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Backscattered peaks
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Close inspection of a gamma-ray spectrum may reveal a wide peak with energy <250keV which does not correspond to a known photon from the source. Such a feature, termed a Backscatter peak, is due to gamma-rays which first interact by Compton scattering with the shielding. Backscattered gamma-rays are those scattered through a large angle (> 120) by the shielding.

The energy dependence of the scattered gamma as a function of angle is found from:

Equation
where m0c2 = rest mass of an electron

Have a go This simulation demonstrates the formation of the backscatter peak. To explore the relationship between the energy transferred to the scattered gamma and the scattering angle go to the graph plotting exercise here...

 

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