Radioactivity

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Radioactivity refers to the spontaneous emission of radioactive particles from unstable nuclei of element isotopes.

There are three types of radioactive emissions, alpha, beta and gamma.

Unstable Nuclei:

The neutron to proton ratio (n/p) is related to the stability of the nucleus.

For atoms of atomic no. 1-20 the n/p ratio is 1 for stable nuclei that do not undergo radioactive decay.

Heavier nuclei, whose atomic number is greater than 20, have an n/p ratio of up to 1.6.

This means that for stable heavy nuclei, there are more neutrons than protons.

There is a band of stability which represents permissible n/p ratio of stable nuclei.

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Alpha radiation:

Has a mass of 4amu.

Has a charge of +2.

It is the nucleus of a helium atom: 2 protons and 2 neutrons.

It is stopped by 0.001cm of metal, a sheet of paper or human skin.

High ionizing power.

Beta radiation:

Has a mass of 1/1823 amu.

Has a charge of -1.

It is stopped by 2cm of aluminium, but penetrates skin and perspex.

Moderate ionizing power.

Gamma radiation:

High energy electromagnetic radiation, similar to X-rays.

Passes through 10cm of aluminium but is stopped by 10cm of lead.

No ionizing power.

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If the n/p ratio is outside the band of stability, radioactive decay occurs.

Beta emission occurs when there are too much neutrons in the nucleus. A neutron is converted to a proton and an electron and the electron is emitted from the nucleus. Hence the neutron number decreases by one and the proton number increases by one.

Alpha emission occurs when there are too many protons in the nucleus. Two protons and two neutrons are emitted from the nucleus, increasing the n/p ratio of the nucleus.