1. X-rays absorption Long Wave IR Visible UV X-rays Gamma rays
Long Wave IR Visible UV X-rays Gamma rays
wavelength (nm)
absorption
X-rays

2. I/Io = e -µ* x = 1/ e µ* x = 1/ e µ  x
Absorption 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

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. µcompd or mixture =  (wt. fraction)element x µelement
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)element x µelement

6. I/Io = e -µ* x = 1/ e µ* x = 1/ e µ  x
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 = µCl = 106
µNaCl = (wt. fraction)Na x µNa + (wt. fraction)Cl x µCl
µNaCl = x x .607 = 76.2

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

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

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

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

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