Modern Physics By Neil Bronks
Mass Number - Number of protons + Neutrons. Atomic Number - Number of protons Atoms C 12 6 In a neutral atom the number of electrons and protons are the same. In Carbon it is……… 6
Hydrogen The simplest atom has one negative electron orbiting one positive proton. The electron is very light compared to the proton. Electron Proton
Helium 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 Neutron Proton
Gravity - Only Positive - Very long range 4 Forces of Nature Electromagnetic – Positive and negative Weak Nuclear- Associated with beta decay • Strong Nuclear – Holds nucleus together - Very Short Range
Dodgy Beard J’ai fais ça ! 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
Safety • Wear Gloves or Apron of lead • Don’t point at anyone • Don’t eat!!!
Alpha Particles • Helium Nuclei • Positive Charge • Heavy so not very penetrating • Very Ionizing 147N +42He178O + 11H
Beta Particle ß • Fast electron from the nucleus • Negative charge • Moderately Penetrating • Moderately ionizing 147N 148O + 0-1 ß
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
Penetrating Power Al foil Paper Concrete
H/W • LC Ord 2007 • Q11
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 Particles in Fields Radioactive Source Cloud Chamber
Ionisation • We can prove that radiation creates ions as we bring a source close to a charge electroscope
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
Solid State Detector This a P-N junction in reverse bias. This creates a huge depletion layer. - + P N A piece of radiation passes through the depletion layer and creates enough carriers to carry one pulse of current.
H/W • LC Ord 2004 • Q10
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
Penetration A Gieger Muller Tube and Counter. Plot the activity against the thickness or the type of barrier
Distance r A Gieger Muller Tube and Counter. Plot the activity against the distance r.
Half Life Time it takes for half the atoms to decay A Gieger Muller Tube and Counter. Plot the activity against the time
Half-Life –time it takes for half the radioactive particles to decay Atoms Not Decayed Time 1 2 3 4
Half Life Calculations 4000 particles time=0 2000 particles time=3s 1 half-life 1000 particles time=6s 2 half-life 500 particles time=9s 3 half-life 250 particles time=12s 4 half-life 125 particles time=15s 5 half-life
Calculations – we use the decay constant λ in our calculations. =0.693/T½ =0.693/3s =0.231s-1
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
Isotopes • Same atomic number different mass number
Uses of Radioactive Isotopes • Medicine – treatment and imaging • Smoke detectors • Food Irradiation • Carbon-14 Dating
Isotopes • Same Atomic number different Mass number
%C-14 time 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
H/W • LC Ord 2005 Q12(d) • LC Higher 2003 Q11
Do I look like Freddie? Rutherford Scattering
Rutherford Scattering – alpha particles fired at gold foil. Most pass unaffected - So the nucleus is very small
Rutherford Scattering – alpha particles fired at gold foil. Nucleus A small number of high energy alphas are Deflected Some reflected completely back - Nucleus totally positive.
Rutherford Scattering – alpha particles fired at gold foil. Nucleus A small number of high energy alphas are Deflected More pass unaffected - So the nucleus is very small Some reflected completely back - Nucleus totally positive.
Proton Accelerated by An electric field Alpha strikes the screen Producing a flash that Is seen with the microscope Alpha Alpha Lithium Target Cockcroft and Walton Hydrogen discharge tube Al p h a Al p h a a
Nobel Prize for Physics • Proton + Lithium 2xAlpha + Energy • Proves Einstein’s Law E=mc2 • First Transmutation by artificial Bombardment of an element Ernest Walton
Binding Energy • The total nucleus weighs less than all its parts • Difference is Mass Defect • Converted to energy to hold the nucleus together • E=mc2
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
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: • mproton = 1.007276 u (u is Atomic mass unit) • mneutron= 1.008665 u • mproton + mneutron = 1.007276 + 1.008665 = 2.015941 u • The mass of the deuteron is: • Atomic mass 2H = 2.013553 u • The mass difference = 2.015941 - 2.013553 = 0.002388 u
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
Use Famous Formula • E=mc2 • E= 3.96x10-30 Kg x (3x108 m/s)2 • E =3.56x10-13 Joules
21H 21H Fusion – The sun and the stars • 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.
Fission • The breaking apart of a heavy nucleus to form smaller products with release of energy. • Caused artificially by the bombardment of the right speed of neutron. • In both fusion and fission the products are lighter than the reactants and the MASS DEFECT is turned into Energy E=mc2
Also produced 3 fast neutrons that can cause another fission and so a chain reaction • Subtract mass in a.m.u. • Convert to kg • Use E=mc2 Uranium-235
Nuclear Equation In the isotope U-238 the neutrons must be slowed down by a moderator - Graphite
Moderators slow down the neutrons to the right speed Control Rods stop the reaction and prevent meltdown Heat to steam to turbine Fuel rods contain the Uranium-235 (Enriched to ensure chain reaction)
H/W • LC Ord 2006 • Q 9