Radium-224

Radium-224 decays by alpha emission with a half-life of 3.63 days, part of the thorium-232 decay series. Including its full decay chain through radon-220, polonium-216, lead-212, bismuth-212 and either polonium-212 or thallium-208 down to stable lead-208, a single 224Ra decay ultimately yields a cumulative 400% alpha and 200% beta emission (an average alpha energy of about 6.75 MeV per decay in the chain).

In current practice, 224Ra is used less as a therapeutic agent in its own right than as the loaded parent of a radionuclide generator: it is retained on a cation-exchange or similar column, from which the short-lived gaseous daughter radon-220 emanates and decays to lead-212, which is then eluted for radiolabelling (the DOE's Radium-224/Lead-212/Bismuth-212 generator product). Some experimental 'dual targeting' constructs have also used 224Ra directly, exploiting the fact that three of its four chain alpha particles decay within about 400 micrometres of the original 224Ra atom, while the longer-lived 212Pb daughter can diffuse further before delivering its own alpha dose.

224Ra is obtained from the decay of thorium-228 (half-life about 1.9 years), which is itself recovered from decaying stocks of uranium-232; the DOE Isotope Program has supplied 224Ra/212Pb/212Bi generators (routinely available in batches up to about 16 mCi, ordered 8-10 weeks in advance) since 2015, drawing on thorium-228 separated at Oak Ridge National Laboratory.