A quantity is subject to exponential decay if it decreases at a rate proportional to its current value. Symbolically, this process can be expressed by the following differential equation, where is the quantity and (lambda) is a positive rate called the exponential decay constant, disintegration constant, rate constant, or transformation constant: The solution to this equation (see derivation below) is: where is the quantity at time , is the initial quantity, that is, the quantity at time . Measuring rates of decay Mean lifetime If the decaying quantity, N(t), is the number of discrete elements in a certain set, it is possible to compute the average length of time that an element remains in the set. This is called the mean lifetime (or simply the lifetime), where the exponential time constant, , relates to the decay rate constant, λ, in the following way: The mean lifetime can be looked at as a "scaling time", because the exponential decay equation can be written in terms of the mean lifetime, , instead of the decay constant, λ: and that is the time at which the population of the assembly is reduced to 1/e ≈ 0.367879441 times its initial value. For example, if the initial population of the assembly, N(0), is 1000, then the population at time , , is 368. A very similar equation will be seen below, which arises when the base of the exponential is chosen to be 2, rather than e. In that case the scaling time is the "half-life". Half-life A more intuitive characteristic of exponential decay for many people is the time required for the decaying quantity to fall to one half of its initial value. (If N(t) is discrete, then this is the median life-time rather than the mean life-time.) This time is called the half-life, and often denoted by the symbol t1/2. The half-life can be written in terms of the decay constant, or the mean lifetime, as: When this expression is inserted for in the exponential equation above, and ln 2 is absorbed into the base, this equation becomes: Thus, the amount of material left is 2−1 = 1/2 raised to the (whole or fractional) number of half-lives that have passed. Thus, after 3 half-lives there will be 1/23 = 1/8 of the original material left. Therefore, the mean lifetime is equal to the half-life divided by the natural log of 2, or: For