Chemistry Reference
In-Depth Information
Figure1.28.
Photoelectric mass-absorption coefficient (
τ
/
ρ
) and mass-scatter coefficient (
σ
/
ρ
)as
functions of the energy
E
of primary photons incident on palladium. The total mass-attenuation
coefficient (
μ
/
ρ
) results from the sum (
τ
/
ρ
)
+
(
σ
/
ρ
), the latter term from the sum of Rayleigh and
Compton scattering (
σ
/
ρ
)
R
+
(
σ
/
ρ
)
C
. Figure from Ref. [57], reproduced with permission from Center
for X-Ray Optics and Advanced Light Source, Lawrence Berkeley National Laboratory,
http://xdb.
lbl.gov
.
Compton scattering in the given energy range; the Rayleigh part is predomi-
nant for energies below 90 keV, the Compton part dominates for energies
above 90 keV. Both effects are relatively minor compared to the photoelectric
absorption. But for energies above 200 keV, the Compton effect becomes the
decisive component of total attenuation. Similar conditions are valid for
elements lighter or heavier than palladium. For light elements like carbon,
the point of balance between Rayleigh and Compton scattering decreases to
10 keV; for heavy elements like lead, it increases to 150 keV [57].
It should be noted that the total mass-attenuation coefficient (
μ
/
ρ
) is mainly
determined and equal to the photoelectric mass-absorption coefficient (
τ
/
ρ
) for
lower photon energies (
<
20 keV). For most elements (
Z
>
14) and energies
between 5 and 20 keV, the quantities (
μ
/
ρ
) and (
τ
/
ρ
) differ by only about 0.01%
up to 3%, relatively. In such cases, only one single set of data may be necessary
for both quantities. For light elements like carbon and energies
>
20 keV,
however,
the
quantities
(
μ
/
ρ
)
and
(
τ
/
ρ
)
are
different
and
have
to
be
distinguished.
In practice, energy-dependent attenuation is used to alter the spectrum of an
X-ray beam. For that purpose, a thin metal sheet called a selective attenuation
filter can be employed. It can easily be inserted into a beam path in order to
reduce a particular spectral peak or an entire energy band with respect to other
peaks or spectral regions.
Search WWH ::
Custom Search