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A-Level Physics > 8: Nuclear Physics > Flashcards

8: Nuclear Physics Flashcards

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1
Q

Rutherford Scattering Experiment (3)

A
  • 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°
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2
Q

Conclusions from Rutherford Scattering (4)

A
  • 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
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3
Q

Properties of Nuclear Radiation (3)

A
  • 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
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4
Q

Experimental Identification of Nuclear Radiation (4)

A
  • 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
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5
Q

Applications of Alpha Radiation (3)

A
  • 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
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6
Q

Applications of Beta Radiation (5)

A
  • 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
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7
Q

Applications of Gamma Radiation (4)

A
  • 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
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8
Q

Required Practical 12

A

Investigation of the inverse-square law for gamma radiation

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9
Q

Required Practical 12 Method (4)

A

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

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10
Q

Inverse-Square Law for γ Radiation

A

I = k / x² where k = n h f / 4 π

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11
Q

Applications of the Inverse-Square Law (4)

A
  • 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
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12
Q

Experimental Elimination of Background Radiation from Calculations

A

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

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13
Q

Background Radiation Origins (5)

A
  • 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)
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14
Q

Random Nature of Radioactive Decay

A

Radioactive decay is completely random - you can’t predict when a nucleus will decay. However, a given nucleus has a constant probability of decaying

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15
Q

Radioactive Decay Equations (2)

A
  • ΔN / Δt = -λ N
  • N = N₀ e^(-λ t)
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16
Q

Activity

A

A = λ N
It is the number of nuclei decaying in a source per unit time

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17
Q

Half-Life Equation

A

T_(1/2) = ln 2 / λ

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18
Q

Half-Life

A

The mean time taken for the number of unstable nuclei to halve

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19
Q

Decay Constant

A

The probability of a nucleus decaying per unit time

20
Q

Graph of N against Z for Stable Nuclei

A

https://chem.libretexts.org/@api/deki/files/38743/2000px-Table_isotopes_en.svg.png?revision=1
Only need to know beta and alpha decay

21
Q

Possible Decay Modes of Unstable Nuclei (4)

A
  • α
  • β⁺
  • β⁻
  • Electron capture
22
Q

Changes in N and Z caused by ____

A

Radioactive decay

23
Q

Excited Nuclei

A

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

24
Q

Closest Approach of Alpha Particles (3)

A
  • 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
25
Q

Electron Diffraction (3)

A
  • 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
26
Q

Typical Value for Nuclear Radius

A

1 fm

27
Q

Coulomb Equation for Closest Approach

A

Eₖ = Qₙᵤ꜀ₗₑᵤₛ qₐₗₚₕₐ / (4 π ε₀ r)

28
Q

Dependence of Radius on Nucleon Number

A

R = R₀ A^(1/3) derived from experimental data

29
Q

Interpretation of ____ for constant ____

A

R = R₀ A^(1/3), density of nuclear material

30
Q

____ applies to all energy changes

A

E = m c²

31
Q

Mass Difference (3)

A
  • 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)
32
Q

Binding Energy (2)

A
  • 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
33
Q

Atomic Mass Unit

A

1 u = 931.5 MeV

34
Q

Average Binding Energy per Nucleon (3)

A
  • 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
35
Q

Fission (2)

A
  • 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
36
Q

Fusion (4)

A
  • 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
37
Q

Thermal Neutrons

A

Nuclear fission can only be induced by a thermal neutron - a neutron, which has been slowed down so it is absorbed by heavy nuclei

38
Q

Chain Reaction

A

Where fission reactions in some rods of uranium release neutrons causing other nuclei to fission

39
Q

Critical Mass

A

The mass of nuclear fuel needed to keep the chain reaction going at a steady rate

40
Q

Moderator (4)

A
  • 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
41
Q

Control Rods (3)

A
  • 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
42
Q

Coolant (4)

A
  • 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
43
Q

Shielding

A

The reactor is surrounded by thick concrete to prevent radiation escaping

44
Q

Emergency Shutdown

A

The control rods are fully lowered into the reactor to slow down the reaction as fast as possible

45
Q

Handling & Storage of Radioactive Waste Materials (7)

A
  • 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

A-Level Physics (9 decks)

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