radiation

RADIATION!

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.

(Noble gases Thoron, ₈₆Rn²²⁰, and Actinon, ₈₆Rn²¹⁹, are also produced, during the chain decay of thorium and actinium)

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 (mGy) of radiation, for minimally ionizing particles. [The SI unit of radiation dose is the "gray" (Gy) which is a radiation dose equivalent to the tissue deposit of 1 J/kg.] Americans actually get about 3.2 mSv per year from all sources. The standard unit for radioactive source strength is the becquerel. One becquerel is defined as the activity of a quantity of radioactive material in which one nucleus decays per second.

Some Links: Dose chart, Background Radiation, Radiation Risks, Fact Sheet.

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.

Nuclear weapons were detonated in the air over the cities of Hiroshima and Nagasaki in Japan in 1945. The health of the survivors, and of their offspring, has been closely monitored. It is perhaps surprising, but no significant adverse health effects have been detected. Death rates due to various diseases such as cancer were normal over the years, and no mutations or excess birth defects were observed, over 70 years and ongoing! [Talk about bad timing... about 70 people were at Hiroshima when the bomb was detonated, and then happened to be in Nagasaki three days later for the second bomb! The most famous is Tsutomu Yamaguchi, who lived to be 93, dying in 2010!] Similarly, there are areas around Kerala, India, where people have lived, for as long as humans have lived in the area, on what is a hugely concentrated natural deposit of Uranium and Thorium, getting doses anywhere from around 5 to 70 mGy. Again, no significant adverse health effects of any kind have been detected in careful, large-scale studies. Other hot sites are in China, in Brazil and in Iran (the hottest of all). Consistent and statistically valid surveys of specific defects that could be due to radiation exposure, in all known hot areas, would be useful, to say the least. But these studies would not be expected to produce astonishing results.

Numbers are in milliGreys.
NUCLEAR TOURISM!

Radioactivity has proven to be enormously useful in medical diagnostics and testing, and in archeological, paleontological, geological and astrophysical dating. Much of what we know about the time-scale of life on earth, the origins of earth and solar system, etc., depends on radioactive decay... a process which in turn depends on a few fundamental physical constants, and is not only completely dependable, but allows large numbers of cross-checks.

Carbon-14 dating in archaeology!

By the way, a 50% lethal dose of radiation delivers about the same total kinetic energy to the body as a rifle bullet, it's just that the rifle bullet does damage at the organic level, while a whole-body radiation dose does damage at the molecular level.

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