Long Wave IR Visible UV X-rays Gamma rays

1. Long Wave IR Visible UV X-rays Gamma rays 1012 106 103 10 1 10-1 10-3 wavelength (nm) absorption X-rays

2. Io I X-rays x I/Io = e -µ* x = 1/ e µ* x = 1/ e µ  x µ* = linear attenuation coefficient µ = mass attenuation coefficient  = density of absorber Absorption

3. Absorption µ - values for elements listed in various texts & International Tables for Crystallography Note that µ changes with X-ray wavelength Higher atomic no. ––> larger µ, in general

4. Absorption What element used for X-ray tube windows? What element used for protection from X-rays?

5. Absorption Io I X-rays x I/Io = e -µ* x = 1/ e µ* x = 1/ e µ  x For compounds & mixtures, calculate µ from µcompd or mixture = (wt. fraction)elementxµelement

6. Absorption Io I X-rays x I/Io = e -µ* x = 1/ e µ* x = 1/ e µ  x Example for NaCl (CuK: (wt. fraction)Na = 23/58.5 = 0.393, (wt. fraction)Cl= 0.607 µNa = 30.1 µCl = 106 µNaCl = (wt. fraction)NaxµNa + (wt. fraction)ClxµCl µNaCl = 0.393x30.1 + 106x.607 = 76.2

7. mass attenuation coefficient Kedge <–– energy wavelength ––> Absorption Mass attenuation coefficient for an element changes with X-ray wavelength (energy) like this: Good news/bad news

8. absorption for element in specimen mass attenuation coefficient X-ray wavelength used for diffraction Kedge <–– energy wavelength ––> Absorption Bad news Absorbed energy re-emitted as fluorescent X-rays Suppose: Then: Absorption high Lots of fluorescence

9. X-ray fluorescence radiation emitted at all angles X-rays detector 2 specimen instead of this: get this: I I 2 2 Absorption Bad news

10. Absorption Good news - -filters -filter materials have atomic nos. 1 or 2 less than anode 50-60% beam attenuation

11. X-rays detector filter specimen Absorption Good news - -filters placing after specimen/before detector filters most of specimen fluorescence