The efficiency of the dismantling process of nuclear facilities can possibly be increased by the use of an active measurement system for the localization of contamination in concrete structures. A fiber optic radiation sensor based on a GAGG scintillation crystal with a high density and atomic number connected to an optical fiber could meet the requirements for this application. This work describes the setup of this measurement system and its properties like the relative energy resolution of 38 % at the gamma-ray energy of 662 keV and a sensitivity regarding the investigated concrete mass of 12.6 s^-1 Bq^-1 g. The main focus lies on the comparison of three different contamination detection analysis methods, the Kolmogorov-Smirnov test regarding the energy distribution or regarding the detector events time difference distribution and a decision threshold based analysis method. A¹³⁷Cs radiation source with an activity of 21 kBq can be detected within 4 min with the Kolmogorov-Smirnov test regarding the time differences and the decision threshold based method with a sensitivity close to 100 %. The analysis method based on the Kolmogorov-Smirnov test regarding the energy distribution only achieves a maximum sensitivity of 78 %. If an approach for the treatment of the natural background count rate variation of concrete is implemented, the Kolmogorov-Smirnov test regarding the time differences with a sensitivity of 100 % detects a contamination within 3 min while the decision threshold based method requires a measurement time of 18 min or more.