Physics OCR B A2 unit 2 listening notes part 2

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  • Created by: Rhys
  • Created on: 28-05-12 17:57

Radioactive Emissions.    'Radioactivity' is a catch-all term for several different emissions from the nuclei of 'radioactive' atoms. There are three main types of radiation: alpha (α), beta and gamma . When radiation occurs, four things must be conserved: •Mass •Charge •Lepton Number •Baryon Number In formulae, mass and charge are shown next to the symbol of the particle. For example, a neutron with mass 1u and no charge is . The charge on a nucleus is equal to the number of protons in the nucleus (electrons can be ignored). The lepton and baryon numbers may be obtained by counting the number of leptons and baryons on either side of the equation, remembering that antiparticles have negative lepton and baryon numbers. alpha Radiation.  Unstable nuclei with a mass greater than 82u emit alpha radiation. This consists of an Helium nucleus, 2 proton and two neutrons. The alpha particle simply splits off from the nucleus. Since the particle has no electrons, it has a charge of +2e. This, combined with its relatively large mass, means that it reacts easily with other particles, ionising them, meaning that it cannot penetrate more than a few centimetres of air. Beta Radiation.  Unstable nuclei with a mass below 82u emit beta radiation. There are two types of beta radiation.   beta minus radiation consists of an electron . This is produced by nuclei with many more neutrons than protons. A neutron changes into a proton, emitting an electron and an antineutrino in order to balance the lepton number.   beta plus radiation consists of an positron . This is produced by nuclei with roughly the same number of neutrons as protons. A proton changes into a neutron, emitting a positron and a neutrino. beta particles also ionise particles, but since they have less charge and mass, they do this less easily, and so they travel further (on average). Both alpha and beta radiation result in the nucleus which emitted them being changed into another element.    Gamma Radiation.  The binding energies of nuclei are quantized - they can only take on certain values. When an electron jumps down an energy level, this energy has to go somewhere - it takes the form of a gamma photon. The structure of the nucleus is not changed by Gamma radiation. Gamma radiation is ionising, but only at the right frequency - the resonant frequency of the things it ionises. gamma radiation travels very far, and only a good thick layer of lead can stop it.     Energy Levels.  As an electron approaches a nucleus from infinity, it becomes 'bound' - it is attached to the nucleus, if you like. In this bound state, the electron occupies what is called an energy level. A nucleus has a discrete number of energy levels, and so electrons bound to a certain nucleus can only take on certain potential energies. These energies are negative by convention. The lowest (most negative) energy level is denoted n=1, the next lowest n=2, and so on. The values of


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