|Photoelectric absorption within the shielding
material can result in characteristic x-ray emission. If these X-rays
escape, they may be detected by the detector. For the common shielding
material lead, this results in the observation of X-ray peaks in the spectrum
at energies between 70-85 keV. These X-rays introduce unwanted
background particularly for low energy gamma-ray
The solution is to use a graded shield. The inner
surface of the lead shield (~10 cm thick) is covered first with a thin layer
of cadmium (~3 mm), then a thin layer of copper (~0.7 mm).
X-rays from the lead produce cadmium X-rays which are absorbed by
the layer of copper.
Any copper fluorescent X-rays produced are typically 8-9 keV in energy
and will be absorbed in the aluminium can surrounding the detector and hence
are not detected.
Use the animation to illustrate the production of x-rays from shielding surrounding the
detector and the effect of introducing a graded shield.
The Bremsstrahlung effect may also be
seen within this set up.