The aim of this study is to characterize the activation of the crocus cavity. For this purpose, the concrete plug of the irradiation channel crossing the cavity shielding was used to measure the activation by gamma spectrometry. In a first time measurements were made after a long cooling time to determine the residual activation. Then, the plug was irradiated at 40Watts for 2 hours to provide a sufficient irradiation to the concrete. At the same time, measurements were led thanks to the dosimetry by thermoluminescence in the cavity. TLDs measurements gave reaction rates in Li6 at strategical positions detailed on this report. TLDs positions were defined in a Cartesian system centred on the centre core. 2D plans based on previous plan were designed to improve the calculation accuracy of the neutrons flux simulations. The analysis shown a very low activity of the concrete plug, below the low level waste thresholds. However results provided by TLDs and the Nagra’s calculations allowed to highlight a greater neutrons fluence in the upper part of the cavity. So, it might be relevant to pursue the gamma ray spectrometry measurements with concrete plugs from other cavity areas where the neutron flux should be higher.