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Nuclear Physics - 2

Nuclear Physics - 2. Prof. Glenn Patrick . Quantum, Atomic and Nuclear Physics, Year 2 University of Portsmouth, 2012 - 2013. Last Week - Recap. Notation units Electron-nucleon scattering Nuclear Size Nuclear Binding Energy Macroscopic description: Liquid Drop Model

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Nuclear Physics - 2

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  1. Nuclear Physics - 2 Prof. Glenn Patrick Quantum, Atomic and Nuclear Physics, Year 2 University of Portsmouth, 2012 - 2013

  2. Last Week - Recap Notation units Electron-nucleon scattering Nuclear Size Nuclear Binding Energy Macroscopic description: Liquid Drop Model Magic Nuclei: Z or N = 2, 8, 20, 28, 50, 82, 126 Spin, magnetic moments and NMR (MRI) Microscopic description: Shell Model Nuclear Structure

  3. Today’s Plan 16 October Nuclear Physics 2 Abundance of elements/nuclei Segre Chart Zone of Stability Stable Nuclei Unstable Nuclei – Mass Parabola Energy Valley, driplines Super-heavy elements, Isle of Stability Radioactivity - Alpha, Beta, Gamma Decays Penetrating Power Radioactive Decay Law Multimodal Decays, Decay Chains Radioactive Dating Copies of Lectures:http://hepwww.rl.ac.uk/gpatrick/portsmouth/courses.htm

  4. Elements- On Earth Abundance of Elements in Earth’s Crust (atom fraction) We normally make ships out of iron and jewellery out of gold for a very good reason. Although there are always exceptions… http://pubs.usgs.gov/fs/2002/fs087-02/

  5. History Supreme • History Supreme: • Gold & Platinum plated! • 100,000 kg. • Cost ~$4.5 billion.

  6. Elements - In the Cosmos Present-day Solar System Composition Lodders, Palme & Gail (2009) arXiv: 0901.1149f Hydrogen by far the most abundant element in Universe, followed by helium: 73% Hydrogen 26% Helium 1% Metals (in astronomy a “metal” is anything other than H or He)

  7. Segre Chart Care – this can be plotted with swapped axes in the text books! Z=N Proton rich or too few neutrons Protons Z Stable nuclei Only ~300 out of ~3100 nuclides Neutron rich Neutrons N

  8. Zone of Stability Nucleonica: Only stable isotopes plotted Protons Z Neutron/proton ratio = 1 Zone of stability Neutron/proton ratio = 2 Neutrons N

  9. Stabile Nuclei All stable nuclei lie within a definite zone of stability. For low Z, most stable nuclei have a neutron/proton ratio of ~1. As Z increases, the zone of stability corresponds to a gradually increasing n/p ratio. More neutrons needed to counter Coulomb repulsion of protons. The heaviest stable isotope was once thought to be Bismuth 209, but this has been found to be slightly radioactive. Now considered to be Lead 208, which has n/p = 1.54 is the most stable nucleus in Nature. It has n/p=1.2, the maximum Binding Energy of 8.7946 MeV/nucleon and it’s magic! Abundance = 3.6% Followed by 58Fe and 56Fe. Iron makes up most of the Earth’s core due to its stability.

  10. Unstable Nuclei – Mass Parabola Unstable nuclei have the wrong proportion of protons and neutrons. The wrong balance of protons & neutrons gives these nuclei too much energy. They correct this by decaying to another nucleus with the same A and with some energy carried away by the decay products. It is a bit like a boulder rolling down a hill.

  11. The Energy Valley Valley of stability Nuclei with lowest total energy Nuclei up the sides of the valley are unstable and will decay until they reach the bottom. In general, the higher up the valley side, the shorter the lifetime.

  12. Driplines Outside drip lines the forces are no longer strong enough to hold nuclei together. Unable to bind A nucleons as one nucleus

  13. Artificial Elements Elements heavier than Uranium 92 not found on Earth as decay time shorter than life of Earth. Have to be made artificially in accelerators. GSI - Darmstadt Only facility that accelerates ions of all chemical elements occurring on Earth. Discovered: Bohrium (107) Hassium (108) Meitnerium (109) Darmstadtium (110) Roentgenium (111) Copernicium (112) SIS Synchrotron Fragment Separator

  14. Isle of Stability “Expedition” to find a predicted “island” of super-heavy elements: a region of increasingly stable nuclei around Z~114 amongst short-lived artificial elements. Due to shell effects : new magic number of Z = 114? 120? 126?… Long lifetimes of minutes or days or years?

  15. Periodic Table (June 2012) International Union of Pure and Applied Chemistry Technetium A=98, Z=43 Minute amounts in Nature Predicted by Mendeleev. Discovered by Segre & Perrier - molybdenum in cyclotron. Naming: 30 May 2012 flerovium (114) livermorium (116) Discovery: 2010 117 and 118 waiting to be named

  16. First X-Rays X-ray picture of the hand of his wife taken by Wilhelm Roentgen on 22 December 1895 The first Nobel Prize in Physics (1901) Roentgen’s X-ray demo using the hand of the anatomist Albert von Killiker - 23 January 1896

  17. Followed by Discovery of Radioactivity Henri Becquerel was studying the properties of X-rays using uranium salts. He found that nearby photographic plates became “fogged”. This radiation was bent by a magnetic field, so not due to X-rays. After processing tons of uranium ore, Marie & Pierre Curie discovered Radium & Polonium.

  18. Alpha, Beta, Gamma Radiation Ernest Rutherford studied radioactivity and found three different types of radiation: α, β and γ

  19. Alpha Decay In the early 20th century, Rutherford et al proved that the alpha particle is the positively charged nucleus of 4He (i.e. it contains 2 protons and 2 neutrons). Large, unstable nucleus Smaller, more stable nucleus Alpha particle Radium example: Energy = 4.8 MeV

  20. Alpha Decay - Quantum Tunnelling •  decay of radioactive nuclei such as uranium is an example of tunnelling. • First proposed by George Gamow in 1928. • The  particle is held inside the nucleus by strong short-range nuclear forces. Outside of the nucleus, the repulsive EM force dominates.

  21. Beta Decay A free neutron does decay. Mean life = 14.7 min. But a free proton decay never been observed to decay. Mean life > 2.1 x 1029 years!

  22. Beta-Minus Decay Beta-Minus No charge Almost massless Beta-minus decay usually occurs with nuclides which have N/Z too large. In the decay, N decreases by 1 and Z increases by 1 (A does not change). Really, this is all to do with the Weak Interaction and quarks changing flavour! Particle physics….

  23. Beta-Plus Decay Anti-particle of the electron Beta-Plus No charge Almost massless Beta-plus decay usually occurs with nuclides which have N/Z too small. In the decay, N increases by 1 and Z decreases by 1 (A does not change).

  24. Beta Decay – 3 Body Process Electron (or positron) has a distribution of energies Means it is a 3 body process rather than 2-body. Evidence for existence of the neutrino

  25. Electron Capture β+ decay not always energetically possible (after all a proton weighs less than a neutron) . Orbital electron (usually from K shell) can provide necessary energy. Electron Capture

  26. Gamma Decays Many alpha and beta decays leave daughter nucleus in an excited state. Often decay to ground state by gamma emission High energy photon(s) emitted (keV – MeV).

  27. Gamma Rays and EM Spectrum Electromagnetic radiation with wavelength of ~10-12 m.

  28. Penetrating Power Simple picture: Lead Paper Aluminium The different penetrating powers are due to the different processes by which heavy particles (like alphas), electrons and photons lose energy. This is a field in itself and the following three slides are just for illustration – just to give you an idea.

  29. Heavy Particles Mean energy loss for protons Mainly ionisation and excitation of atoms. Energy loss in single collision Multiple collisions with electrons & nuclei Corrections

  30. Electrons/Positrons Fractional energy loss in lead as a function of electron energy. Messel & Crawford, 1970

  31. Photons Photon cross-sections showing different contributions (Atomic Photoelectric Effect, Rayleigh Scattering, Compton Scattering, Pair Production off nuclear and electron fields and Photonuclear Reactions).

  32. Radioactive Decay Law Decays are statistical – cannot predict when any particular nucleon will decay. For N nuclei present at time t, the number dN decaying in time dt is proportional to N. Mean lifetime is inverse of decay constant (time for nuclei to reduce by 2.718…) Half life is time for half of nuclei to decay N0/e

  33. Multimodal Decays • Unstable nuclei can often decay via more than one mode (i.e. separate alpha and beta decays). • Each decay mode is random and independent of the other decay modes. • Each mode has it’s own transition probability (i.e. own λ). • For example, Bismuth 212 can decay to both Polonium(Po) and Titanium(Ti) • with a total mean lifetime of 536 secs: 64% 36% Solving for λ1 and λ2

  34. Decay Chains 210Bi decaying 210Po increasing from 210Bi and also itself decaying.

  35. Radioactive Dating

  36. Carbon Dating Cosmic rays produce 14C in the atmosphere by neutron capture: 12C 98.89% 13C 1.11% 14C 0.0000000001% Radioactive. Half-life=5730 years Organic matter absorbs CO2 from the atmosphere, but this stops when they die. The 14C decays from its equilibrium ratio and measuring the proportion of 14C that remains gives the age of sample.

  37. Carbon Dating Activity is defined as number disintegrations per unit of time (e.g. dpm). Specific activity is the amount of radioactivity per unit weight of material. Specific activity standard for 14C is 13.56 dpm/g or 0.226 Bq/g (1950) measured known=5568y (Libby) known IAEA Corrections due to assumptions Not least the assumption of constant 14C content.

  38. Origin and Distribution of 14C Complications: Addition to the air of CO2 by fossil fuels (without 14C) Production of 14C by neutrons released by fission/fusion.

  39. Accelerator Mass Spectrometry (AMS) If sample is large, can do simple counting, but background & time can be a problem. With low abundance/rare isotopes best to use AMS. Strip electrons to make +ve ions Accelerate to few MeV Ion source converts to -ve carbon ions Measures 12C,13C & 14C atoms in sample. Separated by atomic weights Sample, burnt & CO2 converted to graphite. In UK: Oxford University Radiocarbon Accelerator Unit NERC Radiocarbon Laboratory, East Kilbride

  40. 2.5 MV 14C Tandetron – Groningen (NL)

  41. Turin Shroud 24 Mar 2012 26 September 1988 Carbon Dating Tucson 646±31 years old Oxford 750±30 years old Zurich 676±24 years old MEAN 689±16 years old 1262-1384 (95% CL) 10 June 2012

  42. You should now be able to do your Lab Classes even better!

  43. End CONTACT Prof.G.N. Patrick Particle Physics Department Rutherford Appleton Laboratory Didcot, OX12 0QZ Tel: 01235 445343 email: glenn.patrick@stfc.ac.uk

  44. Heavy Particles MeV/c GeV/c TeV/c Stopping power for positive muons on copper Mainly ionisation and excitation of atoms.

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