developments in detector technology for xrd - nist · 20 • to cover a given solid angle we may...
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Developments in detector technology for XRD
Roger Durst
Bruker AXS Inc.
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Detector technology for enhanced speed and accuracy
• Energy discriminating strip detectors
• 2D detectors
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Monochromatization and energy dispersive detectors
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• Before 2000, >90% of all instruments have been equipped with point detectors, > 50% with (secondary) monochromators • Intensity loss ~80-90% with respect to unfiltered rad.
• Today, >90% of all instruments are equipped with PSDs, the majority with Kß-Filters • Intensity loss ~40-60% with respect to unfiltered radiation
• Absorption edges
• Poor filtering of fluorescense
• An energy dispesive detector could in principle improve effective sensitivity by 2-10 times and reduce errors due to absorption edges and fluorescence
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Monochromatization
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Monochromatization Artifacts from Kß-Filter
Pecharsky & Zavalij (2009)
Remnantwhite radiation
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Silicon strip detector principle
• Electron-hole pairs created in depleted silicon by X-ray photoionization • Charge carriers drift to readout
strips
• Key advantages • High counting rate
• Typically of order 106
counts/strip-sec
• Good spatial resolution
• Good energy resolution than other detectors
• Requires optimized readout
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Limitations of energy resolution due to charge sharing effects
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• Energy resolution is accomplished by “counting” the electrons in a strip or pixel
• ∆𝐸 ≥ 2.35 𝐹𝑁
• F=Fano factor, N=# of electron-hole pairs
• Problem, not all electrons are collected in a given strip (or pixel), in general some electrons will diffuse to neighboring strips or pixels: “charge sharing”
• Fall below discriminator level and are thus “lost”
• Lost electrons cause poor energy resolution
Low energy “tail” from charge sharing
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LYNXEYE XE
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• The LYNXEYE XE is the first energy dispersive Si strip detector for home-laboratory X-ray diffraction
• Inter-strip logic to correct for charge sharing
Detector type Compound silicon strip
# strips 192 strips, 75 mm pitch
Active area 14.4 x 16mm
Modes 1D and 0D
Wavelengths Cr, Co, Cu, Mo, and Ag Energy resolution
(Cu) < 680 eV @ 8 KeV
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Incorporation in inter-strip logic improves energy resolution 2-3 times compared to conventional Si detector
Comparable to Graphite monochromator
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LYNXEYE XE Energy Resolution with inter-strip logic
Device Resolution (KeV)
Scintillation counter ~3.5
Traditional silicon strip / pixel ~1.6 - 2.0
Proportional counter ~1.1 - 1.6
LYNXEYE XE <0.68
Graphite monochromator ~0.5 - 1.0
Si(Li) detector (Peltier cooled) <0.2
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Monochromatization Traditional Si Strip
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Monochromatization LYNXEYE XE
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Filtering of fluorescense radiation (Cu)
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LYNXEYE XE Filtering Fe fluoresecence: Iron Ore
• Fluorescense filtered completely • <5% loss of peak intensity
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• Manganese:
• Fluorescense filtered completely at <5% loss of peak intensity
• Iron:
• Fluorescense filtered completely at <5% loss of peak intensity
• Cobalt:
• > 90% Fluorescense filtered at <5% loss of peak intensity
• > 98% Fluorescense filtered at <25% loss of peak intensity
• Nickel:
• > 50% Fluorescense filtered at <5% loss of peak intensity
• > 90% Fluorescense filtered at about 60% loss of peak intensity using an additional primary Ni filter
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Filtering of Fluorescense Radiation (Cu)
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LYNXEYE XE Kβ filtering NIST SRM 1976a (Corundum)
Squ
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LYNXEYE XE, unfiltered radiation LYNXEYE XE, Kß filtering LYNXEYE XE, 0.0125 mm Ni Filter LYNXEYE XE, 0.02 mm Ni Filter
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LYNXEYE XE Kß filtering NIST SRM 1976a (Corundum)
Kα1,2
WL1
Kß3,1 Ni Edge
WL2 + Kβ
Device Remnant Kß Intensity loss
A LYNXEYE XE, Kß filtering 0.8% ~20% B LYNXEYE XE, 0.0125 mm Ni Filter 1.2% ~40% C LYNXEYE XE, 0.02 mm Ni Filter 0.3% ~60%
A B C
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2D detectors for XRD2
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Debye Cone
Sample
Incident Beam
XRD2: Diffraction pattern with both γ and 2θ information
• Integrating over larger γ range improves statistics
• Especially for microdiffraction, mapping or time resolved measurements
• 2D structure of Debye rings gives additional information • Stress, texture, particle size
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What is important for XRD2? Photon-counting detector
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• Photon-counting detectors are preferred
• In XRD2 typically integrate over hundreds or thousands of pixels
• Detectors with a finite noise background (e.g., CCD, IP) result in lower data quality
• CCDs and IPs better for crystallography (where reflections span only a few pixels)
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What is important for XRD2
BIGGER is BETTER
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• To cover a given solid angle we may use a big detector farther away or a small detector closer to the sample
• E.g., a 140 mm detector at D=150 mm and a 28 mm detector at D=30 mm cover the same solid angle
• For a given angular resolution, a smaller detector requires a smaller beam, this means LESS INTENSITY
• However, if the 140 mm detector employs a 500 micron beam, to achieve the same resolution the 28 mm needs to employ a 50 micron beam
• This results in a 100 times loss in intensity for the small detector!
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Types of photon-counting area detectors: Gas and Silicon Pixel Array (Si-PADs)
• Photon counting area detectors employ either conversion in gas (usually Xenon) or conversion in a semiconductor (usually Silicon) • Gas detectors: VANTEC (Bruker), Triton (Rigaku)
• Advantages • Large active areas, no gaps • Lower cost per unit area
• Disadvantages • Lower count rate capability
• Silicon pixel arrays: Pilatus (Dectris), others in development (e.g., Rigaku HPAD)
• Advantages • Higher count rate
• Disadvantages • Higher cost per unit area
Key advantage for lab
Key advantage for synchrotron
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Current generation XRD detectors derived from HEP technology
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Inner tracker: Silicon pixel detectors
Outer muon chambers: Gas (RPC) detectors
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Silicon Pixel Arrays Detectors for XRD2
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• Silicon Pixel Array detector (Dectris Pilatus 2M) proven to collect very high quality XRD2
• Uniquely capable of handling the very high flux at 3rd generation beamlines
• However, large Si-PAD arrays are so far less common for home laboratory use because of the relatively high cost
Courtesy P. Pattinson, ESRF
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Gas detector principle: Xe microgap detector
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• X-rays absorbed in high pressure Xenon
• Entrance window spherical to minimize parallax
• Electrons drift to amplification grid
• Electrons undergo Townshend avalanche multiplication
• Gain >106
• Results in noiseless readout
• Resistive anode protects against discharges (US patent 6340819)
• Readout strips capacitively coupled
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Large active area: 140 mm in dia. Frame size: 2048 x 2048 pixels Pixel size: 68 µm x 68 µm High sensitivity: 80% DQE for Cu No gaps Highly uniform response (<1%) High max linear count rate: 0.9 Mcps Low background noise:<10-5 cps/pix Maintenance-free: no re-gassing Very radiation hard Proven reliability: >500 installed
worldwide
VÅNTEC-500 – Xe microgap detector
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XRD2 data
Multilayer battery anode.
2θ coverage: 70° at 8 cm detector distance
A single frame showing information on phase, stress, texture and grain size
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Fast mapping: Scanning Over a Tooth by XRD2
Embedded Tooth with its Polished
Surface Measurements Taken Vertically, Lengthwise
along the Tooth´s Diameter
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Microdiffraction: Forensic analysis of car paint
video image (for documentation)
integration of data: diffractogram for phase identification
36-0426 (*) - Dolomite - CaMg(CO3)205-0586 (*) - Calcite, syn - CaCO321-1276 (*) - Rutile, syn - TiO2New Frame - File: 2708_07.raw - Start: 11.905 ° - End: 78.926 ° - Step: 0.040 °
Lin (C
ounts
)
0
500
1000
1500
2000
2-Theta - Scale12 20 30 40 50 60 70
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• Energy dispersive detector arrays represents a paradigm change in laboratory X-ray diffraction • Highly effective filtering of fluorescence, white radiation and K-beta
radiation at greatly reduced intensity losses compared to conventional detectors
• No absorption edges associated with metal filters
• Significantly improved intensity, peak-to-background-ratio, lower limits of detection
• XRD2 is a powerful technique for a variety of applications including mapping, microdiffraction, stress, texture
• Gas detectors and silicon pixel array detectors both deliver vey high quality XRD2
• For applications with higher count rates silicon detectors are preferred
• For applications with lower count rates gas detectors deliver comparable data
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Conclusions
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Acknowledgments
• LynxEye XE • W. Dabrowski, T. Fiutowski, P. Wiacek, Univesity of Science and
Technology, Krakow
• J. Fink, H.G. Krane, Bruker AXS
• VANTEC 500 • B. Becker, G. Wachter, S. Medved, Bruker AXS
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1. Mai 2013 31 1. Mai 2013 31 © Copyright Bruker Corporation. All rights reserved