Transcript particlephysics

PARTICLE PHYSICS
Summary
Alpha Scattering & Electron Diffraction
Rutherford’s picture of alpha scattering
nucleus
paths of scattered
alpha particles
The angle of deflection is determined by the K.E. of the
alpha particle. The “no – go “ zone behind the nucleus gets
smaller the larger the K.E. of the alpha particles
Electron diffraction
Utilising wave qualities of electrons
i.e. diffraction
sin  =1.22λ
d
Diameter of nucleus
λ = h (de Broglie)
p
Airy Disk
Airy disk produced by electron
diffraction around a spherical
object (nucleus)
Remember:
 is the angle from 0th order to 1st minimum
No of electrons
Rutherford scattering and electron diffraction
1/r curve predicted by
Rutherford scattering
Actual curve for
electron scattering
Angle
Quantum feature of e- makes it
Ch 17 Probing Deep into Matter
17.2 Scattering and scale
Particle Zoo
Categorisation of sub-atomic particles
Boson
Fermion
• Half integer spin
• Subject to exclusion principle
• Whole integer spin
• Not subject to exclusion principle
• Transmit a force i.e. photons, gluons,
Hadron
Lepton
• Made up of quarks
i.e. protons, neutrons
• Fundamental particle
i.e. electrons, neutrinos
Baryon
Meson
• 3 quarks
i.e. proton, neutrons
• Quark – antiquark
i.e. 0
Flavours of Quark (exam only requires up and down)
(⅔ e) (⅔ e)
(⅔ e)
Charge
-(⅓ e) -(⅓ e)
-(⅓ e)
Quarks
• are never seen on their own
• have anti-versions of themselves
Everything, other than
mass, is opposite
Hadron rules
Quark combinations must have…
• Net integer charge
• Zero colour charge
Ch 17 Probing Deep into Matter
17.1 Creation & annihilation
Particle – Antiparticle
Particles and antiparticles
Same mass, everything else
is opposite
Electron
Antielectron
(Positron)
Charge /C
-e
+e
Spin
-½
½
9.11 x 10-31
9.11 x 10-31
Mass /kg
Annihilation & Creation
What happens if particle-antiparticles meet
+e
-e
Mass destroyed and turned into energy
(& vice versa)
All collisions must:• Conserve energy, linear momentum and charge
• Conserve lepton and baryon number
More of this later
Example interactions
e
+
+
e
+
e- = electron
e+ = positron
 - gamma ray (photon) (no charge)
Mass converted into energy
Example interactions
In theory…(but very rarely happens)
+
e
+
+
e
Energy converted into mass
More commonly…

e
+
+
e
Photon interacts with a nearby nucleus
producing mass
The energy of a photon must be sufficient to create the mass
of a particle & an antiparticle
Erest = mc2
Provided by photon
Electron produced
E = hf
So, if mass is to be created, photon energy
must = 2mc2 (at least)
1 photon producing electron-positron pair
It also works the other way around – knowing the mass of the
particle-antiparticle pair that annihilate you calculate the energy
of the photons produced.
Theoretical requirement for electron neutrino
Beta decay
•  decay occurs due to weak force
• Random radioactive decay event due to
exchange of Z0 or W± boson
• Nuclei with excess n undergo - decay
• Nuclei with excess p undergo + decay
β
Beta decay
In the nucleus a neutron turns into a proton
d
u
u
u
d
d
udd
uud
down quark → up quark
Change in quark charge -⅓ e → ⅔ e = e+
e- must be emitted to conserve charge
Beta decay
1
0
n
1
1
p+
e
0
-1
Interaction must also conserve
baryon and lepton number
e.g. n & p have baryon number = 1
n & p have baryon number = -1
e- has lepton number = 1
e+ ( e- ) has lepton number = -1
Beta decay
1
0
n
1
1
p+
e
0
-1
+ e
0
0
anti electron
neutrino
Charge
0
1 + -1
Baryon
1
1 +
0
Lepton
0
0 +
1 + -1
Problem – doesn’t balance so create new particle
+
β
Beta decay
Work out the decay of a proton into a neutron
1
1
p
1
0
n+
+
e +
0
1
e
0
0
electron
neutrino
Beta decay
Another reason why neutrino’s needed to exist…
Conservation of energy
• Experimentally β particle energy varied –
problem as decay releases fixed amount
• (anti) neutrino has energy left over
Pauli Exclusion Principle
Within an atom, two identical
particles cannot be in the
same quantum state
Applies only to Fermions
– i.e. hadrons and leptons
Explains why objects are solid, periodic table
and electron shells
Spin
A quantum number that comes in lumps of ½
To be aware of…
The closest to identical, two particles can be
is to have all the same quantum numbers
but opposite spin
Think of an alpha particle…
2 protons, 2 neutrons
Both protons/neutrons have identical quantum
states but opposite spins
This makes alpha particles very stable