PARTICLE PHYSICS Summary
Alpha Scattering & Electron Diffraction
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
(de Broglie) sin =1.22λ λ = h d p Airy Disk Diameter of nucleus Electron diffraction Utilising wave qualities of electrons i.e. diffraction Airy disk produced by electron diffraction around a spherical object (nucleus) Remember: is the angle from 0th order to 1st minimum
1/r curve predicted by Rutherford scattering Actual curve for electron scattering Quantum feature of e- makes it Rutherford scattering and electron diffraction No of electrons Angle
Ch 17 Probing Deep into Matter 17.2 Scattering and scale Particle Zoo
Meson • Quark – antiquark • i.e. 0 • Baryon • 3 quarks • i.e. proton, neutrons Categorisation of sub-atomic particles • Fermion • Half integer spin • Subject to exclusion principle • Boson • Whole integer spin • Not subject to exclusion principle • Transmit a force i.e. photons, gluons, • Hadron • Made up of quarks • i.e. protons, neutrons • Lepton • Fundamental particle • i.e. electrons, neutrinos
Charge Flavours of Quark (exam only requires up and down) (⅔e) (⅔e) (⅔e) -(⅓e) -(⅓e) -(⅓e)
Everything, other than mass, is opposite Quarks • are never seen on their own • have anti-versions of themselves Hadron rules Quark combinations must have… • Net integer charge • Zero colour charge
Ch 17 Probing Deep into Matter 17.1 Creation & annihilation Particle – Antiparticle
Antielectron (Positron) Electron -e +e Charge /C -½ ½ Spin Mass /kg 9.11 x 10-31 9.11 x 10-31 Particles and antiparticles Same mass, everything else is opposite
+e -e All collisions must:- • Conserve energy, linear momentum and charge More of this later Annihilation & Creation What happens if particle-antiparticles meet Mass destroyed and turned into energy (& vice versa) • Conserve lepton and baryon number
Example interactions e- + e+ + e- = electron e+ = positron - gamma ray (photon) (no charge) Mass converted into energy
In theory…(but very rarely happens) e- + e+ + More commonly… e- + e+ Photon interacts with a nearby nucleus producing mass Example interactions Energy converted into mass
1 photon producing electron-positron pair 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) 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
u d u u d d uud udd β- Beta decay In the nucleus a neutron turns into a proton down quark → up quark Change in quark charge -⅓ e → ⅔ e = e+ e- must be emitted to conserve charge
n & p have baryon number = -1 e+ ( e- ) has lepton number = -1 Beta decay n p + e- 1 1 0 0 1 -1 Interaction must also conserve baryon and lepton number e.g. n & p have baryon number = 1 e- has lepton number = 1
+ e 0 0 0 1 + -1 anti electron neutrino 1 1 + 0 0 0 + 1 Beta decay n p + e- 1 1 0 0 1 -1 Charge Baryon + -1 Lepton Problem – doesn’t balance so create new particle
0 0 β+ Beta decay Work out the decay of a proton into a neutron p n + e+ + e 1 1 0 1 0 1 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
