Data Acquisition. What choices need to be made?

Transcription

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2 Specimen type and preparation Radiation source Wavelength Instrument geometry Detector type Instrument setup Scan parameters 2

3 Specimen type and preparation Slide mount Front loading cavity Back loading cavity Side drifting cavity Low backgrd plate Several spherical particle techniques 3

4 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 4

5 Preferred orientation 5

6 Preferred orientation 6

7 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 7

8 Specimen type and preparation To get good particle statistics, generally want size < 10 µ Poorly ground sample: 8

9 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 9

10 Radiation sources Lab x-rays Rotating anode x-rays Synchrotron x-rays Constant wavelength neutrons TOF neutrons 10

11 X-rays vs neutrons X-rays - atomic scatt power (ƒ) decreases w/ 2Θ Neutrons - atom scatt cross sections constant w/ 2Θ 11

12 X-rays vs neutrons X-rays - low atomic no. ƒs very small Neutrons - little variation of atom scatt cross sections w/ atomic no. 12

13 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 13

14 X-rays vs neutrons X-rays - usually α 1 -α 2 doublet used (not w/ synchrotron x-rays) 14

15 X-rays vs neutrons 15

16 Radiation sources Lab x-rays relatively low intensity Rotating anode x-rays much higher intensity 16

17 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 17

18 Radiation sources Lab x-rays relatively low intensity Rotating anode x-rays much higher intensity Synchrotron x-rays extremely high intensity monochromatic very high resolution continuously variable wavelength very tiny beam 18

19 Radiation sources Reactor neutrons continuous wavelength distribution monochromator req'd 19

20 Radiation sources Reactor neutrons continuous wavelength distribution monochromator req'd generally low flux, low resolution 20

21 Radiation sources Spallation source (pulsed) time-of-flight (TOF) energy (wavelength) analysis used 21

22 Radiation sources Spallation source (pulsed) time-of-flight (TOF) energy (wavelength) analysis used very high flux, high resolution 22

23 Radiation sources Spallation source (pulsed) time-of-flight (TOF) energy (wavelength) analysis used very high flux, high resolution 23

24 Wavelength Shorter wavelengths more Bragg peaks more peak overlap 24

25 Wavelength Shorter wavelengths more Bragg peaks more peak overlap (keep in mind peak broadening due to sample and/or no. phases present) 25

26 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 26

27 Instrument geometry Choices: a. conventional Bragg-Brentano diffractometer (includes Θ-Θ) b. Guinier camera or diffractometer c. diffractometer w/ curved PSD d. TOF neutron instrument e. 4-circle diffractometer 27

28 Instrument geometry Choices: a. conventional Bragg-Brentano diffractometer (includes Θ-Θ) 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) 28

29 Instrument geometry Choices: a. conventional Bragg-Brentano diffractometer (includes Θ-Θ) 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 29

30 Detector type Conventional scintillation or proportional counter energy resolution not high usually need monochromator 30

31 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 31

32 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 32

33 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 33

34 Instrument setup Divergence and receiving slit sizes 34

35 Instrument setup Divergence and receiving slit sizes Theta-compensating divergence slit keeps irradiated area constant, But changes intensity distribution vs 2Θ 35

36 Instrument setup Divergence and receiving slit sizes 36

37 Instrument setup Divergence and receiving slit sizes 37

38 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 Θ x absorb factor Θ x pref orient factor hkl x extinction factor hkl x F hkl 2 38

39 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 39

40 Scan setup Step size sample dependent - peak widths need 5 observations across top of peak usually Θ 40

41 Scan setup Step size sample dependent - peak widths need 5 observations across top of peak usually Θ Count time longer times > higher intensities > greater precision at some point, little improvement in refinement process for longer count times 41

42 Specimen type and preparation Radiation source Wavelength Instrument geometry Detector type Instrument setup Scan parameters Choose according to objective(s) of experiment 42