A procedure is described for isolation and purification of 225 Ac as a 229 Th daughter decay product using ion exchange. The process includes the following steps: separation of 229 Th from a mixture of 225 Ac and 225 Ra ( 225 Ac precursor) using an anion-exchange resin; separation of 225 Ac and 225 Ra using a cation-exchange resin; purification of 225 Ac to remove inactive impurity cations (primarily Fe) using an anion-exchange resin; final purification of the product on a column with a cation-exchange resin.Monoclonal antibodies and other molecular carriers labeled with α-emitting radionuclides find increasing use in the development of methods for therapy of tumor diseases. With these carriers, radionuclides are delivered directly to the target, separate cell [1][2][3][4][5]. These systems also show promise in therapy of infection diseases [6].The major advantage of α-emitters is that α-particles have extremely short range in biological tissues. High linear energy transfer and short range of α-particles ensure high efficiency of destruction of tumor cells [7].The requirements to an ideal α-emitter are as follows:(1) neither the α-emitter itself, nor its decay products should emit high-energy γ-quanta;(2) the α-emitter half-life should be sufficient for ensuring its isolation and delivery, but not too long to pose hazard to the patient;(3) the chemical properties of the α-emitter should ensure strong binding with various carriers;(4) the α-emitter should be available and cheap.The list of α-emitters potentially suitable for use in radiotherapy is given in Table 1 [7-9].Several methods for producing 225 Ac are known. In particular, 225 Ac can be produced by irradiation of ura-
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