NUCLEAR DECAY PROCESSES One of the first great successes of quantum physics was the prediction of the probability of tunneling of a ⁴He nucleus through the Coulomb barrier of heavy nuclei, which explained the incredible variation in the lifetime of nuclei that underwent so-called α-decay, from 10¹⁸ to 10^-8 seconds!  This is the main process by which nuclei heavier than ²⁰⁸Pb decay, since there is a high probability of [2p,2n] clusters in such nuclei, and their tight binding gives them a high kinetic energy within the nucleus.

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Nuclei with too many neutrons relative to protons, or too many protons relative to neutrons, are unstable to the weak interaction, which can change neutrons to protons or protons to neutrons by having a quark emit a charged weak boson, which then creates a pair.  If the weak boson
has a positive charge, it creates a pair of positron and neutrino; if the weak boson has a negative charge, it creates a pair of electron and anti-neutrino. In heavy nuclei, the probability distribution of the 1s electrons overlaps the nucleus strongly, and the weak interaction process can also occur by a proton capturing an electron and converting to a neutron and neutrino.

Following any type of nuclear decay, the nucleus that results is usually in an excited state, and has to transition to its ground state by emitting photons... with kinetic energies of typically an MeV, these are called gamma rays.



An example of a radioactive decay chain. A very heavy nucleus initiates a chain of sequential decays that eventually ends up at a stable nucleus with many fewer nucleons.


< An example of nuclear medicine... Positron Emission Tomography. A very effective way to spot cancerous growths anywhere in the body.  Any positron emitter with a very short half-life works well. The radioactive atom is chemically incorporated into a biologically active molecule, like glucose.



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