Particles and Radiation

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Matter and Radiation - Inside the Atom

  • Mass of neutron and proton are approx. the same (1.67x10^-27); mass of electron is 2000x smaller (9.11x10^-31).
  • Charge on a proton and electron is the same but opposite (±1.60x10^-19) so an uncharged atom has equal number of protons and elecrons.
  • An isotope has the same number of protons but different number of neutrons.
  • Specific charge is the charge dvided by mass. 
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Matter and Radiation - Stable and Unstable Nuclei

  • The strong nuclear force holds the nucleus together; it overcomes the electrostatic force of repulsion between protons. It has a range of 3-4fm (around same diameter of a small nucleus and is repulsive below 0.5fm to prevent nucleons being pushed together.
  • Radioactive decay releases alpha (2p2n), beta (n->p) or gamma radiation (excess energy). Beta radiation also emits a neutrino to conserve energy.
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Matter and Radiation - Photons

  • Electromagnetic waves ll travel at the speed of light and the wavelength of EM radiation can be found by the equation waveength = c/f. An EM wave consists of a magnetic and electric wave traveling together with perpedicular oscillations in phase with each other.
  • EM waves can be produced when a charged particle loses energy. e.g. when an fast electron  is stopped, slows or changes direction or when an electron in an atom moves energy levels.
  • EM waves can be emitted as 'packets' of energy calles photons. Photons were used to explain the photoelectric effect.
  • Photon energy, E=hf
  • A laser consists  of photons at the same frequency so the power of a laser beam = nhf where n is the number of photons in the beam.
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Matter and Radiation - Particles and Antiparticles

  • An antiparticle is a particle with the same mass but equal and opposite charge.
  • When an antiparticle and particle meet, they annhiliate each other producing a pair of photons. This is used in PET scanners where the pairs of gamma photons are detected and used to build up an image.
  • Positron emission takes place when a proton turns into a neutron. 
  • Annhiliation occurs when the mass of the two particles is converted into radiation energy. Two photons are produced as a result to conserve momentum, therefore the minimum energy (hfmin) of each photon is found by equating the rest energy of the particles. hfmin=E0
  • In pair production, an antiparticle and a particle are produced by a photon. The photon must have sufficient energy to be equal to the rest mass of the two particles. hfmin = 2E0
  • For example, an electron has a rest energy of 0.511MeV so the photon must have energy greater than 2x0.511MeV to produce an electron-positron pair.
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Matter and Radiation - Particle Interactions

  • The weak nuclear force is the cause of beta decay. It is caused by W bosons which have a non-zero rest mass, very short range and are positively or negatively charged. 
  • Sometimes a proton may turn into a neutron by interacting with an electron throught the weak interaction. This is called electron capture and uses a W+ boson.
  • Bosons are known as force carriers as they are exchanged when an interaction occurs. The photon is the electromagnetic 'force carrier'; the W+/- boson is the weak nuclear force's and the pion is the strong nuclear's.
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Quarks and Leptons - The Particle Zoo

  • Muon, heavy electron, is a negatively charged particle with rest mass over 200x greater than an electron. 
  • Pion or pi meson (uu, ud or ud) can be charged in any way and has a rest mass greater than a muon but less than a proton. It is also the carrier of the strong nuclear force.
  • Kaon or K meson (us, us or ds) can be charged any way and has rest mass greater than pion but less than a proton.
  • A kaon can decay into pions, or a muon and an anti neutrino, or antimuon and a neutrino.
  • A charged pion can decay into a muon and an antineutrino, or an antimuon an da neutrino.
  • A neutral pion decays into photons.
  • A muon decays into an electron and an anti neutrino.
  • An antimuon decays into a positron and a neutrino
  • Decays always obey the conservation laws: energy, momentum, charge, lepton number, baryon number.
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Quarks and Leptons - Hadrons

  • Hadrons interact in all four interactions unlike leptons which do not interact in the strong.
  • They are not fundamental and are made up of quarks.
  • Mesons are made of a quark-antiquark pair. 
  • Baryons are made of three quarks, an antibrayon has 3 antiquarks. Proton-uud; neutron-uud  and a ∑ particle is a baryon with a strange quark.
  • The proton is the only stable baryon, a free neutron decays into a proton, similar to β- decay.
  • In β decay a down quark changes to an up quark, and in β decay an up changes to a down.
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Quarks and Leptons - Leptons

  • Leptons and antileptons can interact to produce hadrons. For example, a quark and antiquark may be produced in electron-positron annhiliation.
  • Different types of neutrinos can be produced depending on the lepton in the interaction. 
  • In an interaction between a lepton and a hadron, a neutrino or antineutrino can change into or from a corresponding charged lepton. For example, Ve + n = p + e-
  • In a muon decay, a muon changes into a muon neutrino, an electron and an electron neutrino.
  • Lepton number is conserved in any lepton interaction. Anti-leptons have -1 lepton number and leptons +1.
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Quarks and Leptons - Quarks

  • There are three types of quark (that are needed to know) up, down and strange. They also have corresponding antiquarks. Properties are given in the data sheet
  • Strangeness is always conserved in the strong interaction, but can change by ±1 in the weak interaction. 
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Quarks and Leptons - Conservation Rules

  • Charge and energy are conserved in all interactions.
  • Baryon number and lepton number are also conserved.
  • Strangeness is conserved in the strong interaction but may change by ±1 in the weak.
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Quantum Phenomena - The Photoelectric Effect

  • There are free electrons in a metal, they may be freed if given sufficient energy, Electrons are emitted from the surface of the metal when electromagnetic radiation above a certain frequency is shone on it. This is the photoelectric effect.
  • The threshold frequency is the minimum frequency of radiation needed to liberate an electron. This means the λ must be below a maximum value since λ=c/f
  • Number of electrons emitted is directly proportional to the intensity of the radiation, provided it is above threshold frequency. 
  • There is no delay in photoelectric emission when incident radiation is shone on the surface. Supporting the particle nature of EM radiation.
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Quantum Phenomena - Explaining the Photoelectric E

  • Energy of a photon = hf
  • Ekmax = hf - φ
  • Work function depends on the metal. Minimum energy needed to liberate an electron from the surface of the metal.
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Quantum Phenomena - Electron Interactions

  • Electrons in atoms are held in place by teh electrostatic force of attraction of the nucleus. An electron close to the nucleus has less energy than one further away.
  • The lowest energy state of an electron in an atom is called the ground state. When it gains energy it becomes excited and moves to an excited state. 
  • When excited the atom becomes unstable as there is a vacancy in the lower energy level. When the electron de-excites, it releases energy in the form of a photon. The photon energy is equal to the  difference between energy levels.
  • An electron can also absorb a photon to excite to a higher energy level. but only if the photon energy is exactly equal to the difference in energy levels.
  • Electrons can de-excite directly or indirectly. This explains fluorescence as electrons may be excited by UV photons but de-excite in steps, thereby emitting visible light photons.
  • A fluorescent tube has a fluorescent coating and the tube contains low pressure mercury vapour. Mercury atoms ionise and excite when an electric current is passed through, emitting UV photons, the photons are then absorbed by the coating and deexcite in steps producing visible light.
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Quantum Phenomena - Energy Levels

  • Line spectums show lines where the photons produced are of a specific frequency and wavelength. Each line has a specific energy and energy is different for each line.
  • The line spectra for each element is differnt because the emitted photons are characteristic to the energy levels and the energy levels are unique to each element.
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Quantum Phenomena - Wave-Particle Duality

  • Wave-like nature is observed when diffraction of light takes place. The narrower the gap, or longer the wavelength, the greater the amount of diffraction. Particle nature is observed in the photoelectric effect.
  • De Broglie hypothesised that if light has a dual nature, then maybe matter does too. He put forward an equation defining the wavelength that characterises the wave-like behaviour of matter. It depends on the momentum of the matter. De Broglie λ=h/ρ
  • The dual nature of matter is demonstrated by electrons; they are deflected in a magnetic field (matter) but also are difftacted in a ring pattern when passed through thin metal foil.
  • The beam is produced by attracting electrons froma heated electric filamentto a positively charged plate with a small hole in the centre (thermionic emission). The speed of the electrons can be increased by increasing the potential difference. The higher the velocity of the electrons, the smaller the diffraction rings as the de Broglie wavelength decreases.
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