The present invention provides 223 Ra comprising no more than 0.05 Bq 227 Ac per 1MBq 223 Ra. The 223 Ra may be formed or formable by a method comprising: i) preparing a generator mixture comprising 227 Ac, 227 Th and 223 Ra； ii) loading said generator mixture onto a strong base anion exchange resin； iii) eluting said 223 Ra from said strong base anion exchange resin using a first mineral acid in an alcoholic aqueous solution to give a first eluted 223 Ra solution； iv) loading the 223 Ra of the first eluted 223 Ra solution onto a strong acid cation exchange resin； and v) eluting the 223 Ra from said strong acid cation exchange resin using a second mineral acid in aqueous solution to provide a second eluted solution.
