Development of a new ^<227>Ac counter based on detection ^<219>Rn

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Actinium-227 is a nuclide belong to the ^<235>U decay chain. It's half life (21.7 years) is comparable within that of ^<210>Pb(t_<1/2>=22.7y). Although ^<210>Pb is widely used as a dating tool of lar, coastal and shelf sediments and a tracer of heavy metals in the marine environments, application of ^<227>Ac geochemical problems is limited due to absence of convienient method of determination. The purpose of this study is to develop a new counting method which includes detection of short-lived radon (^<219>Rn and ^<220>Rn) and its decay products by alpha-scintillation cells.The system consists of two successive alpha-scintillation cells and photomultipliers, and counting electronics. Continuous helium gas flow purges ^<219>Rn and ^<220>Rn from MnO_2 fiber on which ^<227>Ac and ^<228>Ra are adsorbed and intuduce them into the alpha-scintillation cells. various experiments were carried out to find optional and prored that the system is useful for simoltaneous determinations of ^<227>Ac and ^<228>Ra.