Matter and Radiation

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1.1 Inside the Atom

  • Atom consists of +vely charged nucleus (made of protons and neutrons), surrounded by -ve electrons
  • Electrons are held in the atom by the electrostatic force of attraction between them and +ve nucleus. 
  • Isotopes are atoms of an element with the same number of protons, different number of neutrons.
  • Proton Number=Z=atomic number
  • Nucleon Number=A=mass number
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1.1 Inside the Atom (cont)

  • Specific charge is defined as charge of a charged particle divided by its mass
  • Units are C/kg
  • All masses needed are given to you, always use +/- 1.6x10^-19 for the charge of any particle for each increment of charge.
  • Specific charge of a nucleus includes all protons and neutrons, and the charge of the protons. 
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1.2 Stable and Unstable Nuclei

  • Strong nuclear force is the force that overcomes electrostatic repulsion between protons in the nucleus, and (except for unstable nuclei) holds protons and neutrons together.
  • Strong force has a range of 3-4 femtometers      (x1-^-15), which is diameter of small nucleus.
  • S.F acts on all protons and neutrons equally, depending on their distance from the centre of the nucleus.
  • It is an attractive force from 0.5fm-3/4fm. Before 0.5fm strong force becomes a repulsive force to stop neutrons/protons colliding.
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1.2 Stable and Unstable Nuclei (cont)

(http://www.cyberphysics.co.uk/graphics/graphs/strongForce.png)

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1.2 Stable and Unstable Nuclei (cont)

  • In Alpha radiation, an unstable nucleus emits an alpha particle (2 protons, 2 neutrons), forming an new nucleus with proton number -2, nucleon number -4. 
  • Beta radiation consists of fast-moving electrons. In beta radiation (Beta + decay) a neutron in the unstable nucleus changes into a proton. A Beta + particle is instantly emitted along with an antineutrino. 
  • Gamma radiation is electromagnetic radiation emitted by unstable nuclei. 
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1.2 Stable and Unstable Nuclei (cont)

(http://www.nuceng.ca/igna/images/Alphae.gif)

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1.2 Stable and Unstable Nuclei (cont)

(http://www.daviddarling.info/images/beta-plus_decay.gif)

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1.3 Photons

  • In a vacuum, all electromagnetic waves travel at the speed of light, c. 
  • speed of EM wave = frequency / wavelength 
  • An electromagnetic wave consists of an electric wave and a magnetic wave which travel together and vibrate perpendicular to eachother and the direction which they're traveling. 
  • They are in phase. 
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1.3 Photons (cont)

(http://www.olympusmicro.com/primer/java/wavebasics/basicwavesjavafigure1.jpg)

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1.3 Photons (cont)

  • A photon is a packet of electromagnetic waves, with a fixed energy amount. This photon energy = h x f.
  • A laser beam emits coheret and monochromatic light. So, all photons emitted have the same frequency and energy.
  • The power of a laser will be the total amount of energy emitted per second, = n x h x f, where n= number of emitted photons per second.  
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1.4 Particles and Antiparticles

  • An antiparticle has exactly the same mass and the opposite charge to its corresponding particle.
  • When a particle and its corresponding antiparticle collide, they annihilate eachother, converting their total mass (energy) into 2 photons.
  • The minimum energy of both photons produced is the rest masses (energies) of the particle/antiparticle pair. So, minimum energy of 1 emitted photon is rest mass of 1 particle or antiparticle. 
  • Any extra energy is converted into kinetic energy. 
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1.4 Particles and Antiparticles (cont)

  • Pair production is when a photon creates a particle and a corresponding antiparticle, and vanishes in the process. 
  • A photon must have a minimum energy (hf) that is enough to create both the particle and the antiparticle at rest. hf=2E.
  • Any extra energy is given to the particle and antiparticle creates for kinetic energy.
  • For calculations always use Joules. 1eV = 1x10^-19 J
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1.4 Particles and Antiparticles (cont)

(http://www.physicscentral.com/explore/action/images/antics-img2.gif)                       (http://tap.iop.org/atoms/particles/534/img_full_47347.gif)

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1.5 How Particles Interact

  • When a single force acts on an object, it changes the momentum of the object.
  • When two particles interact, they exert equal and opposite forces on eachother. Momentum is transferred between these two particles.
  • Exchange particles make this particle interaction possible by creating and exchanging a force between 2 particles. 
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1.5 How Particles Interact (cont)

  • The strong nuclear force holds the neutrons and protons in a nucleus together.
  • There must be another force that causes particle decay and certain particle interactions. This is the weak nuclear force. 
  • Weak nuclear force is weaker than the strong nuclear force so that it does not affect stable nuclei.
  • In most weak interactions, the exchange particles present are W bosons. They have a non-zero rest mass, have a very short range and are charged. 
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1.5 How Particles Interact (cont)

(http://hyperphysics.phy-astr.gsu.edu/hbase/forces/imgfor/neuneu.gif)        (http://www.parrswood.manchester.sch.uk/faculties/science/images/antineutino-proton_collision.gif)    (http://hyperphysics.phy-astr.gsu.edu/hbase/particles/imgpar/decneu.gif)(http://hyperphysics.phy-astr.gsu.edu/hbase/forces/imgfor/neuneu.gif)

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