x-ray absorption coefficient power point note

X-ray absorption coefficient

1. 10
12
10
6
10
3
10 1 10
-1
10
-3
Long Wave IR Visible UV X-rays Gamma rays
wavelength (nm)
X-rays
absorption

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

3. 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
Absorption

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

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

6. I/I
o= e
-µ* x
= 1/ e
µ* x
= 1/ e
µ x
µ
NaCl= (wt. fraction)
Na
xµ
Na
+ (wt. fraction)
Cl
xµ
Cl
µ
NaCl= 0.393x30.1 + 106x.607 = 76.2
I
o I
X-rays
x
Example for NaCl (Cu
K
:
(wt. fraction)
Na
= 23/58.5 = 0.393, (wt. fraction)
Cl
= 0.607
µ
Na
= 30.1 µ
Cl
= 106
Absorption

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

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

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

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

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