A number of radiation-related applications in space such as γ-ray detection demands a flexible and wide-range γ-ray source which can serve as calibrating and testing facility for γ-ray detectors and γ-ray sensitive devices. A dedicated wide energy range γ-ray facility with adjustable and modular segments designed for γ-ray detector calibration and testing was built. Gamma rays are obtained from thermal neutron capture, which is the most suitable and cost efficient way how to provide discrete γ rays with energy above 3 MeV and of reasonable intensity. With appropriate and interchangeable targets the facility can generate γ rays with energy up to 10 MeV. Target materials of elements with high thermal neutron capture cross-section such as chrome, iron, chlorine, nickel and titanium provide suitable spectra. As neutron source 252Cf or 241Am-Be radionuclide sources with emission rate around 106 neutrons/s are used. The emitted fast neutrons are moderated by a modular interchangeable system of elements containing light materials such as graphite, water, heavy water or paraffin. Different geometries and moderator-target combinations have been tested and optimized; the best of them are presented. The facility is optimized with respect to space applications and needs by the European Space Agency (ESA). Performance of the facility is demonstrated by calibration spectra measured by HPGe and scintillator detectors (LaBr3, NaI(Tl)).