Neutron activation is one of the used methods in treatment and distinguish of diseases in which considering energy and number of emitted gamma, type and amount of trace elements of body are determined. Precision of measurement, uniformity of activation rate and received dose are important points in this method. Thus several groups are studied about development of system using in IVNAA method. The goal of this study is optimization of IVNAA system in neutron activation center of Ferdowsi university of Mashhad. For this purpose, the effect of material and thickness of added layer on collimator in increasing of thermal neutron arrived in water and soft tissue phantom and the uniformity of it are checked. Finally, bismuth layer with 3cm thickness on collimator is introduced as the best selection.

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