1

Data Acquisition

What choices need to be made?

2

Data Acquisition

What choices need to be made?

Specimen type and preparation

Radiation source

Wavelength

Instrument geometry

Detector type

Instrument setup

Scan parameters

3

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

4

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

5

Data Acquisition

Preferred orientation

6

Data Acquisition

Preferred orientation

7

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

8

Data Acquisition

Specimen type and preparation

To get good particle statistics, generally want size < 10

Poorly ground sample:

9

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

10

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

11

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

12

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.

13

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

14

Data Acquisition

What choices need to be made?

X-rays vs neutrons

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

15

Data Acquisition

What choices need to be made?

X-rays vs neutrons

16

Data Acquisition

What choices need to be made?

Radiation sources

Lab x-rays

relatively low intensity

Rotating anode x-rays

much higher intensity

17

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

18

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

19

Data Acquisition

What choices need to be made?

Radiation sources

Reactor neutrons

continuous wave-

length distribution –

monochromator

req'd

20

Data Acquisition

What choices need to be made?

Radiation sources

Reactor neutrons

continuous wave-

length distribution –

monochromator

req'd

generally low flux,

low resolution

21

Data Acquisition

What choices need to be made?

Radiation sources

Spallation source

(pulsed)

time-of-flight (TOF)

energy (wavelength)

analysis used

22

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

23

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

24

Data Acquisition

What choices need to be made?

Wavelength

Shorter wavelengths –  more Bragg peaks

more peak overlap

25

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)

26

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

27

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

28

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)

29

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

30

Data Acquisition

What choices need to be made?

Detector type

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

31

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

32

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

33

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

34

Data Acquisition

What choices need to be made?

Instrument setup

Divergence and receiving slit sizes

35

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

36

Data Acquisition

What choices need to be made?

Instrument setup

Divergence and receiving slit sizes

37

Data Acquisition

What choices need to be made?

Instrument setup

Divergence and receiving slit sizes

38

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

39

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

40

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

41

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

42

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