Data Acquisition What choices need to be made?

Data AcquisitionWhat choices need to be made?

Data AcquisitionWhat choices need to be made? Specimen type and preparation Radiation source Wavelength Instrument geometry Detector type Instrument setup Scan parameters

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

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

Data Acquisition Preferred orientation

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: X specimen

Data Acquisition • Specimen type and preparation • To get good particle statistics, generally want size < 10 m • Poorly ground sample:

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

Data AcquisitionWhat choices need to be made? Radiation sources Lab x-rays Rotating anode x-rays Synchrotron x-rays Constant wavelength neutrons TOF neutrons

Data AcquisitionWhat choices need to be made? X-rays vs neutrons X-rays - atomic scatt power (ƒ) decreases w/ 2Q Neutrons - atom scatt cross sections constant w/ 2Q

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.

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

Data AcquisitionWhat choices need to be made? X-rays vs neutrons X-rays - usually a1-a2 doublet used (not w/ synchrotron x-rays)

Data AcquisitionWhat choices need to be made? X-rays vs neutrons

Data AcquisitionWhat choices need to be made? • Radiation sources • Lab x-rays • relatively low intensity • Rotating anode x-rays • much higher intensity

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

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 high resolution

Data AcquisitionWhat choices need to be made? Radiation sources Reactor neutrons continuous wavelength distribution – monochromator req'd

Data AcquisitionWhat choices need to be made? Radiation sources Reactor neutrons continuous wavelength distribution – monochromator req'd generally low flux, low resolution

Data AcquisitionWhat choices need to be made? Radiation sources Spallation source (pulsed) time-of-flight (TOF) energy (wavelength) analysis used

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

Data AcquisitionWhat choices need to be made? Wavelength Shorter wavelengths – more Bragg peaks more peak overlap

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)

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

Data Acquisition Instrument geometry Choices: a. conventional Bragg-Brentano diffractometer (includes Q-Q) b. Guinier camera or diffractometer c. diffractometer w/ curved PSD d. TOF neutron instrument e. 4-circle diffractometer

Data Acquisition Instrument geometry Choices: a. conventional Bragg-Brentano diffractometer (includes Q-Q) 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)

Data Acquisition Instrument geometry Choices: a. conventional Bragg-Brentano diffractometer (includes Q-Q) 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

Data AcquisitionWhat choices need to be made? Detector type Conventional – scintillation or proportional counter energy resolution not high – usually need monochromator

Data AcquisitionWhat 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

Data AcquisitionWhat 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

Data AcquisitionWhat 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

Data AcquisitionWhat choices need to be made? Instrument setup Divergence and receiving slit sizes

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 2Q

Data AcquisitionWhat choices need to be made? Instrument setup Divergence and receiving slit sizes

Data AcquisitionWhat 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: Ihkl = scale factor x mult factorhklx LPQx absorb factorQx pref orient factorhklx extinction factorhklx | Fhkl | 2

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 configuration • larger inherent instrumental errors • extinction effects

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° 2Q

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° 2Q • Count time • longer times ––> higher intensities ––> greater precision • at some point, little improvement in refinement process • for longer count times

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

Data Acquisition What choices need to be made?