3.8 Nuclear physics Flashcards
(142 cards)
1
Q
Describe Thomson’s plum pudding model of the atom.
A
The atom was made up of a sphere a positive charge, with small areas of negative charge evenly distributed throughout.
2
Q
What experiment disproved Thomson’s plum pudding model of the atom?
A
The Rutherford scattering experiment
3
Q
Which model replaced Thomson’s plum pudding model of the atom?
A
The nuclear model
4
Q
Describe the setup of the Rutherford scattering experiment.
A
- An alpha source aimed at gold foil in an evacuated chamber covered in a fluorescent coating
- A microscope that could be moved around the outside of the chamber to observe the path of the alpha particles.
5
Q
What observations were made during the Rutherford scattering, and what was their significance?
A
- Most alpha particles passed straight through the foil with no deflection, suggesting the atom is mostly empty space
- A small amount of particles were deflected by a large angle, suggesting the centre of the atom is positively charged
- Very few particles were deflected back by more than 90˚, suggesting the centre of an atom is very small and very dense
6
Q
What is radiation?
A
Where an unstable nucleus emits energy in the form of EM waves or subatomic particles in order to become more stable
7
Q
What are the three types of radiation?
A
Alpha, beta and gamma
8
Q
What is the range of alpha radiation in air?
A
2-10 cm
9
Q
What is the range of beta radiation in air?
A
Around 1m
10
Q
What is the range of gamma radiation in air?
A
Infinite, following an inverse square law
11
Q
How ionising is alpha radiation?
A
Highly
12
Q
How ionising is beta radiation?
A
Weakly
13
Q
How ionising is gamma radiation?
A
Very weakly
14
Q
Is alpha radiation deflected by electric and magnetic fields?
A
Yes
15
Q
Is beta radiation deflected by electric and magnetic fields?
A
Yes
16
Q
Is gamma radiation deflected by electric and magnetic fields?
A
No
17
Q
What is alpha radiation absorbed/stopped by?
A
Paper
18
Q
What is beta radiation absorbed/stopped by?
A
Aluminium foil (3mm)
19
Q
What is gamma radiation absorbed/stopped by?
A
Several metres of concrete or several inches of lead
20
Q
Why can the types of radiation emitted from a source be easily identified?
A
Due to their differing penetrating powers, by measuring the count rate with a GM tube with different barriers in between it and the source, the each type can be identified.
21
Q
What must be done before measuring the count rate of a source in an experiment?
A
The background count must be measured before the source is present, then taken away from measured values with the source present.
22
Q
What is an application of alpha radiation?
A
Smoke detectors
23
Q
Explain how smoke detectors work.
A
- An alpha source is emitted across a gap in the smoke detector circuit
- The air particles are ionised by the alpha particles colliding with them across the gap
- The electrons move to one plate and the positive ions move to the other, creating an electric current
- When there is smoke in the way, it attracts ions and reduces the current, setting the alarm off
24
Q
Why is alpha radiation used in smoke detectors?
A
- Alpha radiation does not penetrate very far in air, and therefore in the detector, so is safe to use at home
- The source has a long half life so it does not have to be changed frequently
25
What are applications of beta radiation?
- Measuring paper/foil thickness
- Radioactive tracers
26
Explain how beta radiation can be used to measure paper/foil thickness.
- A beta source is placed on one side of the material and a detector on the other
- The thicker it is, the less radiation will get through and so be detected
- The thinner the paper/foil, the more radiation will be let through
- A computer will detect these changes and adjust the roller to change the thickness
27
Why is beta radiation used to measure paper/foil thickness?
Beta radiation can penetrate paper or thin aluminium, but the amount of penetrating will vary sufficiently as thickness changes.
28
How do radioactive tracers work?
- A radioactive chemical in injected into someone’s body
- This decays by beta plus decay, emitting a positron
- When this meets electrons in the body they annihilate to produce 2 identical gamma photons
- Detectors around the body can pick this radiation up and produce an image
29
Do radioactive chemicals used as radioactive tracers have short or long half-lives?
Short, to reduce exposure
30
What is the purpose of radioactive tracers?
To create an image of a patient’s body to help diagnose them.
31
What are applications of gamma radiation?
- Radiotherapy
- Sterilising equipment
32
How does radiotherapy use gamma radiation?
- Ionising radiation kills living cells
- Gamma radiation can be used to kill cancerous cells in a targeted region of the body
- But it will also kills any healthy cells in that region
33
Why does radiotherapy use gamma radiation?
As gamma radiation is highly penetrating and is weakly ionising, it is the safest type of radiation to use in a patient’s body.
34
Why is gamma radiation used in sterilising equipment?
Gamma radiation will kill any microbes and bacteria present on the equipment.
35
What are examples of safety measures used to reduce risk of gamma radiation to medical staff and patients?
- Reduced exposure times
- Shielding
36
Why does the intensity of gamma radiation follow an inverse square law?
As it moves through air it spreads out in all directions equally.
37
How can the inverse square law for gamma radiation be investigated?
- Measure the count rate of a gamma source at different distances from the GM tube, making sure to adjust for the background radiation
- Plotting a graph of corrected count against 1/x² will form a straight line verifying the relationship
38
How must radioactive sources be handled safely?
- Using long handled tongs to move the source
- Storing the source in a lead-lined container when not in use
- Keeping the source as far away as possible from yourself and others
- Never pointing the source towards others
39
How is the corrected count rate found?
Total count rate - background count rate
40
What are sources of background radiation?
- Cosmic rays - enter the Earth’s atmosphere from space
- Artificial sources - caused by nuclear weapons testing and meltdowns
- Radon gas - released from rocks
- Rocks containing naturally occurring radioactive isotopes
41
What kind of process is radioactive decay?
Random
42
What does radioactive decay being a random process mean?
You can’t predict when the next decay will occur.
43
What does λ represent for a given radioactive nucleus?
The decay constant
44
What is the decay constant of a radioactive nucleus?
The probability of a nucleus decaying per unit time.
45
What is the half life of a radioactive substance?
The time taken for the number of nuclei to halve.
46
What kind of decay is radioactive decay?
Exponential
47
How can the half-life of a radioactive substance be determined graphically?
- For a graph of the number of nuclei against time, measure the time taken for the sample size to half
- The modulus of the gradient of the graph of ln(N0) against time
48
What is activity?
The number of nuclei that decay per second
49
How is activity related to half-life?
The time taken for activity to half is equal to the half-life
50
When can the decay constant be used to model the decay of nuclei?
When there is a large number of nuclei in a sample
51
How does the half-life of a radioactive nucleus affect the way it can be used?
- Nuclei with a long half-life, such as carbon-14, which has a half-life of 5730 years, can be used to date organic objects
- Nuclei with relatively short half-lives are used as radioactive tracers in medical diagnosis, such as technetium-99, as it is a pure gamma emitter with a half-life of 6 hours
52
How can carbon-14 be used to date organic objects?
By measuring the current amount of carbon-14 and comparing it to the initial amount, the percentage of which is approximately equal in all living things.
53
How and why must radioactive nuclei with an extremely long half-life be stored?
For example, in steel casks underground, to prevent these nuclei from damaging the environment and the people that may be living around them hundreds of years into the future.
54
Why does a nucleus experience radioactive decay?
- Nuclei are held together by the strong nuclear force
- But protons experience a force of electromagnetic repulsion
- If these forces are out of balance the nucleus will become unstable and experience radioactive decay
55
What are the four reasons why a nucleus might become unstable?
- Too many neutrons
- Too many protons
- Too many nucleons
- Too much energy
56
How does a nucleus with too many neutrons decay?
- Through beta-minus emission
- One of the neutrons in the nucleus changes to a proton and a beta-minus particle and antineutrino is released
- The nucleon number is constant, while the proton number increases by 1
57
How does a nucleus decay if it has too many protons?
- Through beta-plus emission or electron capture
- In beta-plus decay, a proton changes into a neutron and a beta-plus particle and neutrino is released
- In electron capture, an orbiting electron is taken in by the nucleus and combined with a proton forming a neutron and neutrino
- The nucleon number stays constant, while the proton number decreases by 1
58
How does a nucleus decay if it has too many nucleons?
- Through alpha emission
- A helium nucleus is released from the nucleus
- The nucleon number decreases by 4 and the proton number decreases by 2
59
How does a nucleus decay if it has too much energy?
Through gamma emission.
60
When does gamma emission usually occur and why?
After a different type of decay, such as alpha or beta decay because the nucleus becomes excited and has excess energy.
61
What does N represent in nuclear physics?
Number of neutrons
62
What does Z represent in nuclear physics?
Number of protons
63
What does A represent in nuclear physics?
Number of nucleons
64
On an N-Z graph, where does beta minus decay take place?
Above the stable nuclides
65
On an N-Z graph, where does beta plus decay take place?
Below the stable nuclides
66
On an N-Z graph, where does alpha decay take place?
Below the stable nuclides, from around the middle to the very top.
67
Why does the number of neutrons and protons in a stable nucleus not increase uniformly beyond around 20 of each nucleon?
- Beyond this amount the electromagnetic force of repulsion becomes larger than the strong nuclear force keeping the nucleus together
- So more neutrons are needed to increase the distance between protons in order to decrease the magnitude of the electromagnetic force to keep the nucleus stable
68
How is technetium-99m formed?
Beta minus decay
69
What state is technetium-99m formed in?
An excited nuclear state
70
What happens because technetium-99m is formed in an excited nuclear state?
It will emit a gamma photon to reach the ground state.
71
Why is technetium-99m used in medical diagnosis?
- It is a pure gamma emitter
- Gamma rays can be easily detected by a gamma camera
- Gamma rays are very weakly ionising
- It has a half-life of 6 hours
- It can be easily prepared on site
72
How can the nuclear radius of an atom be estimated?
By calculating the distance of closest approach of a charged particle.
73
What is the distance of closest approach of an alpha particle fired at a gold nucleus?
- It will have an initial kinetic energy
- As it moves towards the positively charged nucleus, it will experience an electrostatic force of repulsion and slow down as its kinetic energy is converted to electric potential energy
- The point at which the particle stops and has no kinetic energy is its distance of closest approach
- It’s electric potential energy is equal to its initial kinetic energy due to conservation of energy
74
Why is the distance of closest approach not a very accurate estimate of nuclear radius?
It will always be an overestimate.
75
What is a more accurate method for calculating nuclear radius?
Electron diffraction
76
Why does using electron diffraction give a more accurate of nuclear radius?
Electrons are leptons, so they will not interact with nucleons through the strong nuclear force as an alpha particle would.
77
How can electron diffraction be used to estimate nuclear radius?
- Accelerate electrons at high speeds so their De Broglie wavelength is around 10^-15m
- Directed at a very thin film of material in front of a screen causing them to diffract through the gaps between nuclei and form a diffraction pattern
- Concentric circles, with a central bright spot, which get dimmer moving away from the centre, formed
- Plot graph of intensity against diffraction angle so diffraction angle of the first minimum can be found
- sinθ = 0.61λ/R
78
What is the approximate radius of any nucleus?
1x10^-15 m
79
Describe a graph of nuclear radius against nucleon number (R-A).
Steep gradient for low nucleon numbers, flattens out for larger nucleon numbers.
80
What is the gradient of a graph of ln(R) against ln(A)?
1/3
81
What is the y-intercept of a graph of ln(R) against ln(A)?
ln(R0)
82
Is the density of nuclear material constant?
Yes
83
How can the constant of nuclear density be found for any nucleus?
V=4/3π(R0A^1/3)^3
ρ=m/V
84
What does the value of nuclear density being greater than the density of an atom suggest?
An atom is mostly empty space with most of its mass concentrated in its centre.
85
What does E=mc² describe?
Mass and energy are interchangeable.
86
Does E=mc² apply to all energy changes?
Yes
87
Is the mass of a nucleus or the mass of its constituents greater?
The mass of the nucleus is always lower.
88
What is the mass defect?
The difference between the mass of a nucleus and the mass of its constituents.
89
Why is there a mass defect?
The mass that is lost is converted into energy and released when the nucleons fuse to form a nucleus.
90
What is binding energy?
The energy required to separate a nucleus into its constituents, nucleons
91
What is one atomic mass unit (u)?
- 1/12th of the mass of a carbon-12 atom
- 1.661x10^-27 kg
92
What is nuclear fission?
The splitting of a large nucleus into two daughter nuclei and 2/3 neutrons.
93
How can nuclear fission occur?
Randomly, but it can also be induced.
94
Why is energy released during nuclear fission?
The smaller daughter nuclei has a higher binding energy per nucleon.
95
What is nuclear fusion?
Where two smaller nuclei join together to form one larger nucleus.
96
Why is energy released during nuclear fusion?
The larger nucleus has a much higher binding energy per nucleon.
97
Does nuclear fission or fusion release more energy?
Fusion releases far more energy.
98
Under what condition can nuclear fusion only occur at?
Extremely high temperatures, such as in stars
99
Why can nuclear fusion only occur at extremely high temperatures?
A massive amount of energy is required to overcome the electrostatic force of repulsion between nuclei.
100
What is the binding energy per nucleon?
The binding energy of a nucleus divided by the number of nucleons in the nucleus.
101
Describe a graph of binding energy per nucleon against nucleon number.
- The graph rises steeply at first, reaching a peak around iron (Fe-56)
- There is a sharp peak during the steep rise that doesn’t follow the trend at He-4
- The graph then gradually decreases as nucleon number increases
102
What does the shape of the graph of binding energy per nucleon against nucleon number suggest about nuclei of different nucleon numbers?
- Small nuclei (A<20) have low binding energy per nucleon, so are less stable
- Nuclei smaller than iron can undergo fusion
- Iron has the highest binding energy per nucleon, so is the most stable nucleus
- Nuclei larger than iron can undergo fission and have lower binding energy per nucleon, so are less stable
103
Why does fusion of small nuclei occur?
It increases binding energy per nucleon, releasing energy.
104
Why does fission of large nuclei occur?
It increases binding energy per nucleon, releasing energy.
105
What are the benefits of using nuclear fission in nuclear power plants to create electricity?
- No emission of greenhouse gases
- Reliable for production of power
- Need far less fuel
106
What are the risks of using nuclear fission in nuclear power plants to create electricity?
- The daughter nuclei produced are radioactive, so will need to be stored safely for thousands of years
- Meltdowns in nuclear power plants are a possibility, and can cause devastating harm to the environment
107
What does understanding the nuclear physics behind the production of nuclear power allow for society?
To make informed decisions about how electricity should be generated.
108
How can nuclear fission be induced?
By firing a thermal neutron into a large nucleus, such as uranium-235, causing it to become extremely unstable.
109
Why are thermal neutrons used to induce fission?
- They have a low energy, whereas neutrons with a higher energy rebound away from the nucleus after a collision, not causing a fission reaction
- They cause the nucleus to become extremely unstable
110
Describe induced nuclear fission.
- A thermal neutron is fired into a uranium-235 nucleus, causing it to become extremely unstable
- The unstable nucleus splits into two daughter nuclei of similar mass and at least one neutron is released
- Released neutrons cause more fission reactions, forming a chain reaction, where each fission will cause at least one more fission
111
What is critical mass?
The minimum mass of fuel required to maintain a steady chain reaction in nuclear fission.
112
What does using exactly the critical mass mean?
A single fission reaction follows the last.
113
What does using less than the critical mass mean?
The reaction will eventually stop.
114
What are the key features of a nuclear reactor?
Moderator, control rods, coolant
115
What is the function of the moderator in a nuclear reactor?
Slows down neutrons released in fission reactions to thermal speeds through elastic collisions between the nuclei of the moderator atoms and the fission neutrons.
116
How does the moderator slow down neutrons released in fission reactions to thermal speeds?
Through elastic collisions between the nuclei of the moderator atoms and the fission neutrons.
117
What is the effect of the size of the atoms in the moderator compared to the size of a neutron?
- The closer the moderator atoms are in size to a neutron, the larger the proportion of momentum which is transferred
- So the lower the number of collisions required to get the neutrons to thermal speeds
118
What are examples of materials used as moderators?
Water, graphite
119
Why is water often used as a moderator?
It contains hydrogen, and it’s inexpensive and not very reactive.
120
What is the function of the control rods in a nuclear reactor?
To absorb neutrons in the reactor to control chain reactions, to control the rate at which fission reactions occur to control the amount of energy produced.
121
How can control rods be used to control chain reactions?
The height of the control rods can be controlled
122
What is the condition for which a material can be used for control rods?
Absorb neutrons without undergoing fission
123
What are examples of materials used as control rods?
Boron, cadmium
124
What is the function of the coolant in a nuclear reactor?
To absorb heat released during fission reactions in the core of the reactor, which is then used to make steam which power electricity-generating turbines.
125
What are examples of materials that can be used as a coolant?
Water, molten salt, gas (e.g. helium)
126
Why is water used as a coolant?
It has a high specific capacity, so it can transfer large amounts of thermal energy.
127
What fuel is used in nuclear reactors?
Enriched uranium
128
Describe enriched uranium.
- Formed through the enrichment of mined uranium which consists of 99% U-238, which does not experience fission in order to increase the percentage of U-235 to around 5%
- U-238 absorbs fission neutrons so helps control the rate of fission reactions
129
What is around a nuclear reactor?
Very thick concrete shielding
130
What is the purpose of the very thick concrete shielding around a nuclear reactor?
To block radiation from escaping from the reactor and affecting the workers in the power station.
131
What might happen to the concrete shielding around a nuclear reactor after long term use?
It may become radioactive
132
Why might the concrete shielding around a nuclear reactor become radioactive after long term use?
Neutrons which escape the reactor may enter the shielding nuclei causing them to become unstable and start experiencing beta-minus decay.
133
What is an emergency shut-down?
In an emergency, the control rods are dropped into the reactor core entirely to stop fission reactions from occurring as soon as possible by absorbing all the free neutrons in the core.
134
Why do the two daughter nuclei produced in a fission reaction need to be disposed of responsibly?
These nuclei are usually extremely unstable and have a very high activity.
135
What is high-level waste?
Some of the most dangerous types of nuclear waste, such as spent fuel rods.
136
What is low-level waste?
Waste that contains only short-lived radioactivity, such as tools and gloves.
137
Why can low-level waste be disposed of close to the surface?
It will not take very long to stop being radioactive.
138
Why can high-level waste not be disposed of close to the surface?
It can stay radioactive for thousands of years, posing a danger to the surroundings.
139
How is high-level waste processed?
- Waste is removed and handled remotely, to limit exposure
- Any material removed from the reactor is placed in cooling ponds for up to a year while they may still be producing heat due to radioactive emissions
- Cooling ponds are usually on the same site as the reactor or very close by so the materials don’t have to be transported long distances, increasing the risk of exposure
- Any plutonium or useable uranium is removed from spent fuel rods to be recycled
- Waste is encased in glass and placed in thick steel casts and stored in deep caverns in geologically stable locations
140
Why is radioactive waste stored in geologically stable locations?
So there is no chance of waste coming free of its casing.
141
How are locations to store radioactive waste chosen?
So they make a minimal impact on the environment and people living in the area are consulted about it beforehand.
142
How is it made sure the benefits of nuclear power outweigh the risks?
Risks are minimised as far as possible through safety aspects during the production of energy and the treatment of radioactive waste.
