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, where r is a measurement of the rate of decay, k is the first order rate constant for the isotope, and N is the amount of radioisotope at the moment when the rate is measured.
The rate of decay is often referred to as the activity of the isotope and is often measured in Curies (Ci), one curie = 3.700 x 10" is the initial amount of radioisotope at the beginning of the period, and "k" is the rate constant for the radioisotope being studied.
The worldwide average natural dose to humans is about 2.4 millisieverts (m Sv) per year.
This is four times more than the worldwide average artificial radiation exposure, which in the year 2008 amounted to about 0.6 m Sv per year.
A similar sample of freshly cut wood of the same type of tree had an activity of 15.3 cpm.
Detectable amounts occur naturally in soil, rocks, water, air, and vegetation.
The units of measure for time are dependent upon the unit of measure for the rate constant.
Radon and its isotopes, parent radionuclides, and decay products all contribute to an average inhaled dose of 1.26 m Sv/a.
From these sources it can be inhaled and ingested into the body.
In addition to this internal exposure, humans also receive external exposure from radioactive materials that remain outside the body and from cosmic radiation from space.
By comparing the activity of an archeological artifact to that of a sample of the living organism one can estimate the age of the artifact.
Example: A sample of wood taken from an ancient tomb had an activity of 7.0 counts per minute (decays per minute).
Radon seeps out of these ores into the atmosphere or into ground water; it can also infiltrate into buildings.