3.8 Nuclear Physics Flashcards
Describe the Rutherford scattering experiment.
A beam of alpha particles are fired at a thin gold leaf. A circular detector screen surrounding the gold foil and the alpha source was used to detect alpha particles deflected by any angle.
What are the conclusions of the Rutherford scattering experiment?
Most of the particles passed straight through- the nucleus is small.
Some alpha particles are slightly deflected- the nucleus is positively charged.
Some alpha particles are significantly deflected- the nucleus is massive.
Give a brief outline of the history of the atom.
The Greeks believed all matter was made up of identical atoms.
John Dalton thought that each element was made up of a different type of atom. And the atom couldn’t be broken up.
J.J.Thompson discovered electrons could be removed from from atoms so Dalton’s theory was disproved. He suggested the plum pudding model.
Rutherford suggested atoms didn’t have uniformly distributed charge and density. He discovered the nucleus.
What are the properties of alpha radiation?
The most strongly ionising
Have a range of up to 4cm in air.
Can be absorbed by paper.
What are the properties of beta radiation.
Weakly ionising
Has a range of up to 20cm-3m in air
Absorbed by around 3mm of aluminium
What are the properties of gamma radiation?
Very weakly ionising
Range of over 1km
Can be absorbed by many cm of lead or several m of concrete
What re the uses of alpha radiation?
Smoke alarms- they are very ionising, so they allow current to flow but don’t travel very far.
What are the dangers of alpha radiation?
They can’t penetrate your skin but are dangerous when ingested, they quickly ionise tissue in one area causing a lot of damage.
What are the uses of beta radiation?
Controlling the thickness of a material- the penetration depends on the thickness of the material.
What are the uses of gamma radiation?
Radioactive tracers in medicine- gamma is weakly ionising so it does less damage to the body tissue. A source with a short half life is used.
Treating cancerous tumours- damages all cells so, the beam is rotated.
What are the dangers of gamma radiation?
Can cause cancer
Can cause infertility
Avoid long-term exposure
What is the definition of activity?
The number of decays per unit time.
What is the inverse square law for radiation?
Intensity = k / x^2
What is the experimental verification of inverse-square law?
Set up the apparatus
Turn ont he Geiger count and take a reading of the background count. Do this 3 times and take an average.
Place the counter a distance of d away from the tube.
Record count rate at that distance 3 times and take an average.
Move the counter to a distance of 2d and repeat.
Take readings for multiple distances.
Calculate corrected count rate.
Plot a graph of corrected count rate against distance.
As distance doubles, count rate should fall to 1/4 of original value, supporting inverse square law.
Why does gamma radiation follow inverse square law?
At any distance r from the source, the radiation will be spread over the area of a sphere, 4(pi)(r^2).
When r doubles, the area over which radiation is spread quadruples and intensity decreases by a factor of 4.
What is the application of inverse square law in handling radioactive sources?
Always hold source away from your body as a source becomes significantly more dangerous the closer it gets.
Use tongs when handling to maximise distance.
Keep as far away as possible.
What are the sources of background radiation?
The air- radioactive radon gas
The ground and buildings
Cosmic radiation
Living things
Man-made radiation
What is corrected count rate?
The count rate when you subtract background radiation from it.
How do you determine activity from the count rate of a detector?
Find the corrected count rate
Find the cross sectional area of the detector
Find the area of the sphere at the distance of the detector
Multiply count rate by (Area of sphere) / (cross-sectional area)
Convert answer to Bq
What is the nature of radioactive decay?
Random
Spontaneous
What is the definition of half-life?
The average time it takes for the number of unstable nuclei to halve.
What are the uses of measuring half-life?
Radioactive dating of objects
Medical diagnosis- radioactive tracers must have a long enough half-life to collect data but, radioactivity falls to a safe level quickly.
Why are long half-lives dangerous?
Some substances stay radioactive for a long time, like nuclear waste. So they need to be stored safely to prevent damage to environment and people.
What are the 4 reasons that nuclei decay?
Too many neutrons
Not enough neutrons
Too many nucleons
(Too much energy)
When does beta- emission occur?
In neutron rich nuclei
When do nuclei emit gamma radiation?
When they have to much energy. After alpha or beta decay, nuclei are often in the excited state and release a photon when they de excite.
During electron capture.
In nuclear reactions, what are the conservation rules?
Energy, momentum, charge and nucleon number must be conserved.
When nuclei size increases, why does the line of stability tend away from the N = Z line?
The electrostatic repulsive force between the protons has a long range and it increases on each proton when the proton number increases. The strong force acts between adjacent nucleons and has a short range. So, as Z increases, more neutrons are required to increase the separation of protons, to reduce electrostatic repulsion.
What are the 2 methods of determining nuclear size?
Closest approach of a scattered particle
Electron diffraction
How do you determine nuclear radius using the closest approach method?
When alpha particle approach the nucleus, their kinetic energy is converted to electric potential energy.
They momentarily stop when Ek = Ep
So, Ek = (Qq) /(4piepsilon*r)
This method provides an upper limit to the radius of a nucleus.
How is electron diffraction used to estimate nuclear radius?
If e- are travelling fast enough, their wavelength can be similar to the diameter of a nucleus.
So, the arrangement of atoms behaves like a diffraction grating.
Radius = (0.61*wavelength) / (sin(theta))
When theta is the angle of the first minimum
Wavelength = hc/E
What are the advantages of using electron diffraction to determine nuclear size over the distance of closest approach?
Electrons can get closer to the nucleus ad there’s no electrostatic repulsion.
Electrons are easier to accelerate as they have a higher charge density.
Electrons are easier to produce.
There’s no strong nuclear interaction.
Using alpha particles only provides an upper limit to the radius.
Why is nuclear density constant?
The strong force has a short range and is only present between adjacent nucleons. So, the nucleons are equal distances from each other.
So nuclear density is independent of nucleon number.
Why is carbon dating only valid when a sample is between 200 - 60,000 years old.
Before 200 years have passed, not enough carbon has decayed from activity to fall by a significant amount.
After 60,000 years, as too many half-lives have passed so activity is too close to background.
What is mass deficit?
Total mass of nucleons - mass of a nucleus
Why is there a mass deficit?
The nucleons have less energy when they’re in the nucleus.
What is binding energy?
The energy required to separate all of the nucleons in a nucleus.
How do you calculate binding energy?
Binding energy = mass deficit * c^2
How is fission induced?
When thermal neutrons are absorbed
Creating a chain reaction if you have critical mass
What happens to the mass of a nuclei into fission?
The total mass of the particles formed is less than the mass of the original nucleus. The remaining energy is released in the form a photon or in the Ek or the products.
What is the function of the moderator?
It slows neutrons to thermal speeds as fission neutrons are too fast and can’t be absorbed by the nuclei.
When theta neutrons collide with the moderator, they transfer Ek to the moderator. Momentum is conserved and the collision is elastic.
What materials are typically used for the moderator?
Water
Graphite
What is the purpose of the control rods?
They control the chain reaction by absorbing neutrons so the rate of fission is controlled. In emergency shutdown, the control rods are automatically released to stop the reaction.
What is the purpose of the coolant in a nuclear reactor?
It’s sent around the reactor to remove heat. It is often the same water in the moderator. The heat is used to make steam and power electricity-generating turbines.
What is the critical mass?
The smallest mass where there can be a self-sustaining chain reaction.
Why is critical mass used in a nuclear reactor?
So that there is a steady chain reaction as it has the perfect surface area to volume ratio so the perfect amount of neutrons escape.
What is reactor shielding?
Thick concrete shielding to absorb gamma from the reactor.
What is the source of HLW and how is it dealt with?
Used fuel rods
Stored underwater until it decays into ILW
What is the source of ILW and how is it dealt with?
Decayed HLW and materials contaminated by contact with reactor fuel.
It is chopped up and mixed with concrete or glass and is stored deep underground.
What is the source of LLW and how is it dealt with?
Contaminated equipment, materials and protective clothing.
It is sealed in drums, surrounded by concrete and placed in landfill.
What is the source of HLW how is it dealt with and why? [6 marks]
The (highly radioactive/ most dangerous) waste are the fission fragments from the fission of uranium-235 or from (spent) fuel rods
The waste is initially placed in cooling ponds
Plutonium/uranium is separated to be recycled
High level waste is vitrified/made solid into (pyrex) glass
Then placed in (stainless) steel/lead/concrete cylinders
To be stored deep underground
The waste is (initially) is very hot/generates heat so has to be placed in water/cooling ponds (to remove the heat)
The waste is initially highly radioactive and needs to be screened in water to absorb radiation
The waste is initially highly radioactive and need to be remotely handled to avoid human contact
The waste will be radioactive for thousands of years to long term storage in geologically stable areas must be found.
