The transmission of silicon crystals irradiated by 24 GeV/c protons and reactor neutrons has been measured for photon energies, Eγ , between 0.95 and 1.3 eV. From the transmission data the absorption coefficient α is calculated, and from α(Eγ) the fluence dependence of the band-gap energy, Egap, and the energy of transverse optical phonons, Eph, determined. It is found that within the experimental uncertainties of about 1 meV neither Egap nor Eph depend on fluence up to the maximum fluence of 1 × 1017 cm-2 of the measurements. The value of Egap agrees within about 1 meV with the generally accepted value, if an exciton-binding energy of 15 meV is assumed. A similar agreement is found for Eph. For the extraction of Egap and Eph the second derivative of √α(Eγ) smoothed with a Gaussian kernel has been used.