Nuclear physics

• How do you handle radiation safelty?

  long handled tongs, lead lined containers, keep as far away as possible, dont point towards others
• What is background radiation?

  radon gas (from rocks), artificial sources (nuclear testing), cosmic rays (from space), rocks (containing natural radioactive isotopes)
• Correct count = total count rate - background count rate
• Radioactive decay is random
• Decay constant

  the probability of a nucleus decaying per unit time
• change in number of nuclei (N) over change in time (t), over initial number of nuclei (N) = - decay constant
• Over long periods of time, radioactive decay can be described as exponential decay
• as decay is exponential, the time taken for N to half will be constant, this is the half life
• What is the more accurate way to find half life?

  take logs, and plot a graph of ln (N) against time. ln(N) = (- decay constant x time) + ln (N0)
• How do you derive half life equation ( T ½ = ln 2 / decay constant )?

  by substituting 0.5xN0 for N into logged exponential decay equation
• Half life equation?

  T ½ = ln2/decay constant
• Activity of a radioactive sample is the number of nuclei that decay per second
• Activity is directly proportional to the number of nuclei in the sample. A = (decay constant) N
• Why can you substitute A for N in the exponential equation?

  because activity is directly proportional to the number of nuclei in sample
• Why can time taken for activity to half be equal to half life?

  because activity is directly proportional to number of nuclei in substance, so will be treated the same.
• Decay constant can be used to model the decay of a substance only when there is a large number of nuclei ( as it is measured statistically )
• uses for half life?

  • Carbon-14 dating
  • medical diagnosis (radioactive tracers)
• How should atoms with long half life be stored?

  suitably in a steel cask underground
• why should you find a suitable place to store radioactive decay?

  so as not to effect the environment or harm the people nearby (or could be nearby in the future if long half life)
• What force holds nuclei together?

  strong nuclear force
• Why do protons experience repulsion?

  due to the electromagnetic force from other protons (both positive charge)
• Why may a nucleus become unstable?

  • too many neutrons
  • too many protons
  • too large
  • too much energy
• If a nucleus decays due to being too large, what does it decay via?

  alpha decay. Nucleon no. decreases by 4 and proton no. decrease by 2
• If a nuclei has too many protons…

  decays via beta plus emission, proton changes into a neutron , beta + particle and neutrino released. N = same Z =-1
• If a nucleus has too many protons…

  decays via electron capture, where a low orbiting electron is captured by nucleons, binding w proton to create a neutron and neutrino. N = same, Z = -1
• If a nuclei has too many neutrons?

  decay via beta minus emission. Neutron turns into a proton, releasing a beta- particle and an antineutrino. N= same Z=+1
• If a nucleus has too much energy?

  decay via gamma photon emission. Occurs after another type of decay.
• in the graph of stability, why do number of N / Z not increase uniformly after 20?

  As the electromagnetic force felt by protons becomes larger than Strong nuclear force, more neutrons are needed to increase the distance between protons and decrease magnitude of electromagnetic force ( they don’t feel is as have no charge)
• Distance of closest approach is the point at which a particle stops due to its kinetic energy fully converted into electric potential
• Why do the alpha particles slow down when fired towards nucleus?

  they experience electrostatic repulsion from the positively charged nucleus. Kinetic energy gets transferred into electric potential
• Electric potential = 1/ 4(Pi)(resistivity) x Q1 Q2 / r
• Can calculate the nuclear radius of an atom by finding distance of closest approach (for charged particles)
• Distance of closest approach is always an overestimate
• E = VQ
• Electron diffraction is more accurate than distance of closest approach
• Electrons are leptons and so won’t interact with the nucleons through the strong force ( alpha particles will interact via strong force)
• electron diffraction

  • electrons accelerated to high speeds so that de broglie wavelength is around 10-15m
  • directed to thin film
  • causing them to diffract through gaps of the films nuclei and create diffraction pattern
  • pattern has bright central maxima and gets dimmer as move from centre
• Electron diffraction, Sin ( diffraction of first medium) = 0.61 x de broglie wavelength / radius of nucleus
• Nucleus is held together by strong nuclear force
• Protons experience repulsion due to electrostatic force