Gamma spectrum unfolding for a NaI monitor of radioactivity in aquatic systems: Experimental evaluations of the minimal detectable activity

This paper deals with the experimental evaluation of the minimal detectable activity achievable by unfolding the gamma spectra of a NaI monitor. An aquatic monitor initially developed by the Institut des Radio-Eléments (IRE) is used for the application. Unfolding of the spectra is performed with GRAVEL, a UMG package code, on the basis of a response matrix obtained with MCNP5.1.40. Experimental data have been measured at IRE, in a 20 m3 seawater tank, for known activities of 137Cs mixed with other gamma emitters (40K, 133Ba, 113Sn and 139Ce). Deconvolution allows one to reduce the MDA of 137Cs by an order of magnitude.