The use of an active measurement system to localize contamination in concrete structures of nuclear facilities offers advantages like an easy on-site analysis within short measurement time. A measuring system based on a scintillation crystal connected to a long optical fiber with a large diameter could satisfy the demands of a measuring system for gamma-ray spectroscopy in boreholes. This work describes the developement of a gammaray spectroscopy system using a fiber optic radiation sensor. A prototype was built with a GAGG scintillation crystal, that could reach an energy resolution of 33 % at the 662 keV gamma-ray emission energy of 137 Cs. A minimal time for the detection of a contamination of a 137 Cs source with an activity of 21 kBq and a layer of 6 cm concrete between sensor and radiation source was determined to under one minute with three different evaluation methods. They were based on the decision threshold and the Kolmogorov-Smirnov test corresponding to the energy distribution and the distribution of time differences between detector events. Additionally, it was possible to estimate the distance of a point like source from the sensor by establishing a further analysis parameter comparing different energy regions of a spectrum.