Alpha Decay

Moving GIF of alpha tracks in a cloud chamber

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Alpha emission from U 238 sample

Alpha particles are helium nuclei emitted by quantum tunnelling usually from large unstable nuclei like U 238. Their short, thick ionisation tracks are shown in the cloud chamber picture above as tracks of mist. The 4/2 configuration or 2 protons and 2 neutrons is very stable – think how many multiples of it you can think of – carbon 12, oxygen 16 and so on, which are very common elements. Most of the Earth’s helium is the result of uranium or thorium decays in the Earth’s crust, finding its way to the surface as a by-product of natural gas production.

Nucleon number goes down by 4, proton number by 2.This is effectively an explosive interaction so linear momentum is conserved, so much like a gun recoils when a bullet is fired, the remaining nucleus acquires momentum in the opposite direction to the alpha track.

Am 241 is used in smoke detectors. The smoke prevents the emitted alphas reaching a detector, triggering the alarm

Alphas are heavy and slow-moving, causing massive ionisation of  the air or other intervening medium, about 100,000 ion-pairs per mm path length. Their Relative Biological Effectiveness – a measure of cell damage on irradiation –  due to this increased ionising capability is about twenty times more than betas of the same energy although their range is only a single cell wide.

Each species emits with a characteristic energy. For example, bismuth 210 is an alpha emitter having a half life of just over 5 days and a characteristic energy of 4.69 MeV. Most alphas have energies of between 2.5 and 5.5MeV.

They are deflected by electric fields being attracted to negative charges, also by magnetic fields, according to Fleming’s Left Hand Rule.

They penetrate only a few cm in air, and are therefore stopped by thin paper, so alpha emitters are harmless unless swallowed. Their Relative Biological Effectiveness – a measure of cell damage on irradiation –  due to this increased ionising capability –  is about twenty times more than betas of the same energy. On a more sinister note, the 2006 radiation poisoning and murder of the Russian dissident Alexander Litvinenko was thought to be with plutonium 210, an alpha emitter.

The picture is a representation of quantum tunnelling. The wave is a probability wave of the alpha particle, its amplitude representing the probability of the event. Don’t worry about it for now….

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