magnitude compared to 199Hg. Radium is also promising for a trapped ion optical clock. The radium ion has two subhertz-linewidth transitions from the ground state that could serve as the clock transition in an optical clock. Additionally, radium could be particularly well suited for a transportable optical clock as all transitions necessary for clock operation can be addressed with direct diode lasers. Though radium has no stable isotopes, there are eleven radium isotopes with half-lives longer than one minute that could be compared with high precision on a King plot. Isotope shifts could be measured with high precision on either of the radium ion subhertz-linewidth transitions from the ground state, or on the to intercombination line in neutral radium. The degree of any potential nonlinearities in such a King plot could set bounds on new physics beyond the standard model. Some of the few practical uses of radium are derived from its radioactive properties. More recently discovered radioisotopes, such as cobalt-60 and caesium-137, are replacing radium in even these limited uses because several of these isotopes are more powerful emitters, safer to handle, and available in more concentrated form. The isotope 223Ra (the chloride is under the trade name Xofigo) was approved by the United States Food and Drug Administration in 2013 for use in medicine as a cancer treatment of bone metastasis. The main indication of treatment with Xofigo is the therapy of bony metastases from castration-resistant prostate cancer due to the favourable characteristics of this alpha-emitter radiopharmaceutical. 225Ra has also been used in experiments concerning therapeutic irradiation, as it is the only reasonably long-lived radium isotope which does not have radon as one of its daughters. Radium is still used in 2007 as a radiation source in some industrial radiography devices to check for flawed metallic parts, similarly to X-ray imaging. When mixed with beryllium, radium acts as a neutron source. , radium-beryllium neutron sources are still sometimes used, but other materials such as polonium are more common: about 1,500 polonium-beryllium neutron sources, with an individual activity of , have been used annually in Russia. These RaBeF4-based (α, n) neutron sources have been deprecated despite the high number of neutrons they emit (1.84×106 neutrons per second) in favour of 241Am–Be sources. , the isotope 226Ra is mainly used to form 227Ac by neutron irradiation in a nuclear reactor. Hazards Radium is highly radioactive, and its immediate daughter, radon