Determination of trace levels of uranium and thorium in high purity gadolinium sulfate using the ICP-MS with solid-phase chromatographic extraction resin

The new Super-Kamiokande-Gadolinium (SK-Gd) project is an upgrade of the SuperKamiokande (SK) detector. In the SK-Gd project, 0.2% Gd-2(SO4)(3) is loaded into the 50 kton water tank of the SK. One of the main purposes of the project is to discover supernova relic neutrinos. Neutrino measurements and proton decay searches will also be performed in the SK-Gd. In order to measure solar neutrinos with a low energy threshold of similar to 3.5MeV in the SK-Gd, the main radioactive contaminations, U-238 and Th-232, in (Gd-2(SO4)(3)center dot 8H(2)O) , should be minimized before loading. Our maximum levels for U and Th are 5 mBq (U)/kg (Gd-2(SO4)(3)center dot 8H(2)O) and 0.05 mBq (Th)/kg (Gd-2(SO4)(3)center dot 8H(2)O) . In order to measure such low concentrations of U and Th in (Gd-2(SO4)(3)center dot 8H(2)O) , we developed the solid-phase extraction technique. Using this method, about 90% or more U and Th could be efficiently extracted while Gd was reduced by a factor of about 104. This allowed these radioactivity contaminations to be measured precisely as 0.04 mBq/kg (Gd-2(SO4)(3)center dot 8H(2)O) forUand 0.01 mBq/kg (Gd-2(SO4)(3)center dot 8H(2)O) for Th. We measured three pure (Gd-2(SO4)(3)center dot 8H(2)O) samples using this method and estimated that the purest one contained < 0.04 mBq (U)/kg ((Gd-2(SO4)(3)center dot 8H(2)O) and 0.06 +/- 0.01 mBq (Th)/kg ((Gd-2(SO4)(3)center dot 8H(2)O) by the ICP-MS.