Wds technology june 2015
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1. The document discusses two types of wavelength dispersive spectrometers (WDS): Rowland circle WDS and parallel beam WDS. 2. Rowland circle WDS uses curved diffractors to focus diffracted x-rays, but suffers from low intensities for low energy x-rays when used on SEMs. 3. Parallel beam WDS uses a collimating optic near the sample and flat diffractors, providing excellent intensities across all energies without complicated geometry.
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Wds technology june 2015
- 1. 1 The world leader in serving science Steve Seddio WDS spectrometer technologies: Not all WDS spectrometers are created equally
- 2. 2 What is WDS? nλ = 2d sinθ
- 3. 3 • The result: • Best energy resolution • Best peak-to-background Why WDS? EDS WDS
- 4. 4 The Challenge of WDS • Hypothetical sample in an SEM • For microanalysis, sample is normal to the electron beam
- 5. 5 The Challenge of WDS • X-rays are emitted from the excitation volume in a 3-dimensional hemispherical wavefront
- 6. 6 The Challenge of WDS • Get diverging X-rays to diffract off of a diffractor and then be counted by a detector at a meaningful intensity • Flat diffractors: • Simple geometry • X-rays interact with diffractor with different θ • X-rays continue to diverge after diffraction → low count rates at detector.
- 7. 7 Meeting the Challenge • All WDS spectrometers consist of • Diffractor • Proportional counter (a.k.a., detector) • Flowing P10 gas (90% Ar, 10% CH4) • Sealed Xe • Some WDS spectrometers include an X-ray optic near sample • There are two types of spectrometers that have been developed to meet the WDS challenge • Rowland circle WDS • Parallel beam WDS
- 8. 8 Meeting the Challenge: Intensity • For WDS, the count rate is a function of • X-ray generation • Accelerating voltage • Beam current • Vacuum • Sample composition • Sample preparation • Solid Angle and Optic • Diffractor size • Diffractor distance from sample • Size of optic • Reflectance and/or transmittance of optic • Other • Diffractor “reflectance” • Type of gas in detector • Pressure of gas in detector • Transmittance of detector window • Ambient temperature • Presence / transmittance of spectrometer window
- 9. 9 Rowland Circle WDS: The Classic Solution (c. 1882) • Curved diffractors focus the diffracted X-rays on to detector • Requires complex motion so that the sample, diffractor, and detector remain on a circle of fixed radius
- 10. 10 Rowland Circle WDS: The Classic Solution
- 11. 11 Rowland Circle WDS: The Classic Solution • To maintain RC geometry, diffractor must move away from sample for low-energy X-rays • The result is a decrease in solid angle → low intensity (c/(s×nA)) for low energy X-rays
- 12. 12 Rowland Circle WDS: The Classic Solution • Diffractor rotation affects solid angle
- 13. 13 Rowland Circle WDS: Solid Angle 102030405060 SolidAngle θ Solid angle as a function of θ
- 14. 14 X-ray Energy (keV) 0.1 1 10 SolidAngle(sr) 0.00 0.01 0.02 0.03 0.04 0.05 0.06 Rowland Circle WDS: SEM Solid Angle Mo/B4C 2d = 200 Solid angle as a function of energy Be B O Al Si Ti Fe Cu Sr 210 mm SEM SAdiff = 661 mm2 Moon or Sun
- 15. 15 X-ray Energy (keV) 0.1 1 10 SolidAngle(sr) 0.00 0.01 0.02 0.03 0.04 0.05 0.06 Mo/B4C or C/W 2d = 145 Rowland Circle WDS: SEM Solid Angle Solid angle as a function of energy Be B O Al Si Ti Fe Cu Sr 210 mm SEM SAdiff = 661 mm2
- 16. 16 X-ray Energy (keV) 0.1 1 10 SolidAngle(sr) 0.00 0.01 0.02 0.03 0.04 0.05 0.06 Ni/C or C/W 2d = ~100 Rowland Circle WDS: SEM Solid Angle Solid angle as a function of energy Be B O Al Si Ti Fe Cu Sr 210 mm SEM SAdiff = 661 mm2
- 17. 17 X-ray Energy (keV) 0.1 1 10 SolidAngle(sr) 0.00 0.01 0.02 0.03 0.04 0.05 0.06 Ni/C or Cr/Sc or C/W 2d = ~80 Rowland Circle WDS: SEM Solid Angle Solid angle as a function of energy Be B O Al Si Ti Fe Cu Sr 210 mm SEM SAdiff = 661 mm2
- 18. 18 X-ray Energy (keV) 0.1 1 10 SolidAngle(sr) 0.00 0.01 0.02 0.03 0.04 0.05 0.06 W/Si or C/W 2d = 60 Rowland Circle WDS: SEM Solid Angle Solid angle as a function of energy Be B O Al Si Ti Fe Cu Sr 210 mm SEM SAdiff = 661 mm2
- 19. 19 X-ray Energy (keV) 0.1 1 10 SolidAngle(sr) 0.00 0.01 0.02 0.03 0.04 0.05 0.06 W/Si 2d = 45 Rowland Circle WDS: SEM Solid Angle Solid angle as a function of energy Be B O Al Si Ti Fe Cu Sr 210 mm SEM SAdiff = 661 mm2
- 20. 20 X-ray Energy (keV) 0.1 1 10 SolidAngle(sr) 0.00 0.01 0.02 0.03 0.04 0.05 0.06 TAP 2d = 25.757 Rowland Circle WDS: SEM Solid Angle Solid angle as a function of energy Be B O Al Si Ti Fe Cu Sr 210 mm SEM SAdiff = 661 mm2
- 21. 21 X-ray Energy (keV) 0.1 1 10 SolidAngle(sr) 0.00 0.01 0.02 0.03 0.04 0.05 0.06 PET 2d = 8.742 Rowland Circle WDS: SEM Solid Angle Solid angle as a function of energy Be B O Al Si Ti Fe Cu Sr 210 mm SEM SAdiff = 661 mm2
- 22. 22 X-ray Energy (keV) 0.1 1 10 SolidAngle(sr) 0.00 0.01 0.02 0.03 0.04 0.05 0.06 LiF (200) 2d = 4.027 Rowland Circle WDS: SEM Solid Angle Solid angle as a function of energy Be B O Al Si Ti Fe Cu Sr 210 mm SEM SAdiff = 661 mm2
- 23. 23 X-ray Energy (keV) 0.1 1 10 SolidAngle(sr) 0.00 0.01 0.02 0.03 0.04 0.05 0.06 LiF (220) 2d = 2.848 Rowland Circle WDS: SEM solid angle Solid angle as a function of energy Be B O Al Si Ti Fe Cu Sr 210 mm SEM SAdiff = 661 mm2
- 24. 24 X-ray Energy (keV) 0.1 1 10 SolidAngle(sr) 0.00 0.01 0.02 0.03 0.04 0.05 0.06 Rowland Circle WDS: SEM Solid Angle Solid angle as a function of energy 210 mm SEM SAdiff = 661 mm2 Be B O Al Si Ti Fe Cu Sr LiF (200) 2d = 4.027 PET 2d = 8.742 TAP 2d = 25.757 Mo/B4C 2d = 200 Cr/Sc 2d = ~80
- 25. 25 Rowland Circle WDS: The Electron Microprobe (EPMA) • Electron microscope with typically 5 RC-WDS spectrometers • Spectrometers can concurrently analyze the same or different elements • 2 or 4 diffractors in each spectrometer
- 26. 26 X-ray Energy (keV) 0.1 1 10 SolidAngle(sr) 0.00 0.01 0.02 0.03 0.04 0.05 0.06 Rowland Circle WDS: EPMA Solid Angle Solid angle as a function of energy Be B O Al Si Ti Fe Cu Sr 160 mm EPMA SAdiff = 704 mm2 210 mm SEM SAdiff = 661 mm2
- 27. 27 X-ray Energy (keV) 0.1 1 10 SolidAngle(sr) 0.00 0.01 0.02 0.03 0.04 0.05 0.06 Rowland Circle WDS: EPMA Solid Angle Solid angle as a function of energy Be B O Al Si Ti Fe Cu Sr 160 mm EPMA SAdiff = 704 mm2 210 mm SEM SAdiff = 661 mm2 160 mm EPMA SAdiff = 1320 mm2
- 28. 28 X-ray Energy (keV) 0.1 1 10 SolidAngle(sr) 0.00 0.01 0.02 0.03 0.04 0.05 0.06 Rowland Circle WDS: EPMA Solid Angle Solid angle as a function of energy Be B O Al Si Ti Fe Cu Sr 160 mm EPMA SAdiff = 704 mm2 210 mm SEM SAdiff = 661 mm2 160 mm EPMA SAdiff = 1320 mm2 LiF (200) 2d = 4.027PET 2d = 8.742 TAP 2d = 25.757 Ni/C 2d = ~100 W/Si 2d = 60 W/Si 2d = 45
- 29. 29 Rowland Circle WDS: The Classic Solution • Pros • Excellent energy resolution • Excellent peak-to-background • Cons • Complicated spectrometer geometry • Best results when the chamber is designed for the spectrometer. • X-rays measured with large θ have low intensities • Typically relies on an optical microscope to ensure sample is at proper working distance Not commonly available to the SEM user. Requires a horizontal geometry
- 30. 30 Parallel Beam WDS: A Modern Approach • Parallel beam WDS spectrometer • Collimating optic is located near (~ 20 mm) from sample • Grazing incidence • Polycapillary • Hybrid • Parallel X-ray beam incident on diffractor • No Rowland circle geometry needed • Diffractor is flat
- 31. 31 Parallel Beam WDS: A Modern Approach
- 32. 32 X-ray Energy (keV) 0.1 1 10 SolidAngle(sr) 0.00 0.01 0.02 0.03 0.04 0.05 0.06 Parallel Beam WDS: A Modern Approach Polycapillary Be B O Al Si Ti Fe Cu Sr
- 33. 33 X-ray Energy (keV) 0.1 1 10 SolidAngle(sr) 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 Parallel Beam WDS: A Modern Approach Solid Angle ≠ Intensity Grazing incidence Polycapillary Hybrid Be B O Al Si Ti Fe Cu Sr
- 34. 34 X-ray Energy (keV) 0.1 1 10 EffectiveSolidAngle(sr) 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 Parallel Beam WDS: A Modern Approach Grazing incidence Polycapillary Hybrid Be B O Al Si Ti Fe Cu Sr
- 35. 35 X-ray Energy (keV) 0.1 1 10 EffectiveSolidAngle(sr) 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 Parallel Beam WDS: A Modern Approach Grazing incidence Polycapillary Hybrid 210 mm SEM Be B O Al Si Ti Fe Cu Sr
- 36. 36 Parallel Beam WDS: A Modern Approach • Pros • Excellent energy resolution • Excellent peak-to-background • Excellent intensity • Cons • Cannot accommodate a slit to modestly improve energy resolution with a large intensity cost
- 37. 37 Head-to-Head Comparison: SEM and EMP • Identical accelerating voltage and beam current • PB-WDS with hybrid optic • 1 spectrometer • Sealed Xe detector • Flat diffractors • 160 mm RC-WDS • 5 spectrometers • 3 detectors with 0.1 atm P10 flow • 2 detectors with 1 atm P10 flow • Curved diffractors (Johann and Johansson) X-ray Energy (keV) 0.1 1 10 EffectiveSolidAngle(sr) 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 Seddio and Fournelle 2015
- 38. 38 X-ray Energy (keV) 0.1 1 10 EffectiveSolidAngle(sr) 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 Head-to-Head Comparison: SEM and EMP LiF (200) 2d = 4.027 LiF (220) 2d = 2.848
- 39. 39 Head-to-Head Comparison: SEM and EMP X-ray Energy (keV) 0.1 1 10 EffectiveSolidAngle(sr) 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 LiF (200) 2d = 4.027
- 40. 40 X-ray Energy (keV) 0.1 1 10 EffectiveSolidAngle(sr) 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 Head-to-Head Comparison: SEM and EMP PET 2d = 8.742 LiF (200) 2d = 4.027
- 41. 41 Head-to-Head Comparison: SEM and EMP X-ray Energy (keV) 0.1 1 10 EffectiveSolidAngle(sr) 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 TAP 2d = 25.757 PET 2d = 8.742
- 42. 42 Head-to-Head Comparison: SEM and EMP X-ray Energy (keV) 0.1 1 10 EffectiveSolidAngle(sr) 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 TAP 2d = 25.757
- 43. 43 Head-to-Head Comparison: SEM and EMP X-ray Energy (keV) 0.1 1 10 EffectiveSolidAngle(sr) 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 Ni/C 2d = ~100 W/Si 2d = 60
- 44. 44 Head-to-Head Comparison: SEM and EMP X-ray Energy (keV) 0.1 1 10 EffectiveSolidAngle(sr) 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 Ni/C 2d = ~100 Mo/B4C 2d = 200
- 45. 45 Conclusions • RC-WDS • Excellent solution when microscope is designed primarily for WDS • Yields impractically low intensities on SEMs • PB-WDS • Unrivaled low energy X-ray intensity • High energy X-ray intensities consistent with the best RC-WDS intensities on an SEM
Editor's Notes
- What is WDS? Unlike EDS, in which all X-ray wavelengths are counted concurrently, WDS counts only one wavelength at a time. All WDS relies on Bragg’s Law. The full X-ray continuum is incident on a diffractor. Only X-rays whose wavelength satisfy nλ = 2d sinθ undergo constructive interference and are reflected to a detector.
- Why WDS? Of any microanalytical technique, WDS has the best energy resolution and the best peak-to-background ratio. WDS is able to resolve confusing EDS X-ray interferences.
- The challenge of WDS is to get diverging X-rays to diffract off of a diffractor and then be counted by a detector at a meaningful intensity. The simplest arrangement of sample, diffractor, and detector would be to use a flat diffractor. However, X-rays are incident on the diffractor with different angles. Note that θ at the left of the diffractor is larger than θ at the right. The result would be that different wavelengths would be diffracted across the diffractor. Moreover, X-rays would continue to diverge after diffraction yielding a very low intensity at the detector.
- All WDS spectrometers consist of a diffractor and a proportional counter. In practice, two kinds of proportional counters are used: flowing P10 gas or sealed Xe. Some spectrometers also include an optic located near the sample. In order to meet the challenge of WDS, two main types of spectrometers have been developed: Rowland circle spectrometers and parallel beam spectrometers.
- The intensity measured by a WDS spectrometer is a function of many factors. Some factors are beyond the control of the spectrometer such as accelerating voltage, beam current, SEM vacuum, and sample composition and preparation. However, the design of the spectrometer can influence the measured intensity in many ways. Today, we’ll investigate the effects that diffractor solid angle and the presence of an X-ray optic have on intensity in Rowland circle and parallel beam spectrometers. The factors involved are the diffractor size, the distance of the diffractor from the sample, the size of the optic, and the reflectance and/or transmittance of the optic. Other spectrometer-dependant factors include diffractor reflectance, the type of gas in the detector, the pressure of gas in the detector, the transmittance of the detector window, the ambient temperature, and the presence/transmittance of a window between the SEM chamber and the spectrometer.
- First, let’s examine Rowland circle WDS spectrometers. The concept of the Rowland circle was first developed by Henry Rowland around 1880, and it was applied to X-ray microanalysis about 60 years ago. In order to count monochromatic X-rays, Rowland circle geometry requires sample, diffractor, and detector to be positioned on an imaginary circle of fixed radius, “R.” The diffracted X-rays can be semi-focused on the detector by using Johann diffractors, which are bent to a radius of 2R. True focusing can be achieved by using Johansson diffractors, which are bent to a radius of 2R and then ground to a radius of R. Both diffractor types are commonly used. In practice, the crystal diffractors are made as the Johansson type, and the layered diffractors are made in the Johann type because they are they are difficult to manufacture as Johansson.
- This animation depicts the motion of a Rowland circle spectrometer. The colors represent the relative energies of X-rays with red being low and violet being high. Note that when higher energy X-rays are being analyzed, the diffractor is relatively close to the sample and when lower energy X-rays are being analyzed, the diffractor is relatively far from the sample.
- The distance between the sample and diffractor can be expressed as a function of θ or as a function of wavelength. The solid angle of an object describes how big the object appears to be to an observer and is defined as the surface area of the object divided by the square of the distance between the observer and the object. For example, the sun is very big, but very far. The moon is much smaller than the sun, but much closer to Earth. The sun and the moon have approximately the same solid angle in the sky. It is important to note that the solid angle of a Rowland circle spectrometer is defined by the X-ray being analyzed and cannot simply be improved by moving the spectrometer closer to the sample, as it can with EDS. In the case of a Rowland circle spectrometer, the larger the solid angle of a diffractor, the more X-rays incident on the diffractor, and the more X-rays that can be diffracted to the detector.
- In Rowland circle geometry, the rotation of the diffractor requires that the solid angle of the diffractor be scaled by sinθ. The solid angle of the diffractor can then be expressed as a function of θ or as a function of wavelength.
- For a diffractor with a given 2d-spacing and area, the solid angle as a function of θ would look like this. The solid angles achievable by different Rowland circle spectrometers can be calculated and compared if the Rowland circle radii and diffractor areas are known.
- Here is a plot of the solid angle in steradians as a function of X-ray energy with the location of a few K-alpha lines for reference. First, we will examine the solid angles achievable by Rowland circle spectrometers on SEMs. The radius of the Rowland circle is 210 mm, which is the radius available for Rowland circle WDS systems available on SEMs. The diffractor area is 661 square-mm as is available on that system. The solid angle as a function of X-ray energy is plotted for a many diffractors with different 2d-spacings. Different Rowland circle spectrometer manufacturers offer different diffractors. For the sake of comparison, I have plotted most of them. For reference, the arrow points to the solid angle of the moon or sun.
- Rowland circle spectrometers mounted on SEMs can typically accommodate 4 to 6 diffractors. Users would typically choose to include the diffractors in their system that offer the maximum range of X-ray energies. Such a selection for an SEM may look like this. Again, as we saw in the animation, on a given diffractor, higher energy X-rays can be measured with greater solid angle than lower energy X-rays. For example, note the solid angle of O K-alpha as measured on Cr/Sc compared to that measured on TAP. Another example commonly encountered is the vastly different intensities of Si K-alpha measured on TAP and PET. However, with a Rowland circle spectrometer on an SEM, mechanical limitations prohibit the analysis of Si K-alpha with TAP, whereas TAP is the preferred diffractor with which to analyze Si K-alpha on all microprobe spectrometers.
- The electron microprobe is an electron microscope that is specifically designed for Rowland circle WDS analysis. They typically have 5 Rowland circle spectrometers, which allow the user to analyze multiple or the same elements concurrently on multiple spectrometers. Each spectrometer can hold 2 or 4 diffractors.
- Because the microprobe is specifically designed for Rowland circle WDS, the circle radii on microprobes can be smaller than it is for SEM Rowland circles. Common Rowland circle radii for microprobes are 140 mm and 160 mm, and they are typically available with 300 and 704 square-mm diffractors, respectively. The plot has been amended with solid angle calculations for microprobe with a 160 mm Rowland circle and a 704 square-mm diffractor. The solid angle is strongly improved at all energies and for all diffractors. For a given diffractor, the Rowland circle geometry still yields dramatic changes in solid angle as a function of energy. In Rowland circle systems, there is no way to avoid these dramatic solid angle variations. In order to compensate for the relatively poor intensities achieved at the low-energy spectrometer positions, microprobe manufactures provide two options for improving the overall solid angles of their spectrometers. These enhancements are either using larger diffractors or choosing a smaller Rowland circle radius.
- In red, is the solid angle of the same Rowland spectrometer in blue, but with a diffractor with nearly twice the area, which nearly doubles the solid angle. Again, the reason we care about the solid angle of the diffractor is because it is directly proportional to the intensity measured by the detector. So since these enhanced spectrometers are so much better, why wouldn’t you only use them? For spectrometers with large diffractors, only 2 diffractors fit in a spectrometer at once. Smaller diffractors allow up to 4 diffractors to fit in a spectrometer at once. Additionally, enhanced spectrometers with smaller Rowland circles can only fit on a microprobe where the manufacturer permits.
- Here I have limited the plot to diffractors and spectrometers in an actual university microprobe. Not all diffractors can be used concurrently. Only two spectrometers are enhanced with large crystals. But multiple spectrometers can have the same the same diffractors which effectively adds the solid angles. The same selection of 5 diffractors for the SEM Rowland circle is included in pink. This is why you don’t replace your microprobe with a single RC spectrometer.
- As with all WDS systems, Rowland circle spectrometers, on SEMs and microprobes, yield excellent energy resolution and peak-to-background ratios. However, the complicated geometry of the Rowland circle requires that the microscope chamber be designed especially for the purpose of Rowland circle WDS. On an SEM, the Rowland circle requires many compromises. X-rays measured with large θ have low intensities. This is especially true for low energy X-rays such as B K-alpha. Additionally, most Rowland circle WDS systems include an optical microscope in-line with the electron column to ensure that the sample is at proper working distance. This typically isn’t available to the SEM user. To account for this, most Rowland circle WDS systems are mounted with an inclined or horizontal geometry. Inclined Rowland circle spectrometers are less susceptible to the loss of intensity resulting from small deviations from the analytical working distance. However, rigorous quantitative Rowland circle WDS analysis still requires that standards and samples are both measured at the same working distance. Also, even with a horizontal Rowland circle spectrometer intensity still drops off with deviation from the analytical working distance.
- Parallel beam WDS spectrometers are a modern innovation of the WDS technique. It adds an X-ray optic to the spectrometer that collimates the diverging X-rays and, like an EDS spectrometer, is inserted very close to the sample. There are 3 kinds of collimating optics used in parallel beam WDS spectrometers. Grazing incidence optics are parabolic mirrors that work by the same principles as the mirrors in some telescopes or that can be used to cook food with sunlight. Instead of incoming parallel light being focused to a point, the source of the diverging X-rays are oriented to the focus point of the parabolic mirror and are then collimated. A polycapillary optic works much like a fiber optic. The most recent innovation in collimating X-ray optics is a hybrid in which a polycapillary optic is situated at the center of a grazing incidence optic. With a parallel beam of X-rays, no Rowland circle geometry is necessary. The diffractors can be flat.
- This is an animation of the motion of a parallel beam WDS spectrometer. The full X-ray continuum is collimated by the optic and is incident on the diffractor. Monochromatic X-rays are then diffracted to the detector. The color represents the relative X-ray energy with red being low and violet being high. θ is the angle of the diffractor with the incident X-rays. 4 diffractors are given and the X-ray energies diffracted for that θ are given.
- Let’s investigate the solid angles relevant to parallel beam WDS and compare it with the Rowland circle systems. The solid angle of the diffractors is inconsequential because with a parallel beam, the X-ray flux of the diffractor is not a function of the distance between the diffractor and the sample. It is the solid angle of the X-ray optic that is important because, ideally, every X-ray that is incident on the optic is collimated and incident on the diffractor. Here the solid angle of a policapillary optic is plotted along with the Rowland circle calculations. Because the optic only has one analytical distance from the sample, its solid angle does not change.
- The solid angle of the grazing incidence and hybrid optics have been added to the plot. Note that that the scale of the Y-axis has changed significantly. The hybrid solid angle is the sum of the solid angles of the grazing incidence and polycapillary optics. For Rowland circle WDS, the solid angle of the diffractor is directly proportional to the intensity of the X-rays incident on the diffractor. However, this is not the case for parallel beam WDS because the reflectance and/or transmittance of the optic has a strong effect on the intensity observed at the diffractor.
- Here, the solid angles of the parallel beam optics have been scaled by their reflectance and transmittance. The Y-axis can no longer truly be “solid angle.” I have termed it “effective solid angle” which should be interpreted as the relative intensity observed at the diffractor. Now the calculated solid angles can be meaningfully compared. The effective solid angle of the grazing incidence optic is excellent for X-rays below ~0.5 keV. Above ~0.5 keV, it fails to reflect a meaningful number of X-rays. The polycapillary optic yields a rather consistent effective solid angle of ~0.018 sr between 0–7 keV. The hybrid optic yields an effective solid angle that it the sum of the effective solid angles of the grazing incidence and polycapillary optics. The Rowland circle WDS solid angles are plotted in the background for comparison. It is evident that parallel beam WDS yields the highest X-ray flux at the diffractors for low energy X-rays for a single spectrometer, and microprobe Rowland circle WDS can yield the highest X-ray flux for high energy X-rays.
- However, it is best to compare WDS systems available for the SEMS. The grazing incidence optic dramatically outperforms the SEM Rowland circle below ~0.3 keV but is inferior above that. The polycapillary optic outperforms or matches the effective solid angle of the SEM Rowland circle between ~0-8 keV. Above 8 keV, the SEM Rowland circle outperforms the polycapillary but not dramatically. The hybrid optic optic outperforms or matches the effective solid angle of the SEM Rowland circle between ~0-8 keV and dramaically outperforms the SEM Rowland circle WDS below ~0.3 keV. It is noteworthy that there are few cases in which X-ray interferences occur above ~8 keV.
- Parallel beam WDS yields excellent energy resolution, peak-to-background ratios, and intensities, especially at lower X-ray energies. However, because the X-rays incident on the detector are parallel, a slit cannot be inserted in front of the detector in order to modestly improve energy resolution as can be done with Rowland circle WDS. The problem with slitting the detector is that doing so dramatically decreases the measured intensity.
- In order to confirm the calculated theoretical results, a head-to-head- comparison was done. Intensities on metal standards were measured with a parallel beam spectrometer with a hybrid optic mounted on an SEM and an electron microprobe with 5 160 mm radius Rowland circle spectrometers. When possible, all analytical conditions were kept identical such as samples, accelerating voltage, and beam current. Some differences were unavoidable. The parallel beam system uses a sealed Xe proportional counter. The microprobe uses a proportional counter with 0.1 atm of flowing P10 gas in 3 spectrometers for the measurement or lower energy X-rays and a proportional counter with 1 atm of flowing P10 gas in 2 spectrometers for the measurement or higher energy X-rays.
- Here are he results for measure the intensity of Cu K-alpha. The “LiF*” refers to the LiF220 crystal. The microprobe spectrometer with a large LiF and a high-pressure detector yields more than double the measured intensity obtained on the SEM LiF. Based on effective solid angle, one would expect this difference to be much greater; however, the Xe proportional counter is much more effective at counting high energy X-rays relative to the P10 used by the microprobe. In fact, on the LiF 220 crystal measurements, the SEM out performs the microprobe with a regular sized crystal even though the effective solid angle is slightly greater for the microprobe.
- The Fe K-alpha results are similar to those for Cu. The microprobe slightly outperforms with the large diffractor and high-pressure counter although a greater difference is expected.
- The measurement of Ti K-alpha yields similar results between the microprobe and the SEM.
- The microprobe outperforms the SEM when measuring Si K-alpha on Tap, but not on PET. Remember that on the 210 mm Rowland circle WDS available on an SEM, Si cannot be measured with TAP.
- The Al K-alpha intensity measured on the microprobe is about double that measured in the SEM.
- For C K-alpha, the SEM outperforms the microprobe. Note that the same Ni/C 2d-spacings were not available for this experiment.
- As expected, the SEM strongly outperforms the B K-alpha intensities measured on the microprobe.
- Rowland circle WDS is a tried and true technique that yields excellent results when the microscope is designed specifically for those spectrometers and multiple spectrometers can be fit to the instrument concurrently. On an SEM, Rowland circle WDS yields impractically low intensities. Parallel beam WDS with a hybrid optic unequivocally yields the best intensities for low energy X-rays. It matches or exceeds the intensities for high energy X-rays obtained by Rowland circle WDS on an SEM.