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Description
In in beam gamma ray spectroscopy, the position of the beam spot in the target affects the efficiency of $\gamma$-ray detectors, hence cross sections and angular distributions. Not all solid target experiments allow a determination of the beam spot via camera observation of luminescence during irradiation or via a visual identification after the irradiation.
Here we present a new approach to determine the beam spot position by only using isotropic beam-induced background $\gamma$-lines and a Monte-Carlo description of the setup. By comparing the expected angular distribution (isotropic) for different simulated beamspots and using the reduced chi2 of the angular distribution fit, we created a map of the target showing the most likely beamspot position. This approach was used for the $d(p,\gamma)^3$He-campaign at Felsenkeller, where the $^{19}$F($p,\alpha \gamma)^{16}$O parasitic reaction was used for the beam spot determination.