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Nuclear structure 27.(c) distinguish between nucleon number (mass number) and proton number (atomic number).. 27.(e) use the usual notation for the representation of nuclides. 27.(d) show an understanding that an element can exist in various isotopic forms each with a different number of neutrons 27.(f) appreciate that nucleon number, proton number, and energy and mass are all conserved in nuclear processes 27.(g) represent simple nuclear reactions by nuclear equations of the form 14 7 N 24He 178O 11H 27.(h) show an appreciation of the spontaneous and random nature of nuclear decay. 21.(j) infer the random nature of radioactive decay from the fluctuations in count rate. An atom • Describe the structure of an atom Atomic notation • There are about 256 nuclides in nature which are so stable that they have never been observed to decay. They occur among the 80 different elements which have one or more stable nuclides. • Each nuclide can be identified by characterized by it’s number of protons, its number of neutrons, (and its energy content) • The accepted notation is A Z X • A = mass or nucleon number • Z = atomic number • N = neutron number are related (A = Z + N). Isotopes • Isotopes have the same atomic no, (Z) but different mass number (A) • How could this be? • Some common examples: – H, D and T, – U-235 and U-238, – C-14 and C-12. • Find out the name for nuclides having the same A but different Z, and the same N but different Z . Interactions • • • • What is the charge on a proton? What happens when lots of protons are in close proximity? So how does a nucleus stay together? There must be an attractive force acting on the nucleons which is stronger than the Coulomb force • What about gravity? • The rest mass of a proton is 1.672621637(83)×10−27 kg • Also remember Gravitational force is 1042 weaker than the Coulomb force Nuclear force • The other force is the strong nuclear force • How does it compare to the Coulomb force? • Do you think this is an inverse square law force (field)? • No it must be stronger over shorter distances than the Coulomb force but weaker over distances greater than the diameter of a nucleus. • The decrease is approximately as a negative exponential power of distance N-Z plot (Segrè plot). Neutron number n p + b- n A bb+ B p n + b+ + n Proton number Radioactive decay • The Segre plot shows isotopes which are unstable are far from the line. • These isotopes will naturally decay to become more stable (moving closer to the line) • They can do this by emitting alpha or beta particles. 241 95 4 Am 237 93 Np 2 He. Equation for alpha decay • Nuclear decay processes can be represented by nuclear equations. The word equation implies that the two sides of the equation must ‘balance’ in some way. • Decay processes • alpha source eg americium-241, 241 95 Am 237 93 Np He. 4 2 Beta radiation • • • • • Beta sources are strontium-90, The underlying process is: n –> p + e- + ν Here, ν is an antineutrino. You can translate n –> p + e- into the AZ notation: 90 38 Sr Np 90 39 0 1 e. Gamma radiation • gamma sources eg cobalt-60 • The γ radiation comes from the radioactive daughter of the beta decay. • The daughter is formed in an ‘excited state’ and immediately loses the energy by emitting a gamma ray. • They are only emitted after an alpha or beta decay • All such gamma rays have a well-defined energy. • So a cobalt-60 source which is a pure gamma emitter must be designed so that betas are not emitted. How? • By encasing in metal which is thick enough to absorb the betas but which still allows gammas to escape. Nature of radiations • Use pages 334&5 to complete the table. • Close your book turn over the page and see how much you can do from memory. Spontaneous • Which of the following will cause a nuclide to decay. – Applying huge pressures using high explosives – Applying exremely high temperatures such as at the centre of stars – Accelerating it to speeds near to the speed of light Answer None of them the process is spontaneous and random • If I toss a coin 15 times and each time get a head the next toss is more likely to be – A head – A tail • • • • Answer Still 50/50 the toss is random Nuclear decay is also random You cannot predict exactly when a nucleus will decay To do • • • • Complete the nuclear equations worksheet P4U p347 Q1,2,3,4,8 P4U p370 Q7 Reading – Atomic structure p 328-331. – Nature of radiations p334-335 – Alpha and beta decay equations p338-339 – Background p344-346