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Topic 7: Nuclear WS / edited by JC Becquerel, Marie and Pierre Curie, Ernest Rutherford Rocks containing uranium that gave out some kind of radiation were discovered and investigated. More than one substance was involved (Radium and Polonium were discovered) More than one type of radiation was involved Alpha, Beta, Gamma The properties of the electron were discovered (J.J. Thompson). However there was no evidence for any of the ideas about atom composition. Geiger–Marsden experiment 1909 The alpha particles were observed to occasionally scatter at angles greater than 90°, which is physically impossible unless they are scattering off something more massive than themselves. This led Rutherford to deduce that the positive charge in an atom is concentrated into a small compact nucleus. The majority of the atom is probably empty space as most of alpha particles go through the foil unchanged Isotopes It was also discovered by JJ Thompson and others that atoms of the same element could have different masses. This meant that there had to be both a positive and a neutral component in the nucleus. This model is referred to as the: “Simple Atomic Model” Limitation of the simple model The main problem with this theory was that accelerating charges are known to lose energy. If the orbiting electrons were to lose energy they would spiral into the nucleus. Also, this model does not explain the emission and absorption spectrum. The model does not account for how the protons and neutrons stay together in the nucleus. Atomic energy levels Evidence for electron energy levels comes from emission and absorption spectra. An energy level of 0 corresponds to the electron escaping from the atom. Electrons attached to an atom have negative energy levels. Definitions Nuclide: The name given to a particular species of atom (one whose nucleus contains a specified number of protons and a specified number of neutrons) Isotope: Elements that contain the same number of protons but a different number of neutrons. Nucleon: Protons and neutrons are collectively called nucleons. More Definitions A: Nucleon number – Number of nucleons (protons + neutrons) in the nucleus Z: Proton number – also called atomic number, equal to number of protons in the nucleus N: Neutron number – Number of neutrons in the nucleus N=A–Z Radioactive decay Radioactive decay is a random process and is not affected by external influences. Some nuclei are more stable than others. When an unstable nucleus disintegrates to acquire a more stable state, radiations are emitted. Nuclei also have energy levels! Properties of alpha and beta particles and gamma Property Alpha Beta Gamma Effect on photographic film Yes Yes Yes Appropriate number of ions produced in air 104 per mm travelled 102 per mm travelled 1 per mm travelled Typical material needed to absorb 10-2 mm aluminium, piece of paper A few mm aluminium 10cm lead Penetration ability Low Medium High Typical path length in air A few cm Less than one m Effectively infinite Behaves like a negative charge Not deflected About 108 m/s Speed of light Deflection by E and B fields Behaves like a positive charge Speed About 107 m/s What is ionizing? All three radiations are ionizing, which means that as they go through a substance, collisions occur which cause electrons to be removed from atoms. Atoms that have lost or gained electrons are called ions. When ionisations occur in biologically important molecules, such as DNA, mutations can occur. Biological Effects At the molecular level, an ionisation could cause damage directly to a biologically important molecule such as DNA. This could cause it to cease functioning. Molecular damage can result in a disruption to the functions that are taking place within the cells that make up the organism. As well as potentially causing the cell to die, this could just prevent cells from dividing and multiplying. If malignant cells continue to grow then this is called cancer. Half-Life The time taken for the number (or mass) of radioactive nuclei present to fall to half its value. This length of time is constant at any point in time - showing that radioactive decay is exponential. Artificial (induced) Transmutation Artificial transmutation is the process whereby a nucleus is artificially made from another nucleus. It is different from regular radioactivity in that the reaction is not spontaneous; it is made to happen. When nitrogen gas was bombarded by alpha-particles it was found that there were two products: oxygen gas and positively charged particles. Einstein Mass-Energy Equivalence Relationship If an object increases in energy, then its mass also increases. The relationship between mass and energy is described by Einstein’s famous equation: E=mc2 When energy is released, there is also a decrease in mass of the products. In Einstein’s equation, 1kg of mass is equivalent to 9x1016J of energy. Since mass and energy are equivalent it is sometimes useful to work in units that avoid having to do repeated multiplications by the speed of light. A new possible unit for mass is thus MeV c −2. If 1 MeV c−2 worth of mass is converted you get 1MeV worth of energy. Binding Energy Mass defect: The difference between the mass of a nucleus and the masses of its component nucleons. Binding energy: The amount of energy that is released when a nucleus is assembled from its component nucleons. It comes from a decrease in mass. The binding energy would also be the energy that needs to be added in order to separate a nucleus into its individual nucleons. Mass defect x (Speed of light)2 = Binding Energy Binding energy per nucleon Binding energy per nucleon: is useful measure of the stability of a nucleus binding energy it is the total binding energy divided by the total number of nucleons Fission vs. Fusion Fission: Fission is the name given to the nuclear reaction whereby large nuclei are induced to break up into smaller nuclei and release energy in the process. It is the reaction that is used in nuclear reactors and atomic bombs. A typical single reaction might involve bombarding a uranium nucleus with a neutron. This can cause the uranium nucleus to break up into two smaller nuclei. Fusion: Fusion is the name given to the nuclear reaction whereby small nuclei are induced to join together into larger nuclei and release energy in the process. It is the reaction that fuels all stars.