ИСТИНА

New procedure for chemical separation and purification of 225Ac from irradiated metallic Th targets for targeted radiotherapy of cancer is described. Keywords – actinium-225, thorium, extraction chromatography As a result of irradiation of natural thorium with high-energy protons various fission and activation products of 232Th are formed [1, 2]. Among them 225Ac is formed, which has characteristics that enable to use it in radiotherapy of cancer. There is a growing demand for this radionuclide, while current methods for producing actinium have significant limitations and cannot satisfy it completely. Production of actinium from 233U is limited by its inaccessibility. Actinium-225 may be used either directly for preparation of radioimmunoconjugates or as a mother radionuclide in 213Bi isotope generator. In addition the formation of 223Ra should be noted, which is also a promising α-emitter for medicine. The aim of this work is to develop the new method of chemical separation and purification of large quantities of 225Ac from metallic Th irradiated with high-energy protons. Radiochemical separation of Ac and Ra is a difficult task since a high activity of more than 80 other radionuclides have been observed in the gamma- and alpha-spectrum of Th-target after irradiation. A developed method for chemical isolation of Ac is based on combination of liquid-liquid extraction and extraction chromatography. Irradiated thorium is dissolved in a mixture of concentrated hydrochloric and nitric acids, or in concentrated nitric acid with the addition of catalytic amounts of hydrofluoric acid. Tributylphosphate, trioktylphosphinoxide (TOPO) and di(2-ethylhexyl)orthophosphoric acid (HDEHP) are used in extraction experiments. The behavior of radionuclides, depending on the composition of the aqueous phase is studied. For additional separation of remaining Th AG 1x8 (BioRad) anionite column and extraction chromatographic sorbent TEVA (Eichrom, mixture of trioctyl and tridecyl methyl ammonium chloride as extracting agent) are proposed. Sorption capacity of under experimental conditions (6 M HNO3) for those sorbents is investigated. Sorbents DGA Resin (Eichrom), Ln Resin (Eichrom), TRU Resin (Eichrom), TDi-2 (Karpov Institute) are taken for further chromatographic separations. The sorption behavior of radionuclides, depending on the parameters of the column and acidity of solution is given. It is shown that 225Ac quantitatively adsorbed on Ln Resin and TDi-2 (di(2-ethylhexyl)orthophosphoric acid (HDEHP) as extracting agent) from dilute nitric acid (0.05 M HNO3). Desorption is carried out with acid of higher concentration (3 M). Significant difference in the sorption behavior of 225Ac for Ln Resin and TDi-2 is not observed. In 0.05 M HNO3 most of the fission products (Cs, Ra, Ba, Ag, Pb), are not retained on the column and are eluted in the first few milliliters. After switching to 3 M HNO3, actinium and rare earth elements (La, Nd and Ce) and ruthenium are eluted together in the fraction of 3 ml. On DGA Resin (N,N,N`,N` tetroctyldiglicolamide as extracting agent) Ac absorbed from 6 M nitric acid solution and desorbed with dilute nitric acid (0.01 M HNO3). In this way it is possible to separate actinium fraction from ruthenium, this radionuclide makes some difficulties for the production of final preparation 225Ac. Actinium is eluted together with cerium and lanthanum. To separate Ac(III) from rare earth elements the eluted 3 ml fraction from Ln or DGA Resin (in 3 M HNO3) is loaded onto extraction-chromatographic columns filled with TRU Resin (octylphenyl-N,N-di-isobutyl carbomoylphosphine oxide dissolved in TBP as extracting agent). The possible scheme of separation of 225Ac from the isotopes produced in irradiated thorium target is includes two sequential solvent extractions using HDEHP. After sorption on DGA Resin, for further separation the 3 M nitric acid can be passed through the sorbent TRU Resin. The procedure provides obtaining a pure Ac fraction, containing less than 0.2% 227Ac and no other radionuclides; the chemical yield is 90% or more [3]. [1] S.V. Ermolaev, B.L. Zhuikov, V.M. Kokhanyuk, V.L. Matushko, S.N. Kalmykov, R.A. Aliev, I.G. Tananaev and B.F. Myasoedov. Production of actinium, thorium and radium isotopes from natural thorium irradiated with protons up to 141 MeV, Radiochim. Acta, 100, 1–7, 2012. [2] B.L. Zhuikov, S.N. Kalmykov, S.V. Ermolaev, R.A. Aliev, V.M. Kokhanyuk, Produce of 225Ac and 223Ra from thorium irradiated with protons, Radiochemistry, 2011, 53, 1, p. 66-72. [3] B.L. Zhuikov, S.N. Kalmykov, R.A. Aliev, S.V. Ermolaev, V.M. Kokhanyuk, V.M. Kokhanyuk, I.G. Tananaev and B.F. Myasoedov, A method of producing actinium-225 and radium isotopes and targets for its implementation (Options). Patent № 2373589, Russian Federation, 2009.