The decay of a radioactive species is a random process dependent only on the number of radioactive atoms present at a given time (i.e., the decay rate is a first-order reaction):


Upon integration, the result can be written as



N=number of atoms present at time t;


No=number of atoms present at t=0;


λ=decay constant.

The constant λ is the characteristic decay constant for the radioactive species. The characteristic rate of radioactive decay may conveniently be stated in terms of the half-life (t1/2), which is the time required for an initial number of atoms to be reduced to half that number by decay.

A sample of any radioactive substance which is decaying at the rate of 3.7×1010 disintegration per second is traditionally said to contain one curie (Ci) of radioactivity. A millicurie (mCi) is 10−3 curies and a microcurie (μCi) is 10−6 curies. The microcurie is probably the most frequently used activity unit in the nuclear industry. The SI unit for the activity is Becquerel (Bq). One Bq is equal to one disintegration per second of activity, thus one Ci equals 3.7×1010 Bq.

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