26, Nuclear Physics Flashcards
What does E = mc^2 mean?
Energy = mass * speed of light squared, it means that mass is a form of energy e.g. when two massive particle, anti-particles annihilate they produce gamma rays, it also means energy has mass, e.g. a moving particles is more massive than when at rest.
What does E = mc^2 mean for radioactive decay?
The initial nuclei must have had more rest mass than the components released since the resultant particles have K.E. and photons may have been released.
Why are two photons released in annihilation?
To conserve momentum.
What is pair production?
When a single photon turns into a particle, antiparticle pair when the energy of the photon is higher than the rest mass energy of the pair, excess energy is turned into K.E.
Why is the combined rest mass of a proton and neutron higher than that of deuterium?
Because work has to be done to get past the strong force binding them in the deuterium so it therefore has lower energy than the particles separately and therefore less mass.
What is the name for the difference in mass of a nucleus and the total rest mass of its parts? Also what is the name for the the energy equivalent to this mass?
Mass defect and binding energy.
What element has the most stable isotope?
Iron.
What is induced fission?
When neutrons are shot at a nucleus which then absorbs it and becomes unstable so decays.
Why is induced fission of uranium-235 a chain reaction and how is it controlled?
Because the fission releases 3 neutrons and 1 is required to start it, controlled by absorbing neutrons such that on average only 1 of the 3 released reacts again.
Give a overview of the design of a fission reactor.
The reactor core contains several fuel rods, control rods separate these and a water is often used as a coolant and moderator, this water is pumped into a separate area where it heats water in pipes which evaporates providing K.E. to the turbines.
What are fuel rods?
The radioactive material used, often enriched uranium (mostly U-238 with 2-3% U-235).
What is a moderator?
The substance which reduces the speed of neutrons allowing them to react with the uranium (neutrons must be travelling at similar speeds to the rest of the particles in the core to react, called thermal neutrons), water and carbon are often used.
What is a control rod?
A set of rods which are made of a material which easily absorbs neutrons, they can be raised or lowered to absorb more neutrons such that on average one of the 3 released are able to react again.
What is the environmental impact of a fission reactor?
They produce lots of radioactive waste which is very hard to safely store.
What is fusion?
When two nucleons become bound by the strong force.
