1
Q
Suggest how the idea of atoms came about
A
1- The idea of atoms has been around since the time of the Ancient Greeks in the 5th century BC. Democritus proposed that all matter was made up of little identical lumps called ‘atomos’
2- In 1804 John Dalton put forward a hypothesis that agreed with Democritus that matter was made up of tiny spheres (atoms) that couldn’t be broken up. He reckoned that each element was made up of a different type of atom
3- Nearly 100 years later JJ Thompson discovered that electrons could be removed from atoms. So Dalton’s theory wasn’t quite right (atoms could be broken up)
4- Thompson suggested that atoms were spheres of positive charge with tiny negative electrons stuck in them like fruit in a plum pudding
5- Until this point nobody had proposed the idea of the nucleus. Rutherford was the first to suggest atoms did not have uniformly distributed charge and density
2
Q
What did the Rutherford scattering experiment show the existence of?
A
The Rutherford scattering experiment showed the existence of a nucleus
3
Q
Explain the Rutherford scattering experiment
A
- In 1909 Rutherford and Marsden tried firing a beam of alpha particles at thin gold foil. A circular detector screen surrounding the gold foil and the alpha source was used to detect alpha particles deflected by any angle. They expected that the positively charged alpha particles would be deflected by the electrons by a very small amount if the plum pudding model was true but instead most of the alpha particles just went straight through the foil while a small number were deflected by a large angle. Some were even deflected by more than 90 degrees sending them back the way they came, this was confusing at the time and called for a change to the model of the atom
4
Q
Describe the conclusions that were made from the Rutherford Scattering experiment
A
The results of the Rutherford Scattering experiment suggested that atoms must have a small positively charged nucleus at the centre:
1- Most of the atom must be empty space because most of the alpha particles passed straight through the foil
2- The nucleus must have a large positive charge as some positively charged alpha particles were repelled and deflected by a large angle
3- The nucleus must be small as very few alpha particles were deflected back
4- Most of the mass must be concentrated in the nucleus since the fast alpha particles with high momentum are deflected by the nucleus
5
Q
Explain how to calculate the an estimate for the closest approach of a scattered particle to the nucleus
A
- When you fire an alpha particle at a gold nucleus you know its initial kinetic energy. An alpha particle that ‘bounces back’ and is deflected through 180 degrees will have reversed direction a short distance from the nucleus. It does this at the point where its electric potential energy equals its initial kinetic energy.
- Its just conservation of energy and can be used to find how close the particle can get to the nucleus
6
Q
State the formula used to estimate the closest approach of a scattered particle to a nucleus
A
Ek = Eelec = QgoldQalpha/4πε0r
- ε0 is the permittivity of free space
- r is the distance from the centre of the nucleus in m
7
Q
What is r in the the formula Ek = Eelec = QgoldQalpha/4πε0r?
A
r is the distance between the centres of the scattered particle and the nucleus
8
Q
How do you find the charge of the nucleus?
A
To find the charge of a nucleus you need to know the atoms proton number, Z which tells you how many protons there are in the nucleus. A proton has a charge of +e where e is the size of the charge on an electron so the charge of a nucleus must be +Ze
9
Q
What is the distance of closest approach an estimate for?
A
The distance of closest approach is an estimate of nuclear radius (the radius of the nucleus), it gives a maximum value for it
10
Q
How does the estimate of nuclear radius provided by electron diffraction compare to the estimate given by the distance of closest approach?
A
Electron diffraction gives a much more accurate value for nuclear radii than the distance of closest approach does
11
Q
How many protons and neutrons does an alpha particle have?
A
It has two protons and two neutrons
12
Q
How many protons does a gold nucleus have?
A
79 protons
13
Q
State the formula used to calculate the closest distance of approach of an alpha particle to a nucleus rearranged for r
A
r = Ze*2e/4πε0Ek
- Z is the number of protons in the nucleus
- Ek is the initial kinetic energy of the alpha particle
14
Q
What are the assumptions made with the theory of closest distance of approach?
A
1- The alpha particle is a point charge (it is not)
2- The nucleus does not recoil and gain some kinetic energy (it will)
3- The scattering is by the electrostatic force and the alpha particles do not get close enough to the nucleus to feel the strong force (they probably will if they have a high enough energy)
15
Q
You can use electron diffraction to estimate…
A
Nuclear radius
16
Q
Why is electron diffraction an accurate method for estimating the nuclear radius?
A
Electrons are a type of particle called a lepton. Leptons don’t interact with the strong nuclear force whereas neutrons and alpha particles do so because of this electron diffraction is an accurate method for estimating the nuclear radius
17
Q
Why can electron beams be diffracted?
A
As electrons show wave-particle duality
18
Q
State the formula used to calculate the De Broglie wavelength of a beam of electrons moving at high speeds
A
λ = hc/E
- E is the electron energy in Joules
- h is the planck constant
- c is the speed of light in a vacuum
19
Q
To investigate the nuclear radius what must the wavelength of the electrons be?
A
To investigate the nuclear radius the wavelength of the electrons must be tiny, so the electrons will have a very high energy
20
Q
What will happen if a beam of high energy electrons is directed onto a thin film of material in front of a screen?
A
If a beam of high-energy electrons is directed onto a thin film of material in front of a screen a diffraction pattern will be seen on the screen
21
Q
State the formula used to calculate where the first minimum appears on an electron diffraction pattern
A
Sinθ = 1.22λ/2R
- R is the radius of the nucleus the electrons have been scattered by
22
Q
How do you calculate the radius of a nucleus from an electron diffraction pattern?
A
- Use the equation λ = hc/E to calculate the De Broglie wavelength of the electrons from their energy
- Use the equation Sinθ = 1.22λ/2R to solve for R
23
Q
Why is electron diffraction a more accurate method for estimating nuclear radius than alpha scattering?
A
1- Electrons are not affected by the strong nuclear force
2- Electrons are a better approximation to point charges than alpha particles
3- Electrons penetrate the nucleus but alpha particles do not because they are repelled by the nucleus before they reach it so electrons give a better value for the radius of the nucleus
4- There is less recoil of the nucleus when electrons hit it due to their lower mass. The recoil can complicate calculations
24
Q
What does the intensity of the maxima in an electron diffraction pattern vary with?
A
Diffraction angle
25
Describe the electron diffraction pattern
- The diffraction pattern is very similar to that of a light source shining through a circular aperture - a central bright maximum (circle) containing the majority of the incident electrons surrounded by other dimmer rings (maxima)
26
Describe how the intensity of the electron diffraction pattern varies with the diffraction angle
The intensity of the maxima decreases as the angle of diffraction increases. The graph shows the relative intensity of electrons in each maximum
27
Sketch a general graph of intensity against angle of diffraction for an electron diffraction pattern
*See page 156 in the revision guide*
28
Does the intensity of an electron diffraction pattern ever reach zero?
It never reaches zero but gets very close
29
How does the size of the nuclear radius compare to the atomic radius?
The nuclear radius is very small in comparison to the atomic radius
30
What are the typical approximate values for the radius of an atom and the radius of a nucleus?
By probing atoms using scattering and diffraction methods we know:
1- The radius of an atom is about 0.05nm
2- The radius of a nucleus is about 1fm
31
What is the symbol the nucleon number?
A
32
How does the size of a nucleus change as more nucleons are added to it?
As more nucleons are added to the nucleus it gets bigger
33
What is the relationship between nuclear radius and the nucleon number?
Nuclear radius is proportional to the cube root of the nucleon number
34
Draw a graph of nuclear radius against the cube root of the nucleon number and state their relationship
*See page 157 in the revision guide*
35
State the formula linking nuclear radius and nucleon number
R=R0A^1/3
- R is the nuclear radius
- R0 is a constant
- A is the nucleon number
36
What is the general size of the density of nuclear matter?
The density of nuclear matter is enormous
37
If Mn is the mass of a nucleon derive a formula for the density of a nucleus and what does this formula tell us?
*See page 157 in the revision guide*
- This tells us the density of a nucleus doesn't depend on the mass number and all nuclei have the same density which means that the separation of nucleons must be constant in all nuclei
38
Unstable nuclei are...
radioactive
39
What happens if a nucleus is unstable?
If a nucleus is unstable it will break down to become more stable. Its instability could be caused by having too many neutrons, not enough neutrons or just too much energy in the nucleus
40
What is radioactive decay?
Radioactive decay is the process by which an unstable nucleus decays by releasing energy and/or particles until it reaches a stable form
41
What happens when a radioactive particle hits an atom?
When a radioactive particle hits an atom it can knock off electrons creating an ion so radioactive emissions are also known as ionising radiation
42
Is an individual radioactive decay random?
Yes, an individual radioactive decay is random, it can't be predicted
43
How many types of nuclear radiation are there?
4
44
What are the four types of nuclear radiation?
- Alpha
- Beta-minus
- Beta plus
- Gamma
45
State the constituency of alpha radiation
A helium nucleus, two protons and two neutrons
46
State the constituency of Beta minus radiation
An electron
47
State the constituency of Beta plus radiation
A positron
48
State the constituency of gamma radiation
A short wavelength, high frequency electromagnetic wave
49
Draw the symbols for the four types of nuclear radiation
*See page 158 in the revision guide*
50
What is the relative charge of an alpha particle?
+2
51
What is the relative charge of a positron?
+1
52
What is the relative charge of gamma radiation?
0
53
What is the mass of an alpha particle?
4μ where μ is the atomic mass unit
54
What is the mass of an electron and a positron?
Their masses are negligible
55
What is the mass of gamma radiation?
0
56
Describe an experiment you can do to investigate the penetrating power of radiation types
Different types of radiation have different penetrating powers and so can be stopped by different types of material:
1- Record the background radiation count rate when no source is present
2- Place an unknown source near to a Geiger counter and record the count rate
3- Place a sheet of paper between the source and the Geiger counter. Record the count rate
4- Repeat step two replacing the paper with a 3mm thick sheet of aluminium
Depending on when the count rate significantly decreased you can calculate what kind of radiation the source was emitting
57
State the different features for alpha radiation for the categories: Ionising, speed, penetrating power and affected by magnetic fields
- Alpha radiation is strongly ionising
- Alpha radiation travels slowly
- Alpha radiation is absorbed by paper or a few cm of air
- Alpha radiation is affected by magnetic fields
58
State the different features for Beta minus radiation for the categories: Ionising, speed, penetrating power and affected by magnetic fields
- Beta minus radiation is weakly ionising
- Beta minus decay travels very fast
- Beta minus decay is absorbed by about 3mm of aluminium
- Beta minus decay is affected by magnetic fields
59
State the different features for Beta plus radiation for the categories: Ionising, speed, penetrating power and affected by magnetic fields
- Beta plus decay is annihilated by electrons so it has virtually zero range
60
State the different features for gamma radiation for the categories: Ionising, speed, penetrating power and affected by magnetic fields
- Gamma radiation is very weakly ionising
- Gamma radiation travels at the speed of light
- Gamma radiation is absorbed by many cm of lead, or several metres of concrete
- Gamma radiation is not affected by magnetic fields
61
Explain how the thickness of a material can be controlled using radiation
1- When creating sheets of material like paper, foil or steel, ionising radiation can be used to control its thickness
2- The material is flattened as it is fed through rollers
3- A radioactive source is placed on one side of the material and a radioactive detector on other. The thicker the material, the more radiation it absorbs and prevents from reaching the detector
4- If too much radiation is being absorbed, the rollers move closer together to make the material thinner. If too little radiation is being absorbed they move further apart
62
State the relationship between the ionising properties of alpha and beta radiation
Alpha and Beta particles have different ionising properties
63
Explain why alpha radiation is used in smoke alarms and how it is used
Alpha particles are strongly positive so they can easily pull electrons off atoms. Ionising an atom transfers some of the energy from the alpha particle to the atom. The alpha particle quicky ionises many atoms (about 10000 ionisations per mm in air for each alpha particle) and loses all its energy. This makes alpha sources suitable for use in smoke alarms because they allow current to flow but won't travel very far
64
Why are alpha particles dangerous if they get into the body?
Although alpha particles can't penetrate your skin, sources of alpha particles are dangerous if they are ingested. They quickly ionise body tissue in a small area causing lots of damage
65
Can beta minus particles knock electrons off atoms?
The beta-minus particle has a lower mass and charge than the alpha particle but a higher speed. This means it can still knock electrons off atoms. Each beta particle will ionise about 100 atoms per mm in air, losing energy at each interaction
66
Does beta minus radiation cause more or less damage to the body than alpha radiation once inside?
Beta minus radiation causes much less damage to body tissue as it has a lower number of interactions per mm in air than alpha radiation
67
What is beta radiation commonly used for?
Beta radiation is commonly used for controlling the thickness of a material
68
What are gamma rays mainly used for?
Gamma rays are mainly used in medicine
69
Why is gamma radiation well suited for use in medicine?
Gamma radiation is even more weakly ionising than beta radiation so it will do even less damage to body tissue which makes it suited for use in medicine
70
State the different uses of gamma radiation in medicine
1- Radioactive tracers are used to help diagnose patients without the need for surgery. A radioactive source with a short half-life to prevent prolonged radiation exposure is either eaten or injected into the patient. A detector, ie a PET scanner is then used to detect the emitted gamma rays
2- Gamma rays can be used in the treatment of cancerous tumours - damaging cells and sometimes curing patients of cancer. Radiation damages all cells though, cancerous or not and so sometimes a rotating beam of gamma rays is used. This lessens the damage done to surrounding tissue whilst giving a high dose of radiation to the tumour at the centre of rotation
71
What are some of the disadvantages of using gamma rays in medicine?
- Damage to other healthy cells is not completely prevented however and treatment can cause patients to suffer side effects such as tiredness and reddening or soreness of the skin
- Exposure to gamma radiation can also cause long term side effects like infertility for certain treatments
72
How is the risk to medical staff minimised when using gamma radiation in treatment?
As well as patients, the risk towards medical staff giving these treatments must be kept as low as possible. Exposure time to radioactive sources is kept to a minimum and generally staff leave the room which itself is shielded during treatment
73
What type of radiation are we surrounded by?
We are surrounded by background radiation. Put a Geiger counter anywhere and the counter will click, its detecting background radiation
74
When taking a reading from a radioactive source what do we need to do to ensure our reading is accurate?
When you take a reading from a radioactive source, you need to measure the background radiation separately and subtract it from your measurements
75
What are the 5 main sources of background radiation?
1- The air: Radioactive radon gas is released from rocks. It emits alpha radiation. The concentration of this gas in the atmosphere varies a lot from place to place but its usually the largest contributor to the background radiation
2- The ground and buildings: All rock contains radioactive isotopes
3- Cosmic radiation: Cosmic rays are particles (mostly high energy protons) from space. When they collide with particles in the upper atmosphere they produce nuclear radiation
4- Living things: All plants and animals contain carbon and some of this will be radioactive carbon-14. They also contain other radioactive materials such as potassium-40
5- Man-made radiation: In most areas radiation from medical or industrial sources makes up a tiny, tiny fraction of the background radiation
76
What type of law does the intensity of gamma radiation obey?
The intensity of gamma radiation obeys the inverse square law
77
Explain why the intensity of gamma radiation obeys the inverse square law
- A gamma source will emit gamma radiation in all directions
- This radiation spreads out as you get further away from the source
- This means the amount of radiation per unit area (the intensity) will decrease the further you get from the source
- If you took a reading of the intensity, I, at a distance x, from the source you would find that it decreases by the square of the distance from the source
78
State the equation that demonstrates the inverse square law of the intensity of gamma radiation
I = k/x^2 where k is a constant
79
How can the inverse square law of the intensity of gamma radiation be proved?
- The relationship can be proved by taking measurements of intensity at different distances from a gamma source using a Geiger counter
- If the distance from the source is doubled, the intensity is found to fall to a quarter which verifies the inverse square law
80
What safety precautions can be taken to minimise risk when working with radioactive sources?
- Using a radioactive source becomes significantly more dangerous the closer you get to the source. This is why you should always hold a source away from your body when transporting it through the lab
- Long handling tongs should be used to minimise the radiation absorbed by the body
- For those not working directly with radioactive sources, it's best to just keep as far as way possible
81
Describe the method of the practical used to investigate the inverse square law of radiation
1- Set up the equipment including a Geiger counter, a radioactive source and a metre ruler leaving the source out at first
2- Turn on the Geiger counter and take a reading of the background radiation count rate un counts per second. Do this 3 times and take an average
3- Place the tube of the Geiger counter so it is lined up with the start of the ruler
4- Carefully place the radioactive source at a distance of d from the tube
5- Record the count rate at that distance. Do this 3 times and take an average
6- Move the source so the distance between it and the Geiger counter doubles (2d)
7- Repeat these last two steps for distances of 3d, 4d and so on
8- Once the experiment is finished put away your source immediately, you don't want to be exposed to more radiation than you need to be
9- Correct your data for background radiation. Then plot a graph of corrected count rate against distance of the counter from the source. You should see that as the distance doubles, the corrected count rate drops to a quarter of its starting value supporting the inverse square law
82
Draw the general shape for the graph of corrected count rate against distance from the source for the inverse square law practical
*See page 161 in the revision guide*
83
What is the relationship between the rate of decay for different isotopes?
Every isotope decays at a different rate
84
Can radioactive decay be predicted?
- Radioactive decay is completely random. You can't predict which atom's nucleus will decay when
- Although you can't predict the decay of an individual nucleus, if you take a very large number of nuclei their overall behaviour shows a pattern
85
What is an isotope?
Isotopes of an element have the same number of protons but different numbers of neutrons in their nucleus
86
What is the relationship between the rate of decay for a sample of any particular isotope?
Any sample of a particular isotope has the same rate of decay, ie the same proportion of atomic nuclei will decay in a given time
87
What is the rate of decay measured by?
The rate of decay is measured by the decay constant
88
Define the activity of a sample
The activity of a sample is the number of nuclei (N) that decay each second
89
What is the activity of a sample proportional to?
The activity of a sample is proportional to the size of the sample. For a given isotope, a sample twice as big would give twice the number of decays per second
90
What is activity measured in?
Activity is measured in becquerels (Bq) where 1 Bq = 1 decay per second
91
Define the decay constant
The decay constant (λ) is the probability of a given nucleus decaying per second. The bigger the value of λ the faster the rate of decay. Its unit is s^-1
92
State the formula used to calculate the activity
A = λN
- A is the activity
- λ is the decay constant
- N is the number of nuclei
93
State the formula for calculating the rate of change of nuclei (N)
As activity is the rate of change of N:
ΔN/Δt = -λN
- There is a negative because the number of atoms left is always decreasing
94
How do you calculate the number nuclei if you are given a molar mass?
- If given the molar mass, you need to calculate the number of moles
- Then use N=nNa where N is the number of atoms/molecules, n is the number of moles and Na is Avagadros constant
95
What is the formula used to calculate the number of moles from the molar mass?
Number of moles = mass of substance / molar mass
96
Define half life
The half-life (T1/2) of an isotope is the average time it takes for the number of unstable nuclei to halve
97
The longer the half-life of an isotope ...
the longer it stays radioactive
98
How does the number of undecayed particles decrease?
The number of undecayed particles decreases exponentially
99
What do you need to remember when measuring the activity and half life of a source
When measuring the activity and half life of a source you've got to remember background radiation. The background radiation needs to be subtracted from the activity readings to give the source activity
100
How does the half life change as the number of nuclei you start with changes?
The half-life stays the same. It takes the same amount of time for half of the nuclei to decay regardless of the number of nuclei you start with
101
Explain how to find the half-life of an isotope from a graph of number of unstable nuclei remaining or count rate activity against time
1- Read off the value of count rate (activity) or the particles when t=0
2- Go to half the original value
3- Draw a horizontal line to the curve, then a vertical line down to the x-axis
4- Read off the half life where the line crosses the x axis
5- Check the units carefully
6- Its always a good idea to check your answer. Repeat steps 1-4 for a quarter of the original value. Divide your answer by two. This will also give you the half life. Check that you get the same answer both ways
102
How would you find the negative decay constant for the decay of a radioactive isotope from a graph?
By plotting the natural log of the number of radioactive atoms (or the activity) against time gives a straight line graph. The gradient of the graph is the negative decay constant so gradient = -λ
103
Plot a general graph of ln of the number of radioactive atoms against time for a decaying isotope
*See page 162 in the revision guide*
104
State the formula used to calculate the half life
T1/2 = ln2/λ
105
State the formula used to calculate the number of unstable nuclei remaining
N=Noe^-λt
- No is the number of nuclei originally present
- t is measured in seconds
106
State the formula used to calculate the activity of a sample
A=A0e^-λt
- t is measured in seconds
107
Explain how radioactive substances are used in the radioactive dating of objects
The radioactive isotope carbon-14 is used in radioactive dating. Living plants take in carbon dioxide from the atmosphere as part of photosynthesis, including the radioactive isotope carbon-14. When they die, the activity of carbon-14 in the plant starts to fall with a half life of around 5730 years. Archaeological finds made from once living material can be tested to find the current amount of carbon-14 in them and date them
108
Explain how radioactive substances are used in medical diagnosis
Radioactive tracers are used to help diagnose patients. Technetium-99m is suitable for this use because it emits gamma radiation and has a half life of 6 hours (long enough for data to be recorded but short enough to limit the radiation to an acceptable level) and decays to a much more stable isotope
109
Why are long half-lives dangerous?
Radioactive substances can be dangerous especially if the substances stay radioactive for a long time. Some isotopes found in waste products of nuclear power generation have incredibly long half-lives. This means we must plan ahead about how nuclear waste will be stored, eg. in water tanks or sealed underground to prevent damage to the environment or people not only now but years into the future too
110
Describe the forces acting in the nucleus of an atom and hence why it is easy for a nucleus to become unstable
The nucleus is under the influence of the strong nuclear force holding it together and the electromagnetic force pushing the protons apart. Its a very delicate balance and its easy for a nucleus to become unstable.
111
How can you get a stability graph for a nucleus?
You can get a stability graph by plotting the number of neutrons (N) against the atomic number (Z)
112
Why will a nucleus be unstable?
A nucleus will be unstable if it has:
1- Too many neutrons
2- Too few neutrons
3- Too many nucleons altogether, its too heavy
4- Too much energy
113
In which type of nuclei does alpha emission occur?
Alpha emission happens in very heavy atoms like uranium or radium. The nuclei of these atoms are too massive to be stable
114
What happens to the nuclide notation of an element if an alpha particle is emitted?
The proton number decreases by 2 and the nucleon number decreases by 4
115
In which type of nuclei does beta minus emission occur?
- Beta minus decay is the emission of an electron from the nucleus along with an antineutrino
- Beta decay happens in isotopes that are neutron rich which are nuclei which have more neutrons than protons
- When a nucleus ejects a beta particle one of the neutrons in the nucleus is turned into a proton
116
What is the effect on the nuclide notation of an element if a beta minus particle is emitted?
The nucleon number stays the same and the proton number increases by 1
117
What is the effect on the nuclide notation of an element during beta plus emission?
- In beta plus emission, a proton gets changed into a neutron
- The proton number decreases by one and the nucleon number stays the same
118
In which type of nuclei is gamma radiation emitted from?
Gamma radiation is emitted from nuclei with too much energy
119
When and why is a gamma ray emitted from a nucleus?
After alpha or beta decay, the nucleus often has excess energy, its excited. This energy is lost by emitting a gamma ray
120
What effect does gamma emission have on the constituency of of the nucleus?
During gamma emission, there is no change to the nuclear constituents, the nucleus just loses excess energy
121
What is another way that gamma radiation is produced?
- Another way that gamma radiation is produced is when a nucleus captures one of its own orbiting electrons
- Electron capture causes a proton to change into a neutron. This makes the nucleus unstable and it emits gamma radiation
122
Draw an energy level diagram for an 238U92 nucleus decaying by alpha emission
*See page 165 in the revision guide*
123
What quantities are conserved in nuclear reactions?
- Energy
- Momentum
- Charge
- Nucleon number
124
When drawing a decay equation what must you make sure?
You must make sure that the nucleon and proton numbers on both sides of the equation balance and are equal
125
What does fission mean?
Fission means splitting up into smaller parts
126
What is nuclear fission?
Large nuclei with at least 83 protons are unstable and some can randomly split into two smaller nuclei, this is called nuclear fission
127
What are the two types of nuclear fission and what is the difference between them?
Nuclear fission can be either spontaneous or induced
- Fission is spontaneous if it happens by itself
- Fission is induced if we encourage it to happen
128
State one way that fission can be induced
Fission can be induced by making a neutron enter a 235U nucleus, causing it to become very unstable. Only low energy neutrons (called thermal neutrons) can be captured in this way
129
Why is energy released during nuclear fission?
Energy is released during nuclear fission because the new smaller nuclei have a higher binding energy per nucleon
130
Are large nuclei more likely to undergo spontaneous or induced fission?
- The larger the nucleus the more unstable it will be, so large nuclei are more likely to undergo spontaneous fission
- This means that spontaneous fission limits the number of nucleons that a nucleus can contain, in other words it limits the number of possible elements
131
What do controlled nuclear reactors produce?
-Controlled nuclear reactors produce useful power
- We can harness the energy released during nuclear fission reactions in a thermal nuclear reactor, but its important that these reactions are very carefully controlled
132
Explain how nuclear reactors are used to harness the energy produced from nuclear fission reactions
1- Nuclear reactors use rods of uranium that are rich in 235U as fuel for fission reactions. (The rods also contain a lot of 238U, but that doesn't undergo fission.) These are placed into the reactor remotely which keeps workers as far away from the radiation as possible
2- These fission reactions produce more neutrons which then induce other nuclei to fission, this is called a chain reaction. The neutrons will only cause a chain reaction if they are slowed down, which allows them to be captured by the uranium nuclei. The 235U fuel rods need to be placed in a moderator (for example, water) to further slow down and/or absorb neutrons. These slowed down neutrons are called thermal neutrons
3- This happens through elastic collisions (kinetic energy is conserved) with nuclei of the moderator material. Collisions with particles of a similar mass are more efficient at slowing neutrons down. Water is often used as a moderator because it contains hydrogen, which fits this condition
4- You want the chain reaction to continue on its own at a steady rate, where one fission follows another. The amount of fuel you need to do this is called the critical mass - any less than the critical mass (sub-critical mass) and the reaction will just peter out. Nuclear reactors use a supercritical mass of fuel (where several new fissions normally follow each fission) and control the rate of fission using control rods
133
How are control rods use to control nuclear fission chain reactions?
Control rods control the chain reaction by limiting the number of neutrons in the reactor. These absorb neutrons so that the rate of fission is controlled. These absorb neutrons so that the rate of fission is controlled. Control rods are made up of a material that absorbs neutrons, and they can be inserted by varying amounts to control the reaction rate. In an emergency, the reactor will be shut down automatically by the release of the control rods into the reactor which will stop the reaction as quickly as possible
134
Why is coolant sent around the nuclear reactor after the fission reactions?
Coolant is sent around the reactor to remove heat produced in the fission, often the coolant is the same water that is being used in the reactor as a moderator. The heat from the reactor can then be used to make steam for powering electricity-generating turbines
135
What is a nuclear reactor surrounded by?
A nuclear reactor is surrounded by a thick concrete case which acts as shielding. This prevents radiation escaping and reaching the people working in the power station
136
What happens if the chain reaction in a nuclear reactor is left to continue unchecked?
If the chain reaction in a nuclear reactor is left to continue unchecked, large amounts of energy are released in a very short time. Many new fissions will follow each fission, causing a runway reaction which could lead to an explosion. This is what happens in a fission (atomic) bomb
137
Waste products of fission must be...
stored carefully
138
Compare generating energy from nuclear fission to burning fossil fuels
Although nuclear fission produces lots of energy and creates less greenhouse gases than burning fossil fuels, there are still lots of dangerous waste products
139
What can the waste products of nuclear fission be used for?
- The waste products of nuclear fission usually have a larger proportion of neutrons than nuclei of a similar atomic number - this makes them unstable and radioactive
- The products can be used for practical applications such as tracers in medical diagnosis
140
What is done with material removed from a nuclear reactor after fission reactions?
- When material is removed from the reactor it is initially very hot so it is placed in cooling ponds until the temperature falls to a safe level. This is done remotely, just like the handling of fuel to limit the radiation workers are exposed to
- The radioactive waste is then stored in sealed containers until its activity has fallen sufficiently. Areas for storage are chosen where there will be minimal impact on animals and the environment and any people that live nearby are consulted about the decision to store nuclear waste near them
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What does fusion mean?
Fusion means joining nuclei together
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Why must we take great care when handling the products of fission?
The products may be highly radioactive and so their handling and storage needs great care
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What is nuclear fusion?
- Two light nuclei can combine to create a larger nucleus, this is called nuclear fusion
- A lot of energy is released during nuclear fusion because the new, heavier nuclei have a much higher binding energy per nucleon
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What is the relationship between the fusion of nuclei and energy?
Nuclei need lots of energy to fuse
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Describe the process of nuclei fusion
- All nuclei are positively charged so there will be an electrostatic force of repulsion between them
- Nuclei can only fuse if they overcome this electrostatic force and get close enough for the attractive force of the strong interaction to hold them both together
- About 1MeV of kinetic energy is needed to make nuclei fuse together and thats a lot of energy
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AQA A Level Physics Paper 2
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