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In this study, uranium and thorium concentrations in the sediment samples collected from the Gulf of Izmit in Marmara Sea were determined using neutron radiography method and alpha-autoradiography method. Mean uranium concentrations calculated for the sediment samples collected from the boreholes S2, S3, S5, S8 and KS-2 were found to be 5.99 μg/g (3.58–8.16 μg/g), 4,59 μg/g (2.54–8.22 μg/g), 4.03 μg/g (2.13–6.9 μg/g), 3.71 μg/g (2.36–4.48 μg/g) and 2.81 μg/g (2.16–4.06 μg/g), respectively. The highest uranium concentration in all borehole samples was obtained from borehole S3 with a value of 8.22 μg/g at a depth of 77.00 m, followed by borehole S2 with a value of 8.16 μg/g at 76.95 m depth. These results also confirms the possibility of thermal sources at boreholes S2 and S3. The mean thorium concentrations in boreholes S3 and S8 were found to be 15.26 μg/g (9.60–17.49 μg/g) and 15.76 μg/g (13.89–17.37 μg/g), respectively. The highest thorium concentration in obtained samples from all boreholes was obtained in borehole S3 with a value of 17.49 μg/g at a depth of 77.00 m. [Display omitted] • A total of 90 sediment samples were obtained from different depths of Izmit Gulf. • The uranium activity concentration in sediment samples were determined by the neutron radiography method. • The alpha autoradiography method was used to determine the thorium activity concentration. • The obtained results are in agreement with previously reported studies. [ABSTRACT FROM AUTHOR]