Module 6.4 - Nuclear and Particle Physics Flashcards
Who proposed the plumb pudding model of the Atom?
J.J. Thomson
What was the set up for Rutherford’s scattering experiment?
- A stream of alpha particles from a radioactive source is fired at a very thin sheet of gold foil
- The particles are scattered by the gold foil and can be detected as a flash of light on a florescent screen
- The number of flashes per given time are recorded at different scattering angles
What was the expected result for the scattering experiment form the plumb-pudding model?
The plumb pudding model predicted that all the alpha particles would be detected within a small angle of the beam.
What were the conclusions made form the alpha particle experiment and their respective observations?
The atom is mostly empty space - most of the alpha particles went straight through the foil
The atom must have a large positively charged center - some of the alpha particles were deflected through large angles
The nucleus is tiny - Very few particles were deflected > 90*
Most of the mass is in the nucleus - alpha particles with high momentum were deflected
What is the nuclear model of the atom?
- In every atom there is a positive nucleus containing neutrons and protons. Orbiting this nucleus are negatively charged electrons.
- The charge of the electron is opposite that of the proton
- The nucleus only makes up a tiny part of the atom (1/10000th the size)
- Nucleus makes up almost all the mass of the atom
What is another name for the mass number of an atom?
The nucleon number
What is an isotope?
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons
What is the diameter of an atom?
~ 0.1 nm
What is the equation for nuclear radius?
R = r0 * A^(1/3)
r0 = 1.4 fm
R α A^1/3
What is the volume of a nucleus?
V = 4/3 pi*r^2
At what distance does the strong nuclear force become attractive?
0.5 fm
At what distance can the strong nuclear force no longer hold nucleons together?
3 fm
The strong nuclear force becomes too weak to counter the electrostatic repulsion
How does the force exerted by the strong nuclear force change with distance?
At small separations ( < 0.5fm ) it is repulsive. It then becomes attractive at > 0.5fm reaching a maximum attractive value and then falling rapidly to 0. After 3fm it can no longer counter the electrostatic force pushing protons apart.
How does the strong nuclear force vary between nucleons?
Experiments have shown that the strong nuclear force works equally between all nucleons
What are the relative strengths of the four forces?
Strong nuclear - 1
Electromagnetic - 10^-3
Weak nuclear - 10^-6
Gravitational - 10^-40
What is a hadron?
Hadrons are particles that are affected by the strong nuclear force
What is a lepton?
Leptons are fundamental particles that don’t feel the strong nuclear force
Define the term fundamental particle?
Fundamental particle - A particle with no internal structure and hence which can’t be divided into smaller parts
What is the difference between particles and antiparticles?
Antiparticles have the same mass as particles but the opposite charge
What are the properties of neutrinos?
Neutrinos have 0 mass and 0 charge. They only interact via the weak nuclear force
Give an example of a lepton and hadron?
Hadrons:
- Protons, Neutrons
Leptons:
- Electrons, Neutrinos
Why were anti-particles predicted?
When Paul Dirac wrote an equation obeyed by electrons he found a mirror image solution that predicted the existence of a particle like the electron but with opposite electric charge
Quark structure of a proton?
proton: uud
Quark structure of a neutron?
neutron: udd
What happens when a particle and anti-particle annihilate?
When an antiparticle and particle annihilate:
- All the mass of the particle and antiparticle gets converted into energy, in the form of a PAIR of photons
How can you calculate the maximum wavelength of a photon produced by annihilation?
- Annihilation produces a photon pair
- The combined energy of the photons will equal the combined energy of the particles so:
2Emin = 2mc^2
Emin (one photon) = mc^2 - This is the MINIMUM energy of the photon as it assumes particles have no kinetic energy, only mass
- E = hc/l for maximum wavelength
What is the law for antimatter production?
When energy is converted to mass you get EQUAL amounts of matter and anti-matter.
If a an extra proton is produced when two protons collide an extra antiproton is too.
Why are electron-positron pairs most often produced in pair production?
electron-positron pairs have the lowest mass so are the easiest to produce via pair production
What are the requirements for pair production?
Pair production only happens if a photon has enough energy to produce that much mass. Pair production also tends to happen near a nucleus as it makes it easier to satisfy the law of conservation of momentum.
How do you calculate the minimum energy needed by a photon for pair production?
Minimum energy is the combined energy of the two particles (produced) at rest.
A particle and antiparticle have the same rest mass so:
Emin = 2Mc^2
What are the two types of hadron and what is their structure?
Baryon - three quarks
Meson - quark - antiquark pair
What is meant by ‘quark confinement’?
A quark can never be found alone. If a proton is hit with enough energy to separate a quark the energy just gets converted into a quark-antiquark pair, producing a meson.
What does the weak nuclear force do?
The weak nuclear force is responsible for beta decay
What happens in β- decay?
In β- decay:
- A neutron turns into a proton
- An electron and anti-neutrino are created
- (A down quark turns into an up quark via the weak interaction)
What happens in β+ decay?
In β+ decay: - A proton turns into a neutron In β- decay: - An positron and neutrino are created - (An up quark turns into an down quark via the weak interaction)
State the quark equations for β+ and β- decay
β+: u > d + e+ + v
β-: d > u + e- + vbar
Which conservation laws must be considered when looking at particle decay?
spin, baryon number, lepton number, CHARGE
Define radioactive decay?
Radioactive decay - The decay of an unstable nucleus by releasing energy/particles (nuclear radiation) until it reaches a stable form
What can make a nucleus unstable?
- Too many neutrons
- Too few neutrons
- To many nucleons (in total)
- To much energy in nucleus
What is meant by radioactive decay being ‘random’?
- You can’t predict when a nucleus in an isotope will decay or which nucleus will decay next
What is meant by radioactive decay being ‘spontaneous’?
Spontaneous:
- The decay of nuclei is NOT affected by external factors
What is the penetrating power of the four types of radiation?
Alpha: a few cm of air/sheet of paper
Beta-: 3mm of aluminium
Beta+: basically 0 as annihilated by electrons
Gamma: a few cm of lead/several meters of concrete
What precautions should you take when handling radioactive sources for a practical?
- Should be kept in a lead lined box when not in use
- Should only be picked up with long-handled tongs
- Always keep a safe distance form source
- Do not point source at anyone
Which type of radiation has the least ionizing power and which has the most?
Least: gamma
most: alpha
What is conserved during radioactive decay?
Charge, nucleon number, energy,momentum
In what types of nuclei does alpha emission happen?
Alpha emission happens to nuclei that are too massive (have too many nucleons)e.g. uranium, radium
How do proton and nucleon numbers of an atom change when an alpha particle is emitted?
When an alpha particle is emitted:
- Proton number decreases by 2
- Nucleon number decreases by 4
In what types of nuclei does gamma emission happen?
Gamma emission happens in nuclei with excess energy called ‘excited’ nuclei. Gamma decay usually happens just after alpha or beta decay
In what types of nuclei does beta- emission happen?
Beta- happens in neutron-rich isotopes
What does the decay equation for beta-/+ look like?
The electron is replaced by a β particle symbol.
e.g:
Re > Os + β + vbar
Define activity (of an isotope)
Activity is the number of active nuclei that decay per second
What is the equation for activity?
A = λN A = - ΔN/Δt
What is the unit for activity?
Activity - Bequerels (Bq)
1 Bq = 1 decay per second
Define half life
The half-life of an isotope is the average time it takes for the number of undecayed nuclei to halve.
What is the equation for half-life?
t½ = ln(2)/λ
What is the equation for activity/undecayed nuclei at time t?
X = X0e^(-λt)
Define carbon dating?
Carbon dating - A method for finding the age of organic material by comparing the activities or carbon-14 ratios of dead material vs a similar living material.
How does radioactive dating using carbon-14 work?
- Living plants take in Co2 from the atmosphere for photosynthesis. This mainly is stable carbon-12 but also includes a small amount of the radioactive isotope carbon-14
- Animals then take the carbon in when they eat the plants
- All living things contain the same ratio of carbon-12 to carbon-14, and this ratio has stayed almost constant throughout history
- When they die the activity of carbon-14 starts to fall
- Old once-living material can be tested to find the current ratio of carbon-14 to carbon-12 in them. This can be compared to a similar sample of living material to find how long the material has been dead for
What is the half life of carbon-14?
5730 years
What are some limitations of carbon dating?
- It assumes the ratio of carbon-12 to carbon-14 atoms has remained constant over time which may not be the case due to recent rise in co2 emissions. volcanic eruptions and nuclear tests can affect this too.
- The tiny amounts of Carbon-14 present in organisms means the activity is extremely small - hard to filter out background radiation
Why can carbon dating not be used to date rocks? What alternative is there?
carbon-14 has a too short half life. The decay of rubidium-87 is used instead
What is mass defect?
The mass of a nucleus is less than the mass of it’s individual parts. This difference is called mass defect
What is binding energy?
Binding energy is the energy needed to separate all the nucleons in a nucleus. (measured in MeV)
Why is binding energy = mass defect?
As nucleons join together the total mass decreases. This lost mass is converted to energy and released. The energy released is equivalent to the mass defect. If you put this energy back in the nucleons will have enough energy to be separate again, hence mass defect = binding energy.
Just be careful because binding energy is in eV and mass defect in u
How do you calculate binding energy per nucleon?
Binding energy per nucleon = binding energy/nucleon number
MeV = B/A
Where are the most stable nuclei found on a binding energy per nucleon vs nucleon number scatter graph?
Around the maximum point of the line of best fit, at A = 56
Where on a binding energy per nucleon does fusion and fission occur?
Fusion - before the peak at N = 56 (Fe)
Fission - after the peak at N = 56 (Fe)
How do you calculate the energy released by fusion?
Energy released = binding energy after fusion - binding energy before fusion
How do you calculate the energy released by fission?
Energy released = binding energy after fission - binding energy before fission (except this time you’re going back in nucleon number)
What is spontaneous / induced fission?
If an unstable nucleus splits into two smaller nuclei on its own spontaneous fission has occurred. If we encouraged it to split (using a neutron) it is induced fission.
How can we make a U-235 nucleus undergo induced fission?
By firing a low energy ‘thermal neutron’ at it. The nucleus will absorb the neutron causing it to become an unstable U-236 nucleus and decay.
Why is there a limit to the number of elements possible?
The larger a nucleus, the more unstable it is and the more likely it is to undergo spontaneous fission and split. This limits the number of nucleons a nucleus can contain and therefore the number of elements.
Why does fission release energy?
The new smaller nuclei have a higher combined binding energy than the nucleus that split to create them so the excess energy is released.
Why are net energy gain fission reactors so hard to create?
Nuclei can only fuse if they have enough energy to overcome the electrostatic repulsion between them, after which they get close enough for the strong nuclear force to bind them.
This means fusion requires much higher temperatures and pressures than fission, where matter becomes plasma. Such conditions are generally only found inside of stars.
Why does fusion release energy?
Fusion reactions release a lot of energy as the nucleus created has a much higher binding energy per nucleon than the nuclei used to make it
How are fusion reactions sustained in stars?
The energy released in other fusion reactions helps maintain the high temperatures needed for fusion reactions to occur.
If the temperature falls fusion reactions will stop occurring, there is a critical point where fusion is self sustaining.
What are the benefits of fusion over fission?
- No harmful waste produced
- Fusion produces more energy per mole of reactant than fission, despite an individual fusion reaction releasing less energy than an individual fission one
What are the pros/cons of a nuclear power plant?
Pros:
- Fission does not produce co2 so does not contribute global warming like fossil fuels
- Fission provides a continuous energy supply unlike renewables
Cons:
- Waste products are highly radioactive so difficult to handle and store
- Accidents / natural disasters could cause an uncontrolled chain reaction to occur
- Because of safety precautions building / decommissioning a nuclear reactor is expensive and time consuming
How is nuclear waste handled and what are some drawbacks of this?
1) The fuel rods are placed in a cooling pond until temperatures fall to a safe level
2) Waste is stored in sealed containers in specialist facilities until its activity has fallen sufficiently
Drawbacks:
- It can take many years for the activity to fall
- If radioactive waste leaks it will contaminate the environment and could contaminate water supplies
What is the basic structure of a nuclear reactor?
fuel rods with control rods in between, all suspended in a MODERATOR (usually water)
How do nuclear reactor work?
1) Nuclear reactors use U-235 rich uranium fuel rods
2) Fission reactions from U-235 produce fast moving neutrons that are slowed down by the moderator and induce fusion in more U-235 nuclei creating a chain reaction
3) The moderator heats up when slowing/absorbing these neutrons
4) The hot moderator (or coolant) is used to heat water which turns to steam and turns a turbine
5) this also cools the moderator (or coolant) which is returned to the reactor
Why is the choice of moderator important in a nuclear reactor?
The moderator must slow down neutrons enough that they can cause further fission. Only low energy ‘thermal neutrons can cause reactions
Why do only low energy neutrons induce fission?
Slow neutrons can be captured by the U-235 nucleus easily compared to fast moving neutrons that just bounce off
How do control rods work?
Control rods are made up of a material that absorbs neutrons. They can control the rate of chain reactions occurring by limiting the number of neutrons in the reactor. They can be inserted by varying amounts to control the reaction rate. (fully in to emergency stop as quick as possible)
