Determination of the Spectral and Intensity Distribution of the Mono-Energetic Gamma-Ray Beam at HI$\gamma$S Using a Large Volume HPGe Detector

The High-Intensity Gamma-ray Source facility (HI$\gamma$S) provides $\gamma$-ray beams with small energy spread ($1-3$ \%) suitable for investigating the response of nuclides to dipole radiation. Nuclear resonance fluorescence experiments require precise knowledge of the intensity and the spectral distribution of the incident $\gamma$-ray beam in order to deduce the scattering cross section. We present a method for determining the energy spectrum of the $\gamma$-ray beam using a large volume HPGe detector. We monitor the $\gamma$-ray flux during in-beam experiments by measuring the intensity of $\gamma$-rays scattered through small angles from a copper plate. The results are compared to flux determinations derived from strong narrow resonances with known scattering cross sections in various nuclides in $\gamma$-ray scattering experiments.