131 INTRODUCTION A method of thin source (i.e. less than the alpha particle range) nrenaration of samples utilizing a ZnS screen and phototube counting was developeu. The counting procedures are the same as for tne method using thick sources described by Turner et. al. (1), and applied by several investigators [Turner (2); Cherry (3), (4); Hasson and Cherry (5); Shannon and Cherry (6)]. Applications of the thin source counting technique have been made by Osanov and Popov (7) and Curtis and Heyd (8). Estimation of the range of the alpha particles in the sample and the detection screeen, and inhomogeneity problems in source preparation constitute the greatest errors in the accurate determination of alpha concentrations in sclids. These problems were reviewed by Cherry (9), who concluded that the alpha particle ranges in non gaseous media are complicated and have not been well defined for general application. The range of an alpha particle in any medium is a function of the atomic number (Z) of the medium, and the energy (£) of the alpha particle emitted. Hence, in environmental samples, the mean value of each of these two variables, wnich is determined from the composite proportions of the individual components (Z and E), is needed since only one nuclide and/or one elemental absorber is rarely present. In the coralline atoll of Bikini and matrix of the absorber is constant from sample to sample. The Z of calciu.. carbonate, which constitutes the sediments of coralline environments, on an atom fraction basis, is 10.0. EXPERIMENTAL METHODS A dried subsample (5-15%) of each sediment sample was ground to a