Knowledge-20 NE

• Stable nuclei lie along the central orange belt that curves upwards:

  • light stable nuclei have approximately the same number of protons and neutrons
  • heavy stable nuclei have more neutrons than protons to help bind the neutron together.
• Emitters:

  • alpha emitters tend to be large nuclei because the strong nuclear force is unable to overcome the electrostatic force of repulsion between protons.
  • beta minus emitters occur to left of the stability belt where isotopes are neutron -rich
  • beta plus emitters occur to right of the stability belt where isotopes are proton -rich
• A radioactive decay series , when a radioactive isotope decays into an isotope which itself is unstable , can be represented on the N-Z graph.
• the emission of alpha or beta radiation can leave a nucleus in an excited state. the nucleus loses energy and returns to its ground state by emitting gamma radiation.
• the energy changes in a nucleus can be shown using a nuclear energy level diagram.
• Metastable states:

  Excited states that last for a significant period are called metastable states.
• Metastable:

  Isotopes in a metastable state can be useful in medicine because they can be separated and used as a source that emits only gamma radiation.
• Estimating the nuclear radius:

  The maximum size of a nuclear radius can be estimated from the distance of closest approach of an alpha particle.
• Estimating the nuclear radius:

  At the distance of closest approach to a nucleus of charge Q, the electric potential energy of the alpha particle will be equal to its kinetic energy:

  • Ek = 1/4 pie e0 x Q Q / r
  • r = Q Q / 4 pie e0 x Ek
• Measuring nuclear radius:

  Electron diffraction can be used to determine the radius of nuclei more accurately.
• Measuring nuclear radius:

  High energy electrons have a de Broglie wavelength small enough that they are diffracted by the nuclei of atoms. Graphs can show how the intensity varies with the angle for electron diffraction by a nucleus.
• Measuring nuclear radius:

  • the angle of first minimum is given by sin0 = 1.22 x de broglie wavelength / d
  • where d is the diameter of the nucleus
• Data from electron diffraction experiments show that the radius of a nucleus R is related to its nucleon number A by:

  • R = R0 x A^1/3
• The equation R = R0 x A^1/3 can be used to show that all nuclear material has the same density:

  • assuming nucleus is spherical its volume is,
  • V = 4/3 pie R^3 = 4/3 pie (R0 x A^1/3)^3 = 4/3 pie r^3 A
  • substituting the equation for density:
  • p = m / 4/3 pie r^3 A
  • p = Au / 4/3 pie r^3 A = p = u / 4/3 pie r^3 = p = 3u /4 pie r^3 A
  • p = 3.4x10^17
• The mass m of any object increases or decreases when it gains or loses energy E:

  • E = mc^2
  • this applies to all energy changes
• Atomic and nuclear masses are often expressed in atomic mass units where :

  • 1 u = 1/12 mass of a carbon 12 atom
  • 1 u = 1.661x10^-27
  • 1 u = 931.5 Mev
• The mass defect:

  in any change where energy is released the total mass after the change is always less than the total mass before the change because some energy is converted to released energy.
• The mass defect:

  The mass of a nucleus is less than the total mass of the individual nucleons of which it is formed.
• The mass defect:

  the mass defect of a nucleus is the difference between the mass of a separated nucleons and the mass of the nucleus. This is equal to the energy released when a nucleus forms from separate protons and neutrons.
• Nuclear fission is the process by which a large unstable nucleus splits into two smaller nuclei.
• Nuclear fusion is the process by which small nuclei join to form a larger nucleus.
• energy is released when nuclear fission and fusion take place because the resulting nuclei have higher binding energy per nucleon.
• Induced nuclear fission is the splitting of heavy nuclei by firing slow-moving thermal neutrons at them.
• Thermal neutrons are those made in thermal equilibrium with the surroundings.
• A chain reaction can occur when the neutrons released by the fission of a nucleus go on to induce fission in other nuclei.
• Critical mass is the minimum mass a fissile material (able to undergo fission) must have fir a self-sustaining chain reaction to occur.
• The binding energy of a nucleus is the work that would need to be done to separate it into its constituent nucleons , and is equal to the mass defect:

  • binding energy of nucleus = mass defect x c^2 = mc^2
• The binding energy per nucleon is measure of the stability of a nucleus - the greater the binding energy the more stable the nucleus is.
• The change in binding energy per nucleon is equal to the energy released from fission or fusion.
• Iron -56 has the highest binding energy per nucleon, so is the most stable nucleus.

  • binding energy per nucleon is increased and so energy is released by fusion of small nuclei.
  • binding energy per nucleon is increased and so energy is released by fission of large nuclei.
• Induced nuclear fission is used to create a controlled chain reaction in nuclear reactors in power stations.
• Control rods:

  • absorb neutrons produced in fission and are raised from and lowered into the reactor to control the rate of fission.
• Coolant:

  • needs high specific heat capacity to absorb lots of the energy released in fission, and to be easily pumped round the reactor to be replaced.
• moderator:

  • has similar mass and kinetic energy to fast moving neutrons released in fission, slow the neutrons so they become thermal neutrons , and have a higher probability of being absorbed.
• Safety features of reactors:

  • the reactor is surrounded by thick concrete walls to absorb the radiation emitted.
  • fuel rods are radioactive so they are inserted and removed from the reactor by a remote devices.
  • in emergencies control rods are automatically inserted into the core to bring fission to a stop.
• Disposal of fuel rods:

  • spent rods are hot and radioactive , they are unstable and emit radiation.
  • remote handling rods are used to remove fuel rods and placed in cooling ponds
  • after cooling fuel rods are reprocessed so they can be used in practical applications.
• Advantages and disadvantages of nuclear power:

  • nuclear power can generate large amounts of electricity to meet high power needs
  • dont produce greenhouse gasses
  • large amounts of nuclear waste are produces and probability of reactor accidents.
• The rutherford scattering experiment:

  • a beam of alpha particles were directed at a thin gold foil.
  • Occurs in a vacuum so that no collisions between air particles and alpha particles can occur.
  • The experiment was done in order to determine structure of an atom.
• Due to the nucleus repelling alpha particles and causing deflection it was deduced that the nucleus had a positive charge.