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Chapter 17 – Probing Deep into Matter
17.1 Creation and Annihilation
Learning outcomes

particles can be created and annihilated in matter–antimatter pairs.

particles and their antiparticles have opposite charges and lepton or baryon numbers.

electromagnetic interactions arise from the exchange of photons.

electrons are fermions, which obey the Pauli exclusion principle.

leptons are particles like the electron, neutrino and their antiparticles. Baryons are particles
like the proton and neutron and their antiparticles.

total energy, momentum and charge are conserved in interactions, as are lepton and
baryon number.

particles and their antiparticles have opposite charges and lepton or baryon numbers.
Lesson 1: Bubble chambers and pair production
Objectives:
- all the words for particles and interactions: matter, antimatter, electron, proton,
neutron, lepton, baryon, neutrino, meson
- all the conservation laws – momentum, energy, charge
Starter Activity Reading 20T 'Three poems about particles'
Reading 30T 'Early ideas about elementary particles'
What happened with beta decay and the neutrino.
Reading 10T 'The discovery of beta decay'
SoftAct 10S 'Bubble chamber photographs' as a demo
Cloud chambers - how to 'see' particles
http://www.ap.smu.ca/demos/content/modern/alpha_particles_in_a_cloud_chamber/
alpha_particles_in_a_cloud_chamber.html
Pair production -what does it mean? Particles and their classification:
lepton (light) number/particles = electrons, neutrinos (or anything equal to or lighter than the
electron)
baryon (heavy) number/particles = everything else (protons, neutrons, everything made of quarks
that is heavier (or equal to) the proton))
Dis 20O 'Making PET scans'
Dis 30O 'Conserved quantities in electron-positron annihilation'
Dis 40O 'Pair creation and annihilation'
Dis 50O 'Quantum fields create and destroy particles'
SAQ 10S 'Things that don't change'
Lesson 2/3:. Interactions and particles
Objectives: - there are matter particles and force particles, fermions and bosons
- which forces are mediated by which particles]
- Feynmann diagrams are a neat way of showing interactions
Starter: SAQ 30S 'Creation and annihilation'
A new way to look at fields: The forces between things are due to exchange of particles:
SoftAct 30S 'Interactions in particle physics'
Book page 177/178 and discuss Feynmann diagrams
http://www2.slac.stanford.edu/vvc/theory/feynman.html
There is a difference
Dis 90O 'Identical particles - bosons and fermions
Thats why you don't fall through the floor.
Fermions (electron, proton, neutron, neutrino..) are matter particles (spin = 1/2)
Bosons (photons, gluons, W and Z particles and gravitons(?)) are field particles (spin = 0)
Relativity and particle physics - who ordered that?
SoftAct 40S 'Time and the Muon'
SoftAct 50S 'Identifying particles using Lancaster Particle Physics software'
and.. Why did the chicken cross the road?
http://www.xs4all.nl/~jcdverha/scijokes/2.html#chicken_11
Homework:
Comp 50C 'Creation from annihilation'
MC 40M 'Particles and interactions'
MC 60M 'Keeping momentum and energy unchanged'
Extension: a) Reading 50T Text to Read 'Where did all the antimatter in the Universe go?'
b) Reading 60T Text to Read 'Symmetry and conservation laws'
12.2 How big are nuclei?
Learning outcomes

scattering experiments reveal the structures of atoms, nuclei and nucleons; the smaller the
scale the greater the energy needed

atoms have tiny dense positively charged nuclei, made of protons and neutrons packed
together at high density

quarks with fractional electric charges combine in threes to form neutrons, protons and
other particles, all called baryons

quarks combine in particle–antiparticle pairs to form mesons.

the strong ‘colour’ force between quarks is carried by gluons, which like photons are
bosons. Particles which ‘feel’ this force are called hadrons.

at high energies, scattering experiments create a large number of new kinds of particle
Lesson 4 How big are nuclei?
Objectives
- scattering experiments involve firing particles at matter
- what they do tells us what matter is made of
- Rutherford did it with alpha particles to find out about the atom
The history of ideas about the atom and how scientists have found out.
http://www.emsb.qc.ca/laurenhill/science/models.html
How do you find out?
Exp 70E 'Probing arrangements'
Dis 110O OHT 'Alpha particle scattering experiment'
SoftAct 80S 'Probes scattered by a target'
SoftAct 90S 'Many probes scattered by a target'
SoftAct 100S 'Where scattered probes go'
Dis 120O OHT 'Rutherford's picture of alpha particle scattering'
Run Rutherford Scattering applet from:
http://phet.colorado.edu/new/get_phet/simlauncher.php
Summary notes on distance of distance of closest approach and sketches of the shape of the path
of a scattered alpha particle. A charged particle needs energy of several MeV to get close to a
nucleus.
Lesson 5: Calculating distances etc.
Objectives:
- you can work out how far a particle can get to a nucleus
Look in book page 184 or Display Material 130O OHT 'Distance of closest approach'
SAQ 80S 'Rutherford scattering: directions of forces'
SAQ 70S 'Rutherford scattering: energy and closest approach'
May want to start next lesson…
Lesson 6 Inside the nucleus
Objectives:
- nuclei are very dense
- the volume of a nucleus is proportional to the number of nucleons
- you need big energies to go deep into matter
- nuclei are made of quarks there are rules for putting them together to make
hadrons and mesons
Starter: How dense is dense?
Work out the density of the nucleus of Helium. Work out the mass of a dice sized piece of it.
SoftAct 130S 'The density of nuclear matter'
+ look at page 187 or Dis 140OOHT 'Density of nuclear matter'
The volume of a nucleus is proportional to the number of nucleons - link to the mass of neutron
stars.
Probing smaller and smaller things requires larger and larger accelerators and higher energies.
Geiger and Marsden's apparatus fitted on a table top; a detector at CERN is a big as a small house
Dis 150O OHT 'Deep inelastic scattering'
Look at page 188 in the book or Dis160 OOHT 'Quarks and gluons'
Pres 60P 'Quark models'
SAQ 110S 'Putting quarks together'
SAQ 90S: Electrons measure the size of nuclei
Homework
1.Questions page 181
2. SoftAct 110S 'The Livingston curve'
3. SoftAct 120P Presentation 'The funding of particle physics'
4. Reading 80T Text to Read 'Tracking particles'
Extension: a) SAQ 100S Scattering and scale
b) Reading 70T Text to Read 'Towards a nuclear atom'
c) Comp 120C 'Finding parts of protons'
d) Reading 80T: Text to Read 'Tracking particles'
12.3 The music of the atoms
Learning outcomes
 electrons confined in a region of space can be modelled as standing waves, with wavelengths
determined by the size and shape of the confining region
 the de Broglie wavelength is

h
p
 discrete atomic energy levels correspond to discrete electron standing waves in an atom.
 eletrons can make quantum jumps between allowed energy levels, emitting or absorbing a
photon whose energy is given by E = hf = Einitial – Efinal.
 the energy level spacings in hydrogen are given by
En 
13.6 eV
n2
where n is the principal quantum number.
Lesson 7: Standing waves in atoms
Objectives
- standing waves have certain wavelengths
- we can think of electron waves as fitting into an atom as a standing wave
- if so they can only have certain wavelength, frequencies and energies.
Standing waves on a string again.
Electron diffraction expt. How do you resolve fact that electrons are wavy with them having orbits?
Looking back at standing waves
Exp 140E 'Standing waves - for electrons?'
+ look at page 193
Electrons in atoms have particular wavelengths, so have particular energies.
Work out k.e. and p.e. (see page 94).
SoftAct 150S 'Sizing up a hydrogen atom'
Lesson 8: Energy levels and transitions
Objectives
- only certain energies are allowed
-
electrons make transitions between them by absorbing or emitting photons.
If you look at gas tubes through diffraction glasses you see line spectra.
The upshot being that only discrete energy levels are allowed - see page 94
get out the tubes and look at the lines.
SAQ 150S 'Spectra and energy levels'
Dis 200O OHT 'Standing waves in atoms'
Dis 210O OHT 'Size of the hydrogen atom'
SAQ 170S 'Carrots and guitar strings'
Dis 220O OHT 'What the world is made of'
Homework:
SAQ 160S 'How small could a hydrogen atom be?'
Data 180D 'Products of the Big Bang'
Comp 140C 'How Niels Bohr began quantum theory'
Extension: a) Reading 40T: Text to Read 'Quantum theory in the twentieth century'