6.4 Nuclear and Particle Physics Flashcards
What experiment evidenced the nucleus?
Alpha-particle scattering experiment
-Stream of alpha particles at gold foil, some went through, some deflect back at larger then expected angle.
What is the relative size of an atom and nucleus?
Nucleus is 10,000th the size of an atom
Diameter of atom: 0.1nm
Diameter of nucleus: 1*10^-14
What is the relative size of a proton and neutron?
and what is their mass?
2000 times more massive then an electron
1 atomic mass unit
What is the nucleon number the same as?
Mass number (A)
What is an isotope?
atom with the same number of protons but different number of neutrons
What does having more or less neutrons mean for the atom?
- Chemical properties remain same
- Nucleus becomes unstable, may be radioactive
What does each symbol represent in
R=ro A^1/3
ro=constant (about 1.4fm)
A=Nucleon number
What is the rough nuclear density of any element?
10^17 Kg/m^-3
What conclusions can be drawn from the fact nuclear density is > atomic density?
- Most of an atoms mass is in its nucleus
- The nucleus is small compared to an atom
- An atom contain lots of empty space
What is the strong nuclear force?
acts between all nucleons and all quarks and counteracts the repulsive electrostatic forces between protons in the nucleus
It is attractive at small distances (up to ~3 fm) and repulsive at incredibly small distances (below ~0.5 fm) and has a limited range.
What are the 4 facts about the strong nuclear force that experiments arrived to?
- has a short range (attractive to 3fm)
- Strength of strong force falls quickly beyond the distance
- Nuclear force works equally between all nucleons (p-p, n-p, n-n)
- At small separations, force is repulsive otherwise it would crush (0.5fm)
What is a Hadron and what are examples of it?
particle that feels the strong nuclear force and weak nuclear force.
Examples include proton and neutrons
What is a Lepton and what are 2 types of them?
Fundamental particle that doesn’t feel the strong force.
They interact via weak nuclear force and gravity
Examples include electrons (e-) and neutrinos (v)
What is a neutrino?
A lepton, with 0 charge and 0 mass.
What are the 4 fundamental forces?
Gravitational force
Electromagnetic force
The weak nuclear force
The strong nuclear force
What is the quark model of a proton?
uud
What is the quark model of a neutron?
udd
What is the charge of an up quark?
+2/3
What is the charge of a down quark?
-1/3
What is the charge of a strange quark?
-1/3
What is the charge of an anti up, anti down and anti strange quark?
Anti up -2/3
Anti down +1/3
Anti strange +1/3
What is the equation for beta - decay In terms of the quark model?
d —> u + e + antineutrino
What is the equation for beta + decay?
In terms of the quark model?
u —> d + e + neutrino
What are baryons?
Particles made from a combo of 3 quarks
What are mesons?
Particles made from 1 quark and 1 antiquark
What is the gravitational force?
acts on particles with mass. It is always attractive, has an infinite range but is very weak.
What is the weak nuclear force?
The weak nuclear force is the force responsible for beta decay. It acts to change quark types over very small distances
What is antimatter?
Every particle has a corresponding antiparticle. The particle and antiparticle will have equal mass but opposite charge and so are attracted to each other.
What type of particle is a neutron?
A hadron that is unstable and will decay into a proton. It’s an example of beta- decay caused by weak nuclear force
n —> p + e- + antineutrino
What is radioactive decay?
spontaneous breakdown of an atomic nucleus resulting in the release of energy and matter from the nucleus.
It is a random process meaning that it is impossible to predict which of a number of identical nuclei will decay next.
Describe alpha radiation?
Speed:
Ionising ability:
Penetrating power:
Stopped by:
Speed: slow
Ionising ability: high
Penetrating power: low
Stopped by: paper
Describe beta particle
Speed:
Ionising ability:
Penetrating power:
Stopped by:
Speed: fast
Ionising ability: medium
Penetrating power: medium
Stopped by: 3mm aluminium
Not effected by magnetic field
Describe gamma radiation
Speed:
Ionising ability:
Penetrating power:
Stopped by:
Speed: speed of light
Ionising ability: low
Penetrating power: high
Stopped by: several cm’s lead
Techniques and procedures used to investigate the absorption of α- particles, β-particles and γ-rays by appropriate materials.
- Set up Geiger counter- Geiger muller tube- material -radiation -radioactive source
- Measure background count (wait 30s and repeat 3 times and find mean)
- Divide count by time interval to get bg count rate
- record count rate with different materials, state types of observation
- Repeat with different source
Give the equation 212Po84 alpha decay
212Po84 —> 208Pb82 +4He2
Give the equation for beta minus decay of carbon-14
14C6 —>14N7 + 0Beta-1
Give the equation for the beta plus decay of Nitrogen-12
12N7 —> 12C6 + 0beta+1
What’s the equation to work out the activity of a source?
activity of a source; decay constant (lambda) of an isotope;
A = lambdaN
Define half life
And What is the equation for the half life of an isotope?
Average time it takes for the number of undeclared nuclei to halve
(Lambda)t1/2 = ln(2)
What causes a nucleus to be unstable?
Too many neutrons
Too few neutrons
Too many nucleons in total (too heavy)
Too much energy in the nucleus
What is conserved in nuclear reactions?
Charge, energy, momentum
What causes alpha emission?
When nuclei are to massive to be stable
Why does beta minus decay happen?
When isotopes have way more neutrons then protons
Why does beta plus decay happen?
When isotopes have way more protons then neutrons
Why does gamma radiation happen?
When the nucleus has excess energy
What are the techniques and procedures used to determine the half-life of an isotope such as protactinium
- Shake bottle (containing uranium salt and 2 solvents
- Wait for liquid to separate (protactinium are top, uranium salt bottom), point Geiger muller tube to top layer
- As soon as liquid separates record count rate (how many counts in 10s). Remeasure the count rate at sensible intervals
- Wait 10 mins before checking background count rate
- Subtract background rate from readings. Plot graph of count rate against time and work out half life
Explain the simulation of radioactive decay using dice
The dice represent undecayed nuclei and if they land on a particular number, say 6, they have ‘decayed’.
Record how many dice you start with and throw them all, removing any that decay, then throw again.
Record graph of number of dice remaining against time
graphical methods and spreadsheet modelling of the equation (delta N)/(delta t)=-lambda N
.
What is radioactive dating, e.g. carbon-14 dating
Plants absorb CO2, including radioactive carbon-14
Animals eat plants, all living things have the same percentage of carbon-14
When they die activity of carbon-14 falls (half-life 5730yr)
You can test the once living materials for its amount of carbon 14 to see the age
What are some safety precautions when handling radioactive sources?
Sources should be kept in lead-lined box
Only picked up with long handled tongs or forceps
Don’t point it at anyone and keep a safe distance from them
What is the mass defect?
The mass of a nucleus is less then the mass of its constituent parts, mass defect is the difference between them.
What happens as nucleons join together?
Nucleons join together and total mass decreases, the lost mass is converted in to energy and released
What is the binding energy?
The energy needed to separate all of the nucleons in a nucleus, it is equivalent to the mass defect
How do you find binging energy per nucleons (MeV)
Binding energy/nucleon number (A)
What does a high binding energy per nucleon mean?
More energy is needed to remove the nucleon from the nucleus
Where do the most stable nuclei occur on. A binding energy per nucleon - nucleon number graph
At the max point on the curve (nucleon number 56=iron)
What happens when energy is converted into mass? In terms of matter
You get equal amounts of matter and antimatter
What happens when you fire 2 protons at each other at high speed?
Lots of energy at point of impact, energy might be converted into more particles.
If an extra proton is formed there will always be an anti proton to go with it. It’s called pair production.
What is the minimum energy needed for a photon to undergo pair production?
Energy equal to the energy at rest of particles produced.
Particles and anti particles have same mass so…
E=2mc^2
Where does pair production usually happen and why?
Near the nucleus, which helps conserve momentum
What is the most common pair produced in pair production?
Electron-position pairs, due to relatively low mass
What is annihilation?
When a particle meets its anti particle
All the mass of the anti particle and particle convert to energy in the form of a pair of photons
How are mesons made?
Blast a proton with energy, the energy gets changed into more quarks and antiquarks, it’s pair production.
What limits the number of nucleons that a nucleus contains?
Spontaneous fission, limiting number of elements that there are.
How can fission be induced?
Fire a neutron at a uranium nucleus causing it be unstable. Only low energy neutrons (thermal neutrons) can be captured this way.
What is the basic structure of a fission reactor and how does it all work?
Uranium fuel rods
Fission reactions produce more neutrons inducing other nuclei to fission (chain reaction)
Thermal neutrons used (slow enough for uranium nuclei to catch them)
Fuel rod placed in moderator (like water) to absorb/slow down neutrons
Control rods absorb neutrons to slow down rate of fission
Coolant (often same as water in moderator) removes heat produced from fission
Heat from reactor is used to make steam for powering turbine for electricity
What is meant by the critical mass of fuel?
So chain reaction continues on its own an a steady rate
What would happen if you left a nuclear reactor unchecked?
Lots of energy released in a short time.
Many new fissions will follow each fission, causing a runaway reaction leading to an explosion
This occurs in fission (atomic) bomb
What are the benefits of nuclear fission reactors?
Doesn’t produce CO2
Continuous energy supply
What does the splitting of a large nuclei into 2 mean in terms of energy?
Nucleon number of the 2 nuclei are smaller then the original nucleus, so an increase in binding energy per nucleon.
So energy is also released in fission (but less compared to fusion)
How is nuclear waste handled?
The material is initially very hot, so placed in cooling ponds
Then stored in sealed containers until activity falls
What poses a risk to nuclear reactors and what precautions are taken?
Leaks can occur of the waste contamination water supplies
Accidents and natural distress can occur
So building and decommissioning nuclear plants is very time consuming and expensive. Location is very important
What is nuclear fusion?
2 light nuclei combine creating a larger nucleus
They only fuse if they have energy> the electrostatic repulsion between them and get close enough for the strong interaction to bind them
So fusion requires much higher temperatures and pressure.
This occurs in stars and matter turns into a state called plasma.
Lots of energy released (as heavier nuclei have much higher binding energy per nucleon). The energy maintains the temperature.
What does combining small nuclei in nuclear fusion mean in terms of binding energy?
Binding energy per nucleon increases drastically, so a lot of energy is released in fusion
Compare the energy released in fusion and fission
Fusion gives more energy per nucleon
Fission gives more energy per reaction
In practice, fusion occurs at a much lower temperature. Suggest a reason why.
Some nuclei will be travelling faster / have greater (kinetic) energy (to overcome electrostatic repulsion and hence cause fusion).
