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Graded shield
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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 measurements.

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.

Have a go 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.


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1999, 2000 The University of Liverpool, Department of Physics

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