8: Nuclear Physics Flashcards
Rutherford Scattering Experiment (3)
- A stream of alpha particles was fired at a sheet of very thin gold foil
- If the plum pudding model was accurate, the alpha particles would have been detected within a small angle of the beam
- However, most passed straight through and some were deflected at angles greater than 90°
Conclusions from Rutherford Scattering (4)
- Atoms must be mostly empty space as most alpha particles passed straight through
- The nucleus must have a large positive charge as some alpha particles are repelled or deflected at large angles
- The nucleus must be tiny as very few alpha particles are deflected at angles greater than 90°
- Most of the mass must be in the nucleus since the fast alpha particles are deflected by the nucleus
Properties of Nuclear Radiation (3)
- Alpha has high ionising power, is slow, absorbed by paper or a few cm of air and affected by magnetic fields
- Beta has weak ionising power, is fast, absorbed by ~3 mm of aluminium and affected by magnetic fields
- Gamma has very weak ionising power, travels at the speed of light, absorbed by many cm of lead or several m of concrete and not affected by magnetic fields
Experimental Identification of Nuclear Radiation (4)
- Place a Geiger-Müller tube near an unknown source and record the count rate
- Place a sheet of paper between the source and tube and record the count rate
- Replace the paper with aluminium foil and record the count rate
- Depending on the material, if any, that reduced the count rate, you can identify the type of radiation
Applications of Alpha Radiation (3)
- Alpha sources are used in smoke alarms as they allow current to flow, by ionising atoms in air, but won’t travel very far
- When smoke is present, the alpha particles can’t reach the detector, setting the alarm off
- Although alpha particles cannot penetrate skin, if ingested, they ionise the body tissue, causing lots of damage
Applications of Beta Radiation (5)
- Beta particles ionise fewer atoms than alpha does, causing less damage to body tissue
- When creating sheets of material, beta radiation can be used to control its thickness
- The material is flattened as it is fed through rollers. A radioactive source is placed on one side and a detector on the other
- The thicker the material, the more radiation it absorbs and prevents from reaching the detector
- If too much radiation is absorbed, the rollers move closer to make the material thinner and vice versa
Applications of Gamma Radiation (4)
- Gamma radiation is less ionising than beta so does less damage to body tissue
- Radioactive tracers help diagnose patients. A source, with a short half-life to prevent prolonged exposure, is inserted into the patient. A detector then detects the emitted gamma rays
- Gamma rays can be used to treat cancerous tumours. Radiation damages healthy cells as well. Patients can suffer, possibly long-term, side effects
- The risk towards medical staff must be minimised. Exposure times are kept low and staff leave the room during treatment
Required Practical 12
Investigation of the inverse-square law for gamma radiation
Required Practical 12 Method (4)
https://www.cyberphysics.co.uk/practical_experiments/diagrams/ISL.png
1. Record the count rate on the GM tube to measure the background radiation count rate
2. Set up the apparatus in the diagram, noting the distance X
3. Record the count rate
4. Repeat step 3, increasing the distance of X
Inverse-Square Law for γ Radiation
I = k / x² where k = n h f / 4 π
Applications of the Inverse-Square Law (4)
- From the inverse-square law, using a radioactive source becomes more dangerous the closer you get to the source
- This is why the source is held away from the body
- Long handling tongs should be used to minimise radiation absorbed by the body
- Those, who aren’t working with the source, should stay far away
Experimental Elimination of Background Radiation from Calculations
When you take a reading of the count rate from a radioactive source, you need to measure the background radiation count rate and subtract it from your measurement
Background Radiation Origins (5)
- The air (radioactive radon gas released by rocks)
- The ground and buildings (rocks)
- Cosmic radiation (cosmic rays colliding with the upper atmosphere producing radiation)
- Living things (carbon-14 radioisotope)
- Man-made radiation (medical or industrial sources)
Random Nature of Radioactive Decay
Radioactive decay is completely random - you can’t predict when a nucleus will decay. However, a given nucleus has a constant probability of decaying
Radioactive Decay Equations (2)
- ΔN / Δt = -λ N
- N = N₀ e^(-λ t)
Activity
A = λ N
It is the number of nuclei decaying in a source per unit time
Half-Life Equation
T_(1/2) = ln 2 / λ
Half-Life
The mean time taken for the number of unstable nuclei to halve
Decay Constant
The probability of a nucleus decaying per unit time
Graph of N against Z for Stable Nuclei
https://chem.libretexts.org/@api/deki/files/38743/2000px-Table_isotopes_en.svg.png?revision=1
Only need to know beta and alpha decay
Possible Decay Modes of Unstable Nuclei (4)
- α
- β⁺
- β⁻
- Electron capture
Changes in N and Z caused by ____
Radioactive decay
Excited Nuclei
After alpha or beta decay, the nucleus often has excess energy - it’s in an excited state. This energy is lost by emitting a gamma ray
Closest Approach of Alpha Particles (3)
- In Rutherford’s scattering experiment, an alpha particle deflected through 180° will have stopped a short distance from the nucleus
- The alpha particle does this at the point where its electric potential energy equals its initial kinetic energy
- You can calculate the distance between their centres, at which the alpha particle stops, and if the alpha particle had high enough energy, assume this is the nuclear radius
Electron Diffraction (3)
- A beam of moving electrons has an associated de Broglie wavelength
- A thin film can be used as a diffraction grating for the beam and measurements taken from the pattern to calculate the nuclear radius
- Textbook Page 366 Figure 4
Typical Value for Nuclear Radius
1 fm
Coulomb Equation for Closest Approach
Eₖ = Qₙᵤ꜀ₗₑᵤₛ qₐₗₚₕₐ / (4 π ε₀ r)
Dependence of Radius on Nucleon Number
R = R₀ A^(1/3) derived from experimental data
Interpretation of ____ for constant ____
R = R₀ A^(1/3), density of nuclear material
____ applies to all energy changes
E = m c²
Mass Difference (3)
- As nucleons join together, the total mass decreases
- This lost mass is converted to energy and released
- The amount of energy released is equivalent to the mass difference (or mass defect)
Binding Energy (2)
- The energy required to separate a nucleus into individual nucleons is the same as the energy released when the separate nucleons combine to form the nucleus
- This energy is called the binding energy and is equivalent to the mass difference
Atomic Mass Unit
1 u = 931.5 MeV
Average Binding Energy per Nucleon (3)
- Average binding energy per nucleon = Binding energy / Nucleon number
- Textbook Page 393
- Before Fe, fusion reactions release energy and after Fe, fission reactions release energy
Fission (2)
- Nuclear fission is when an unstable large nucleus randomly splits into two smaller nuclei
- Energy is released because the new, smaller nuclei have a higher average binding energy per nucleon
Fusion (4)
- Nuclear fusion is when two light nuclei combine to produce a larger nucleus
- A lot of energy is released because the new, heavier nucleus has a much higher average binding energy per nucleon
- All nuclei are positively charged so there will be electrostatic repulsion between them
- So lots of energy must be put in to get the nuclei to collide and fuse
Thermal Neutrons
Nuclear fission can only be induced by a thermal neutron - a neutron, which has been slowed down so it is absorbed by heavy nuclei
Chain Reaction
Where fission reactions in some rods of uranium release neutrons causing other nuclei to fission
Critical Mass
The mass of nuclear fuel needed to keep the chain reaction going at a steady rate
Moderator (4)
- Fuel rods are placed in a moderator, which slows down neutrons so they can be absorbed by nuclei
- The moderator slows down neutrons through elastic collisions with nuclei of the moderator material
- When neutrons collide with particles of similar mass, they slow down more efficiently
- Water is used as it contains hydrogen, which has a similar mass to a neutron
Control Rods (3)
- Nuclear reactors use a supercritical mass of fuel and control the rate of fission using control rods
- They absorb neutrons so that the rate of fission is controlled
- They are made up of a material that absorbs neutrons, such as boron
Coolant (4)
- Coolant is sent around the reactor to remove heat produced by fission
- The material should be a fluid and efficient at transferring heat
- The coolant is often the same water in the moderator
- The heat from the reactor produces steam for powering electricity-generating turbines
Shielding
The reactor is surrounded by thick concrete to prevent radiation escaping
Emergency Shutdown
The control rods are fully lowered into the reactor to slow down the reaction as fast as possible
Handling & Storage of Radioactive Waste Materials (7)
- The most dangerous waste are fission fragments from spent fuel rods
- The waste is initially very hot so placed in cooling ponds and highly radioactive so needs to be screened here
- Plutonium and uranium are separated to be recycled
- High level waste is vitrified as it may leak in liquid form, then placed in concrete containers to be stored deep underground
- The waste is highly radioactive so needs to be remotely handled
- The waste will be radioactive for thousands of years so needs to be stable in container hence need to vitrify and needs to be stored in geologically stable areas
- Transporting waste presents a potential danger to the public so waste is enclosed in impact resistant casings
