potassium 40

po·tas·si·um 40 (40K),

A naturally occurring (0.0117%) radioactive potassium isotope; beta emitter with half-life of 1.26 billion years; chief source of natural radioactivity of living tissue.
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An average, Human has about 30 milligrams of potassium 40 that has a half life of 5730 years [2].
The results of this study shows that average concentration of potassium 40 in soils sampled from Ardabil is more than standard level of Atomic Energy.
According to doctoral student, Daniel Madigan, who studies the migration patterns of tuna at Stanford University, they didn't bring much, the levels were far lower than, for instance, levels of naturally occurring potassium 40 that have existed in the ocean for centuries, but the radioactivity was enough to survive the fishes' migration east to North America from the Western Pacific, which they undertake when they're around a year old.
Radiometrics: This technique breaks the natural gamma ray spectrum into individual components: potassium 40 (K), uranium 214 (U), thorium 208 (T) and total gamma (T).
Some independent scientists think that 64uSv from tritium is more dangerous than 64uSv from potassium 40.
Potassium 40 and radon were also detected, but at levels similar to those occurring in nature, they said.
Potassium, in general, is a combination of three isotopes among which only the isotope Potassium 40 is radioactive and its half -life is 1/28 x [10.
Thus all the heat from radioactive decay comes from the crust and mantle - about eight terawatts from uranium 238 (238U), another eight terawatts from thorium 232 (232Th), and four terawatts from potassium 40 (40K).