Modern Physics

By Neil Bronks

Atoms

Mass Number Number of protons + Neutrons.

6 12 C 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.

Proton Electron

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.

Helium

Electron

 Gravity - Only Positive - Very long range Electromagnetic – Positive and negative 4 Forces of Nature  Strong Nuclear – Holds nucleus together - Very Short Range Weak Nuclear Associated with beta decay

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  238

U

92  234

Th

90  2 4

He

 Helium Nuclei  Positive Charge  Heavy so not very penetrating  Very Ionizing

14 7 N + 4 2 He



17 8 O + 1 1 H

Beta Particle

ß

234 90

Th

 234

Pa

91  0  1      Fast electron from the nucleus Negative charge Moderately Penetrating Moderately ionizing

14 7 N



14 8 O + 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    Paper Al foil Concrete

H/W

 LC Ord 2007  Q11

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

http://www.absorblearning.com/m edia/attachment.action?quick=br& att=838

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 1 2 3 4 Time

Half Life Calculations 4000 particles 2000 particles time=0 time=3s 1000 particles time=6s 500 particles time=9s 250 particles 125 particles time=12s time=15s 1 half-life 2 half-life 3 half-life 4 half-life 5 half-life

Calculations – we use the decay constant λ in our calculations.



=0.693/T

½ 

=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

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

H/W

 LC Ord 2005 Q12(d)  LC Higher 2003 Q11

Rutherford Scattering

Do I look like Freddie?

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

Accelerated by An electric field Hydrogen discharge tube Proton Alpha strikes the screen Producing a flash that Is seen with the microscope Alpha  Lithium Target Alpha 

Nobel Prize for Physics

 Proton + Lithium  2xAlpha + Energy  Proves Einstein ’ s Law E=mc 2  First Transmutation by artificial Bombardment of an element 3 7

Li

 1 1

p

 2 4   2 4  

Energy

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=mc 2

 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 consists of one proton and one neutron atom, and . The masses of the constituents are:     m proton = 1.007276 u (u is 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

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=mc 2  E= 3.96x10

-30 Kg x (3x10 8 m/s)  E = 3.56x10

-13 Joules 2

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.

2 1 H 2 1 H

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=mc 2

Also produced 3 fast neutrons that can cause another fission and so a chain reaction 1. Subtract mass in a.m.u.

2. Convert to kg 3. Use E=mc 2 Uranium-235

Nuclear Equation

238

U

92  0 1

n

 97

Kr

36  139

Ba

56  3 0 1

n



Energy

In the isotope U-238 the neutrons must be slowed down by a moderator Graphite

Moderators slow down the neutrons to the right speed Fuel rods contain the Uranium-235 (Enriched to ensure chain reaction) Control Rods stop the reaction and prevent meltdown Heat to steam to turbine

H/W

 LC Ord 2006  Q 9

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 Matter combining with anti-matter to form energy in the form of e-m radiation 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 Matter turns Into energy To find frequency of wave E = 2h.f

Wave made by Anihilation

  A proton and a Anti constituents are: proton. The masses of the  

m

m proton proton = 1.007276 u (u is Atomic mass unit ) + 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

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

Use Famous Formula

 E=mc 2  E= 3.34x10

-27 Kg x (3x10 8 m/s)  E = 3.01x10

-10 Joules 2

Use Planks Equation

 E=hf  h= planks constant  f= frequency  3.01x10

-10 Joules = (6.6x10

-34 js)x(f)  f= 3.01x10

-10 Joules / 6.6x10

-34 js = 4.56x10

23 Hz

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

Annihilation and Production p + p +  +  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 . 4x10 9 = 4x10 -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.

H/W

 LC Higher 2003 10(a)

Quarks - Inside the Hadrons  6 Quarks  6 Anti-Quarks – Opposite Signs UP

+2/3

DOWN

-1/3

STRANGE

-1/3

CHARMED

+2/3

TOP

+2/3

BOTTOM

-1/3

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

Baryon Meson

Baryons Hadrons

Particle Zoo

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

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

Beta decay

 In β− decay, the weak force converts a neutron into a proton while emitting an electron and an antineutrino  n 0 → p + +

e

+ ν e  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

0 1

n

 1 1

p

 0  1   0 0 

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

Detectors

H/W

 LC Ord 2002 Q11  LC Higher 2004 10(a)