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Particles, Radiation and Quantum Phenomena

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... Exchange Particle Exchange Beta Decay Detecting Particles Detecting Particles Refraction Refraction Critical Angle Fibre Optics ... Examples of optical fibre ... – PowerPoint PPT presentation

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Title: Particles, Radiation and Quantum Phenomena


1
Particles, Radiation and Quantum Phenomena
2
Contents
  • Particles
  • Rutherford Scattering
  • Constituents of the Atom
  • Four Force Model
  • Quarks and Antiquarks
  • Particles and Antiparticles
  • Particle Families
  • Particle Exchange
  • Beta Decay
  • Detecting Particles
  • Electromagnetic Radiation and
  • Quantum Phenomena
  • Refraction
  • The Photoelectric Effect
  • Wave Particle Duality
  • Spectra and Energy Levels

3
Rutherford Scattering
Rutherford proposed a model of the atom to
consist of A heavy, positively charged Nucleus
at the centre, with a much lighter, negatively
charged electron field in orbit around it.
The Rutherford Scattering experiment consists of
a piece of gold foil, which is bombarded with
positively charged alpha particles
4
Rutherford Scattering
Rutherford found the following occurred
Most alpha particles are undeflected
A few alpha particles are slightly deflected
A few alpha particles bounce off Nucleus
5
Constituents of the Atom
For most elements, a sample contains a mixture of
different versions. These have the same number of
protons, and electrons, but a different number of
neutrons.
Isotopes
Nuclide
Nuclide
e electron () p proton () n neutron
3e
3e
3p 3n
3p 4n
6
7
Nucleon number
Li (Lithium)
Li (Lithium)
3
3
Proton number
Mass of proton 1.00728 u Charge of proton
1.60x10-19 C Mass of neutron 1.00867
u Charge of electron - 1.60x10-19 C Mass of
electron 0.00055 u Diameter of an atom
10-10 m 1 u (unified atomic mass unit)
1.66x10-27 kg Diameter of a nucleus 10-14 m
6
Constituents of the Atom
Nuclide - This is a particular version of an
atom. The previous example shows a simple model
of the two naturally occurring nuclides of
Lithium, along with the symbols used.
Nucleon Number A - This number is made up of the
total number of protons and neutrons in the
nucleus, also called nucleons. Once called the
mass number.
Proton Number Z - This is the number of protons
in the nucleus, and also the number of electrons
in a neutral atom. Once called the atomic number.
Isotopes - These are atoms with the same proton
number, but different nucleon numbers. They have
the same electron arrangement and, therefore, the
same chemical properties
7
Four Force Model
Force Strength Range Comments
Strong interaction Very Strong 1x10-15m Affects protons and neutrons only
Weak interaction Weak 1x10-18m Affects all particles
Gravitational forces Very Weak Infinite range Affects all objects with mass
Electromagnetic forces Strong Infinite range Affect all objects with static or moving charge
8
Quarks and Antiquarks
Quarks and Antiquarks are the fundamental
particles. They are what make up nucleons, i.e.
protons and neutrons, as well as other particles
There are 6 types of quark, of which two occur in
protons and neutrons. These are shown below in
the examples. These quarks are known as UP (u)
and DOWN (d). They have charges of 2e/3 and
-1e/3 repectively.
u
u
u
d
d
d
Nucleons containing UP and DOWN quarks
9
Quarks and Antiquarks
The 6 quarks along with their antiquarks, make up
the quark family. The properties of antiquarks
are similar to those of the corresponding quark,
but with the opposite sign.
Quark / Antiquark Symbol Symbol Charge/e Charge/e Baryon number, B Baryon number, B Strangeness, S Strangeness, S
up u u 2/3 -2/3 1/3 -1/3 0 0
down d d -1/3 1/3 1/3 -1/3 0 0
charm c c 2/3 -2/3 1/3 -1/3 0 0
strange s s -1/3 1/3 1/3 -1/3 -1 1
top t t 2/3 -2/3 1/3 -1/3 0 0
bottom b b -1/3 1/3 1/3 -1/3 0 0
10
Particles and antiparticles
Most types of particles have a corresponding
antiparticle. This has the same rest mass, but at
least one property which is opposite to that of
the particle. For example
Particle
Antiparticle
Positron (antielectron)
Electron
Proton
Antiproton
Spin
Neutrino
Antineutrino
When a particle and its antiparticle meet, in
most cases they annihilate each other and their
mass is converted into energy.
11
Particle Families
Hadrons Hadrons
Baryons Spin ½ or 3/2 Mesons Spin 0 or 1
Generation Leptons (not Strong force) Spin ½ Leptons (not Strong force) Spin ½
1 Electron e - Electron-neutrino ?e
2 Muon µ - Muon-neutrino ?µ
3 Tau t - Tau-neutrino ?t
Proton p
Neutron n
Lamda ?
Sigma-plus S
Sigma-zero S 0
Omega-minus O -
Others
Pion-zero p 0
Pion-plus p
Kaon-plus ?
Kaon-zero ? 0
Others
e
0
nucleus
-e
0
0
e
-e
0
0
e
e
0
-e
0
Charge
0
-e
Charge
12
Particle Exchange
The interaction between particles that results in
attractive and repulsive forces is due to
continual exchange of exchange particles. They
have a short existence on borrowed energy, and
are often referred to as virtual particles.
The diagram below is a Feynman Diagram of two
electrons interacting. The straight lines show
the paths of the electrons, and the squiggly line
shows the virtual photons that move between them.
This is an example of the electromagnetic force
interactions.
13
Particle Exchange
There is a similar interaction for the other
forces. For Gravitational Forces, the exchange
particles are called gravitons but these havent
been observed yet.
The Strong Force is what holds quarks together in
a nucleon, and their exchange particles are
called gluons.
n
n
p
n
n
p
p0
p
p -
n
n
p
n
n
p
The above Feynman diagrams show the interactions
that are responsible for the Strong Force between
nucleons. These are called pions or pi-mesons and
are some of the Strong Force interactions. The -
0 represent the charge.
14
Particle Exchange
For Weak Force interactions, there is an exchange
of one of three kinds of particles called
intermediate vector bosons. The symbols for these
are W, W -, Z0 and like pions the - 0
represent the charge.
p
e-
?e
e-
e-
p
W
p -
W-
?e
?e
?e
n
n
e-
The above Feynman diagrams show the
neutino-neutron interaction, ß- decay
interaction, and the electron-antineutrino
collision interaction, respectively.
15
Beta Decay
Beta Decay is shown here in more detail
This interaction causes a quark in the neutron to
change from down, d, to up, u, in the proton and
emits a W- particle which then decays into an
electron and antineutrino.
16
Detecting Particles
Detectors are needed to reveal the paths of the
particles produced in experiments using particle
accelerators.
Bubble Chamber Filled with liquid hydrogen, the
pressure is suddenly released so that the
hydrogen is ready to vaporise. Charged particles
entering the chamber ionise the hydrogen. This
triggers vaporisation, so a trail of bubbles is
formed along the track of each particle.
17
Detecting Particles
Drift Chamber A gas filled chamber, containing
typically thousands of parallel wires. Incoming
particles cause a trail of ionisation in the gas.
Their track is calculated electronically by the
length of time ionisation electrons take to drift
to the nearest sense wires. A computer processed
the signals and displays the tracks graphically.
Particle
18
Refraction
When waves cross a boundary between two
materials, there is a change of speed called
refraction. If the direction of the wave is at an
angle other than the normal line, then a change
in direction occurs.
Angle of incidence
Angle of refraction
The change in speed is described by the
refractive index, n of the material.
n is given by n cv / cm where cv is the
speed of light in a vacuum, and cm the speed of
light in the material in question. n is unitless.
19
Refraction
The refractive index between two materials is
1n2 It is the ratio of speed of light in
material 1, to the speed of light in material 2,
and is given by 1n2 c1/c2 n1/n2
Snells Law states sini /sinr c1/c2
1n2 From Snells law, the angle of refraction
can be calculated if the refractive index of the
two materials is known. The refractive index can
be calculated from the sin of the angles of the
light measured in the materials.
20
Critical Angle
When light speeds up as it crosses a boundary,
the change in direction is away from the normal
line. For a specific angle of incidence, called
the critical angle, the angle of refraction is
90o. The diagram shows a the stages of
refraction, from normal refraction, to the
critical angle, to total internal reflection.
21
Fibre Optics
Optical Fibres utilise total internal reflection
at the boundaries of the fibre to carry
information digitally in the form of light. The
diagram below shows a straight section of optical
fibre, but the cable can bend and the light will
continue to travel down the fibre, as long as the
critical angle is achieved.
The cables can be clad in a material of a lower
refractive index than the core, and this lowers
the critical angle. Examples of optical fibre
usage communications for phone lines, and
medicine for endoscopy.
22
Photoelectric Effect
The Photoelectric effect provides evidence that
electromagnetic waves have particle-like
behaviour.
In the photoelectric effect, electrons are
emitted from a metals surface when it absorbs
electromagnetic radiation. The diagram to the
left shows this.
There are no electrons emitted below a certain
frequency, called the threshold frequence, fo ,
which is different for different metals. Above
this frequency, electrons are emitted with a
range of kinetic energies up to a maximum,
(½mv2)max
23
Photoelectric Effect
The wave model cannot explain the Photoelectric
effect. The explanation for this relies on the
concept of the photon, a quantum packet of
energy. So EM radiation is given by short bursts
of energy.
The relationship between the energy of the
photon, E, and its frequency is given by E
hf where h is Plancks constant h 6.63x10-34
Js
The energy of a photon can be measured in Joules
or Electronvolts. One electronvolt is the energy
transfer when an electron moves through a
potential difference of 1 volt, given by 1 eV
1.60x10-19 J
24
Photoelectric Effect
The minimum energy required for the electron to
escape the metal is called the work function, F
If energy is less than F, then no emission occurs
Emission is possible when hf F
The photoelectric equation relates the maximum
kinetic energy of the emitted electrons to the
work function and the energy of each
photon hf F (½mv2)
At the threshold frequency, the minimum frequency
that can cause an emission is zero, so the
equation becomes hfo F
25
Wave Particle Duality
Waves show particle-like behaviour, and particles
show wave-like behaviour.
All particles have an associated wavelength
called the de Broglie wavelength ?
The relationship between the particles
wavelength and its momentum, p, is given by the
de Broglie equation ? h/p h/mv
26
Spectra and Energy Levels
The loss of energy from an electron gives line
spectra, and is different for each material. When
current is passed through hydrogen gas, the
hydrogen spectrum is given as below
Line spectrum are unique for each element, and
for each isotope of that element.
27
Spectra and Energy Levels
An energy level diagram shows the amounts of
energy that electrons have at each level in an
atom. The energies are measured from a zero
equivalent to a single free electron.
The diagram shows the energy levels in a hydrogen
atom. An orbiting electron has less energy than a
free electron, so the energies are shown as
negatives relative to the ground state. An
electron with the minimum possible energy is in
the ground state higher energy levels are called
excited states.
When an electron moves from an energy level E1 to
a lower energy level E2 the energy of the photon
emitted is given by hf E1 E2
28
Summary
  • Particles
  • Rutherford Scattering
  • Constituents of the Atom
  • Four Force Model
  • Quarks and Antiquarks
  • Particles and Antiparticles
  • Particle Families
  • Particle Exchange
  • Beta Decay
  • Detecting Particles
  • Electromagnetic Radiation and
  • Quantum Phenomena
  • Refraction
  • The Photoelectric Effect
  • Wave Particle Duality
  • Spectra and Energy Levels
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