along electron channel  125 . An interior space of the vacuum enclosure, including sample chamber  110  and X-ray generation site  115 , can be under ultra-high vacuum, while exterior environment  103  can be an indoor room environment at one atmosphere air pressure. 
       FIG.  1 B  is a horizontal section through a midplane of sample chamber  110 , showing a common octagon configuration having symmetry axes  112 ,  114  and eight ports. In some examples, two opposed ports  181 ,  185  can be used for a sample loader and a sample manipulator, while other ports  182 - 184 ,  186 - 187  can be variously used for a cold finger, instrumentation access, or auxiliary tools; or can be unused. In normal operation, each of ports  181 - 187  can provide a vacuum-tight connection to associated equipment, or can simply be sealed closed. Channel  180  of sample chamber  110  can be coupled to pump coupler  130  at intake aperture  132 . For clarity of illustration, flanges, gaskets, or other coupling details are omitted from  FIGS.  1 A- 1 B . The intersection of symmetry axes  112 ,  114  can lie on column axis  105 . 
     Terminology 
     The term “additive manufacturing” (sometimes, “3-D printing”) refers to processes for fabricating objects using layer-by-layer material deposition, with the object shape defined by computer-directed deposition of the material rather than by a pre-formed mold. That is, additive manufacturing fabricates shape without use of a mold, although a substrate can be used as a base upon which additional material layers are deposited. 
     The term “atomic number” (Z) refers to the number of protons in one atomic nucleus of an elemental material. For a composition of multiple elements (e.g., an alloy, a compound, a mixture, or a composite of one material interspersed with or within another material), an “average atomic number” Zavg can be defined as 
     
       
         
           
             Z 
             a 
             v 
             g 
             = 
             
               
                 
                   
                     
                       
                         ∑ 
                         i 
                       
                       
                         
                           f 
                           i 
                         
                         
                           Z 
                           i 
                         
                         
                             
                           k 
                         
                       
                     
                   
                 
               
               
                 1 
                 / 
                 k 
               
             
           
         
       
     
      where the subscript i denotes a respective element in the composition having atomic number Zi, and fi is the fraction of protons in the composition belonging to atoms of element i. That is, ∑ i ƒ i  = 1. k and ⅟k are positive exponents; k=1 denotes a simple average atomic number, while k=2.94 denotes a Khan average atomic number, sometimes dubbed “effective atomic number”. For an elemental material, the average atomic number is simply the atomic number of that element. While atomic number is an integer, average atomic number need not be an integer. A low-Z material has average atomic number less than equal to 14. In some examples, aluminum (Z=13) can be used as a low-Z cladding over the shell of an X-ray shield. A high-Z material has average atomic number greater than or equal to 50. 
     In the context of X-rays or X-ray shielding, the term “block” refers to an X-ray interacting with an X-ray shield resulting in absorption or inelastic scattering. Inelastic scattering is a process whereby one or more photons are generated, each with lower energy than the incident X-ray, and the original X-ray is extinguished. Non-interacting incident X-rays can be said to “pass” through the shield. Passing can include elastic scattering. The term block can be contingent on the normal operation of an apparatus. To illustrate, a given X-ray shield could block 99% of X-rays generated by a 10 keV electron beam (while passing the other 1%), 90% of X-rays generated by a 100 keV beam (while passing the other 10%), or 50% of X-rays generated by a 1 MeV beam (while passing the other 50%). 
     The term “bottle” refers to a sealable container with at least one port coupling the interior of the container to its exterior. A bottle remains a bottle when the at least one port is closed. A “vacuum bottle” is a bottle which can be evacuated (e.g., through the port) to hold a vacuum in the interior of the bottle, with discrete objects optionally situated in the vacuum. An “inverse vacuum bottle” is a bottle which can hold material sealed from vacuum exterior to the bottle. A bottle can have any of a wide range of shapes, and is not limited to shapes that are cylindrical or shapes that have a narrow neck leading to a port. Some bottles of interest herein can have twisted or helical shapes, with one, two, or more ports. 
     The term “chamber” refers to a space or volume inside an enclosure. The presence of one or more ports does not preclude an enclosure from defining a chamber. A “sample chamber” is a chamber, often within an electron microscope or other analytic equipment, in which a sample can be placed for analysis under normal operation. 
     An “electron microscope” is a type of analytic equipment in which a sample is illuminated by an electron beam, and resulting particles or electromagnetic radiation are used to form an image. A scanning electron microscope (SEM) images a sample surface based on reflected, secondary, or backscattered particles or radiation from the sample surface on which the electron beam is incident. Because beam interactions detected by a SEM occur at or near this surface, a SEM can operate on samples of arbitrary thickness. In contrast, a transmission electron microscope (TEM) images a sample surface based on transmitted electrons (including scattered electrons). A TEM operates on samples of about 10-150 nm thickness, which can be mounted on a grid for mechanical support and thermal conductivity; in turn the grid can be held in a sample holder. A TEM can provide magnifications up to and exceeding 50 million, while SEM magnifications are usually limited to about 2 million. In this disclosure, scanning transmission electron microscopes (STEM), which perform imaging of transmitted electrons, are considered to be TEMs. The electron beam in an electron microscope can be generated in an electron gun, and accelerated, focused, or steered through a series of stages toward a sample chamber. Commonly, the electron gun, intermediate stages, the sample chamber, and downstream imaging stages can be arranged as a columnar structure dubbed an “electron microscope column”, or simply “column”. A longitudinal axis of the column is dubbed a “column axis.” 
     The term “enclosure” refers to a structure defining an interior space (e.g., a chamber). While some enclosures described herein are sealed, this is not a requirement, and other enclosures can have one or more ports allowing matter to freely move between interior and exterior spaces. Particularly, some described enclosures can have ports that are initially open, but then are temporarily closed (e.g., for leak testing) or permanently closed (e.g., prior to putting the enclosure in service). 
     The term “filling,” applied to a chamber (or shell), is understood to mean that at least 50% by volume of the chamber (or interior space of the shell) is occupied by a filling material, such as X-ray shielding material. 
     The term “fluid” refers to matter in a liquid or gas phase that can assume the shape of a surrounding enclosure. A fluid can be homogeneous or inhomogeneous. 
     The term “isolated,” in context of enclosures, refers to two spaces lacking a clear path connecting the two spaces. An enclosure with no open ports can serve to isolate its interior volume from an exterior space. The presence of leaks in an enclosure does not preclude the spaces from being isolated. 
     The term “leak,” as a noun, refers to an unintended path through a wall of an enclosure. As a verb, the term refers to an act of passing through a leak. A path through a wall can be considered to be a leak if its leak rate is between 10 -3  mbar•l/s and a predetermined leak rate threshold (such as 10 -12  mbar•l/s). 
     The term “leak rate” refers to a rate at which a material passes through a surface under a given pressure difference, and is commonly measured in units similar to mbar•liter/second (shortened to mbar•l/s). Air can pass through a 10 µm diameter hole in a thin wall, across a one atmosphere pressure difference, at about 10 -2  mbar•l/s. Leak rates of interest herein for ultra-high vacuum systems can be in a range 10 -3  to 10 -12  mbar•l/s, for He gas at 25° C. with 1000 mbar (one atmosphere) pressure difference. 
     The term “port” refers to a macroscopic opening in an enclosure. Commonly, a port can have a transverse spatial extent in a range 0.1 mm to 10 cm, sometimes 1 mm to 1 cm, but this is not a requirement. A port can be circular, but this is not a requirement, and ports having square, oval, elongated slot shapes, or other shapes can also be used. A “pumping port” is a port of a vacuum enclosure coupled to a vacuum pump. 
     The term “pump coupler” refers to a section of a vacuum enclosure coupling a main chamber of the vacuum enclosure to a vacuum pump. An interface region between the pumping port and a main chamber within a vacuum enclosure is termed an “intake aperture” of the pump coupler. The interface between the pump coupler and the vacuum pump can be a pumping port of the vacuum enclosure, sometimes termed an “outlet aperture” of the pump coupler. That is, fluid atoms or molecules extracted by a vacuum pump from the main chamber can pass from the main chamber through the intake aperture into the pump coupler, and thence through the outlet aperture into the vacuum pump. 
     The term “resin” refers to a viscous fluid. A resin is termed “loaded” when it serves as a medium holding suspended particles, such as particles of an X-ray shielding material. Some resins can be epoxy resins or other curable resins that cure to a rigid solid, but this is not a requirement. Other resins can retain their viscous fluid properties persistently. 
     The term “rigid” refers to an enclosure or other solid object having a determinate shape under normal usage. To illustrate, a glass bottle is rigid, while a common plastic bag is not. Minute shape deformations due to vibration, thermal expansion, changes in pressure differential between interior and exterior of an enclosure, or similar effects do not preclude an object being considered rigid. 
     The term “seal,” as a verb, refers to an act of closing a port of an enclosure. The port can be closed, e.g., by welding a cap over the port. When all ports of an enclosure are sealed, the interior volume of the enclosure can be isolated from exterior space. 
     The term “shell” refers to a thin-walled enclosure. A shell can be rigid, but this is not a requirement. A bellows structure, whether made of stainless steel or another material, can be a flexible shell. Some shells of disclosed examples can have wall thicknesses in a range 30 µm to 3 mm, 100 µm to 1 mm, or about 0.3 mm. Wall thickness can vary between different portions of a shell. To illustrate, a shell of median thickness 0.3 mm can have a base section about 1 mm thick and a flange section about 3 mm thick. 
     The term “vacuum” refers to a condition of a chamber having a fluid pressure below 10 -3  mbar. “Ultra-high vacuum” (UHV) refers to pressures below 10 -9  mbar. 1 mbar is about 10 2  Pa, 10 2  N/m 2 , or 0.75 torr. 
     The term “vacuum conductance” refers to a rate of mass flow between two planes in a vacuum, divided by the pressure difference between the planes. To illustrate, the two planes can be the intake and outlet apertures of a pump coupler. Under conditions of molecular flow, mass flow through the pump coupler can be proportional to the pressure difference between intake and outlet apertures. Generally, the presence of objects (e.g., an X-ray shield) within the pump coupler, bends, non-uniformities in cross-section of the pump coupler, or surface roughness can cause the vacuum conductance of a pump coupler to be lower than for a straight smooth-bore empty pump coupler of same or similar dimensions. 
     The term “vacuum enclosure” refers to a structure configured to isolate vacuum inside the enclosure from an environment not under vacuum. In some examples, the environment can be air, but this is not a requirement and other gas or liquid environments can be used. Alternatively, the environment outside a vacuum enclosure can be a higher pressure vacuum than inside the enclosure. A vacuum enclosure can be formed by a mix of one or more shells, other structures, ports, or gaskets, in any combination. 
     The term “vacuum pump” refers to an apparatus operable to extract fluid atoms or molecules from an interior space of a vacuum enclosure to which it is coupled. Vacuum pumps found in UHV systems can include turbomolecular pumps, cryopumps, ion pumps, or getters, which can be assisted by mechanical, diffusion, or other types of roughing pumps. 
     The term “welding” refers to a process for joining two solid objects, in which the material of both objects is temporarily heated or liquefied proximate to the joint. For metal objects, heating can be accompanied by liquefaction, e.g., with a welding torch, or heating can be performed without liquefaction, e.g., with acoustic energy in ultrasonic welding. For non-metals, e.g., polymers, heating or liquefaction can be performed with acoustic energy or with solvents. Some common welding processes, such as tungsten-inert-gas (TIG) or metal-inert-gas (MIG) welding, can use a filler material. Other welding processes, such as laser welding or ultrasonic welding, can omit a filler material. 
     The term “within” means wholly contained inside of. Thus, an innovative X-ray shield can be deployed within a vacuum enclosure, and can be removed from the vacuum enclosure. After removal of the X-ray shield, the vacuum enclosure can be resealed and evacuated, intact and in a same configuration with or without the X-ray shield. 
     The term “X-ray” refers to electromagnetic radiation having a wavelength in a range about 10 pm to 10 nm or photon energy in a range about 100 eV to 100 keV. X-rays are often generated through the interaction of a high-energy particle beam with stationary material, but this is not a requirement. X-rays can also be generated through interaction of higher energy photons (e.g., gamma rays or other X-rays) with matter, or through radioactive decay. In some disclosed examples, X-rays can arise within an electron microscope, for example at a site where an electron beam is incident upon a sample. X-rays can also be generated by incidence of a focused ion beam on a sample. 
     The term “X-ray generation site” refers to a location in an apparatus where, under normal operation of the apparatus, X-rays are expected to be generated. For example, a sample stage or a sample chamber within an electron microscope can be an X-ray generation site. A plate or block defining an aperture through which an electron beam passes can also be an X-ray generation site. Thus, the X-ray generation site is present within the apparatus when the apparatus is switched off or when no sample is present. 
     The term “X-ray shield” (or simply “shield”) refers to a device incorporating an X-ray shielding material and configured to block some X-rays. The blocked X-rays can be generated during normal operation of an apparatus associated with the X-ray shield. 
     The term “X-ray shielding material” refers to a material having average atomic number at least 30, or in a range 30 to 100. Some X-ray shielding materials of interest in this disclosure include lead (atomic number Z=82), tungsten (Z=74), or tin (Z=50). Antimony (Z=51), tantalum (Z=73), bismuth (Z=83), or depleted uranium (Z=92) can also be used. Common structural materials such as stainless steel (Zavg=29) or aluminum (Z=13) are not considered X-ray shielding materials herein. 
     Example X-Ray Shield Deployments 
       FIG.  2    is a section view  200  illustrating X-ray shielding deployed external to a vacuum enclosure.  FIG.  2    depicts a representative electron microscope generally similar to that of  FIG.  1 A . X-ray generation site  215  is situated along column axis  205  and within sample enclosure  210 , the latter coupled to pump  240  via pump coupler  230 , in a configuration similar to that described in context of  FIGS.  1 A- 1 B . The illustrated apparatus includes electron optics  222 ,  228  and pole pieces  224 ,  226 , also similar to that of  FIG.  1 A . 
     In  FIG.  2   , X-ray shield  250  is provided outside ( 203 ) the vacuum enclosure in a conventional arrangement. Because shield  250  is not within the interior vacuum space, no provision for vacuum compatibility is required. However, shield  250  surrounds and is larger than pump coupler  230 , and the mass of shield  250  can be considerable. 
       FIG.  3    is a section view  300  of an apparatus with an X-ray shield deployed inside a vacuum enclosure. Particularly, innovative X-ray shield  350  can be placed within the vacuum enclosure, e.g., within sample chamber  310  and in proximity to X-ray generation site  315 . Other components in  FIG.  3   , including components  322 ,  324 ,  326 ,  328  along an electron column having axis  305 , as well as pumping port components coupler  330  and pump  340 , are generally similar to similarly numbered components described in context of  FIG.  1 A  and are not described further. 
       FIG.  4    is a section view  400  of an apparatus with another X-ray shield deployed inside a vacuum enclosure. Particularly, innovative X-ray shield  450  can be placed within the vacuum enclosure, e.g., within pump coupler  430 . Other components in  FIG.  4   , including components  410 ,  422 ,  424 ,  426 ,  428  along an electron column having axis  405 , as well as pump  440 , are generally similar to similarly numbered components described in context of  FIG.  1 A  and are not described further. 
     Numerous variations and extensions of the disclosed method can be implemented. In some examples, an X-ray shield can be placed partly within sample chamber  310  and partly within pump coupler  330 , i.e., straddling intake aperture  332 . In other examples, multiple X-ray shields can be deployed within a vacuum enclosure, variously disposed between sample chamber  310 , pump coupler  330 , or proximate to other ports (e.g. similar to  181 - 187  of  FIG.  1 B ). In further examples, a combination of X-ray shields can be deployed inside a vacuum enclosure (similar to  350  or  450 ) and outside the vacuum enclosure (similar to  250 ). The shapes depicted for innovative X-ray shields  350 ,  450  are merely exemplary, and innovative X-ray shields can be fabricated in a wide range of shapes or sizes to suit geometric or functional constraints of a given application. 
     First Example Method 
       FIG.  5    is a flowchart  500  of a first example method for fabricating an X-ray shield. In this method, a shell is fabricated, leak-tested, filled, and sealed, to obtain an X-ray shield suitable for deployment within an ultra-high vacuum enclosure. 
     At process block  510 , a shell can be fabricated. The shell can define a chamber and can have one or more ports. At block  520 , the shell can be tested to verify that the shell is free of leaks. Then, at block  530 , the tested shell can be filled with an X-ray shielding material and, at block  540 , the port(s) of the shell can be sealed. 
     Numerous variations and extensions of the disclosed method can be implemented. Fabrication at block  510  can be performed by additive manufacturing, and can include one or more of direct metal laser sintering (DMLS), selective laser melting (SLM), electron beam melting (EBM), or binder jetting (BJ). 
     The shell can be rigid. The shell material can include stainless steel. The shell can have a median wall thickness of about 0.3 mm, or in a range 0.05 to 2.0 mm, or 0.1 to 1.0 mm. 
     Prior to leak testing, the chamber inside the shell can be temporarily isolated from the environment outside the shell. For example, one port can be sealed with a cap and another port can be coupled to a Helium leak detector. Temporary isolation can be maintained for the duration of leak testing  520  and can be undone thereafter so that the port(s) can be used for filling the shell at process block  530 . 
     Leak testing  520  can be performed to a predetermined leak rate threshold, meaning that a measured leak rate above the threshold results in failing the test, while a measured leak rate below the threshold results in passing the test. The predetermined leak rate threshold can be an ultimate sensitivity of the Helium leak detector, or another leak rate value higher than the ultimate sensitivity. To illustrate, a leak detector can have a sensitivity of 10 -9  mbar•l/s and the predetermined threshold can be 10 -7  mbar•l/s. Then, a shell having a detected leak rate of 10 -8  mbar•l/s can pass the leak test and can be deemed free of leaks. As another illustration, the predetermined threshold can be equal to the sensitivity of an instant leak detector, so that any detected leak causes a shell to fail the leak test, and an absence of any detectable leak is required to pass the leak test. 
     In some examples, a first port can be used during filling  530  to introduce an X-ray shielding material into the chamber, while a second port can be used to release pre-existing fluid (e.g., air) displaced from the chamber as the chamber is filled. In other examples, the second port can be connected to a pump to evacuate the chamber prior to filling, to avoid trapped air pockets and ensure a void-free X-ray shield. In further examples, more than one port can be used for either the filling function or the release function. Still further, filling  530  can be performed using a single port. The single port can be connected to two valves, one valve being opened for chamber evacuation by a pump, and the other valve being opened for filling from a supply reservoir. Alternatively, the single port can be used simultaneously for filling, through a feeding tube inserted into the port, and for release of displaced fluid, through a portion of the port not blocked by the feeding tube. 
     he X-ray shielding material can include at least 50% by weight of a high-Z metal, such as lead. Filling at block  530  can include introducing the metal into the chamber in a molten state. The X-ray shielding material can be in the form of metal powder suspended within a resin. 
     Sealing at block  540  can include welding a respective cap onto each of the one or more ports of the shell. 
     The shell can have an average atomic number Z1 and the method can be extended to cladding the shell with another material having average atomic number Z2 &lt; Z1. 
     Example Procedure for Reducing X-ray Emission 
     In some examples, the disclosed technologies can be applied to reduce X-ray emission from an electron microscope or from other X-ray generating equipment. The electron microscope or other equipment can have an X-ray generation site situated within a vacuum enclosure. An X-ray shield can be manufactured by the methods of  FIG.  5    or  FIG.  7   , as shown in the respective figure or using any of the variations or extensions described herein, in any combination. Then, the X-ray shield can be secured within an interior volume of the vacuum enclosure. In some examples, the X-ray shield can be positioned within a pump coupler, e.g., along a path between the X-ray generation site and a vacuum pump, and can be oriented to block greater than a first threshold fraction of X-rays emitted through an intake aperture of the pump coupler and parallel to a longitudinal axis of the X-ray shield. In varying examples, the first threshold fraction can be 50%, 80%, 90%, 95%, 98%, or 99%. 
     Example X-ray Shield 
       FIGS.  6 A- 6 D  are views  601 - 604  of a first example X-ray shield according to the disclosed technologies. Views  601 - 602  are cutaway views of the X-ray shield. For clarity of illustration, X-ray shielding material has been omitted from views  601 - 602 , but will be described below. 
       FIG.  6 A  shows a shell  610  having a curved twisted surface with wall  612  and defining a chamber  620 . Also part of shell  610  are base  614  and mounting flange  616 . In some examples, at least wall  612  can be fabricated using an additive manufacturing technique. In varying examples: one or both of base  614  and flange  616  can also be fabricated by additive manufacturing, e.g. in a same process operation as wall  612 ; one of base  614  and flange  616  can be provided as a base upon which wall  612  is additively manufactured; or one or both of base  614  and flange  616  can be fabricated separately from wall  612  and joined to wall  612  in a separate process operation. Accordingly, base  616 , wall  612 , and flange  614  can be formed of a same material or of different materials, in any combination. In some examples, base  616 , wall  612 , and flange  614  can all be stainless steel. In  FIG.  6 A , chamber  620  is shown hollow, to better illustrate the structure of the instant X-ray shield. As deployed for service, chamber  620  can be filled with an X-ray shielding material. XYZ coordinate axes  608  are also shown. 
       FIG.  6 B  is another view  602  through a midplane of flange  616 , showing a structure of port  622  sealed (temporarily or permanently) by cap  632 . Generally, cap  632  can be regarded as part of a completed X-ray shield but not as part of shell  610 . In  FIG.  6 B , dashed line  609  indicates a farthest extent of shell  610  in the direction of cap  632  (the Z direction of coordinate axes  608  shown in  FIG.  6 A ). 
       FIGS.  6 C- 6 D  are semi-transparent and cutaway views  603 - 604  of pump coupler  630  showing X-ray shield  650  (not in cutaway view, and filled with X-ray shielding material) positioned within pump coupler  630 . Flange  636  can be considered a part of pump coupler  630 . 
     The elongate twisted shape of shell  610  as illustrated can advantageously be used in or near a pump coupler (similar to X-ray shields  350 ,  450  of  FIGS.  3 - 4   ). On one hand, the azimuthal sweep of the twisted shape about the Z axis of  608  blocks a high fraction of X-rays parallel to the Z axis and within the transverse extent of shell  610 . On the other hand, the narrow transverse profile and gentle helical sweep of shell  610  provide low impedance to molecular flow through an associated pump coupler. 
     The illustrated shape of shell  610  is merely exemplary. In some examples, the azimuthal twist of shell  610  can be about 195° from flange  616  to base  614 , or in a range 180° to 210°. In other examples, a variety of transverse profiles and azimuthal sweeps can be used. A shell having a cross-shaped transverse section (that is with four arms) can be fabricated with only about 100° of azimuthal twist, or in a range 90° to 110°. Alternatively, a shell can be fabricated with a single vane (e.g. extending from about port  622  to edge  627  relative to view  602 ) rotating approximately 390° about the Z-axis from flange to base. In further examples, the curved surfaces of shell  610  can be approximated by a set of planar surfaces. 
     Second Example Method 
       FIG.  7    is a flowchart  700  of a second example method according to the disclosed technologies. This method incorporates some of the variations or extension described for  FIG.  5   . At process block  710 , a rigid shell can be fabricated by additive manufacturing. The shell can define a chamber and can have one or more ports. At process block  720 , to facilitate leak testing at block  730 , the shell’s interior and exterior can be temporarily isolated from each other. Block  720  can be performed by capping ports shut or coupling ports to a He leak detector, in any combination. At block  730 , the shell can be tested to verify that the shell is free of leaks to a predetermined leak rate threshold. 
     Once verified that the shell is free of leaks, at optional process block  740 , a low-Z cladding can be applied to the exterior of the leak-tested (and leak-free) shell. At block  750 , the tested shell can be filled with an X-ray shielding material. In some examples, block  750  can be performed using block  752 , by injecting molten metal (e.g., molten lead or molten tin) into an interior chamber of the shell. In other examples, block  754  can be used to perform block  750 . Particulate metal in a liquid can be injected into the interior chamber of the shell. In some examples, the particulate metal can be grains or powder of a metal such as lead or tungsten. 
     Once filled, the shell can be sealed at block  760 . In some examples, block  762  can be used to perform block  760 , by welding a cap onto each port of the filled shell. Other techniques can also be used. As another example, a shell port can be fabricated as a tube, and the tube can be crimped to close the port and then welded to seal the closed port. Fabrication of the X-ray shield can be complete after block  760 . 
     At block  770 , the fabricated X-ray shield can be secured within an interior volume of a vacuum enclosure. At block  780 , the vacuum enclosure, with X-ray shield inside, can be pumped to an ultra-high vacuum. Thus, the X-ray shield having an ultra-high vacuum on its outside, and having a possibly vacuum-incompatible material (such as lead or a resin) on its inside, can be regarded as an inverse vacuum bottle as described herein. 
     In variations of this method, block  740  can be performed later, e.g. between process blocks  750  and  760 , or between blocks  760  and  770 . 
     Example Apparatus 
       FIG.  8    is a block diagram of an apparatus  800  incorporating an X-ray shield. Apparatus  800  has a vacuum enclosure  802  in which an X-ray generation site  815  is present. X-ray shield  850  can also be situated within vacuum enclosure  802  and can include an inverse vacuum bottle  852  containing X-ray shielding material  854 . 
     In some examples, apparatus  800  can be an electron microscope having a column axis. Apparatus  800  can include a pump coupler (similar to  130  of  FIG.  1 A ). X-ray shield  850  can be positioned within the pump coupler, oriented to block at least 80% of X-rays that are emitted from X-ray generation site  820  through an intake aperture of pump coupler. In further examples, X-ray shield  830 , positioned within the pump coupler, can reduce a vacuum conductance of the pump coupler by at most 20%. The pressure within vacuum enclosure  802  can be held below 10 -9  mbar. X-ray shielding material  854  can be lead, or can include at least 50% by weight of lead. Inverse vacuum bottle  852  can incorporate stainless steel. In additional examples, inverse vacuum bottle  852  can be clad with a low-Z material having average atomic number less than or equal to 14. X-ray shield  850  can incorporate a twisted elongate member. 
     Third Example Method 
       FIG.  9    is a flowchart  900  of a third example method according to the disclosed technologies. At process block  910 , an inverse vacuum bottle containing an X-ray shielding material can be placed inside vacuum enclosure of an electron microscope. Then, at process block  920 , the vacuum enclosure can be pumped down to a pressure less than or equal to 10 -9  mbar. 
     In some examples, the vacuum enclosure can include a pump coupler and, at block  910 , the inverse vacuum bottle can be secured within the pump coupler. 
     Additional Example X-ray Shields 
       FIGS.  10 A- 10 B  are views  1001 - 1002  of a second example X-ray shield according to the disclosed technologies. In contrast to the example of  FIGS.  6 A- 6 D , the instant shield  1050  has two ports  1022 . Both  FIGS.  10 A- 10 B  show the shell  1010  (e.g., as fabricated at a process block similar to  510 ). Port caps and X-ray shielding material are omitted from  FIGS.  10 A- 10 B  for clarity of illustration. 
       FIG.  10 A  is an oblique view of shield  1050 .  FIG.  10 B  is a cutaway view depicting wall  1012  and chamber  1020 , arranged in an elongated twisted shape from base  1014  to flange  1016 . The depicted shape is illustrative, and other shapes can be used. XYZ coordinate axes  1008  are also shown. 
       FIG.  11    is a concept view  1100  illustrating a shape of a third example X-ray shield  1150 . Like other examples disclosed herein, shield  1150  has a shell  1110  enclosing a chamber  1120 . Walls  1112  of shell  1110  are shown as solid lines. Endcaps, ports, flanges, or a filling shielding material are omitted from  FIG.  11    for simplicity of illustration. Shield  1150  has a transverse cross-section which is thinner near longitudinal axis  1105  than away from axis  1105 . Redistributing shielding material away from axis  1105  can improve shielding for X-rays that are away from or divergent from axis  1105 , and can remove excess shielding material on-axis that provides only marginal benefit. Thus, the illustrated configuration can improve shielding effectiveness of a given mass of X-ray shielding material. Shield  1150  is illustrated having approximately 270° of azimuthal twist, but this is not a requirement and the illustrated concept can be applied to shields of other twists or other shapes. 
       FIGS.  12 A- 12 C  are concept views  1201 - 1203  illustrating a shape of a fourth example X-ray shield  1250 . Mechanical details such as a shell, a chamber, and ports are omitted from  FIG.  12    for simplicity of illustration, and can be similar to other examples disclosed herein.  FIG.  12 A  shows a profile view of shield  1250 , which has about 360° of twist about longitudinal axis  1205 .  FIG.  12 B  shows an oblique view of shield  1250 , while  FIG.  12 C  shows shield  1250  fitted inside a cylindrical pipe  1230 , which can be part of a pump coupler. The additional twist of shield  1250  relative to other disclosed examples can provide improved shielding, e.g. a greater percentage of X-rays blocked, particularly those having angles divergent from axis  1205 , as compared to a shield with less twist. 
       FIG.  13    is a concept view  1300  illustrating a shape of a fifth example X-ray shield  1350 . Whereas the shape of shield  1250  has two intertwined helical edges  1257  about axis  1205 , shield  1350  has one helical edge  1357  about axis  1305  and one straight edge  1355  generally collinear with axis  1305 . Compared with shield  1250 , shield  1350  can provide comparable shielding effectiveness for X-rays parallel to axis  1305  with lower vacuum impedance. Shield  1350  has a helical twist of approximately 390° about axis  1305 . 
       FIGS.  14 A- 14 D  are concept views  1401 - 1404  illustrating a shape of a sixth example X-ray shield  1450 . Mechanical details such as a shell, a chamber, and ports are omitted from  FIG.  14    for simplicity of illustration, and can be similar to other examples disclosed herein.  FIG.  14 A  shows a profile sectional view of shield  1450  which has an elongated twist member  1453  and a sleeve member  1456 .  FIG.  14 B  shows an end view of shield  1450 , while  FIG.  14 C  shows an oblique view of shield  1450 , and  FIG.  14 D  shows a cutaway section of shield  1450 . Like other disclosed examples, interior volume  1420  within twist member  1453  can be filled with X-ray shielding material. Twist member  1453  has about 360° of twist about longitudinal axis  1405 , like shield  1250 . In some examples, sleeve  1456  can be a double-wall pipe (e.g. an annular hollow shell) surrounding the same or a different chamber as inside twist member  1453 . Such a configuration can advantageously improve shielding for X-rays emitted at angles divergent from axis  1405 , in conjunction with a high level of shielding provided by twist member  1453  for X-rays parallel to axis  1405 . In other examples, sleeve  1456  can be a single-wall pipe, which may not significantly increase X-ray shielding effectiveness of shield  1450 , but which can improve mechanical rigidity or facilitate installation of shield  1450  within a pump coupler or other vacuum enclosure. 
     A Generalized Computer Environment 
       FIG.  15    illustrates a generalized example of a suitable computing system  1500  in which described examples, techniques, and technologies for controlling manufacture of an X-ray shield can be implemented. The computing system  1500  is not intended to suggest any limitation as to scope of use or functionality of the present disclosure, as the innovations can be implemented in diverse general-purpose or special-purpose computing systems. The computing system  1500  can control an additive manufacturing process, another manufacturing process, a leak-testing process, a pumping process, or operation of an electron microscope, or associated instrumentation; or can acquire, process, output, or store measurement or operational data. 
     With reference to  FIG.  15   , computing environment  1510  includes one or more processing units  1522  and memory  1524 . In  FIG.  15   , this basic configuration  1520  is included within a dashed line. Processing unit  1522  can execute computer-executable instructions, such as for control or data acquisition as described herein. Processing unit  1522  can be a general-purpose central processing unit (CPU), a processor in an application-specific integrated circuit (ASIC), or any other type of processor. In a multiprocessing system, multiple processing units execute computer-executable instructions to increase processing power. Computing environment  1510  can also include a graphics processing unit or co-processing unit  1530 . Tangible memory  1524  can be volatile memory (e.g., registers, cache, or RAM), non-volatile memory (e.g., ROM, EEPROM, or flash memory), or some combination thereof, accessible by processing units  1522 ,  1530 . The memory  1524  stores software  1580  implementing one or more innovations described herein, in the form of computer-executable instructions suitable for execution by the processing unit(s)  1522 ,  1530 . For example, software  1580  can include software  1581  for controlling an additive manufacturing process, software  1582  for controlling a coating process, software  1583  for controlling leak testing, or other software  1584 . The inset shown for software  1580  in storage  1540  can be equally applicable to software  1580  elsewhere in  FIG.  15   . The memory  1524  can also store control parameters, calibration data, measurement data, or database data. The memory  1524  can also store configuration and operational data. 
     A computing system  1510  can have additional features, such as one or more of storage  1540 , input devices  1550 , output devices  1560 , or communication ports  1570 . An interconnection mechanism (not shown) such as a bus, controller, or network interconnects the components of the computing environment  1510 . Typically, operating system software (not shown) provides an operating environment for other software executing in the computing environment  1510 , and coordinates activities of the components of the computing environment  1510 . 
     The tangible storage  1540  can be removable or non-removable, and includes magnetic disks, magnetic tapes or cassettes, CD-ROMs, DVDs, or any other medium which can be used to store information in a non-transitory way and which can be accessed within the computing environment  1510 . The storage  1540  stores instructions of the software  1580  (including instructions and/or data) implementing one or more innovations described herein. Storage  1540  can also store image data, measurement data, reference data, calibration data, configuration data, or other databases or data structures described herein. 
     The input device(s)  1550  can be a mechanical, touch-sensing, or proximity-sensing input device such as a keyboard, mouse, pen, touchscreen, or trackball, a voice input device, a scanning device, or another device that provides input to the computing environment  1510 . The output device(s)  1560  can be a display, printer, speaker, optical disk writer, or another device that provides output from the computing environment  1510 . Input or output can also be communicated to/from a remote device over a network connection, via communication port(s)  1570 . 
     The communication port(s)  1570  enable communication over a communication medium to another computing entity. The communication medium conveys information such as computer-executable instructions, audio or video input or output, or other data in a modulated data signal. A modulated data signal is a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media can use an electrical, optical, RF, acoustic, or other carrier. 
     A data acquisition system can be integrated into computing environment  1510 , either as an input device  1550  or coupled to a communication port  1570 , and can include analog-to-digital converters or connections to an instrumentation bus. An instrumentation control system can be integrated into computing environment  1510 , either as an output device  1560  or coupled to a communication port  1570 , and can include digital-to-analog converters, switches, or connections to an instrumentation bus. 
     In some examples, computer system  1500  can also include a computing cloud  1590  in which instructions implementing all or a portion of the disclosed technology are executed. Any combination of memory  1524 , storage  1540 , and computing cloud  1590  can be used to store software instructions and data of the disclosed technologies. 
     The present innovations can be described in the general context of computer-executable instructions, such as those included in program modules, being executed in a computing system on a target real or virtual processor. Generally, program modules or components include routines, programs, libraries, objects, classes, components, data structures, etc. that perform particular tasks or implement particular data types. The functionality of the program modules can be combined or split between program modules as desired in various embodiments. Computer-executable instructions for program modules can be executed within a local or distributed computing system. 
     The terms “computing system,” “computing environment,” and “computing device” are used interchangeably herein. Unless the context clearly indicates otherwise, neither term implies any limitation on a type of computing system, computing environment, or computing device. In general, a computing system, computing environment, or computing device can be local or distributed, and can include any combination of special-purpose hardware and/or general-purpose hardware and/or virtualized hardware, together with software implementing the functionality described herein. 
     General Considerations 
     As used in this application and in the claims, the singular forms “a,” “an,” and “the” include the plural forms unless the context clearly dictates otherwise. Additionally, the term “includes” means “comprises.” Further, the term “coupled” does not exclude the presence of intermediate elements between the coupled items. Furthermore, as used herein, the terms “or” and “and/or” mean any one item or combination of items in the phrase. 
     The systems, methods, and apparatus described herein should not be construed as limiting in any way. Instead, the present disclosure is directed toward all novel and non-obvious features and aspects of the various disclosed embodiments, alone and in various combinations and sub-combinations with one another. The disclosed systems, methods, and apparatus are not limited to any specific aspect or feature or combinations thereof, nor do the disclosed systems, methods, and apparatus require that any one or more specific advantages be present or problems be solved. The technologies from any example can be combined with the technologies described in any one or more of the other examples. Any theories of operation are to facilitate explanation, but the disclosed systems, methods, and apparatus are not limited to such theories of operation. 
     Although the operations of some of the disclosed methods are described in a particular, sequential order for convenient presentation, it should be understood that this manner of description encompasses rearrangement, unless a particular ordering is required by specific language set forth below. For example, operations described sequentially may in some cases be rearranged or performed concurrently. Moreover, for the sake of simplicity, the attached figures may not show the various ways in which the disclosed systems, methods, and apparatus can be used in conjunction with other systems, methods, and apparatus. Additionally, the description sometimes uses terms like “produce,” “provide,” or “test” to describe the disclosed methods. These terms are high-level abstractions of the actual operations that are performed. The actual operations that correspond to these terms will vary depending on the particular implementation and are readily discernible by one of ordinary skill in the art. 
     In some examples, values, procedures, or apparatus are referred to as “lowest”, “best”, “minimum,” or the like. It will be appreciated that such descriptions are intended to indicate that a selection among a few or among many alternatives can be made, and such selections need not be lower, better, less, or otherwise preferable to other alternatives not considered. 
     Theories of operation, scientific principles, or other theoretical descriptions presented herein in reference to the apparatus or methods of this disclosure have been provided for the purposes of better understanding and are not intended to be limiting in scope. The apparatus and methods in the appended claims are not limited to those apparatus and methods that function in the manner described by such theories of operation. 
     Any of the disclosed methods can be controlled by, or implemented as, computer-executable instructions or a computer program product stored on one or more computer-readable storage media, such as tangible, non-transitory computer-readable storage media, and executed on a computing device (e.g., any available computing device, including tablets, smart phones, or other mobile devices that include computing hardware). Tangible computer-readable storage media are any available tangible media that can be accessed within a computing environment (e.g., one or more optical media discs such as DVD or CD, volatile memory components (such as DRAM or SRAM), or nonvolatile memory components (such as flash memory or hard drives)). By way of example, and with reference to  FIG.  15   , computer-readable storage media include memory  1524 , and storage  1540 . The terms computer-readable storage media or computer-readable media do not include signals and carrier waves. In addition, the terms computer-readable storage media or computer-readable media do not include communication ports (e.g.,  1570 ). 
     Any of the computer-executable instructions for implementing the disclosed techniques as well as any data created and used during implementation of the disclosed embodiments can be stored on one or more computer-readable storage media. The computer-executable instructions can be part of, for example, a dedicated software application or a software application that is accessed or downloaded via a web browser or other software application (such as a remote computing application). Such software can be executed, for example, on a single local computer (e.g., any suitable commercially available computer) or in a network environment (e.g., via the Internet, a wide-area network, a local-area network, a client-server network, a cloud computing network, or other such network) using one or more network computers. 
     For clarity, only certain selected aspects of the software-based implementations are described. Other details that are well known in the art are omitted. For example, it should be understood that the disclosed technology is not limited to any specific computer language or program. For instance, the disclosed technology can be implemented by software written in Adobe Flash, C, C++, C#, Curl, Dart, Fortran, Java, JavaScript, Julia, Lisp, Matlab, Octave, Perl, Python, Qt, R, Ruby, SAS, SPSS, SQL, WebAssembly, any derivatives thereof, or any other suitable programming language, or, in some examples, markup languages such as HTML or XML, or with any combination of suitable languages, libraries, and packages. Likewise, the disclosed technology is not limited to any particular computer or type of hardware. Certain details of suitable computers and hardware are well known and need not be set forth in detail in this disclosure. 
     Furthermore, any of the software-based embodiments (comprising, for example, computer-executable instructions for causing a computer to perform any of the disclosed methods) can be uploaded, downloaded, side-loaded, or remotely accessed through a suitable communication means. Such suitable communication means include, for example, the Internet, the World Wide Web, an intranet, software applications, cable (including fiber optic cable), magnetic communications, electromagnetic communications (including RF, microwave, infrared, and optical communications), electronic communications, or other such communication means. 
     In view of the many possible embodiments to which the principles of the disclosed subject matter may be applied, it should be recognized that the illustrated embodiments are only preferred examples of the disclosed subject matter and should not be taken as limiting the scope of the claims. Rather, the scope of the claimed subject matter is defined by the following claims. We therefore claim all that comes within the scope and spirit of these claims.