Modern Physics By Neil Bronks

C Atoms Mass Number - Number of protons + Neutrons. 12 6 Atomic Number - Number of protons 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

4 Forces of Nature (Order of strength) Strong Nuclear – Holds nucleus together - Very Short Range Electromagnetic – Positive and negative 4 Forces of Nature (Order of strength) Weak Nuclear- Associated with beta decay Gravitational - Only Positive - Very long range

J’ai fais ça ! Radiation Dodgy Beard 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!!!

Nuclear Equations Top and bottom must add up Top is mass number Bottom is atomic number Proton H Neutron electron And Alpha

Alpha Particles  147N +42He 178O + 11H Helium Nuclei Positive Charge Heavy so not very penetrating Very Ionizing 147N +42He 178O + 11H

Beta Particle ß 147N  148O + 0-1 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

 

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   Particles in Fields Cloud Chamber 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

Click here for internet demo

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.

Geiger Muller Tube

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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 Plot the activity against the 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 demo from internet click here

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.

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

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

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

Calculations (a) Using the formula =0.693/T½ 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/T½ =0.693/(14.263x24x60x60) = 6.62 x 10-7 s-1

Calculations = 6.62 x 10-7 s-1 (b) Using Activity =dN/dt = -N 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 6x1023 = 2 x 6x1023 Activity = 6.62 x 10-7 x 12x1023 = = 7.3 x1017 Bq

Isotopes Same atomic number different mass number

Isotope pp

Uses of Radioactive Isotopes Medicine – treatment and imaging Smoke detectors Food Irradiation Carbon-14 Dating

Isotopes Same Atomic number different Mass number

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

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Rutherford Scattering Do I look like Freddie? Rutherford Scattering

Rutherford on internet

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.

Cockcroft and Walton Hydrogen discharge tube Alpha  Proton Accelerated by An huge electric Field (700000v) Alpha strikes the screen Producing a flash that Is seen with the microscope Alpha  Al p h a Al p h a a Lithium Target

Internet explanation

Nobel Prize for Physics Proton + Lithium  2xAlpha + Energy Proves Einstein’s Law E=mc2 First Transmutation by artificial Bombardment of an element Ernest Walton

E=mc2 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

E=mc2 E= 3.96x10-30 Kg x (3x108 m/s) 2 E = 3.56x10-13 Joules Use Famous Formula E=mc2 E= 3.96x10-30 Kg x (3x108 m/s) 2 E = 3.56x10-13 Joules

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. 21H

Fission 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. 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 (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 Fuel rods contain the Uranium-235 (Enriched to ensure chain reaction)

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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

Annihilation e+ e- e+ + e- 2hf (2g) 2 photons conserve momentum ? Matter combining with anti-matter to form energy in the form of e-m radiation Annihilation e+ e- e+ + e- 2hf (2g) 2 photons conserve momentum ?

Matter combining with anti-matter to form energy in the form of e-m radiation Annihilation e+ e- 2 photons conserve momentum An electron and a positron collide to make energy. All the mass of the electrons gets turned into gamma waves So Energy E=mc2 To find frequency of wave E = 2h.f Matter turns Into energy

Wave made by Anihilation A proton and a Anti proton. The masses of the constituents are: mproton = 1.007276 u (u is Atomic mass unit) mproton + manti = 1.007276 + 1.007276 = 2.014552 u The mass difference = 2.014552 u To use this in a calculation we covert to kg

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

E=mc2 E= 3.34x10-27 Kg x (3x108 m/s) 2 E = 3.01x10-10 Joules Use Famous Formula E=mc2 E= 3.34x10-27 Kg x (3x108 m/s) 2 E = 3.01x10-10 Joules

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 = 4.56x1023 Hz In practice this is low as KE from particles increases this.

A matter and anti-matter pair being created by energy from an e-m wave Pair Production 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 = 2mc2

Annihilation and Production + 0 - 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 . 4x109= 6x10-10 J) As Energy to make 3 Pions is E=mc2 =(3x 2.4842x10-28xcxc) =6.7x10-11 J Subtracting we find the KE after collision.

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Quarks - Inside the Hadrons 6 Anti-Quarks – Opposite Signs UP +2/3 STRANGE -1/3 TOP DOWN -1/3 CHARMED +2/3 BOTTOM -1/3

Feels strong nuclear force Hadrons Baryons Mesons 3 quarks Quark+anti-quark Proton Pion uud ud Feels strong nuclear force

Baryon Meson

Hadrons on internet

Particle Zoo Subject to all forces Leptons Hadrons Baryons Mesons Fundamental particles 3 Quarks Quark + Anti-quark Do not feel Strong Nuclear Force Proton uud Pion ud

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 n0 → p+ + e- Pauli said there must be a new particle called a neutrino

Beta decay In β− decay, the weak force converts a neutron into a proton while emitting an electron and an antineutrino n0 → p+ + e- + νe This explains loss in energy and momentum. Pauli proposed it’s existence in 1930 but was not discovered until 1956 as it is so weakly interacting with other particles.

Nuclear Formula

Particle Accelerators-Linear Very high Voltage electric fields Electro-magnetic attraction pulls particles down.

Circular Accelerators Particles spiral in fields (Flemings Left hand rule) Cyclotron- We put the field at right angles e more power with oscillating field

CERN Particles can travel in opposite directions and double the collision energy

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

Detectors

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H/W Roundup (Yeh har) 2007 q11 2004 q10 2003 q11 2005 q12(d) 2006 q 9