About these slides • These slides are used as part of my lessons and shouldn’t be considered comprehensive – There’s no excuse for.
Download ReportTranscript About these slides • These slides are used as part of my lessons and shouldn’t be considered comprehensive – There’s no excuse for.
About these slides • These slides are used as part of my lessons and shouldn’t be considered comprehensive – There’s no excuse for not turning up to lessons! • These slides use material from elsewhere on the assumption of fair/educational use – If you own the copyright to any of this material and want it credited/removed please contact me • These slides may contain errors – Use at your own risk! This work is licensed under a Creative Commons Attribution-NonCommercialShareAlike 3.0 Unported License. Particles and Radiation Constituents of the Atom Particle Interactions Stable and Unstable Nuclei Classification of Particles (I) Particles, antiparticles and photons Classification of Particles/Quarks Conservation Rules Constituents of the atom Structure of an atom • There are three particles that make up an atom • They are: – Proton – Neutron – Electron Structure of an atom (II) Particle Proton Neutron Electron Mass (kg) 1.672 x 10-27 1.674 x 10-27 9.109 x 10-31 Charge (C) 1.602 x 10-19 0 -1.602 x 10-19 Particle Proton Neutron Electron Relative Mass 1 1 1/1836 Relative Charge +1 0 -1 Specific Charge Particle Proton Neutron Electron Specific Charge is the amount of charge on a particle per kg of that particle Mass (kg) 1.672 x 10-27 1.674 x 10-27 9.109 x 10-31 Particle Proton Neutron Electron Charge (C) 1.602 x 10-19 0 -1.602 x 10-19 Specific Charge (Ckg-1) 9.581 x 107 0 -1.759 x 1011 Specific Charge (II) • We can calculate a specific charge for an ion 24 • For example 12 Mg has a charge of +3.2x10-19C and a mass of 3.98x10-26kg • What is its specific charge? Question • A Cu atom loses two electrons. ion formed: 63 29 For the – Calculate its charge in Coulombs – State the number of nucleons it contains – Calculate its specific charge in Ckg-1 Question • Calculate the mass of an ion that has a specific charge of 1.20x107Ckg-1 and a negative charge of 3.2x10-19C – The ion has 8 protons, how many neutrons and electrons does it have? History of Atomic Structure In Ancient Greece…. 5th century B.C. cheese But… • Another big cheese got in the way • Aristotle dismissed the idea of atoms (he thought everything was made from a combination of earth, air, fire and water • So for the next 2000 years atoms were ignored 1661 • Robert Boyle proposed that there were some substances that could not be made simpler • These are what we now call the chemical elements 1803 • John Dalton proposed the following (based on experiment, not on philosophy) – All matter is made of atoms – Atoms of an element are identical – Each element has different atoms – Atoms of different elements combine in constant ratios to form compounds – Atoms are rearranged in reactions 1896 • Henry Becquerel discovered radioactivity • This means the atom cannot be indivisible 1897 • JJ Thomson discovered the electron • Given that they have a negative charge and very little mass he realised that atoms must contain some positively charged material The plum pudding model + + + - + + + + + + + - 1911 • Ernest Rutherford performs an experiment where he fires alpha particles at a thin gold foil • Most of them pass straight through BUT some bounce almost straight back • “as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you” Nucleon Number and Isotopes • Write a definition for: – Proton Number, Z – Nucleon number, A • Explain the existence of isotopes Copy and complete the table Element Symbol Protons Neutrons Electrons Lithium Li 3 4 3 Carbon Sodium Aluminium Pb Ti Zn 72 15 0 Tungsten Nuclide Notation • Any nuclide (atomic species) can be written so as to show its proton number, Z and nucleon number, A as follows: A Z X • Any set of nuclides with the same proton number are called isotopes Stable and unstable nuclei Fundamental Forces • There are four fundamental forces: – Gravity – Electromagnetic force – Strong nuclear force – Weak nuclear force • The electromagnetic force will try and push the protons in a nucleus away from each other • This doesn’t happen because of the strong nuclear force Strong Nuclear Force • Only hadrons feel the strong nuclear force – We’ll deal with what a hadron is in more detail soon. For now just know that neutrons and protons are hadrons, electrons aren’t • It is a very short range force – Attraction is felt below 3 femtometres – Repulsion is felt below 0.5 femtometres Belt of stability • As mass number increases the stability of atoms is related to the ratio of neutrons to protons in the nucleus • Atoms with too many neutrons or protons will decay to form more stable atoms Radioactive decay • Bismuth (Z = 83) is the stable nucleus with the highest number of protons • All atoms higher than this will undergo radioactive decay • We can write equations for this decay – and you’ll need to know how Alpha decay • An alpha particle is sometimes described as a helium nucleus 4 2 – It contains 2 protons and 2 neutrons • If something decays by alpha decay the total number of nucleons must be preserved on each side of the equation A Z X Y A4 Z 2 4 2 • The original nucleus is the parent, the new one is called the daughter Complete this: Th Ra 229 90 Beta decay • In b decay a neutron in the nucleus changes into a proton • This produces a b- particle (an electron) and an antineutrino, n̅, which are emitted instantly – A neutrino has no charge and very little mass – An antineutrino is the corresponding antiparticle A A 0 Z Z 1 1 X Y b n Gamma radiation, g • Gamma radiation is electromagnetic radiation • It is emitted by an unstable nucleus with too much energy as a result of alpha or beta decay • Gamma radiation has no mass or charge Strong nuclear force or electromagnetic force? • • • • Does not affect a neutron Has a limited range Holds the nucleons in a nucleus Tends to make a nucleus unstable Complete this: 65 28 Ni Cu b n Question • 213 83 Bi decays by emitting a beta particle, then an alpha particle, then another beta particle • Write the decay equations for each stage • How many neutrons and protons are there in the final atom? Particles, antiparticles and photons True or false? • Light is a wave – True! • Light is a particle – Also true!? Photons • Photons are a manifestation of something called wave-particle duality – We’ll cover this in more detail later • This states that electromagnetic radiation (of which light is a part) can be both a wave and a particle at the same time • A photon is a packet of electromagnetic waves Photon energy • The energy of a photon, E, can be calculated from its frequency using the Planck constant, h • The energy of the photon is given by E = hf where h = 6.63 x 10-34 Js • Note that since f = c/l this can also be written E = hc/l Question • Calculate the frequency and energy of a photon of wavelength 590nm Question • What is the energy of a photon with l = 430nm? Particles, antiparticles and photons Particle energies • Up until now we have expressed energy in terms of Joules, J • This isn’t very useful when talking about particles – The energies involved are tiny • Instead we use the electron volt, eV, and more particularly millions of electron volts, MeV 1MeV = 1.60 x 10-13J Electron Volts • One electron volt is defined as the energy transferred when an electron is moved through a potential difference of 1V Antimatter Particles and antiparticles • Antimatter was first predicted by Paul Dirac in 1928 • He said that there must be an antiparticle equivalent to every particle and that they must have the following properties: – Exactly the same rest mass as its particle – Exactly opposite charge (if the particle has a charge) – Will annihilate the particle and itself if they meet Rest mass and rest energy • Einstein stated that a particle at rest (stationary) has a rest mass (m0) and a corresponding rest energy (given by E=m0c2) • This rest energy can’t normally be unlocked, but if a particle and antiparticle annihilate, two photons are produced, each with half the combined energy of the two particles Annihilation photon antiparticle particle photon • Two photons are produced so momentum is conserved • Each photon has a minimum energy hfmin equal to the rest energy of the particle, E0 hfmin = E0 Pair production • If a photon has sufficient energy then it can change into a particle and its corresponding antiparticle • Note that for this to happen hfmin = 2E0 Pair production calculation • So if an electron (and therefore a positron) has a rest energy of 0.511 MeV the minimum energy of a photon needed for pair production of an electron and a positron is 2 x 0.511 MeV hfmin = 2 x 0.511 MeV = 1.022 MeV • A photon with less energy than this could not produce an electron and a positron What’s the energy in Joules? What’s the minimum frequency photon needed? Positron emission • The positron is the anti-electron • It is produced during positron decay by a nucleus with too many protons • A proton changes into a neutron and a positron and a neutrino are emitted • Can you predict the decay equation? A Z X Y b n A Z 1 0 1 Discovery of the positron • A cloud chamber allows us to see the path left by ionising particles • The addition of a magnetic field means that charged particles will curve • The direction of curve tells us the sign of the charge • The amount of curve tells us the mass Question • If the rest energy of a proton is 1.501 x 10-10 J what is it’s rest energy in MeV? • What is the minimum energy, in MeV, required of a photon to create a protonantiproton pair? Question • The rest energy of an electron is 0.511MeV • State the minimum energy of each photon created when a positron and an electron annihilate each other Question • A positron created in a cloud chamber in an experiment has 0.158MeV of kinetic energy. It collides with an electron at rest, creating two photons of equal energies as a result of annihilation • Calculate the total energy of the positron and the electron • Show that the energy of each photon is 0.590 MeV Particle Interactions Forces • Forces affect the momentum of an object – (momentum is the mass of an object multiplied by its velocity) • When two objects interact they exert equal and opposite forces on each other (Newton’s third law) • If no other forces act then momentum is transferred Thought experiment • If a skateboarder throws a basketball away from themselves they will move backwards with the same momentum as the ball • If another skateboarder then catches the ball they will also move away • This is a model for a repulsion interaction between particles Exchange particles • Richard Feynman came up with the idea of an exchange particle that transferred momentum and/or charge between two particles – In effect the exchange particle is responsible for the force • The electromagnetic force between two charged objects is due to the exchange of virtual photons – They’re virtual because we can’t see them, if we did then we’d stop the force working Exchange Particles Force Acts on Relative strength* Range Exchange particle Gravitational Everything with mass 10-40 Infinity graviton Electromagnetic Charged particles 10-2 Infinity photon Strong nuclear Quarks 1 10-15 m gluon Weak nuclear Quarks and 10-5 leptons 10-17 m intermediate vector bosons (Z0, W+ and W- particles) Particle Interactions ✔ ✔ Feynman diagrams • Feynman diagrams represent the interaction between two particles • The lines don’t represent the paths of the particles p p g p Time p Weak nuclear force • We’ve already met the strong nuclear force – It holds nucleons together • The weak nuclear force is responsible for b- and b+ decay when a neutron changes into a proton and vice-versa • The exchange particle for weak interactions is called the W boson – There is also a Z boson, but you don’t need to know about it W bosons • W bosons are different to photons – They have a non-zero rest mass – They have a very short range (<0.001fm) – They have a charge (W+ boson and W- boson exist) Neutron-neutrino interaction • If a neutron and a neutrino interact the neutron is turned into a proton and a bparticle is emitted • We can think of this as two separate processes: neutron proton + carrier particle carrier particle + neutrino b- particle • For charge to be conserved the carrier particle must have a negative charge: Wboson Feynman diagram p n W b- n Proton-antineutrino interaction • If a proton and an antineutrino interact the proton is turned into a neutron and a b+ particle is produced • The W+ boson carries the charge (note again this is conserved) n p W b+ n̅ • If the W boson doesn’t meet another particle then it will decay • We see this in beta decay b- decay • We know that in b- decay a neutron turns into a proton and an electron and an antineutrino are produced b- p n W n̅ b+ decay • In b+ decay a proton turns into a neutron and a __________ and a ___________ are produced • Draw the Feynman diagram Other weak interactions • You need to know two other weak interactions – Electron Capture – Electron-proton collision Electron capture • If a nucleus is proton rich it can either undergo b+ decay (as we’ve already seen) or electron capture • In electron capture a proton interacts with an inner shell electron outside the nucleus n p W n e- Proton-electron collision • If a proton collides with an electron at very high speed then the same interaction can occur ① • If the electron has sufficiently high speed then a W- exchange could happen ②: ① n p W ② n n e- e- W- n p Sketch the Feynman diagram • For the electromagnetic force between – Two protons – A proton and an electron – Two electrons Sketch the Feynman diagram • For – b+ decay – b- decay Sketch the Feynmann Diagram • For the interaction between – A neutron and a neutrino – A proton and an antineutrino Question • What is the range of a W boson? • Given it can’t travel faster than the speed of light what is its estimated lifetime? Classification of Particles Classifying Particles Particles Hadrons Mesons Leptons Baryons Electron, e- Pion, p and many others... Muon, m Kaon, K Neutron, n Proton, p Electron neutrino, ne Muon neutrino, nm Fundamental particles • Leptons are fundamental, or elementary, particles – They can’t be broken down further • Hadrons (mesons and baryons) aren’t fundamental particles – They are made of quarks • More on them later… Anti-particles • All particles have a corresponding antiparticle Particle Corresponding anti-particle Electron, e- Positron, e+ Muon Neutrino, nm Anti-muon neutrino, n̅m Electron Neutrino, ne Anti-electron neutrino, n̅e Proton, p Anti-proton, p ̅ Neutron, n Anti-neutron, n̄ Pion, p+, p-, p0 Pion, p-, p+, p0 Kaon, K+,K-,K0 Kaon, K-,K+,K̄0 – Plus others you don’t need to worry about at the moment Leptons • Leptons are elementary • Leptons interact through the weak interaction – And the electromagnetic interaction if they are charged • Leptons are not subject to the strong interaction • You need to know about electrons, muons and neutrinos Muons, m • Muons are sometimes referred to as ‘heavy electrons’ – They have a negative charge but over 200 times the rest mass of an electron • Muons decay into electrons and antineutrinos or positrons and neutrinos – We’ll look at this in a bit Neutrinos, n • Neutrinos are all around us, billions are passing through the earth as we speak • There are different types, or ‘flavours’, of neutrino • Muon neutrinos, nm, are produced in muon decays • Electron neutrinos, ne, are produced in beta decay Generations • Electrons and electron neutrinos are first generation leptons • Muons and muon neutrinos are second generation leptons • When leptons decay or are produced neutrinos of the same generation are involved Lepton numbers • We have already seen how particle interactions have to conserve charge – if the particles going in have a charge the products must have the same charge • Momentum is also conserved – the total momentum of the products equals that of the original particles • A third property that is conserved is lepton number Lepton Numbers • A lepton has a lepton number of +1 • An anti-lepton has a lepton number of -1 • Any other particle has a lepton number of 0 • We look at the total lepton number on each side of an interaction – it needs to be the same Lepton Interactions • Leptons can change into other leptons through the weak interaction • They can also be produced or annihilated in particle-antiparticle interactions • We have seen an example of a Leptonhadron interaction: p n W b- n Lepton Numbers • n + ne p + e0+1 0+1 • n + ne p̅ + e+ 0+1 0 + -1 allowed not allowed Muon decay • Remember: A lepton can decay into another lepton • A muon can decay into a muon neutrino – an electron and an electron antineutrino are created to conserve charge m- e- + n̅e + nm Charge Lepton Number -1 1 -1 1 0 -1 0 +1 Forbidden decays • Some decays are forbidden: m- e- + n̅e + n̅m Charge -1 -1 0 0 Lepton Number 1 1 -1 -1 Total Lepton Number = -1 Charge Lepton Number m- e- + n̅m + ne -1 -1 0 Muon 0 can’t decay into an electron neutrino 1 1 -1 Electrons +1 can only be created with an electron antineutrino Question • State one similarity, and one difference between: – An electron and a muon – An electron neutrino and a muon neutrino Question • From what you have learnt so far predict the decay of an antimuon, m+ m+ n̅m + e+ + ne • If the antimuon has no kinetic energy how much energy is removed by the other particles? Use the data booklet to help you. Question • What is the charge and lepton number of: Charge – A muon neutrino 0 – An antimuon +1 – A positron +1 – An electron antineutrino 0 Lepton Number 1 -1 -1 -1 Hadrons • Hadrons can interact through the strong interaction • They can also interact through the electromagnetic interaction if they are charged • Hadrons decay through the weak interaction (apart from the proton, which is stable) Baryons • Baryons are protons and any other hadron that decays into a proton (directly or indirectly) • You will need to know how a neutron decays into a proton – We’ve done this, draw the Feynman- diagram b here p n W n̅ Quark composition • Baryons are not fundamental particles • They are made up of three quarks • There are 6 types of quark in total, but you only need to know about 3 of them: – Up – Down – Strange Quark composition • You need to know the quark composition of a neutron and a proton: • A Neutron is dowN, dowN, up • A Proton is uP, uP, down Progress so far ✔ ✔ ✔ ✔ ✔ Anti-baryons • Anti-baryons are made from the antiparticle equivalent quarks • So what’s the composition of an antiproton and an anti-neutron? Baryon Number • Like the lepton number for leptons there is a baryon number for baryons • Similar rules apply – Baryons have a baryon number of +1 – Anti-baryons have a baryon number of -1 – Mesons and leptons have a baryon number of 0 • As for the lepton number the baryon number must be conserved in a reaction Baryon Number • Since baryons are made of three quarks this effectively means that – quarks have a baryon number of +⅓ – antiquarks have a baryon number of -⅓ – leptons have a baryon number of 0 Quark properties • You will need to know how to use the following quark properties (given on the data sheet) Up u Charge Q Strangeness S Quarks Down Strange d s +⅔ -⅓ -⅓ 0 0 -1 Antiquarks Up Down Strange u̅ d̅ s̅ -⅔ +⅓ +⅓ 0 0 +1 Strangeness • Like charge strangeness is conserved – But only in strong interactions • Strangeness is assigned based on the behaviour of particles when they decay • It is dependent on the strange quark • You will be expected to be able to calculate strangeness from quark composition, or say whether an interaction is allowed Applying your knowledge • What is the strangeness of a neutron? • What is the strangeness of a proton? Strange baryons • You do not need to remember, but may come across in exams, the S particle – a baryon with a strange quark in its composition • Given this information about S particles can you work out their quark composition? Particle Baryon Number (B) Charge (Q) Strangeness (S) S0 S+ S- +1 +1 +1 0 +1 -1 -1 -1 -1 S particle composition? Up u Charge Q Strangeness S Particle Quarks Down Strange d s +⅔ -⅓ -⅓ 0 0 -1 Antiquarks Up Down Strange u̅ d̅ s̅ -⅔ +⅓ +⅓ 0 0 +1 S0 Baryon Charge (Q) Number (B) +1 0 Strangeness (S) -1 S+ +1 +1 -1 S- +1 -1 -1 Mesons • Mesons consist of two quarks – One quark and one antiquark • The mesons for up, down and strange quarks are: Mesons • Note that – Each pair of charged mesons is a particleantiparticle pair – There are two uncharged K mesons, the K0 meson and the K̅0 meson – The antiparticle of any meson is a quark-antiquark pair, and is therefore another meson – A p0 meson is any quark-corresponding antiquark combination and is its own antiparticle Questions • For the next questions try not to look back through your notes • Instead, if you’re not sure about a quark composition, try and calculate it from the information given in the question and the datasheet Questions • What is the quark composition of: – A proton – A neutron Questions • Given its strangeness work out the quark composition of the following hadrons: – p0 (S=0) – An antiproton (S=0) – A K- meson (S=-1) – A S0 baryon (S=-1) Question • Draw the Feynman diagram in terms of quarks for b+ decay Question • A S- particle has a strangeness of -1 what is its quark composition? • A K+ meson is composed of a strange antiquark and an up quark use this information to describe the following reaction in terms of quarks and anti quarks: p- + p K+ + S- Conservation Rules ✔ ✔ ✔ ✔ Energy and Charge • Conservation of energy and conservation of charge apply to all changes in science – Not just particle antiparticle interactions and decays • Don’t forget that rest energy needs to be taken into account Lepton Numbers • Remember that a lepton has a number of +1, an antilepton has a number of -1 and any other particle has a number of 0 • Also remember that the conservation applies to each branch of the lepton family e.g. m+ e+ + n̅m + ne Baryon numbers • Baryons are applied numbers in a similar way to leptons • A baryon has a number of +1, antibaryon is -1 and any other particle is 0 – Remember this means that we can give a quark a baryon number of + 1/3 and an antiquark a baryon number of -1/3 Are these observed? • Use baryon numbers to decide if these reactions are observed or not: p + p̅ p+ + pp + p̅ p + p- p + p p + p + p + p̅ p + p̅ p̅ + p+ Strangeness • Strangeness is only conserved in strong interactions, not weak • If a particle contains a strange quark it has a strangeness of -1, if it contains a strange antiquark it has a strangeness of +1 e.g. p- + p K0 + L0 where L0 is a strange baryon called the neutral lambda particle which contains a strange quark