Presentation on theme: "Prof. Glenn Patrick Quantum, Atomic and Nuclear Physics, Year 2"— Presentation transcript:
1Prof. Glenn Patrick Quantum, Atomic and Nuclear Physics, Year 2 This lecture…Prof. Glenn PatrickQuantum, Atomic and Nuclear Physics, Year 2University of Portsmouth,
2Last Week - Recap Notation units Electron-nucleon scattering Nuclear SizeNuclear Binding EnergyMacroscopic description: Liquid Drop ModelMagic Nuclei: Z or N = 2, 8, 20, 28, 50, 82, 126Spin, magnetic moments and NMR (MRI)Microscopic description: Shell ModelNuclear Structure
3Today’s Plan 16 October Nuclear Physics 2 Abundance of elements/nuclei Segre ChartZone of StabilityStable NucleiUnstable Nuclei – Mass ParabolaEnergy Valley, driplinesSuper-heavy elements, Isle of StabilityRadioactivity - Alpha, Beta, Gamma DecaysPenetrating PowerRadioactive Decay LawMultimodal Decays, Decay ChainsRadioactive DatingCopies of Lectures:
4Abundance of Elements in Earth’s Crust Elements- On EarthAbundance 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…
5History Supreme History Supreme: Gold & Platinum plated! 100,000 kg. Cost ~$4.5 billion.
6Elements - In the Cosmos Present-day Solar System CompositionLodders, Palme & Gail (2009)arXiv: fHydrogen by far the most abundant element in Universe, followed by helium:73% Hydrogen26% Helium1% Metals(in astronomy a “metal” is anything other than H or He)
7Care – this can be plotted with swapped axes in the text books! Segre ChartCare – this can be plotted with swapped axes in the text books!Z=NProton richor too few neutronsProtons ZStable nucleiOnly ~300 out of ~3100 nuclidesNeutron richNeutrons N
8Zone of Stability Protons Z Neutrons N Neutron/proton ratio = 1 Nucleonica:Only stable isotopes plottedProtons ZNeutron/proton ratio = 1Zone of stabilityNeutron/proton ratio = 2Neutrons N
9Stabile NucleiAll 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 graduallyincreasing n/p ratio.More neutrons needed to counter Coulomb repulsion of protons.The heaviest stable isotope was once thought to beBismuth 209, but this has been found to be slightly radioactive.Now considered to be Lead 208, which has n/p = 1.54is the most stable nucleus in Nature.It has n/p=1.2, the maximum Binding Energy of 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.
10Unstable 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.
11The 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.
12Driplines Outside drip lines the forces are no longer strong enough to hold nuclei together.Unable to bind A nucleons as one nucleus
13Artificial ElementsElements heavier than Uranium 92 not found on Earth as decay time shorter than life of Earth. Have to be made artificially in accelerators.GSI - DarmstadtOnly 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 SynchrotronFragment Separator
14Isle 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?
15Periodic Table (June 2012) International Union of Pure and Applied ChemistryTechnetiumA=98, Z=43Minute amounts in NaturePredicted by Mendeleev.Discovered by Segre & Perrier- molybdenum in cyclotron.Naming: 30 May 2012flerovium (114)livermorium (116)Discovery: 2010117 and 118waiting to be named
16Nobel Prize in Physics (1901) First X-RaysX-ray picture of the hand of his wife taken by Wilhelm Roentgen on 22 December 1895The firstNobel Prize in Physics (1901)Roentgen’s X-ray demo using the hand of the anatomist Albert von Killiker - 23 January 1896
17Followed 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.
18Alpha, Beta, Gamma Radiation Ernest Rutherford studied radioactivity and found three different types of radiation: α, β and γ
19(i.e. it contains 2 protons and 2 neutrons). Alpha DecayIn 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, unstablenucleusSmaller, more stablenucleusAlpha particleRadium example:Energy = 4.8 MeV
20Alpha 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.
21Beta Decay A free neutron does decay. Mean life = 14.7 min. But a free proton decay never been observedto decay. Mean life > 2.1 x 1029 years!
22Beta-Minus Decay Beta-Minus No chargeAlmost masslessBeta-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 andquarks changing flavour! Particle physics….
23Beta-plus decay usually occurs with nuclides which have N/Z too small. Anti-particle ofthe electronBeta-PlusNo chargeAlmost masslessBeta-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).
24Beta Decay – 3 Body Process Electron (or positron) has a distribution of energiesMeans it is a 3 body process rather than 2-body.Evidence for existence of the neutrino
25Electron Capture 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
26Gamma DecaysMany alpha and beta decays leave daughter nucleus in an excited state.Often decay to ground state by gamma emissionHigh energy photon(s) emitted (keV – MeV).
27Gamma Rays and EM Spectrum Electromagnetic radiationwith wavelength of ~10-12 m.
28Penetrating Power Simple picture: Paper Aluminium 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.
29Mean energy loss for protons Heavy ParticlesMean energy loss for protonsMainly ionisation and excitation of atoms.Energy loss in single collisionMultiple collisionswith electrons & nucleiCorrections
30Fractional energy loss in lead as a function of electron energy. Electrons/PositronsFractional energy loss in lead as a function of electron energy.Messel & Crawford, 1970
31Photon cross-sections showing different contributions PhotonsPhoton cross-sections showing different contributions(Atomic Photoelectric Effect, Rayleigh Scattering, Compton Scattering, Pair Production off nuclear and electron fields and Photonuclear Reactions).
32Radioactive Decay Law N0/e Decays are statistical – cannot predict when any particular nucleon will decay.For N nuclei present at time t, the number dN decaying in time dtis proportional to N.Mean lifetime is inverse of decay constant(time for nuclei to reduce by 2.718…)N0/eHalf life is time for halfof nuclei to decay
33Multimodal DecaysUnstable 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
34Decay Chains 210Bi decaying 210Po increasing from 210Bi and also itself decaying.
36Cosmic rays produce 14C in the atmosphere by neutron capture: Carbon DatingCosmic rays produce 14C in the atmosphere by neutron capture:12C %13C 1.11%14C %Radioactive. Half-life=5730 yearsOrganic 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.
37Carbon DatingActivity 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 dpm/g or Bq/g (1950)measuredknown=5568y (Libby)knownIAEACorrections due to assumptionsNot least the assumption of constant 14C content.
38Origin 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.
39Accelerator 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 ionsAccelerate to few MeVIon source converts to-ve carbon ionsMeasures 12C,13C & 14C atoms in sample.Separated by atomic weightsSample, burnt & CO2converted to graphite.In UK: Oxford University Radiocarbon Accelerator UnitNERC Radiocarbon Laboratory, East Kilbride