The nucleus of an atom contains positively charged protons and neutrons, which have no charge. The number of protons determines what element the atom is; hydrogen atoms have one, oxygen atoms have eight, and so on. Most elements have multiple isotopes - variants of the element that have the same number of protons but different numbers of neutrons.
Actinium-225, with 89 protons and 136 neutrons, has a half-life of about 10 days. This means half the atoms will have decayed within 10 days, three-quarters within 20 days, seven-eighths within 30 days, etc.
Each time the atom decays, it releases an alpha particle, but there are more alpha particles to come. Actinium-225 decays into radioactive francium-221, which also releases an alpha particle when it decays, this time to astatine-217. All in all, the decay chain from a single actinium-225 atom releases four alpha particles, with radioisotopes along the way having half-lives from 46 minutes down to about four millionths of a second, before reaching bismuth-209, which is essentially stable.
Alpha particles release radiation that is energy-dense but travels only a short distance. In targeted alpha therapy, an alpha particle is carried by another molecule directly to a cancer cell. There, it releases powerful radiation that is largely confined to the diseased cell, with minimal damage to surrounding cells. This radiation can create breaks in DNA- breaks too severe for the cell to repair, ultimately leading to its death.
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