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.
