Decay Chains

When an atom undergoes radioactive decay the isotope produced is not necessarily stable itself. If it is not then that can undergo radioactive decay. This can carry on in what are called decay chains.

Uranium 238 Decay Chain

In the diagram above the atomic mass is along the bottom (204-240) and the atomic number up the side.

Starting with Uranium-238 with a halflife of 4.5billion years (4.5×109 – if you do not understand this notation see here) and decays via alpha decay. The decay produce is Thorium-234 which has a halflife of 24.1 days and decays to Protoactinium-234 via beta decay. This has a halflife of 6.75 hours and decays to Uranium-234 via beta decay.

Notice that alpha decay results in the loss of 4 units of mass and and two protons – i.e. the atomic mass drops by 4 and the atomic number drops by 2. Therefore the arrow points down to the left. Beta decay does not change the atomic mass but increases the number of protons (since a neutron turns into a proton) and so the arrow points straight up.

If we want to calculate how much radioactivity something will emit then we often have to take into account the radioactive emissions of all the ‘daughter products’.

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