Presentation on theme: "Nuclear Physics - 2 Quantum, Atomic and Nuclear Physics, Year 2 University of Portsmouth, 2012 - 2013 Prof. Glenn Patrick."— Presentation transcript:
Nuclear Physics - 2 Quantum, Atomic and Nuclear Physics, Year 2 University of Portsmouth, Prof. Glenn Patrick
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 Todays 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:
4 Elements- On Earth Abundance of Elements in Earths 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…
5 History Supreme History Supreme: Gold & Platinum plated! 100,000 kg. Cost ~$4.5 billion.
6 Elements - In the Cosmos 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) Lodders, Palme & Gail (2009) arXiv: f Present-day Solar System Composition
7 Segre Chart Neutrons N Protons Z Z=N Stable nuclei Only ~300 out of ~3100 nuclides Neutron rich Proton rich or too few neutrons Care – this can be plotted with swapped axes in the text books!
8 Zone of Stability Neutron/proton ratio = 2 Neutron/proton ratio = 1 Zone of stability Nucleonica: Only stable isotopes plotted Neutrons N Protons Z
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 MeV/nucleon and its magic! Abundance = 3.6% Followed by 58 Fe and 56 Fe. Iron makes up most of the Earths 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 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) Fragment Separator Elements heavier than Uranium 92 not found on Earth as decay time shorter than life of Earth. Have to be made artificially in accelerators. SIS Synchrotron
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 Naming: 30 May 2012 flerovium (114) livermorium (116) Discovery: and 118 waiting to be named Technetium A=98, Z=43 Minute amounts in Nature Predicted by Mendeleev. Discovered by Segre & Perrier - molybdenum in cyclotron.
16 First X-Rays Roentgens X-ray demo using the hand of the anatomist Albert von Killiker - 23 January 1896 X-ray picture of the hand of his wife taken by Wilhelm Roentgen on 22 December 1895 The first Nobel Prize in Physics (1901)
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 20 th century, Rutherford et al proved that the alpha particle is the positively charged nucleus of 4 He (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 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 years!
22 Beta-Minus Decay Beta-Minus No charge Almost massless Really, this is all to do with the Weak Interaction and quarks changing flavour! Particle physics…. 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).
23 Beta-Plus Decay No charge Almost massless Anti-particle of the electron Beta-Plus 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 ~ m.
28 Penetrating Power PaperAluminium Lead 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. Simple picture:
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 N 0 /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 its 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 λ 1 and λ 2
34 Decay Chains 210 Bi decaying 210 Po increasing from 210 Bi and also itself decaying.
35 Radioactive Dating
36 Carbon Dating Cosmic rays produce 14 C in the atmosphere by neutron capture: Organic matter absorbs CO 2 from the atmosphere, but this stops when they die. 12 C 98.89% 13 C1.11% 14 C % Radioactive. Half-life=5730 years The 14C decays from its equilibrium ratio and measuring the proportion of 14 C that remains gives the age of sample.
37 Carbon Dating Specific activity is the amount of radioactivity per unit weight of material. Specific activity standard for 14 C is dpm/g or Bq/g (1950) Activity is defined as number disintegrations per unit of time (e.g. dpm). IAEA known=5568y (Libby) measured known Corrections due to assumptions Not least the assumption of constant 14 C content.
38 Origin and Distribution of 14 C Complications: Addition to the air of CO 2 by fossil fuels (without 14 C) Production of 14 C by neutrons released by fission/fusion.
39 Accelerator Mass Spectrometry (AMS) In UK: Oxford University Radiocarbon Accelerator Unit NERC Radiocarbon Laboratory, East Kilbride If sample is large, can do simple counting, but background & time can be a problem. With low abundance/rare isotopes best to use AMS. Sample, burnt & CO 2 converted to graphite. Measures 12 C, 13 C & 14 C atoms in sample. Separated by atomic weights Ion source converts to -ve carbon ions Accelerate to few MeV Strip electrons to make +ve ions
MV 14C Tandetron – Groningen (NL)
41 Turin Shroud 24 Mar June September 1988 Carbon Dating Tucson646±31 years old Oxford750±30 years old Zurich676±24 years old MEAN689±16 years old (95% CL)
42 You should now be able to do your Lab Classes even better!
43 CONTACT Prof. G.N. Patrick Particle Physics Department Rutherford Appleton Laboratory Didcot, OX12 0QZ Tel: End
44 Heavy Particles MeV/c GeV/c TeV/c Stopping power for positive muons on copper Mainly ionisation and excitation of atoms.