26 - Nuclear Physics Flashcards
What are the two interpretations of Einstein’s energy mass equation?
Energy has mass
Mass is a form of energy
What happens when an electron positron pair annihilate?
Their total mass is transformed into energy in the form of two gamma photons.
What happens in pair production?
A single photon vanishes and its energy creates a particle anti particle pair.
What is mass defect?
The difference between the mass of the separated nucleons and the nucleus of the atom.
What is binding energy?
The energy required to completely separate a nucleus into protons and neutrons.
What does binding energy per nucleon tell us?
How tightly bound a nucleus is.
How stable the nucleus is.
What does a graph of binding energy per nucleon against nucleon number show us?
That for lighter elements, fusion releases energy.
For heavier elements, fission releases energy.
What happens in induced fission?
Uranium 235 absorbs a slow moving neutron .
UNSTABLE U-236
Splits into daughter nuclei and three fast neutrons
Compare the total binding energy before and after nuclear fission.
The total binding energy after is greater than the total binding energy before.
What is the energy released in nuclear fission a combination of?
Kinetic energies of daughter nuclei and neutrons, energy of gamma photons and neutrinos.
What is the rate of growth of a chain fission reaction?
3^n
How many slow neutrons are preserved after each fission reaction in a nuclear reactor?
1
What are fuel rods made up of.
2-3% U-235
Rest U-238
What is the job of the moderator?
To slow down the fast neutrons that are produced in a fission reaction so that they have a low enough kinetic energy to be absorbed by another U-235 nucleus and continue the chain reaction.
What is the job of the coolant?
To remove the thermal energy from the reactants of each fission reaction.
Give some key features of a moderator?
Cheap
Readily available
Relatively low specific heat capacity
Must not absorb neutrons
What is a commonly used coolant/moderator in a nuclear reactor?
Water
What are control rods commonly made from?
Boron or Cadmium
What is the job of control rods.
To absorb neutrons and ensure that exactly one neutron survives after each reaction.
What is nuclear fusion?
When smaller nuclei combine to make larger nuclei.
How close do nuclei need to be to go through nuclear fusion and why?
a few fm (mx10^-15)
This is so that the strong nuclear force can attract them into a larger nucleus.
What are the stages of fusion in a proton proton cycle?
p + p = H + e+ +v
H + p = He₃
He₃ + He₃ = He₄ + 2p
