8.1 Radioactivity Flashcards
Describe the Rutherford scattering experiment set up.
- Beam of alpha particles directed at thin gold foil
- In vacuum so that no collisions with air particles occured
- Movable detector would observe scattering angle
What were the observations of the Rutherford scattering experiment?
- Most alpha particles passed through the gold foil
- Some alpha particles were deflected by large angles
- A few alpha particles were reflected back
What were the conclusions of the Rutherford scattering experiment?
- An atom is mostly empty space and the nucleus is small
- The nucleus is positively charged
- The nucleus is very dense
What are the three types of radiation?
- Alpha
- Beta
- Gamma
What is an alpha particle?
2 protons and 2 neutrons
(aka a Helium nucleus)
What is a beta particle?
Electron
(or a positron)
What is a gamma particle?
Very high energy photon
(aka gamma ray)
Rank radiation from most ionising to least.
- Alpha
- Beta
- Gamma
Rank radiation from fastest to slowest
- Gamma
- Beta
- Alpha
Rank radiation from most penetrating to least.
- Gamma
- Beta
- Alpha
What are the charges of the three types of radiation?
- Alpha is +2e
- Beta is -e (or +e)
- Gamma is 0
Which types of radiation are effected by magnetic fields?
Alphs and beta
What type of radiation can be stopped by a few centimeters of air or paper?
Alpha
What type of radiation can be stopped by a few millimeters of aluminium?
Beta
(and by extension, alpha)
What type of radiation can be stopped by a few centimeters of lead or a few meters of concrete?
Gamma
(and by extension alpha and beta)
What type of radiation follows the inverse square law?
Gamma
What does the inverse square law state?
Intentisty is inverse;y proportional to the square of the distance from the source.
What are some safety procedures that need to be followed?
- Never directly handle sources
- Use long-armed tong to increase distance from source
- Display signage warning others sources are in use
- Keep time source is being used to a minimum
- Store sources in lead boxes when not in use
What are some sources of background radiation?
- Radon gas
- Rocks
- Comic rays
- Nuclear weapons testing
- Nuclear disasters
How is corrected count calculated?
Corrected count = Total count - Background count
What does it mean for radioactive decay to be random in nature?
Which nucleus will decay and when is unpredictable and determined by chance.
Define decay constant.
The porbability of a nucleus decaying per second.
What is the equation for rate of change of nuclei number?
What do the letters represent in this equation?
Define Activity and what is its equation?
Rate of decay of radioactive nuclei in a given isotope.
What do the letters represent in this equation?
What is the equation for radioactive decay involving time?
What do the letters represent in this equation?
What is the equation for activity involving time?
What do the letters represent in this equation?
Define half life?
Time taken for half the nuclei in a radioactive sample to decay.
What is the equation for half life?
What do the letters represent in this equation?
What are some applications of radioactivity?
- Dating (eg Carbon dating)
- Medical tracers (eg Tc-99m)
What is a N-Z graph and what does it show?
A graph of neutron number vs proton number. Showing the stability curve of nuclei.
For light nuclei (Z=0-20) what is the relationship between N and Z?
N=Z
How does the constituents of stable nuclei change as Z increases beyond 20?
- Greater neutron to proton ratio
- Extra neutrons needed to balance strong nuclear force against the Coulomb repulsion between protons
Where are alpha emitters found on the N-Z graph and why?
- Occur beyond Z=60 (more neutrons than protons)
- Strong nuclear force is unable to overcome the Coulomb repulsion
Where are beta minus emitters found on the N-Z graph and why?
- Occur left of stability line
- Nuclei are neutron rich (too many neutrons), becomes more stable by converting a neutron to a proton
(and emits a beta minus and an anti-neutrino)
Where are beta plus emitters found on the N-Z graph and why?
- Occur right of stability line
- Nuclei are proton rich (too many protons), becomes more stable by converting a proton to a neutron
(and emits a beta plus and a neutrino)
How can alpha decay be represented graphically?
How can beta minus decay be represented graphically?
How can beta plus decay be represented graphically?
On an N-Z graph, where does electron capture lie in the same region as?
Beta plus emitters.
How can alpha decay be represented in equation form?
How can beta minus decay be represented in equation form?
How can beta plus decay be represented in equation form?
How can electron capture be represented in equation form?
Why would a gamma photon be emitted after an unstable nucleus emits an alpha or beta particle?
Daughter nucleus is formed in an excited state.
(ie has too much energy)
What are nuclear energy levels similar to?
Electron energy levels.
What are two ways of finding nuclear radius?
- Distance of closest approach
- Electron diffraction
Describe closest approach method?
- Alpha particle approaches target nucleus from far away
- Alpha particle is slowed down by Coulomb repulsion
- Alpha particle stops at closest approach
- Alpha particle accelerates away from target nucleus due to Coulomb repulsion
(KE is converted into Eelectrical PE)
What equations do you equate to find distance of closest approach?
Initial KE = EPE at closest approach
What do the letters represent in this equation?
How is electron diffraction used to find nuclear radius?
- Electron beam fired at thin sheet of specific element
- Electrons diffract passing through sheet
- Diffraction pattern produced on target screen
- Angle of first minimum used to calculate nuclear radius
What is the equation for nuclear radius from electron diffraction?
What do the letters represent in this equation?
Draw the graph of intensity vs angle for electron diffraction by a nucleus.
What is the order of magnitude of a nuclei’s radius?
10-15m
What is the equation for nuclear radius?
What do the letters represent in this equation?
Does nuclear density depend on nuclear radius?
No
Nuclear density is constant
What is the mass-energy equivalence equation?
What do the letters represent in this equation?
What is the mass defect?
Difference in mass between a nucleus and the sum of its constituent nucleons in an unbound state.
Why is there a mass defect?
Because energy is needed to bring the constituent parts of a nucleus together.
-OR-
Because energy is released when breaking apart a nucleus into its constituent parts.
What is binding energy?
Energy that nust be added to separate the nucleons of the nucleus.
-OR-
Energy that is liberated when nucleons bond together.
- Why is energy released when a nucleus forms from seperate neutrons and protons?
- And what is the energy released equal to?
- The nuclear stong force does work on the nucleons to pull them together.
- Energy released is the binding energy.
What is the binding energy per nucleon of a nucleus?
Average work done per nucleon to remove all the nucleons from a nucleus.
What does binding energy per nucleon also tell us about a given nucleus?
Its stability.
A higher binding energy => more stable nucleus
Draw a graph of binding energy per nucleon vs mass number.
Annotate your graph showing where Iron would be and the regions where fusion and fission occur.
- Heavy nuclei fission towards Iron
- Light nuclei fuse towards Iron
What is nuclear fission?
- A large unstable nucleus splits into 2 smaller daughter nuclei.
- Binding energy per nucleon increases and hence releases energy.
(Also releases neutrons)
What is nuclear fusion?
- 2 small nuclei fure together to form a larger nucleus.
- Binding energy per nucleon increases and hence releases energy.
Why is fusion difficult on Earth?
Strong Coulomb repulsion between positively charged nuclei requires a lot of energy to overcome i.e. very high temperatures.
What is induced fission?
When a nucleus absorbs a (thermal) neutron and splits into 2 daughter nuclei, 2 or 3 neutrons, and energy. These neutrons then go on to induce further fission events.
What are two examples of nuclear fuel?
- U-234
- Pu-239
What are thermal neutrons?
Neutrons moving slow enough to be absorbed by the nuclear fuel.
What is a chain reaction?
Where one fission event induces further fission events.
What is critical mass?
The smallest mass of fissile material required for a chain reaction to be sustained.
What is the function of a moderator?
To slow fast neutrons downs through elastic collisions with moderator atoms, so that the neutrons can induce fission events.
What are two examples of moderator material?
- Water
- Graphite
What factors affect the choice of moderator material?
Size of atom being close to that of a neutron and transfer of KE is most efficient if this is the case.
What is the function of the control rods?
They stop the chain reaction from being out of control.
They absorb neutrons so that only 1 of the neutrons released in each reaction can go on to be absorbed by another uranium.
If not then the nuclear reactor would overheat as too many reactions would happen at once.
What is an example of a control rod material?
- Boron
What is the function of the coolant?
To transfer heat from the reactor to a heat exchanger, to boil water to turn turbines to generate electricity.
What are two examples of coolant material?
- Water
- Carbon Dioxide gas
Which waste products from a nuclear reactor are the highest risk?
Spent fuel rods.
What is the reactor core vessel made from and why?
(in terms of safety)
Thick steel
* To absorb beta, neutron, and some gamma rays from the core.
* To withstand high pressure and temperature in the core
What is the reactor building made of and why?
Very thick concrete
* Absorb any radiation that escapes the reactor core vessel
What is an emergency shutdown?
Emergency shutdown inserts all control rods fully into the core to stop fission completely.
How are fuel rods inserted and removed from the reactor?
They are inserted and removed with remote handling devices.
Which waste products from a nuclear reactor poses the highest risk?
Spent fuel rods.
How is high-level radioactive waste dealt with?
- Removed remotely
- Stored in sealed containers in cooling ponds for 1 year
- Encased in glass in thick steel casks and stored in deep caverns in geologically stable locations
How is low-level radioactive waste dealt with?
Sealed in metal drums and buried in large trenches.
