C26 - Nuclear Physics Flashcards
What are the 2 interpretations of Einstein’s mass-energy equation?
1) Mass is a form of energy. E.g. the interaction of an electron-positron pair will result in annihilation, and the entire mass of the particles is transformed into 2 gamma photons.
2) Energy has mass. Change in mass is related to change in energy. E.g. a moving ball has/gains kinetic energy, implying that its mass is greater than its rest mass).
What’s pair production?
The replacement of a single photon with a particle and a corresponding antiparticle of the same total energy.
A singe photons vanishes and its energy creates a particle and corresponding antiparticle.
What is the mass defect?
The difference between the mass of the completely separated nucleons and the mass of the nucleus.
What is the binding energy (of a nucleus)?
The minimum energy required to completely separate a nucleus into its constituent protons and neutrons.
How is the binding energy of a nucleus calculated?
Binding energy = mass defect of nucleus * c^2
What’s conserved in natural radioactive decay?
Mass-energy
Since energy is released, there must be an accompanying decrease in mass.
(E.g. for alpha decay, this means that the total mass of the alpha particle and daughter nucleus must be less than the mass of the parent nucleus).
- This decease in mass is equivalent to the energy released.
What does a higher binding energy suggest?
The greater the binding energy per nucleon, the more tightly bound the nucleons are with the nucleus, and so the more stable the nucleus is.
Fe-56 is the most stable isotope.
How is the average binding energy per nucleon affected by A (atomic mass)?
For nuclei with A < 56, the binding energy per nucleon increases as A increases.
For nuclei with A > 56, the binding energy per nucleon decreases as A increases.
What does a graph of nucleon number A, against binding energy (MeV) per nucleon show?
(Roughly upwards curve)
- For nuclei with A < 56, the binding energy per nucleon increases as A increases.
- For nuclei with A > 56, the binding energy per nucleon decreases as A increases.
- He-4 nucleus (alpha particle) has abnormally greater binding energy per nucleon than its immediate neighbours. (Similar for C12 and O16 too).
- Iron 56 is most stable isotope (highest point on curve).
- Energy is released in natural radioactive decay. Total binding energy of the parent nucleus is less than the binding energy of the daughter nucleus and alpha particle. The difference is due to energy released as kinetic energy.
- In fusion, 2 low-A nuclei join to form a higher A nucleus. The newly formed nucleus has a much greater BE than initial nuclei therefore E is released.
- In fission, a high-A number nucleus splits into 2 lower A number nuclei. Energy is released because the 2 nuclei produced have higher BE than the parent nucleus.
What’s induced fission?
Nuclear fission occurring when a nucleus becomes unstable on absorbing another particle (e.g. a neutron).
What’s a thermal neutron?
A neutron in a fission reactor with mean kinetic energy similar to the thermal energy of particles in the reactor core (a.k.a. Slow neutrons).
What happens during induced fission (of uranium-235)?
The uranium-235 isotope undergoes fission easily by absorbing a slow neutron (thermal neutron).
(Uranium-238 nuclei are more likely to capture/absorb the neutrons than undergo fission.)
The uranium-235 nucleus captures a thermal neutron and becomes a highly unstable nucleus of uranium-236. This then splits (in less than a microsecond), forming daughter nuclei.
Three fast neutrons are also produced. (Total proton and neutron number conserved). (Results in chain reaction).
How do energy and mass change after a fission reaction?
Total mass of particles after is always less than the total mass before.
This is because of the energy released in the reaction. Total binding energy of the particles after fission is greater than the total binding energy before it (the difference between being the energy released).
Energy released in a single fission reaction is a combination of kinetic energy of the particles produced and the energy of photons and neutrinos emitted.
What are the main components of a fission reactor?
Fuel rods (containing enriched uranium) are evenly spaced out within a steel-concrete vessel, known as the reactor core.
A coolant is used to remove the thermal energy produced from the fission reactions within the fissile fuel.
The fuel rods are surrounded by a moderator (slows fast moving neutrons) and control rods, which can be moved in and out of the reactor core.
What’s the purpose of a moderator in a nuclear fission reaction?
To slow down the fast neutrons produced in fission reactions.
Fast moving neutrons bounce off the massive uranium nuclei with negligible loss of Ek.
However, when the collide elastically with protons in water, or carbon nuclei, they transfer Ek and slow down.
(Water and carbon are good moderators. Sometimes the moderator is also the coolant).
