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Beta Particles

Beta particles have a mass which is half of one thousanth of the mass of a proton and carry a single negative charge.

What are beta particles?

Beta minus particles (β-) are electrons from the nucleus and are ejected by some radionuclides during a form of radioactive decay called beta-decay. The emission of the electron's antiparticle, the positron or beta plus particle (β+), is also called beta decay. Beta-decay normally occurs in nuclei that have too many neutrons to achieve stability. It occurs commonly in the radioactive products of nuclear fission and occurs in natural radioactive decay chains following one or more alpha-decays

What are the properties of beta particles?

Beta particles have a mass which is half of one thousanth of the mass of a proton (Figure 1) and carry a single negative charge. Beta-particles are emitted with a continuous energy spectrum ranging from near zero energy up to a maximum energy specific to each radionuclide. (The actual radioactive decay process will always produce a certain fixed amount of energy but in beta-decay, the energy is split randomly between the beta particle and an almost undetectable, uncharged, particle called the neutrino). Beta particles are much less ionising that alpha particles and generally do less damage for a given amount of energy deposition.

Mass of an electron

Who discovered beta particles?

Henri Becquerel is credited with the discovery of beta particles. In 1900, he showed that beta particles were identical to electrons. The term electron was coined in 1891 by the Irish physicist, George Stoney, to describe the basic unit of electricity. Later in 1897 in Cambridge, Joseph John Thomson had shown that cathode rays were particles with a negative electric charge and much smaller than an atom. These particles were later named electrons

What causes some radionuclides to emit beta particles?

Beta minus particle (β-) emission occurs when the ratio of neutrons to protons in the nucleus is too high. An excess neutron transforms into a proton and an electron. The proton stays in the nucleus and the electron is ejected energetically.

This process decreases the number of neutrons by one and increases the number of protons by one. Since the number of protons in the nucleus of an atom determines the element, the conversion of a neutron to a proton actually changes the radionuclide to a different element.

Often, gamma ray emission accompanies the emission of a beta particle. When the beta particle ejection doesn't rid the nucleus of the extra energy, the nucleus releases the remaining excess energy in the form of a gamma photon.

The decay of technetium-99, which has too many neutrons to be stable, is an example of beta decay. A neutron in the nucleus converts to a proton and a beta particle. The nucleus ejects the beta particle and some gamma radiation. The new atom retains the same mass number, but the number of protons increases to 44. The atom is now a ruthenium atom.

Beta decay

What are the effects of exposure to beta particles?

Beta-particles, being less ionising than alpha-particles, can travel though many centimetres or even metres or air and though millimetres of skin or tissue. Sufficient intensity of beta-radiation can cause burns, rather like severe sunburn. If beta-emitting radionuclides are inhaled or ingested, they can also do damage to living cells and internal organs.

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