Characterization of detectors using the photon tagger NEPTUN for energies up to 20 MeV

A new setup for the characterization of γ - ray detectors has been installed at the NEPTUN photon tagger facility of TU Darmstadt. The tagging technique used at NEPTUN provides a quasi monoenergetic photon source up to about 20 MeV by selecting single γ - ray energies within a bremsstrahlung spectrum. The energy is freely selectable by changing the tagging condition. The detector response function (DRF) of γ - ray detectors for quasi monoenergetic incident photons can be measured. This allows to investigate DRFs of various photon detectors as a function of the incident γ - ray energy. Simulations of DRFs that are intensively used in the analysis of nuclear physics experiments can be tested and compared to experimental data. The experimental setup is presented and the measurement of the DRF of a large volume high-purity Germanium detector is described as an example.