data acquisition - penn state college of earth and …ryba/coursework/rietveld... · 2 data...
TRANSCRIPT
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Data AcquisitionWhat choices need to be made?
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Data AcquisitionWhat choices need to be made?
Specimen type and preparation
Radiation source
Wavelength
Instrument geometry
Detector type
Instrument setup
Scan parameters
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Data AcquisitionWhat choices need to be made?
Specimen type and preparation
Slide mount
Front loading cavity
Back loading cavity
Side drifting cavity
Low backgrd plate
Several spherical particle techniques
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Data AcquisitionWhat choices need to be made?
Specimen type and preparation
Slide mount
Front loading cavity
Back loading cavity
Side drifting cavity
Low backgrd plate
Several spherical particle techniques
Preferred orientation is worst prep problem
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Data AcquisitionPreferred orientation
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Data AcquisitionPreferred orientation
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Data AcquisitionWhat choices need to be made?
Specimen type and preparation
Slide mount
Front loading cavity
Back loading cavity
Side drifting cavity
Low backgrd plate
Several spherical particle techniques
Low angle problem - fixed divergence slit:
specimen
X
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Data Acquisition
Specimen type and preparation
To get good particle statistics, generally want size < 10 µ
Poorly ground sample:
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Data AcquisitionWhat choices need to be made?
Specimen type and preparation
Slide mount
Front loading cavity
Back loading cavity
Side drifting cavity
Low backgrd plate
Several spherical particle techniques
Neutron diffraction requires larger specimens
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Data AcquisitionWhat choices need to be made?
Radiation sources
Lab x-rays
Rotating anode x-rays
Synchrotron x-rays
Constant wavelength neutrons
TOF neutrons
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Data AcquisitionWhat choices need to be made?
X-rays vs neutrons
X-rays - atomic scatt
power (ƒ) decreases w/
2Θ
Neutrons - atom scatt
cross sections constant
w/ 2Θ
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Data AcquisitionWhat choices need to be made?
X-rays vs neutrons
X-rays - low atomic no.
ƒs very small
Neutrons - little variation
of atom scatt cross
sections w/ atomic no.
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Data AcquisitionWhat choices need to be made?
X-rays vs neutrons
X-rays - low atomic no.
ƒs very small
Neutrons - little variation
of atom scatt cross
sections w/ atomic no.
magnetic spin – use for
magnetic structure detn
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Data AcquisitionWhat choices need to be made?
X-rays vs neutrons
X-rays - usually α1-α2 doublet used (not w/ synchrotron x-rays)
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Data AcquisitionWhat choices need to be made?
X-rays vs neutrons
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Data AcquisitionWhat choices need to be made?
Radiation sources
Lab x-rays
relatively low intensity
Rotating anode x-rays
much higher intensity
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Data AcquisitionWhat choices need to be made?
Radiation sources
Lab x-rays
relatively low intensity
Rotating anode x-rays
much higher intensity
Synchrotron x-rays
extremely high intensity
monochromatic
continuously variable wavelength
very tiny beam
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Data AcquisitionWhat choices need to be made?
Radiation sources
Lab x-rays
relatively low intensity
Rotating anode x-rays
much higher intensity
Synchrotron x-rays
extremely high intensity
monochromatic
continuously variable wavelength
very tiny beam
very highresolution
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Data AcquisitionWhat choices need to be made?
Radiation sources
Reactor neutrons
continuous wave-
length distribution –
monochromator
req'd
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Data AcquisitionWhat choices need to be made?
Radiation sources
Reactor neutrons
continuous wave-
length distribution –
monochromator
req'd
generally low flux,
low resolution
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Data AcquisitionWhat choices need to be made?
Radiation sources
Spallation source
(pulsed)
time-of-flight (TOF)
energy (wavelength)
analysis used
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Data AcquisitionWhat choices need to be made?
Radiation sources
Spallation source
(pulsed)
time-of-flight (TOF)
energy (wavelength)
analysis used
very high flux,
high resolution
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Data AcquisitionWhat choices need to be made?
Radiation sources
Spallation source
(pulsed)
time-of-flight (TOF)
energy (wavelength)
analysis used
very high flux,
high resolution
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Data AcquisitionWhat choices need to be made?
Wavelength
Shorter wavelengths – more Bragg peaks
more peak overlap
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Data AcquisitionWhat choices need to be made?
Wavelength
Shorter wavelengths – more Bragg peaks
more peak overlap
(keep in mind peak broadening due to sample
and/or no. phases present)
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Data AcquisitionWhat choices need to be made?
Wavelength
Shorter wavelengths – more Bragg peaks
more peak overlap
(keep in mind peak broadening due to sample
and/or no. phases present)
X-rays – most atom types have very strong absorption
of characteristic wavelengths
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Data Acquisition
Instrument geometry
Choices:
a. conventional Bragg-Brentano diffractometer (includes Θ-Θ)
b. Guinier camera or diffractometer
c. diffractometer w/ curved PSD
d. TOF neutron instrument
e. 4-circle diffractometer
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Data Acquisition
Instrument geometry
Choices:
a. conventional Bragg-Brentano diffractometer (includes Θ-Θ)
b. Guinier camera or diffractometer
c. diffractometer w/ curved PSD
d. TOF neutron instrument
e. 4-circle diffractometer
Generally want good resolution & high intensity – can be
obtained w/ all but (c) above, & (a) w/reactor neutrons (CW)
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Data Acquisition
Instrument geometry
Choices:
a. conventional Bragg-Brentano diffractometer (includes Θ-Θ)
b. Guinier camera or diffractometer
c. diffractometer w/ curved PSD
d. TOF neutron instrument
e. 4-circle diffractometer
Generally want good resolution & high intensity – can be
obtained w/ all but (c) above, & (a) w/reactor neutrons (CW)
Instrument geometry affects instrument file
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Data AcquisitionWhat choices need to be made?
Detector type
Conventional – scintillation or proportional counterenergy resolution not high – usuallyneed monochromator
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Data AcquisitionWhat choices need to be made?
Detector type
Conventional – scintillation or proportional counterenergy resolution not high – usuallyneed monochromator
Also common – solid state detector – very high energy resolution – monochromator not needed
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Data AcquisitionWhat choices need to be made?
Detector type
Conventional – scintillation or proportional counterenergy resolution not high – usuallyneed monochromator
Also common – solid state detector – very high energy resolution – monochromator not needed
Neutrons – He counter
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Data AcquisitionWhat choices need to be made?
Detector type
Conventional – scintillation or proportional counterenergy resolution not high – usuallyneed monochromator
Also common – solid state detector – very high energy resolution – monochromator not needed
Neutrons – He counter
What about image plates? – poor resolution, hi bkgrd
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Data AcquisitionWhat choices need to be made?
Instrument setup
Divergence and receiving slit sizes
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Data AcquisitionWhat choices need to be made?
Instrument setup
Divergence and receiving slit sizes
Theta-compensating divergence slit keeps irradiated area constant,But changes intensity distribution vs 2Θ
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Data AcquisitionWhat choices need to be made?
Instrument setup
Divergence and receiving slit sizes
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Data AcquisitionWhat choices need to be made?
Instrument setup
Divergence and receiving slit sizes
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Data AcquisitionWhat choices need to be made?
Instrument setup
Divergence and receiving slit sizes
Use of monochromator changes polarization correctionin LP factor
Integrated intensities of Bragg reflections:
Ihkl = scale factor x mult factorhkl x LPΘ x absorb factorΘ xpref orient factorhkl x extinction factorhkl x | Fhkl | 2
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Data AcquisitionWhat choices need to be made?
Scan setup
Scan range
no. of reflections – want >5 x no. parameters refined wavelength dependent low angle reflections may not be useful due to
specimen configurationlarger inherent instrumental errorsextinction effects
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Data AcquisitionWhat choices need to be made?
Scan setup
Step size
sample dependent - peak widths need 5 observations across top of peak usually 0.01 - 0.05° 2Θ
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Data AcquisitionWhat choices need to be made?
Scan setup
Step size
sample dependent - peak widths need 5 observations across top of peak usually 0.01 - 0.05° 2Θ
Count time
longer times ––> higher intensities ––> greater precision at some point, little improvement in refinement process for longer count times
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Data AcquisitionWhat choices need to be made?
Specimen type and preparation
Radiation source
Wavelength
Instrument geometry
Detector type
Instrument setup
Scan parameters
Choose according to objective(s) of experiment