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We consider the thorium distributions that are coincident with the distribution of ejecta after the Mare Imbrium impact occurs on the lunar surface and derive a simple model on the sphericaJ target to predict the thickness of Imbrium ejecta deposits as a function of distance from the centre of the Imbrium basin.Then we use the result of Lunar Prospectors gamma ray experiment to test the hypothesis that the distribution of thorium on the lunar surface has an origin from the Mare Imbrium.