Wet preparation of radiotherapy sources

A method of accumulating radium radionuclides, comprising:

• providing a first solution including thorium radionuclides and a thorium-binding extractant, wherein the first solution does not bind to radium, such that radium atoms resulting from decay of the thorium diffuse out of the first solution;
• allowing a portion of the thorium radionuclides in the first solution to decay into radium atoms;
• allowing the radium atoms to diffuse out of the first solution;
• and
• collecting radium atoms resulting from the decay that diffused out of the first solution, from out of the first solution,
• wherein providing the first solution comprises introducing the first solution into a chamber with a second solution, which does not mix with the first solution, such that the decay radium atoms diffuse into the second solution, and
• wherein the first solution is lighter than the second solution.