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Slide 2 Modern Physics By Neil Bronks Slide 3 Atoms C 12 6 Mass Number Mass Number - Number of protons + Neutrons. Atomic Number Atomic Number - Number of protons In a neutral atom the number of electrons and protons are the same. In Carbon it is 6 Slide 4 Hydrogen Electron Proton The simplest atom has one negative electron orbiting one positive proton. The electron is very light compared to the proton. Slide 5 Helium Neutron In this atom we see two neutrons and two protons forming the nucleus. The Neutron has no charge but is the same mass as the proton. Electron Proton Slide 6 4 Forces of Nature (Order of strength) Gravitational - Only Positive - Very long range Weak Nuclear- Associated with beta decay Strong Nuclear Holds nucleus together - Very Short Range Electromagnetic Positive and negative Slide 7 Radiation Decay of nucleus by the emission of a particle or a ray. Discovered by Henri Becquerel Units 1 Bq is one decay per second Natural happens without outside bombardment Artificial happens due to bombardment Jai fais a ! Dodgy Beard Slide 8 Safety Wear Gloves or Apron of lead Dont point at anyone Dont eat!!! Slide 9 Nuclear Equations Top and bottom must add up Top is mass number Bottom is atomic number Proton H Neutron electron And Alpha Slide 10 Alpha Particles Helium Nuclei Positive Charge Heavy so not very penetrating Very Ionizing Very Ionizing 14 7 N + 4 2 He 17 8 O + 1 1 H Slide 11 Beta Particle Fast electron from the nucleus Negative charge Moderately Penetrating Moderately ionizing 14 7 N 14 8 O + 0 -1 Slide 12 Gamma Ray High energy e-m wave (A Photon) No charge - not deflected by field Very penetrating Need lead to stop most of them Not very ionizing Release energy after reaction Slide 13 Penetrating Power Paper Al foil Concrete Slide 14 Slide 15 Slide 16 H/W LC Ord 2007 Q11 Slide 17 Particles in Fields Charged particles move in a circular path as the force is always at right angles to the direction of motion- Fleming's Left Hand Rule Radioactive Source Cloud Chamber Slide 18 Slide 19 Click here for internet demo Slide 20 Ionisation We can prove that radiation creates ions as we bring a source close to a charge electroscope Slide 21 Ionising Power Alpha is heaviest and so does most damage poison with Polonium Beta is only moderately ionising Gamma is only slightly ionising but difficult to stop Slide 22 Solid State Detector PN This a P-N junction in reverse bias. This creates a huge depletion layer. + - A piece of radiation passes through the depletion layer and creates enough carriers to carry one pulse of current. Slide 23 Geiger Muller Tube Slide 24 H/W LC Ord 2004 Q10 Slide 25 Experiments All experiments the same stick a DETECTOR in front of a source and count the decays. Move it away for distance and plot Time for half life and plot Put things in front for penetration Slide 26 Penetration A Gieger Muller Tube and Counter. Plot the activity against the thickness or the type of barrier Slide 27 Distance A Gieger Muller Tube and Counter. Plot the activity against the distance r. r Slide 28 Half Life A Gieger Muller Tube and Counter. Plot the activity against the time Time it takes for half the atoms to decay Slide 29 Half-Life time it takes for half the radioactive particles to decay Atoms Not Decayed Time 1234 Slide 30 Half life demo from internet click here Slide 31 Half Life Calculations 1000 particlestime=6s 2 half-life 500 particles time=9s 3 half-life 250 particlestime=12s 4 half-life 125 particlestime=15s 5 half-life 4000 particlestime=0 2000 particlestime=3s 1 half-life Slide 32 Calculations we use the decay constant in our calculations. =0.693/T =0.693/3s =0.231s -1 Slide 33 Activity Calculations Rate of Decay = x number you started with dN/dt = - x N Start with 4000 particles and =0.231 Activity = 4000 x 0.231=924 Bq Slide 34 Calculations 1) You start with 100 grams of sulfur-35, which has a half life of 87.51 days. How much time will it take until only 12.5 grams remain? How many half lives? 100>50>25>12.5 so 3 half lives Time = 3 x 87.51 = 262.53 days Slide 35 Calculations 2) You measure the radioactivity of a substance, then when measuring it 120 days later, you find that it only has 25 % of the radioactivity it had when you first measured it. What is the half life of that substance? How many half lives 100%>50%>25% 2 half lives =120 days 1 half life = 60 days Slide 36 Calculations 3) Your professor gives you 64g of phosphorus- 32 (half life = 14.263 days). (a) What is its decay constant ? (b) What is its activity (Rate of Decay)? (a) Using the formula =0.693/T =0.693/(14.263x24x60x60) = 6.62 x 10 -7 s -1 Slide 37 Calculations 3) Your professor gives you 64g of phosphorus- 32 (half life = 14.263 days). (a) What is its decay constant ? (b) What is its activity (Rate of Decay)? = 6.62 x 10 -7 s -1 (b) Using Activity =dN/dt = - N N= Moles x 6x10 23 = 2 x 6x10 23 Activity = 6.62 x 10 -7 x 12x10 23 = = 7.3 x10 17 Bq Slide 38 Isotopes Same atomic number different mass number Slide 39 Isotope pp Slide 40 Uses of Radioactive Isotopes Medicine treatment and imaging Smoke detectors Food Irradiation Carbon-14 Dating Slide 41 Isotopes Same Atomic number different Mass number Slide 42 Carbon-14 Dating At death all animals contain the same ratio of C-14 to C-12 The rate of decay of C-14 is fixed The C-14 left tells us how long ago it died %C-14 time Slide 43 H/W LC Ord 2005 Q12(d) LC Higher 2003 Q11 LC H 2007 12(d) Slide 44 Rutherford Scattering Do I look like Freddie? Slide 45 Rutherford on internet Slide 46 Rutherford Scattering alpha particles fired at gold foil. very small Most pass unaffected - So the nucleus is very small Slide 47 Rutherford Scattering alpha particles fired at gold foil. Nucleus Deflected A small number of high energy alphas are Deflected totally positive. Some reflected completely back - Nucleus totally positive. Slide 48 Rutherford Scattering alpha particles fired at gold foil. Nucleus Deflected A small number of high energy alphas are Deflected very small More pass unaffected - So the nucleus is very small totally positive. Some reflected completely back - Nucleus totally positive. Slide 49 Cockcroft and Walton Alpha pha Lithium Target Alpha Hydrogen discharge tube Accelerated by An huge electric Field (700000v) Proton Alpha strikes the screen Producing a flash that Is seen with the microscope Slide 50 Internet explanation Slide 51 Nobel Prize for Physics Proton + Lithium 2xAlpha + Energy Proves Einstein s Law E=mc 2 First Transmutation by artificial Bombardment of an element Ernest Walton Slide 52 Binding Energy The total nucleus weighs less than all its parts Mass Defect Difference is Mass Defect Converted to energy to hold the nucleus together Converted to energy to hold the nucleus together E=mc 2 E=mc 2 Slide 53 As Iron is the most stable as you go towards it you release energy So Carbon-12 is lighter than 12 protons The difference is the binding energy Slide 54 Binding Energy of a Deuteron A deuteron is the nucleus of a deuterium atom, and consists of one proton and one neutron. The masses of the constituents are:deuteriumprotonneutron m proton = 1.007276 u (u is Atomic mass unit ) Atomic mass unit m neutron = 1.008665 u m proton + m neutron = 1.007276 + 1.008665 = 2.015941 u The mass of the deuteron is: Atomic mass 2 H = 2.013553 u The mass difference = 2.015941 - 2.013553 = 0.002388 u Slide 55 Convert to Kg Multiply by conversion factor 1u = 1.66x10 -27 Kg Mass = m = (0.002388) x 1.66x10 -27 Mass = m = 3.96x10 -30 Kg Slide 56 Use Famous Formula E=mc 2 E=mc 2 E= x 2 E= 3.96x10 -30 Kg x (3x10 8 m/s) 2 E = E = 3.56x10 -13 Joules Slide 57 Fusion The sun and the stars Fusion is the joining together of 2 light nuclei to make one nucleus with release of energy. Fusion is the joining together of 2 light nuclei to make one nucleus with release of energy. Caused by a super fast collision at high temperature in a magnetic bottle. Caused by a super fast collision at high temperature in a magnetic bottle. 21H21H 21H21H Slide 58 Fission The breaking apart of a heavy nucleus to form smaller nucleus with release of energy. The breaking apart of a heavy nucleus to form smaller nucleus with release of energy. Caused artificially by the bombardment of the right speed of neutron. MASS DEFECT In both fusion and fission the products are lighter than the reactants and the MASS DEFECT is turned into Energy E=mc 2 Slide 59 1.Subtract mass in a.m.u. 2.Convert to kg 3.Use E=mc 2 Also produced 3 fast neutrons that can cause another fission and so a chain reaction Uranium-235 Slide 60 Slide 61 Slide 62 Nuclear Equation In the isotope U-238 the neutrons must be slowed down by a moderator - Graphite Slide 63 Fuel rods contain the Uranium-235 (Enriched to ensure chain reaction) Moderators (Graphite) slow down the neutrons to the right speed Control Rods (Boron Steel) absorb neutrons to stop the reaction and prevent meltdown Heat to heat exchanger prevents Radiation escaping steam to turbine Slide 64 H/W LC Ord 2006 Q 9 Slide 65 Leptons Fundamental particles 1/1846 of an a.m.u. Does not feel the strong nuclear force Matter Electron, Muon, Tau, Anti-matter Positron, Anti-Tau Anti-matter first suggested by Paul Dirac Slide 66 Slide 67 Annihilation e+e+ e-e- Matter combining with anti-matter to form energy in the form of e-m radiation 2 photons conserve momentum ? e + + e - 2hf ( ) Slide 68 Annihilation e+e+ e-e- An electron and a positron collide to make energy. All the mass of the electrons gets turned into gamma waves So Energy E=mc 2 To find frequency of wave E = 2h.f Matter turns Into energy Matter combining with anti-matter to form energy in the form of e-m radiation 2 photons conserve momentum Slide 69 Wave made by Anihilation A proton and a Anti proton. The masses of the constituents are:protonAnti m proton = 1.007276 u (u is Atomic mass unit)Atomic mass unit m proton + m anti = 1.007276 + 1.007276 = 2.014552 u The mass difference = 2.014552 u To use this in a calculation we covert to kg Slide 70 Convert to Kg Multiply by convertion factor 1u = 1.66x10 -27 Mass = m = (2.014552) x 1.66x10 -27 Mass = m = 3.34x10 -27 Kg Slide 71 Use Famous Formula E=mc 2 E=mc 2 E= x 2 E= 3.34x10 -27 Kg x (3x10 8 m/s) 2 E = E = 3.01x10 -10 Joules Slide 72 Use Planks Equation E=hf h= planks constant 3.01x10 -10 Joules = (6.6x10 -34 js)x(f) f= 3.01x10 -10 Joules / 6.6x10 -34 js = 4.56x10 23 Hz In practice this is low as KE from particles increases this. Slide 73 Slide 74 Slide 75 Pair Production e+e+ e-e- An electron and a positron are created from a gamma ray. (We can also get a proton and an anti-proton) We do the calculation in reverse To find energy of wave E = h.f As we get 2 electrons E = 2mc 2 A matter and anti-matter pair being created by energy from an e-m wave Slide 76 Annihilation and Production p+p+ p-p- New particles are produced from the KE of the colliding protons They must conserve charge If we carry in 4Gev (1.6x10 -19. 4x10 9 = 6x10 -10 J) As Energy to make 3 Pions is E=mc 2 =(3x 2.4842x10 -28 xcxc) =6.7x10 -11 J Subtracting we find the KE after collision. p+p+ p-p- +0-+0- Slide 77 H/W LC Higher 2003 10(a) Slide 78 Quarks - Inside the Hadrons 6 Quarks 6 Anti-Quarks Opposite Signs UP +2/3 STRANGE -1/3 TOP +2/3 DOWN -1/3 CHARMED +2/3 BOTTOM -1/3 Slide 79 Hadrons BaryonsMesons 3 quarksQuark+anti-quark ProtonPion uudud Feels strong nuclear force Slide 80 Baryon Meson Slide 81 Hadrons on internet Slide 82 Subject to all forces Particle Zoo Hadrons Leptons Fundamental particles Do not feel Strong Nuclear Force Baryons 3 Quarks Proton uud Mesons Quark + Anti-quark Pion ud Slide 83 Ghost Particle Mystery By 1930 most of the particle physics world was understood However the decay of the neutron to a proton producing a beta particle did not obey Einstein's Law n 0 p + + e - Pauli said there must be a new particle called a neutrino Slide 84 Beta decay In decay, the weak force converts a neutron into a proton while emitting an electron and an antineutrino n 0 p + + e - + e This explains loss in energy and momentum. Pauli proposed its existence in 1930 but was not discovered until 1956 as it is so weakly interacting with other particles. Slide 85 Nuclear Formula Slide 86 Particle Accelerators-Linear Very high Voltage electric fields Electro-magnetic attraction pulls particles down. Slide 87 Circular Accelerators Particles spiral in fields (Flemings Left hand rule) Cyclotron- We put the field at right angles e more power with oscillating field Slide 88 CERN Particles can travel in opposite directions and double the collision energy Slide 89 Magnets force particles in circular path so stay in tube Circular more compact High velocity needed to overcome repulsion Vacuum to avoid collisions increase mean free path More velocity more KE so more new particles made Slide 90 Detectors Slide 91 Slide 92 H/W LC Ord 2002 Q11 LC Higher 2004 10(a) Slide 93 H/W Roundup (Yeh har) 2007 q11 2004 q10 2003 q11 2005 q12(d) 2006 q 9 2003 10(a)