1 Data Acquisition What choices need to be made?

2 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 counter energy resolution not high – usually need monochromator

31 Data Acquisition What choices need to be made? Detector type Conventional – scintillation or proportional counter energy resolution not high – usually need 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 counter energy resolution not high – usually need 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 counter energy resolution not high – usually need 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 correction in LP factor Integrated intensities of Bragg reflections: I hkl = scale factor x mult factor hkl x LP  x absorb factor  x pref orient factor hkl x extinction factor hkl x | F hkl | 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 configuration larger inherent instrumental errors extinction 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 ° 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 ° 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