Nuclear Physics Flashcards
What ideas does the equation E = mc^2 propose
1) Mass is a form of energy - the interaction of an electron-positron pair illustrates this idea well - the particles completely destroy each other and the entire mass of the particles is transformed into two gamma photons
2) Energy has mass - the change in mass of an object is related to the change in its energy. A moving ball has kinetic energy implying that its mass is greater than its rest mass.
How do unstable nuclei decay
By emitting either particles or photons
Explain nuclei decay using Einstein’s Mass-Energy Equation
Mass-energy is conserved
Energy is released in radioactive decay, there must be an accompanying decrease in mass.
Total mass of the parent particle/nucleus and the daughter nucleus must be less than the mass of the parent nucleus.
The decrease in mass is equivalent to the energy released.
What happens when positrons and electrons meet
They’re antiparticles they will annihilate each other.
Their entire mass is transformed into energy in the form of two identical gamma photons.
Minimum energy released = 2mc^2 - If interacting particles have kinetic energy, than the energy of each photon would be even greater
What happens in pair production
A single photon vanishes and its energy creates a particle and a corresponding antiparticle
What can be used to determine the mass of particles accurately
A mass spectrometer
How can nucleons be separated
By doing work to overcome the strong nuclear force, external energy must be supplied to make this happen. Since energy and mass are equivalent, the total mass of the separated nucleons must be greater than the mass of the nucleus
Define Mass Defect
Difference between the mass of the completely separated nucleons and the mass of the nucleus
Define the binding energy of a nucleus
Minimum energy required to completely separate a nucleus into its constituent protons and neutrons
Equation for binding energy of a nucleus
B.E = mass defect of nucleus x c^2
Whaen is a nucleus more stable
When it has a greater Binding energy per nucleon
What is induced fission
Uranium-235 absorbs slow neutrons, becoming unstable, and splitting up into two approximately equal halves plus fast neutrons.
What are slow neutrons also known as and why
Thermal neutrons because their mean kinetic energy is similar to the thermal energy of particles in the reactor core.
Why are neutrons used to split a Uranium-238 nuclei
U-238 Nuclei are more likely to capture the neutrons than to undergo fission.
Why are slow neutrons also known as thermal neutrons
Their mean KE is similar to the thermal energy of particles in the reactor core
What is the total mass of the particles after a fission reaction
Less than the total mass of the particles before the reaction. the difference in mass corresponds to the energy released in the reaction.
What is the total binding energy of particles after a fission reaction
Always greater than the total binding energy before it. Difference in binding energies is equal to the energy released.
When is a chain reaction possible
When fast neutrons produced in U-235 fission reactions can be slowed down in order to instigate further fission reactions in other U-235 nuclei. These 3 neutrons start 3 more reactions
After n generations of fission events, how many neutrons will there be
3^n
What happens in chain reactions
Growth in neutron numbers will be exponential.
Rate of energy release will also grow exponentially with time.
What is inside a fission reactor
Fuel rods spaced evenly within a reactor core
A coolant, moderator and control rods
What is the reactor core
A steel-concrete vessel
What is a coolant used for
To remove the thermal energy produced from fission reactions within the fissile fuel
What are fuel rods surrounded by
The moderator
How can control rods be moved
In and out the core
What do fuel rods contain
Enriched uranium
What is the role of the moderator
To slow down the fast neutrons produced in fission reactions
What must be the material for a moderator
Cheap and readily available. Must not absorb the neutrons in the reactor.
What are good candidates for a moderator and why
Water and carbon as whenfast-moving neutrons collide elastically with protons in water or carbon nuclei, they transfer significant kinetic energy and slow down
What is the moderator in many rectors
The coolant
In a pressurised water reactor, the water acts both as a moderator and a coolant
What is the role of the control rods
To ensure that exactly one slow neutron survives per fission reaction.
What are the materials of control rods
A material whose nuclei readily absorb neutrons.
Most commonly boron or cadmium.
What is the position of the control rods
Automatically adjusted to ensure that exactly one slow neutron survives per fission reaction
What is done to slow down or completely stop nuclear fission
Rods are pushed further into the reactor core
What is nuclear fusion
A process in which small nuclei are combined to make larger nuclei
What is the only way to make nuclei fuse
To bring them within a few 10^-15m, so that the short-range strong nuclear force can attract them into a larger nucleus
All nuclei have a positive charge so they will repel each other. At high temperatures, they can get close enough to absorb each other through the strong nuclear force
Two protons fusion reaction
1 1 2 0
p + p -> H + e + v
1 1 1 1
Proton + proton -> Deuterium + positron + neutrino
Why aren’t there no power stations using fusion yet
Maintaining high temperatures for long enough to sustain fusion and on confining the extremely hot fuel within a reactor
