Data Acquisition What choices need to be made?
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Transcript of Data Acquisition What choices need to be made?
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Data Acquisition
What choices need to be made?
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Data Acquisition
What 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 Acquisition
What 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 Acquisition
What 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 Acquisition
Preferred orientation
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Data Acquisition
Preferred orientation
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Data Acquisition
What 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
Poorly ground sample:
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Data Acquisition
What 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 Acquisition
What 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 Acquisition
What choices need to be made?
X-rays vs neutrons
X-rays - atomic scatt
power (ƒ) decreases w/
2
Neutrons - atom scatt
cross sections constant
w/ 2
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Data Acquisition
What 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 Acquisition
What 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 Acquisition
What choices need to be made?
X-rays vs neutrons
X-rays - usually 1-2 doublet used (not w/ synchrotron x-rays)
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Data Acquisition
What choices need to be made?
X-rays vs neutrons
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Data Acquisition
What 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 Acquisition
What 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 Acquisition
What 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 high resolution
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Data Acquisition
What choices need to be made?
Radiation sources
Reactor neutrons
continuous wave-
length distribution –
monochromator
req'd
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Data Acquisition
What 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 Acquisition
What choices need to be made?
Radiation sources
Spallation source
(pulsed)
time-of-flight (TOF)
energy (wavelength)
analysis used
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Data Acquisition
What 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 Acquisition
What 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 Acquisition
What choices need to be made?
Wavelength
Shorter wavelengths – more Bragg peaks
more peak overlap
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Data Acquisition
What 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 Acquisition
What 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 -)
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 -)
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 -)
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 Acquisition
What choices need to be made?
Detector type
Conventional – scintillation or proportional counterenergy resolution not high – usuallyneed monochromator
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Data Acquisition
What 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 Acquisition
What 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 Acquisition
What 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 Acquisition
What choices need to be made?
Instrument setup
Divergence and receiving slit sizes
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Data Acquisition
What 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 2
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Data Acquisition
What choices need to be made?
Instrument setup
Divergence and receiving slit sizes
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Data Acquisition
What choices need to be made?
Instrument setup
Divergence and receiving slit sizes
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Data Acquisition
What 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 LP x absorb factor xpref orient factorhkl x extinction factorhkl x | Fhkl | 2
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Data Acquisition
What 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 Acquisition
What 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° 2
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Data Acquisition
What 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° 2
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 Acquisition
What 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