RADIOACTIVITY!

α decay... 4He clusters penetrate the Coulomb barrier.


β decay: an excess neutron changes into a proton plus an electron plus an anti-electron neutrino, via the weak nuclear force, or an excess proton changes into a neutron, a positron (anti-electron) and electron neutrino.

Following either α or β decay, the resulting nucleus is generally in one of its excited states, and it has to de-excite to the ground state by emitting photons. Since nuclear energy levels are generally an MeV apart, these γ-ray photons have a million times the kinetic energy of a photon of visible light.


Unless you eat the radioactive material, the only emission from a radioactive nucleus that can reach you across space is the γ-ray photons!  Solid matter is almost completely transparent to them.


A millisievert (mSv) is defined as "the average accumulated natural background radiation dose to an individual for 1 year, exclusive of radon, in the United States." 1 mSv is the dose produced by exposure to 1 milligray (mG) of radiation.  Americans actually get about 3.2 mSv per year from all sources.


The whole-body exposure threshold for acute hematopoietic syndrome or "radiation sickness" is 500 mGy. A dose of ~3,000 mGy produces an acute gastrointestinal syndrome that can be fatal without major medical intervention, and a dose of ~ 5,000 mGy is considered the human LD 50 / 30, that is, the lethal dose for 50% of the population in 30 days, even with treatment. These are acute thresholds: the same dose fractionated over a series of exposures or over a longer time may produce less injury, as the body has a chance to repair damage between exposures.

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