The Rn-222 isotope can be produced using the Hahn emanation source. The solution of 226Ra(II) ions is mixed with FeCl2 solution, and then iron hydroxide is precipitated with ammonia solution that contains no carbonate. The precipitate

obviously contains radium ions, which disintegrate to radon. At the optimal Ra: Fe = 1:55 ratio, about 95% of radon gas is emanated. As the precipitate ages, the quantity of the emanated radon decreases. When that happens, the precipitate is suspended in water; iron hydroxide is separated by extraction with ether. Radium remains in the aqueous phase, from where it can be precipitated again by iron chlo­ride and ammonia. It is very important that the ammonia solution should not con­tain carbonate ions; otherwise, radium carbonate is formed, which cannot be separated from iron hydroxide.

A scheme of an emanation source is shown in Figure 8.5. Radon can be col­lected in the ampoule (P). Radon gas has been used in medical therapy to replace the toxic Ra-226.