Abstract: A terminally sterilized isotope generator for producing an alpha-emitting Lead-212 (212Pb) daughter isotope by emanation of Radon-220 (220Rn) gas from Radium-224, comprising a closed retainer assembly including a parent-isotope chamber for receiving a parent isotope, a daughter-isotope chamber for collecting the 212Pb daughter isotope, and a gas permeable membrane separating the parent-isotope chamber from the daughter-isotope chamber, wherein the parent isotope is naturally decaying into 220Rn within the parent-isotope chamber, wherein the gas permeable membrane allows the 220Rn to passively pass therethrough, wherein the 220Rn is spontaneously decaying into 212Pb within the daughter-isotope chamber. An eluent is delivered in the daughter-isotope chamber, to elute in a liquid form the 212Pb generated in a gaseous form; and a collection container collects the 212Pb daughter-isotope eluted.