# Activity and the Decay Constant

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## Activity and the Decay Constant

These equations often confuse students but they are actually based on a few very simple ideas. Understand those and it all becomes easier.

**1.** Radioactive decay is a random, spontaneous event and cannot be predicted. However, if there are many of these decays, then the rate of decay can be predicted using statistics as described in the decay equations.

**2.** The number of decays you will measure each second (the activity, A) from a sample depends on the number of atoms in the sample, N.

**Look:**

Here are two blocks of exactly the same radioisotope. The chance of an atom decaying from one is exactly the same as in the other **but** there are **twice** as many atoms in the 2kg block. So there will be twice as many decays per second in the 2kg block.

Note that the activity of a sample is measured in **bequerel, Bq.**

**1Bq = 1 decay per second.**

So in an equation this would be:

A ∝ N

A = λN

**Where**

l = the constant of proportionality, called **the Decay Constant**.

Units: s^{-1}, although sometimes quoted as hours ^{-1} or even years ^{-1}.

The decay constant gives you an idea of how quickly or slowly a material will decay.

A large value of λmeans that the sample will decay quickly