We have developed a function to describe the response of an 8-cm³ germanium detector to photon energies as low as 6 keV, going up to 120 keV. Detection effects like the Ge X-ray escape and Compton scattered photons in the neighborhood of the detector were treated. This study was based on the fit of analytic functions to the features of the experimental spectra, thus revealing the parameters and their dependence on the photon energy. Our response function has 13 parameters and its validity is shown by the reproduction of the parameter-generating spectra and through the independent fit of an ¹⁵²Eu spectrum.