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In this work some samples from the area around Miri Lake, a place of high natural radioactivity in Sudan, were analysed by laser fluorimetry for the determination of uranium concentration in soil and biological materials. Gram quantities of samples were ashed. The residue, after fuming with hydrofluoric acid, nitric acid, and perchloric acid, were taken up in a Ca(N0 3 ) 2 salting solution, and the uranium was extracted with methylisobutylketone (MIBK). After the stripping of uranium from MIBK solution into 0.001 Μ HN0 3 , it was measured with a laser fluorimeter type Scintrex UA-3. At present this technique has not yet been widely used in the field concerned [1, 2].

Natural radioactivity / Uranium / Thorium / Sediments / Marine / Red seaSummary. Alpha-emitting isotopes of uranium and thorium in surface marine sediments collected from the harbours at Port Sudan and Sawakin on the Sudanese coast of the Red Sea have been studied using radiochemical separation procedures and alpha-particle spectrometry. Activity concentrations of 238 U, 235 U, 234 U, 232 Th, 230 Th and 228 Th were measured. The ranges of activities observed for 238 U and 232 Th were 2.71Ϫ13.42 Bq/ kg and 3.62Ϫ18.73 Bq/kg from Port Sudan and 3.87Ϫ 24.79 Bq/kg and 1.13Ϫ10.04 Bq/kg dry weight from Sawakin, respectively. The behaviour of these decay-series nuclides was examined using daughter/parent and non-isotopic specific activity ratios in sediments. Simple regression analysis showed insignificant correlation between uranium and thorium activities and the organic matter content in sediments. The results are compared with those from coastal waters of other oceans.

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