8 Nuclearr Flashcards
1
Q
What did the Rutherford scattering demonstrate?
A
existence of a nucleus.
2
Q
What was the set-up for the alpha scattering experiment?
A
- monoenergetic alpha particles were fired at a thin gold foil
- zinc sulphide screen flashed when alpha particles hit it
- vacuum
3
Q
What were the paths of the particles in the scattering experiment?
A
- most passed straight through
- some displayed a small deflection
- 1 in 10000 were deflected by angles > 90°
4
Q
What did the results from the alpha scattering experiment show?
A
The atom must contain a small concentrated positive charge with mass
5
Q
What does it mean, that radioactive decay is spontaneous?
A
The rate cannot be changed by heating/cooling, dissolving in acid etc.
6
Q
What will NOT change the rate of radioactive decay?
A
- heating/cooling
- dissolving in acid
- applying pressure
- applying a magnetic or electric field
7
Q
What happens in alpha decay?
A
A nuclei decays into a new nuclei and emits an alpha particle
8
Q
What happens in beta minus decay?
A
A nuclei decays into a new nuclei by changing a neutron into a proton and electron
9
Q
What happens in gamma decay?
A
After alpha or beta decay, surplus energy is sometimes emitted
10
Q
What are the properties of gamma radiation?
A
High frequency, short wavelength. move at 3x10^8 ms. stopped by lead
11
Q
What is the most ionising type of radiation?
A
Alpha
12
Q
Why can alpha only travel a few cm in air?
A
It is highly ionising
13
Q
Why do alpha particles ionise air?
A
To gain the electrons they need to become a helium atom
14
Q
What can alpha radiation be blocked by?
A
A sheet of paper or few cm of air
15
Q
What can beta radiation be blocked by?
A
A few mm of aluminium
16
Q
What can gamma radiation be blocked by? (2)
A
- several metres of concrete
- several centimetres of lead
17
Q
Why does each beta particle travel a different distance?
A
It has a range of energies
18
Q
Why does gamma radiation intensity decrease?
A
They spread out
intensity ↓ as beam area ↑
19
Q
Brief outline of an experiment to verify the 3 types of radioactive emission?
A
- measure activity of background radiation
- place geiger count within 2cm of source then measure count rate again
- deduct backgound count - does reading change when tube is moved to distance of 10cm?
- leave tube at this distance and place aluminium instead - count rate ↓ then beta
- repeat with lead sheet - count rate should drop to background count
20
Q
What are some sources of background radiation?
A
- radon gas from ground
- human body and food
- rocks
- cosmic rays
- artificial sources (e.g. medical, nuclear power and weapons)
21
Q
How should sources of radiation be stored?
A
In a lead box
22
Q
What are some steps for safe handling of radioactive sources?
A
- use handling tool e.g. tongs
- use lowest activity source possible
- keep 2m away from others
23
Q
What are alpha particles used in?
A
Smoke alarms
24
Q
Why are alpha particles used in smoke alarms?
A
Allow current in air to flow, but don’t travel very far
25
How do smoke alarms work?
* alpha particles ionise many atoms and lose energy quickly
* allow current to flow
* when smoke present, alpha particles can't reach detector and this sets alarm off
26
What is beta radiation used in?
Control thickness of sheets of material e.g. paper, Al foil or steel
27
What is gamma radiation used in?
* radioactive tracers - help diagnose patients without need for surgery
* treatment of cancerous tumours
28
What is the unit for activity?
Bq = Becquerels
29
What is half life?
The time taken for half of the radioactive nuclei to decay into other nuclei
30
How does carbon dating work?
* whilst living, plants take in CO2
* small fraction of carbon atoms is radioactive C-14
* ratio of C-14 to C-12 increases with time
* enables age of plant to be calculated
31
Why do larger nuclei have more neutrons than protons?
Extra neutrons help to bind nucleons together without introducing the repulsive electrostatic forces than protons would
32
Why are very large nuclei, with more neutrons than protons, often unstable?
Strong nuclear force between nucleons is unable to overcome the electrostatic force of repulsion between the protons
33
In a N-Z graph, where do B- emitters lie?
To the left of the stability belt
34
In a N-Z graph, where do B+ emitters lie?
To the right of the stability belt
35
On an N-Z graph, what does electron capture lie in the same region as?
B+ emission
36
On an N-Z graph, which region does electron capture take place in?
To the right of the stability belt
37
When might an unstable nucleus emit gamma radiation?
When the ‘daughter’ nuclei is formed in an excited state after it emits an alpha or beta particle or undergoes electron capture
38
What is binding energy?
The energy required to separate an atom into its constituent parts
39
How would you calculate the binding energy of an atom?
* add up masses of constituent parts
* take away mass on periodic table
* multiply mass difference by unified mass constant
* E=mc² in J
* change to MeV
40
What does a graph of binding energy per nucleon against nucleon number reveal?
The stability of the elements
41
On a graph of binding energy per nucleon against nucleon number, which are the most stable elements?
Those with a nucleon number around 56 Fe
42
Which type of elements release energy from fusion versus fission?
* smaller nucleon number - fusion
* high nucleon number - fission
43
Why do we know energy is released in fusion?
Binding energy per nucleon increases. Mass defect is greater. Energy has been released
44
Why do we know energy is released in fission?
As a heavy nucleus split binding energy of each fragment is greater. Mass defect is greater therefore energy has been released
45
What is fusion?
The process by which light nuclei join together forming heavier nuclei
46
Where does fusion happen?
In stars
47
When, in fusion, will nuclei fuse?
When they overcome the electrostatic repulsion
48
What happens, in fusion, after nuclei overcome the electrostatic repulsion?
The strong nuclear force holds them together
49
What is induced nuclear fission?
The process by which energy is released when a radioactive isotope is forced to split
50
What is used in induced nuclear fission and why?
Uranium 235 - long half life and abundance mean it is found in large quantities
51
How is nuclear fission undergone?
The radioactive nucleus absorbs a slow neutron, causing it to become unstable and split
52
Why is energy released in induced nuclear fission?
Due to change in mass
53
What does the chain reaction that is nuclear fission consist of?
* when a nucleus is split, more neutrons are released
* these can then split other uranium nuclei
* the process keeps going
54
In induced nuclear fission, why do neutrons need to be slowed down?
* allows them to be more easily absorbed by fissile nuclei
* so that they are in thermal equilibrium with the moderator hence the term ‘thermal neutron’
* This ensures neutrons can react efficiently with the uranium fuel
55
In induced nuclear fission, what are neutrons slowed down using?
A moderator
56
In induced nuclear fission, why do extra neutrons need to be absorbed?
So the reaction stays at a constant rate
57
In induced nuclear fission, how are extra neutrons absorbed?
Using control rods
58
What is the critical mass of a fuel?
The minimum mass required to establish a self-sustaining chain reaction
59
What does the reactor core contain?
* fuel rods
* control rods
* coolant
60
What is the coolant in a nuclear reactor?
Water at high pressure
61
What is the reactor core connected to in a nuclear reactor?
A heat exchanger, via steel pipes
62
What is function of the control rods?
To absorb neutrons
63
What does the depth of the control rods control?
The number of neutrons in the core
64
What happens if the control rods are pushed in further?
They absorb more neutrons so that the number of fission events per second is reduced
65
What condition must be true, in a nuclear reactor, for a chain reaction to occur?
The mass of the fissile material (e.g. U-235) must be greater than a minimum mass (the critical mass)
66
Why does the mass of the fissile material need to be greater than the critical mass for a chain reaction to occur?
* some fission neutrons escape form the fissile material without causing fission
* if mass of fissile material < critical mass, too many fission neutrons escape as SA to mass ratio is too high
67
What are the safety features of nuclear reactors? (4)
* reactor core is a thin steel vessel
* core is in a building with thick concrete walls
* every reactor has an emergency shut down system
* the sealed fuel rods are inserted and removed from the reactor by remote handling devices
68
How is the reactor core being a thick steel vessel a safety feature?
* to withstand high pressure and temperatures in the core
* absorbs beta emission and some gamma radiation and neutrons from the core
69
How is the reactor core being in a building with thick concrete walls a safety feature?
Absorb neutrons and gamma radiation that escape from the reactor vessel
70
How is every reactor having an emergency shut-down system a safety feature?
Control rods are inserted completely into the core to stop fission when needs be
71
How is radioactive waste categorised?
High, intermediate or low level, depending on its activity
72
Example of high level radioactive waste?
Spent fuel rods
73
How are spend fuel rods stored (high level waste)?
* stored underwater in cooling ponds for a year as they continue to release heat
* then stored in sealed containers in deep trenches
74
How is intermediate level radioactive waste stored?
Sealed in drums that are encased in concrete then stored in special buildings with walls of reinforced concrete
75
How is low level radioactive waste stored?
Sealed in metal drums and buried in large trenches
76
Examples of low level radioactive waste?
Lab equipment and protective clothing
77
Describe how ideas atoms have changed over time.
* The idea of atoms has been around since the time of Ancient Greeks -> Proposed by Democritus
* In 1804, John Dalton suggested that atoms couldn’t be broken up and each element was made of a different type of atom
* Nearly 100 years later, JJ Thomson showed that electrons could be removed from atoms
* Thomson suggested that that atoms were spheres of positive charge with negative electrons in them like a plum pudding
* Rutherford suggested the idea of a nucleus - that atoms did not have uniformly distributed charge and density
78
What was the original model for atom structure?
Plum pudding model
79
Describe the plum pudding model.
Atoms are made of positive charge with electrons stuck in them like plum pudding.
80
Who suggested an alternative to the plum pudding model?
Rutherford
81
Which experiment showed the existence of a nucleus in atoms?
Rutherford scattering
82
Describe the Rutherford scattering experiment.
* Beam of alpha particles from radioactive source is fired at thin gold foil.
* Circular, fluorescent defector screen surrounding gold foil (and the alpha source) was used to detect alpha particles deflected at any angle.
* Most of the alpha particles went straight through the foil, but a small proportion were deflected by a large angle (up to 90°).
83
Why is the foil used very thin?
ensures that an alpha particle was deflected by a 'single' collision
84
Describe the main conclusions of the Rutherford scattering experiment.
Atoms must have a small, positively-charged nucleus at the centre:
* Most of the atoms must be empty space, since most of the alpha particles passed straight through the foil
* Nucleus must have a large positive charge, since positively-charged alpha particles were repelled and deflected by a large angle
* Nucleus must be small, since most of the alpha particles passed straight through the foil (very few deflected by > 90°)
* Most of the mass must be in the nucleus, since positively-charged alpha particles were repelled and deflected by a large angle by the nucleus.
85
What does the Rutherford scattering experiment tell us about the empty space in the atom?
Most of the atom must be empty space, since most of the alpha particles passed straight through the foil
86
What does the Rutherford scattering experiment tell us about the charge of the nucleus?
Nucleus must have a large positive charge, since positively-charged alpha particles were repelled and deflected by a large angle
87
What does the Rutherford scattering experiment tell us about the size of the nucleus?
The nucleus is small, since most of the alpha particles passed straight through the foil
88
What does the Rutherford scattering experiment tell us about the distribution of mass in the atom?
Most of the mass must be in the nucleus, since positively-charged alpha particles were repelled and deflected by a large angle
89
How did Rutherford and Kay discover EVIDENCE for the existence of a neutron?
Fired high energy alpha particles at different gases.
Thought there was only protons in the nucleus.
If there was only protons, you’d expect high mass (massive) nuclei to have very high charges (compared to lower mass nuclei).
But the charges observed were lower than expected.
Must be another part in the nucleus: he called in “proton-electron doublet” - it was actually the neutron.
90
When an alpha particle is fired at a nucleus, what can be assumed at the point at which it’s direction of travel is reversed?
Initial kinetic energy = Electric potential energy
(This is because all of the initial kinetic energy that the alpha particle was fire with has been converted into potential energy)
91
Describe how you can estimate the closest approach of a scattered particle to a nucleus, given the initial kinetic energy.
* Equate the initial kinetic energy that the particle was fired with with the potential energy of the particle at the turning point. This is from Coulombs law.
* Initial kinetic energy = Electric potential energy
* Ek = Qgold x Qalpha / 4πε₀r
* Calculate r
92
An alpha particle with initial kinetic energy of 6.0MeV is fired at a gold nucleus. Estimate the radius of the nucleus by finding the closest approach of the alpha particle to the nucleus.
* Initial kinetic energy = 6.0 x 10^6 MeV = 9.6 x 10^-13 J
* This equals electric potential energy, so:
* 9.6 x 10^-13 = Qgold x Qalpha / 4πε₀r
* 9.6 x 10^-13 = (79 x 1.60 x 10^-19) x (2 x 1.60 x 10^-19) / 4π x 8.85 x 10^-12 x r
* r = 3.8 x 10^-14 m
* This is a maximum estimate for the radius.
93
What are the two methods of estimating nuclear radius and which is better?
* Closest approach of scattered particle
* Electron diffraction
Electron diffraction gives more accurate values.
94
Why can electron beams be diffracted?
Like other particles, they show wave-particle duality and have a de Broglie wavelength.
They are also used because they are lighter = better to accelerate
95
In order to use electron diffraction to determine nuclear radius, what must the electrons’ energy be and why?
High, because the wavelength must be very small in order for diffraction to be observed due to the tiny nucleus.
96
In order to use electron diffraction to determine nuclear radius, of what order must the electrons’ wavelength be?
10^-15
97
What is the equation for the first minimum on the diffraction pattern caused by high-energy electron diffraction?
sinθ ≃ 1.22λ / 2R
Where:
* θ = Angle from normal (°) or scattering angle
* λ = de Broglie wavelength
* R = Radius of nucleus the electrons have been scattered by (m)
(Note: Not given in exam and can’t be derived!)
Or Rsinθ ≃ 0.61λ
98
Describe how electron diffraction can be used to estimate nuclear radius.
* Beam of high-energy electrons is directed at a thin film in front of a screen
* λ = hc / E
* Diffraction pattern is seen
* Look at the first minimum:
* sinθ = 1.22λ / 2R
99
A beam of 300 MeV electrons is fired at a piece of thin foil, and produces a diffraction pattern on a fluorescent screen. The first minimum of the diffraction pattern is at angle of 30° from the straight-through position. Estimate the radius of the nuclei the electrons were diffracted by.
* E = 300 MeV = 4.8 x 10^-11 J
* λ = hc / E = 6.63 x 10^-34 x 3.00 x 10^8 / 4.8 x 10^-11 = 4.143 x 10^-15 m
* R = 1.22λ / 2sinθ = 1.22 x 4.143 x 10^-15 / 2sin(30) = 5.055 x 10^-15 m = 5 fm
100
Describe the diffraction pattern for a beam of high-energy electrons directed at a thin foil.
Similar to light source shining through circular aperture:
* Central bright maximum (circle)
* Surrounded by other dimmer maxima (rings)
* Intensity of maxima decreases as angle of diffraction increases
This shows intensity for each maximum:
101
What is the approximate radius of an atom?
5 x 10^-11 m
102
Describe the graph of radius of nucleus against nucleon number.
* Starts at origin
* Curve, starting with strep gradient and then becoming shallower
As more nucleons are added, the nucleus gets bigger
103
What do we assume when calculating nuclear radius?
Assume there are no gaps between nucleons.
Assume the nucleons are spherical.
104
What type of nuclei are radioactive?
Unstable nuclei
105
What things can cause a nucleus to be unstable?
* Too many neutrons
* Not enough neutrons
* Too many nucleons altogether
* Too much energy
106
What is radioactive decay?
When an unstable nucleus releases energy and/or particles until it reaches a stable form.
107
Why are radioactive emissions also known as ionising radiation?
When a radioactive particle hits an atom, it can knock off electrons, creating an ion.
108
What does gamma radiation follow and what is it's range?
Gamma radiation has an infinite range and follows an inverse square law
109
Describe how you can investigate the penetrating power of different radiation types.
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 Geiger counter. Record the count rate.
4) Repeat step 2 replacing the paper with 3mm thick aluminium.
5) Count rate - background rate = corrected count rate
6) Look at when the corrected count rate significantly decreased. From this, work out what kind of radiation is emitted.
110
For an alpha particle, describe the ionising power, speed, penetrating power and whether it is affected by a magnetic field.
* Ionising power = Strong
* Speed = Slow
* Penetrating power = Absorbed by paper or a few cm of air
* Affected by magnetic field
111
For a beta-minus particle, describe the ionising power, speed, penetrating power and whether it is affected by a magnetic field.
* Ionising power = Weak
* Speed = Fast
* Penetrating power = Absorbed by 3mm of aluminium
* Affected by magnetic field
112
For a beta-plus particle, describe the ionising power, speed, penetrating power and whether it is affected by a magnetic field.
Annihilated by electron - so virtually 0 range.
113
Q
For a gamma ray, describe the ionising power, speed, penetrating power and whether it is affected by a magnetic field.
* Ionising power = Very weak
* Speed = Speed of light
* Penetrating power = Absorbed by many cm of lead or several m of concrete
* Not affected by magnetic field
114
How can a magnetic field show you the type of radiation?
Charged particles (alpha and beta) are deflected in a circular path.
Beta deflects more because it is lighter.
Positive charge goes one way, negative goes the other.
Curvature of path can show mass and charge
115
How can material thickness by controlled using radiation? (4)
* A material is flattened as it is fed through rollers
* Radioactive source is placed on once side of the material and a radioactive detector is placed on the other
* The thicker the material, the more radiation it absorbs and prevents from reaching the detector
* If too much radiation is being absorbed, the rollers move closer together to make the material thinner (and vice versa)
116
Why do alpha particles not travel very far?
They are strongly positive so quickly ionise many atoms and lose their energy to the atom.
117
Why are alpha particles suitable for use in smoke alarms?
They allow current to flow, but have a short range.
When smoke is present, the alpha particles can’t reach the detector and this sets the alarm off.
118
When are alpha particles dangerous?
When they are ingested, because they cannot penetrate skin, but quickly ionise body tissues, causing damage.
119
Give a use of beta radiation.
Controlling the thickness of a material in production.
120
Compare the speed of alpha and beta particles.
Beta particles are faster
121
Compare the danger between alpha and beta
Beta has lower mass and charge than alpha but can still ionise electrons.
Lower number of interactions (100 atoms per mm compared to 10,000 atoms per mm) means beta causes less damage to body tissues.
122
Why is gamma used in medicine?
| What does it prevent the use of?
Weakly ionising compared to alpha and beta = do less damage to body tissue.
Prevents the need of surgery to help diagnose patients.
123
How can gamma rays be used as a tracer in medicine? (2)
* Radioactive source with a short half-life is injected or eaten by patient
* Detector (e.g. a PET scanner) is then used to detect emitted gamma rays
124
What are some sources of background radiation?
1) The air - radioactive radon gas from rocks (alpha)
2) Ground and buildings
3) Cosmic radiation
4) Living things
5) Man-made radiation - medical e.g. x-rays, internal e.g. food, nuclear waste, fallout, air travel
125
Why is the air a source of background radiation?
* It contains radon gas released from rocks
| * Radon is an alpha emitter
126
Why is the ground and buildings a source of background radiation?
All rock contains radioactive isotopes
127
Why is cosmic radiation a source of background radiation?
* Cosmic rays are particles from space
| * When they collide with the upper atmosphere, they produce nuclear radiation
128
Does the inverse square law apply for all radioactive sources?
Yes
129
How can the inverse square law for radioactive source be applied to safety?
The radioactive source becomes significantly more dangerous the closer you hold it to your body, so keeping a large distance from the source is important
130
What does the graph for corrected count rate from a radioactive source against distance look like?
1/x² graph
131
What does a graph of: Number of unstable nuclei remaining, N, against time, t, look like?
132
What does a graph of ln(Number of unstable nuclei remaining) against time look like?
What is the Y intercept?
133
Describe the stability graph for nuclei. (5)
* Number of neutrons (N) is plotted against number of protons (Z)
* N = Z dotted line is added for reference. It goes diagonally to the top right, through the origin.
* Line of stability starts along the N = Z line, then curves upwards away from it.
* Area above line of stability is β⁻-emitters. It gets gradually wider.
* Area below line of stability is first β⁺-emitters and then α-emitters. It gets gradually wider. The α-emitter area starts at earlier on the lower side of the area.
134
Describe when each type of radioactive emission may occur.
```
* α - Heavy nuclei
* β⁻ - Neutron-rich nuclei
β+ - Proton-rich nuclei
Electron capture - proton-rich nuclei
* γ - Nuclei with too much energy
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