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Generalization for Primary Beam Spectral Variations
In practice use five energy bins
Beam hardening accounted for in terma and approximated by
precalculated correction factors (Papanikolaou 1993)
Papanikolaou N, Mackie T R, Meger-Wells C, Gehring M and Reckwerdt P 1993 Investigation of the
convolution method for polyenergetic spectra Med. Phys. 20 1327–36.