Transmitted photon spectra of low-energy gamma rays through nuclear grade graphite

Transmitted γ-ray spectra of 241Am, 57Co and 203Hg point sources are measured through different thicknesses of nuclear grade graphite (density 1.6 g/cm3) with a collimated NaI(Ti) detector assembly, and unfolded by the iterative technique. Irrespective of the energy of the primary photon entering the medium, the spectra show a multiple-scatter peak at 50 keV energy. The intensity of the peak decreases exponentially with the thickness of the medium. Measured build-up factors are compared with the calculated ones available in the literature. (author)