Matter and Radiation

Matter and Radiation

1. Inside the Atom

   • Nucleon is a proton or neutron in the nucleus of an atom
   • Proton/atomic number has symbol Z
   • Nucleon/mass number has symbol A
   • Nuclide is a type of nucleus
   • Specific charge = charge/mass

2. Stable and Unstable Nuclei

   • Strong nuclear force overcomes electrostatic force to hold protons and neutrons together
     • Range of 3-4 femtometres, similar to size of small nucleus
     • Attractive force from 0.5 - 4 fm
       • Repulsive below 0.5 to prevent protons and neutrons being pushed together
   • Alpha radiation consists of alpha particles, two protons and two neutrons
   • Beta radiation consists of fast-moving electrons
     • Antineutrino is emitted when a neutron decays into a proton
   • Gamma radiation is an Em radiation emitted by an unstable nucleus

3. Photons

   • Em wave consists of an electric and magnetic wave oscillating at right angles to each other in phase
   • Em waves are emitted when a fast-moving electron is stopped/slows down/changes direction
     • Or an electron in a shell of an atom moves to a different shell of lower energy
   • Em waves are emitted in packets known as photons
   • Photoelectric effect is the emission of electrons from a metal surface when light is directed at the surface
   • photon energy, E = hf
     • Therefore power of a laser beam = nhf
       • n is the number of photons passing a point in a second
   • A laser beam consists of photons of same frequency

4. Particles and antiparticles

   • When antimatter and matter particles meet they annihilate and radiation is released
     • Used in positron emitting tomography (PET) scanners
       • Positron-emitting isotope is administered to the patient
         • Some of it reaches the brain via the blood system
           • Each positron travels a couple millimetres before meeting an electron and annihilating
             • Annihilation releases two gamma photons
               • Sensors detect the photons, an image of the brain is gradually built up
   • positron emission occurs when a proton decays to a neutron
   • Pair production is the opposite of annihilation
     • A photon with sufficient energy can change into a particle-antiparticle pair
       • Minimum energy of photon needed = hfmin = 2Eo
         • Eo is the rest mass of the particle and antiparticle
   • 1 MeV = 1.6x10^-13 J

5. Particle Interactions

   • Electromagnetic force is due to the exchange of virtual photons
   • Weak nuclear force
     • responsible for B+ and B- decay
       • Both a new particle and a new antiparticle are created in each type of decay
     • W- boson decays into a B- particle and an antineutrino
       • W+ boson decays into a B+ particle and a neutrino
     • A neutrino can interact with a neutron and change it into a proton
       • A B- particle (an electron) is created and emitted as a result
     • W bosons are an exchange particle
       • they have a non-zero rest mass, a range of less than 0.001 fm, and are charged
     • An antineutrino can interact with a proton and make it change into a neutron
       • A B+ particle (a positron) is created and emitted as a result
   • Electron capture
     • a proton in a proton-rich nucleus can turn into a neutron through the weak interaction
       • proton interacts with an inner-shell electron from outside the nucleus
       • This can also happen when a proton and electron collide at high speed
       • W+ boson changes electron into a neutrino
     • Unstable atoms use this to become more stable
   • The photon and W boson are known as force carriers because they are exchanged when the em force and wnf act

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