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A quasi-monoenergetic photon field with energy of >200 keV was obtained using a radioactive source with a backscatter layout. The backscatter layout allows the calibration of a dosimeter for two different energies with a single radioactive source. The setup was optimized through calculations and experiments to minimize the low energy component and to obtain field uniformity. The optimal backscatter layout was found to comprise a 1 ' 1 m 2 steel floor, a radioactive source placed 20 cm above the floor, and a 10-cm cubic lead block placed directly above the source. Under the proposed layout and using a 208-MBq Cs-137 source, a monoenergetic photon field with 190 + 9.6 keV (FWHM) and a dose rate of 3.18 + 0.18 µSv/h was obtained within a 10 ' 10 ' 10 cm 3 area, at a distance of 15 to 25 cm above the lead block.

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