Particle physics

• Rutherford's alpha scattering experiment:

  • A narrow beam of alpha particles, all of the same KE, from a radioactive source were targeted at a thin piece of gold foil, which was only a few atomic layers thick
  • The alpha particles were scattered by the foil & detected on a zinc sulfide screen mounted in front of a microscope
  • Each alpha particle hitting this fluorescent screen produced a tiny speck of light
  • The microscope was moved around in order to count the number of alpha particles scattered through different values of the angle theta per minute, for theta from zero to almost 180
• Observations & conclusions:

  • Most of the alpha particles passed straight through the thin gold foil with very little scattering. About 1 in every 2000 alpha particles were scattered
  • Very few of the alpha particles - about 1 in every 10000 - were deflected through angles of more than 90 degrees
  • Most of the atom is empty space with most of the mass concentrated in a small region - the nucleus
  • The nucleus has a positive charge, it repelled the few positive alpha particles that came near it
  • initial kinetic energy of alpha particle = electrical potential energy at distance, d
• • The nucleus of an atom contains positive protons & uncharged neutrons
  • The proton & the neutron have approximately the same mass
• Nucleon --> Either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons on a nucleus defines that atom's mass number (nucleon number)
• • A proton has a charge of +e, where e is the elementary charge
  • A neutral atom has the same number of electrons & protons
• Isotopes --> the same element that have the same number of protons but different number of neutrons. All isotopes of an element undergo the same chemical reactions
• Nuclear size & density:

  • The radius of the nucleus depends on the nucleon number A of the nucleus
  • R = r0A^1/3
  • r0 has an approximate value of 1.2fm (1fm = 10^-15 m )
  • All nuclei have a density of about 10^17 kgm^-3
• The repulsive electrostatic force F is given by:

  F = Qq / 4 x pi x echelon x r^2
• Strong nuclear force --> keeps protons & neutrons together in the nucleus. It acts between all nucleons. It is a very short range force, effective over just a few femtometres.
• Antimatter:

  • Every particle has a corresponding antiparticle, if the twp meet they completely destroy each other in a process called annihilation, where the masses of both particle & antiparticle are converted onto a high energy pair of photons
  • An antiparticle has the opposite charge to the particle (if the particle has charge) and exactly the same rest mass as the particle
• • The antiparticle of the electron is the positron
  • A positron has mass 9.11 x 10^-31 Kg like an electron, & charge +1.60 x 10^-19 - the opposite of the charge on an electron
  • Antiproton, antineutron & antineutrino are the antiparticles of the proton, neutron & neutrino respectively
  • Most antiparticles are symbolised by a bar over the letter for the particle
• Weak nuclear force --> responsible for inducing beta decay within unstable nuclei
• Fundamental forces:

  • strong nuclear --> experienced by nucleus
  • Electromagnetic --> experienced by static & moving charged particles
  • Weak nuclear --> responsible for beta decay
  • Gravitational --> experienced by all particles with mass
• Fundamental particles --> a particle that has no internal structure & hence can't be divided into smaller bits
• Hadrons --> particles & antiparticles that are affected by the strong nuclear force. E.g: protons, neutrons & mesons. Hadrons, if charged, also experience the electromagnetic force. Hadrons decay by the weak nuclear force
• Leptons --> particles & antiparticles that aren't affected by the strong nuclear force. E.g: electrons, neutrinos & muons. Leptons, if charged, also experience the electromagnetic force.
• Hadrons & quarks:

  • Quarks together with leptons, are the building blocks of all matter
  • Considered to be fundamental particles
  • Any particle that contains quarks is called a hadron
  • The standard model of elementary particles requires 6 quarks & their 6 anti-quarks

  
• • All hadrons experience the strong nuclear force
  • It is the individual quarks that are bound together within the particle by the attractive strong nuclear force
  • Force is so strong it may not be possible to separate the individual quarks
• • A proton consists of 3 quarks - up, up, down (uud)
  • The total charge of the proton is the sum of the individulal charges of the quarks
  • Proton charge Q = (+2/3)e + (+2/3)e + (-1/3)e = +1e
• Mesons & baryons:

  • Baryons are any hadrons made with a combination of 3 quarks
  • Protons & neutrons & antiprotons = baryons
  • Mesons are the hadrons made up with a combination of a quark & an anti-quark
• Neutrinos --> fundamental particles that carry no charge & may have a tiny mass
• There are 3 types of neutrinos:

  • The electron neutrino Ve
  • Muon neutrino Vmu
  • Tau neutrino
• • Each neutrino also has its antiparticle ( antineutrino)
• Beta decay:

  • Alpha radiation --> emission of helium nuclei
  • Beta radiation --> emission of either electrons or positrons
  • Gamma radiation --> emission of high - energy gamma photons
  • The force responsible for beta decay is the weak nuclear force
• • In beta negative decay, a neutron in an unstable nucleus decays into a proton, an electron, and an electron antineutrino
  • The nucleon number A & proton number z and total charge are conserved
• • In beta positive decay, a proton decays into a neutron, a positron, and an electron neutrino (Ve)
  • Nucleon & proton number are conserved, as well as total charge
• • Charge is a quantity that must be conserved in any reaction or decay involving charged particles