Concentration of Th isotopes for manganese nodules at various levels of their external manganese rich layers

The nodules used in this study were collected from two cruises in the western North Pacific by box coring (KH-79-4) and dredging (GH-78-1). After being dried at 110°C each sample extracted from the top, side and bottom of the nodule at various penetration levels were radiochemically analyzed for their Thorium isotopes contents by digesting about 100mg of a nodule sample without application of the successive separation treatment. Thorium in the solution was purified with an anion exchange resin and chemical separation techniques, and electrodeposited onto a silver disc from a hydrochloric acid and ethanol solution. The alpha activities of 228Th, 230Th and 232Th were counted with a spectrometer equipped with a surface barrier silicon detector. The chemical yield was determined by counting the 0 activity of 234Th added before the analysis. The yield ranged from 80 to 95%.