In the radiation therapy with ions, no satisfying method exists for real-time range monitoring. The PET monitoring, which is implemented in a few facilities for this purpose, has its drawbacks. This is why new methods are under investigation, like the detection of promptly emitted gamma - photons. These photons are emitted all along the ion path from excited target and projectile particles, with a maximum of the emission in the region of the Bragg peak for the most photon energies. By this maximum, the Bragg peak depth and with that, the ion range should be identified. This circumstance was investigated using the Gate simulation framework. The angle distribution of the photons was measured with the result, that more photons are detected in forward direction than in backward direction for a majority of the photon energy ranges. The ion range was calculated by detecting the prompt photons outside of a water target using cylindrical lead collimators and cylindrical detectors. The finite collimator length, the septal penetration and absorption effects in the target were taken into account by a response function, which linked the photon production with the photon detection. The accuracy of the calculation of the ion range largely depended on the errors of this response function and the relative error varied between 0 % and 5 %, depending of the primary ion energy.