Theory
Scattering
Atoms
αβγ decay
Decay Series
Law
Half-life
Geiger
Carbon-14
Fission
Chain Reaction
Nuclear Energy
Nuclear reactors
Fusion

The bigger the sample, the faster the decay.

Although radioactivity is a random process, there is a certain underlying order to it.

A radioactive material decays exponentially.

A high level of activity from a radioactive source suggests a strong source.

The Half-life is the time taken for half the radioactive nuclei in a sample to decay.

The half-life is a constant value for a particular isotope ...

....it is indendent of the size of the sample or the time at which measurements are taken.

Half-life may be measured by measuring activity using a Geiger counter.

The Half-life of aradioactive material is the time taken for half the radioactive nuclei in a sample to decay.
It can be shown that T = 0.693/. By analysing the formula it can be seen that the value of T depends only on the decay constant and is therefore independent of the number of atoms present N, or the time t, at which the measurements were taken.
The concept of Half-life can be understood by studying the diagram below.

text

Measurement of Half-life
The number of radioactive nuclei in a sample falls exponentially over time. Likewise, the activity of the sample diminishes exponentially with time. Therefore, to measure the half-life, we measure the time taken for the activity to drop by half. After a further half-life the activity will have dropped by a half (to a quarter of the original value) and after a further half-life, the activity will have dropped by a half (to one eight of the original value).
The half-life of different radioactive substances vary greatly, as can be seen from a few sample values in the table below.

Radium 226 1620 years
Radon 222 3.8 days

Polonium 212

0.0003 milli-seconds

Measurement of Half-life
The number of radioactive nuclei in a sample falls exponentially over time. Likewise, the activity of the sample diminishes exponentially with time. Therefore, to measure the half-life, we measure the time taken for the activity to drop by half. After a further half-life the activity will have dropped by a half (to a quarter of the original value) and after a further half-life, the activity will have dropped by a half (to one eight of the original value).