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


Specimen type and preparation

Radiation source

Wavelength

Instrument geometry

Detector type

Instrument setup

Scan parameters

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Data Acquisition



Slide mount

Front loading cavity

Back loading cavity

Side drifting cavity

Low backgrd plate

Several spherical particle techniques

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Data Acquisition



Slide mount


Back loading cavity


Low backgrd plate


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



Slide mount


Back loading cavity


Low backgrd plate


Low angle problem - fixed divergence slit:

specimen

X

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Data Acquisition


To get good particle statistics, generally want size < 10

Poorly ground sample:

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Data Acquisition



Slide mount


Back loading cavity


Low backgrd plate


Neutron diffraction requires larger specimens

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Data Acquisition


Radiation sources

Lab x-rays

Rotating anode x-rays

Synchrotron x-rays

Constant wavelength neutrons

TOF neutrons

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Data Acquisition


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


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


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


X-rays vs neutrons

X-rays - usually 1-2 doublet used (not w/ synchrotron x-rays)

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Data Acquisition


X-rays vs neutrons

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Data Acquisition


Radiation sources

Lab x-rays

relatively low intensity


much higher intensity

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Data Acquisition


Radiation sources

Lab x-rays




Synchrotron x-rays

extremely high intensity

monochromatic

continuously variable wavelength

very tiny beam

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Data Acquisition


Radiation sources

Lab x-rays




Synchrotron x-rays

extremely high intensity

monochromatic

continuously variable wavelength

very tiny beam

very high resolution

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Data Acquisition


Radiation sources

Reactor neutrons

continuous wave-

length distribution –

monochromator

req'd

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Data Acquisition


Radiation sources

Reactor neutrons

continuous wave-

length distribution –

monochromator

req'd

generally low flux,

low resolution

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Data Acquisition


Radiation sources

Spallation source

(pulsed)

time-of-flight (TOF)

energy (wavelength)

analysis used

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Data Acquisition


Radiation sources

Spallation source

(pulsed)


energy (wavelength)

analysis used

very high flux,

high resolution

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Data Acquisition


Radiation sources

Spallation source

(pulsed)


energy (wavelength)

analysis used

very high flux,

high resolution

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Data Acquisition


Wavelength

Shorter wavelengths – more Bragg peaks

more peak overlap

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Data Acquisition


Wavelength


more peak overlap

(keep in mind peak broadening due to sample

and/or no. phases present)

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Data Acquisition


Wavelength


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:






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:






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


Detector type

Conventional – scintillation or proportional counterenergy resolution not high – usuallyneed monochromator

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Data Acquisition


Detector type


Also common – solid state detector – very high energy resolution – monochromator not needed

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Data Acquisition


Detector type



Neutrons – He counter

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Data Acquisition


Detector type



Neutrons – He counter

What about image plates? – poor resolution, hi bkgrd

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Data Acquisition


Instrument setup

Divergence and receiving slit sizes

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Data Acquisition


Instrument setup


Theta-compensating divergence slit keeps irradiated area constant,But changes intensity distribution vs 2

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Data Acquisition


Instrument setup


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Data Acquisition


Instrument setup


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Data Acquisition


Instrument setup


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


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


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


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



Radiation source

Wavelength

Instrument geometry

Detector type

Instrument setup

Scan parameters

Choose according to objective(s) of experiment

Data Acquisition What choices need to be made?

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