1
Q
What is meant by activity?
A
- The rate of decay of the radioactive nuclei in a given isotope
- It is proportional to the total number of undecayed nuclei in the sample
2
Q
What is activity measured in?
A
Becquerels
3
Q
Alpha decay:
What is it?
How ionising?
Stopped by?
A
- The emission of an alpha particle (2 protons and 2 neutrons) from an unstable nucleus (usually one with too many nucleons) to make it more stable.
- Alpha radiation is strongly ionising and is stopped by a few centimetres of air or a sheet of paper.
4
Q
Beta decay:
What is it?
How ionising?
Stopped by?
A
- The emission of a beta particle when a proton turns into a neutron (or vice versa) in an unstable nucleus.
- Beta minus radiation is weakly ionising.
- stopped by 3mm of aluminium foil but beta plus radiation is immediately annihilated by an electron
5
Q
What is binding energy?
A
The amount of energy required to split a nucleus into all its separate constituent nucleons. It is equivalent to the mass defect.
6
Q
What is a chain reaction?
A
The process by which neutrons released by a fission reaction induce further fissile nuclei to undergo fission.
7
Q
What can closest approach be used for?
A
Estimating a nuclear radius (by firing a alpha particle at it)
8
Q
What is contamination?
A
The introduction of radioactive material to another object. The object is consequently radioactive.
9
Q
What is meant by the critical mass?
A
The minimum mass of fissile material required in a fission reactor for a chain reaction to be sustained.
10
Q
What is electron capture?
A
A process that occurs in proton-heavy nuclei, in which an electron is drawn into the nucleus, causing a proton to transition into a neutron. An electron neutrino is also produced.
11
Q
Describe the process of fission
A
- An unstable nucleus absorbs a thermal neutron
- The nucleus of fuel splits into two smaller daughter nuclei.
Releasing two or three fast moving neutrons, and a lot of energy.
12
Q
What is meant by fusion?
A
The joining of two smaller nuclei to form a larger nucleus and to release energy.
13
Q
Gamma radiation:
What is it?
How ionising?
Stopped by?
A
- The emission of Gamma rays from an unstable nucleus that has too much energy.
- Gamma radiation is very weakly ionising but requires several centimetres of lead to be stopped.
14
Q
Define half-life
A
The average time it takes for the number of radioactive nuclei in a sample to halve
15
Q
What is irradiation?
A
The exposure of an object to radiation. The exposed object does not become radioactive.
16
Q
What is the mass defect?
A
The difference in mass between a nucleus and the sum of the masses of its constituent nucleons.
17
Q
What is radioactive dating? What isotope is commonly used?
A
- The use of radioactive isotopes with known half-lives to date objects
- The isotope that is usually used is Carbon-14
18
Q
What does Rutherford scattering show?
A
- The existence and nature of the nucleus.
- That the nucleus is small, dense and positively charged
19
Q
3 observations and conclusions from Rutherford scattering experiment
A
Most alpha particles passed straight through the foil with no deflection
- atom is mostly empty space (and not a uniform density)
A small amount of particles were deflected by a large angle
- centre of the atom is positively charged
Very few particles were deflected back by more than 90 degrees
- centre of the atom was very dense - charge is concentrated in centre
20
Q
Applications of beta radiation
A
Thickness measurements of paper and aluminium foil
21
Q
Which radiation follows inverse square law?
A
Gamma radiation
22
Q
How to verify inverse square law of gamma radiation?
A
- Measure the count rate of a gamma source at different distances from the GM tube (making sure to adjust for the background radiation)
- then plot a graph of corrected count against 1/x^2 , which will form a straight line verifying the above equation.
23
Q
Danger of radiation if gets in body
A
Can cause mutations and damage to cells (which can lead to cancer etc.)
24
Q
3 sources of background radiation
A
● Radon gas - which is released from rocks
● Cosmic rays - which enter the Earth’s atmosphere from space
● Artificial sources - caused by nuclear weapons testing and nuclear meltdowns
25
How can waste with a long half-life be stored?
Underground in steel casks (to prevent damage to environment and people in future)
26
Decay that happens if nucleus has too many neutrons
Beta minus decay (so that a neutron changes into a proton)
27
Decay that happens if nucleus has too many protons
Beta-plus emission **or** electron capture
28
Decay that happens if nucleus has too many nucleons
Alpha emission
29
What happens if nucleus has too much energy?
Gamma emission (usually occurs after a different type of decay, as nucleus becomes excited)
30
4 reasons technetium-99m is useful in medical diagnosis
- It only emits gamma rays (which are weakly ionising)
- Half-life (6hrs) short enough to not remain in body for too long after the medical examination
- Half-life (6hrs) long enough to complete the diagnosis
- Can be made near to the hospital
31
How to estimate nuclear radius using closest approach
* All kinetic energy is converted into electric potential energy
* (gives an overestimate - doesn't include distance between alpha particle and nucleus, which won't touch)
32
2 ways to calculate nuclear radius
- Distance of closest approach
- Electron diffraction
33
Why is energy released during nuclear fission?
The smaller daughter nuclei have a higher binding energy per nucleon than the original unstable nucleus
34
Why is energy released during nuclear fusion?
The larger nucleus has a **much** higher binding energy per nucleon
35
After what element do elements begin to undergo fission as opposed to fusion? Why?
Iron - it has the highest binding energy per nucleon
36
How to calculate energy released during fission?
Mass defect = Mass before - Mass after
Mass defect * 931.5MeV (or use E = mc^2)
37
How do you induce fission in uranium?
By firing a thermal neutron into the uranium nucleus, making the nucleus extremely unstable
38
What is a thermal neutron?
A slow moving neutron, which low energy
39
What is the purpose of moderators?
To slow neutrons released in fission reaction to thermal speeds, using **elastic collisions**, so that the neutrons are absorbed by the uranium
40
What is the purpose of control rods?
To absorb neutrons in the reactor in order to control chain reactions
41
What is the purpose of coolant (in a nuclear reactor)?
* To absorb the heat released during fission reactions in the core of the reactor
* This heat then makes steam which powers electricity-generating turbines
42
Example materials for moderators, control rods and coolants
- Moderator - Water (inexpensive, not very reactive, similar in mass to neutron), graphite
- Control rods - Boron and Cadmium
- Coolant - Water (high SHC), helium
43
What happens in an emergency shut-down?
The control rods are dropped into the reactor core entirely in order to stop fission reactions from occurring as soon as possible by absorbing all the free neutrons in the core
44
How to safely insert fuel rods into reactor?
**Remotely** to limit the worker’s exposure to radiation
45
What is shielding (in a nuclear reactor)?
**Very thick concrete** shielding around the nuclear reactor, which blocks radiation from escaping from the reactor and affecting the workers in the power station
46
3 safety features at a nuclear reactor
- Remote handling of fuel
- Shielding
- Emergency shut-down
47
What is the source of the most dangerous waste?
Fission fragments from the fission of **spent** uranium-235 fuel rods
48
How is radioactive waste treated?
- Must be placed in cooling ponds for up to five years
- Uranium/plutonium is separated to be recycled
- High level waste is vitrified (made solid) and then placed in thick steel casks **deep** underground
49
Describe the Rutherford scattering experiment
- Beam of **alpha particles** is directed at a **thin gold foil**
- Occurs in a **vacuum** so that no collisions between air particles and alpha particles can occur
- Experiment was done in order to determine **structure of atom**
50
How is electron diffraction used to determine the diameter of an atom?
- electron beam fired at thin sheet of desired atom (only several atoms thick)
- diffraction pattern produced on screen behind
- using the angle of the **first minimum** we can use equations to calculate the diameter
51
Why is there a mass defect?
Binding energy is released when a nucleus is formed. When a nucleus is formed, some of its mass is converted into energy, hence atomic mass is less than mass of constituent particles
51
cm of air to stop alpha particles
4cm
52
cm of air to stop beta particle
40cm
53
How far can gamma rays travel in air?
About 1km
54
Define atomic mass unit
1/12 mass of **atom** of Carbon 12-6
55
Why a high temperature for nuclear fusion?
- Nuclei need to be close together for strong nuclear force to be involved
- But the electrostatic force is repulsive and tries to prevent this
- If temperature is high, then nuclei have high kinetic energy to overcome this repulsion
55
Why a high temperature for nuclear fusion?
- Nuclei need to be close together for strong nuclear force to be involved
- But the electrostatic force is repulsive and tries to prevent this
- If temperature is high, then nuclei have high kinetic energy to overcome this repulsion
56
How do you reduce the power output of nuclear reactor?
**Insert** control rods **further** into the nuclear reactor, so **more** neutrons are absorbed, reducing further fission reactions
57
All nuclei have approximately the same density. State two conclusions about nucleons in the nucleus that can be deduced from this fact
- Nucleons have a constant separation
- Neutrons and protons have similar masses
58
What happens to U-235 and U-238 in nuclear reactors when they absorb neutrons?
- U-235 absorbs thermal neutrons, and splits into two smaller nuclei, giving out neutrons
- U-238 absorbs a neutron, but doesn't undergo fission
59
4 advantages of nuclear power
- Greater energy output per unit mass of fuel (than fossil fuels)
- Little green house gas emissions (so less global warming effects)
- Some (but not all) nuclear power stations can adjust their output quickly
- Nuclear power can be produced continuously (whereas renewables are dependent on sunlight/wind etc)
60
3 safety procedures when dealing with radioactive sources in schools in colleges
- Using **long handled tongs** to move the source (to maximise distance between source and person)
- Stand behind a lead absorber (for shielding)
- Storing the source in a **lead-lined container** when not in use (to limit exposure time)
61
Why is carbon dating unsuitable if a sample is less than 200 years old?
It is difficult to measure accurately the small drop in activity
62
Why is carbon dating unsuitable if a sample is more than 60,000 years old?
The activity would be very small/comparable to the background
63
Define the Avogadro constant
The number of atoms in 12g of carbon-12 (or the number of molecules in one mole of substance)
64
What is the dominant force between two protons?
Coulomb repulsion
65
4 problems with dealing with dangerous waste and how they are dealt with
* Waste is initially very hot and radioactive → placed in cooling ponds
* High level waste may leak in liquid form → vitrified and placed in steel barrels
* the waste will be radioactive for thousands of year → stored somewhere geologically stable (deep underground)
* transporting waste presents a danger to public → processed onsite or nearby
66
How does carbon dating work for fossils?
* All living organisms have the same amount of carbon-14 atoms as a percentage of all carbon isotopes
* Once the organism dies, it no longer absorbs carbon from the atmosphere.
* Carbon-14 is **radioactive** and so will decay over a **known half-life**.
* The **older** a fossil is, the **fewer carbon-14 isotopes** it will contain and the **less radiation** it emits.
67
What happens when U-238 interacts with neutrons in a nuclear reactor?
U-238 absorbs/scatters neutrons
68
Why are spent fuel rods more dangerous than unused fuel rods?
* Unused fuel rods only emit alpha radiation (which is weakly penetrating)
* Spent fuel rods contain fission fragments with a larger proportion of neutrons than nuclei of a similar atomic number
* Making them unstable, and emitters of beta and gamma radiation, which are strongly penetrating
69
Which method of estimating the nuclear radius is more accurate?
* Electron diffraction
* Electrons are leptons, so don't interact with the strong nuclear force (unlike alpha particles)
70
Where does the first minimum occur in electron diffraction?
sin θ = 1.22λ / 2R
(where R is the radius of the nucleus the electron has been scattered by)
71
What is θ in the equation *sin θ = 1.22λ / 2R*?
The angle at which the **first minimum** occurs in electron diffraction.
72
What does the graph of intensity against angle for electron diffraction look like?
Like the single source pattern (centre peak twice the width and much more intense that subsidiary maxima *unless a log graph has been plotted*)
73
1. Radius of a typical atom
2. Radius of a typical nucleus
1. 0.05nm (5E-11m)
2. 1fm (1E-15m)
74
1. Radius of a typical atom
2. Radius of a typical nucleus
1. 0.05nm (5E-11m)
2. 1fm (1E-15m)
75
What word best describes the decay of an individual radioactive nucleus?
Random
76
Which of alpha, beta, gamma radiation are affected by magnetic fields?
Alpha and beta
77
What is likely to happen after an alpha/beta/electron capture decay?
A gamma ray is emitted, as the nucleus often has excess energy
(gamma ray *always* emitted in electron capture)
78
What do you need to draw for rutherford scattering
* Alpha source
* Thin gold foil
* Circular detector
* labelled as a **vacuum**
79
What is the decay constant?
The probability a given nucleus will decay per second
80
3 desired features of materials for a moderator in a fission reactor
* small/light nuclei
* should not become radioactive when bombarded with electrons
* available in large amounts (for low cost)
81
Applications of gamma radiation
* Thickness measurements of steel
* Medical radioactive tracers
82
Describe the location of the line of stability for an N/Z graph
Straight line N=Z until N=20, then curves upwards and becomes steeper
83
If writing an equation for electron capture, what two things should you not forget on the RHS?
* The electron neutrino
* The gamma ray photon released
84
What were Geiger and Marsden expecting to see in the alpha particle scattering experiment?
* The flashes all occurring within a small angle of the beam
* Because the positively-charged alpha particles would be deflected a small amount by the electrons (in the plum pudding model)
85
Why must the **coolant** in a nuclear reactor be a gas or liquid?
So it can be pumped around the reactor (e.g. water!)
86
Why must the **coolant** in a nuclear reactor have a high specific heat capacity?
So it can absorb more heat energy for a small increase in temperature
87
What is the likely initial mode of decay for fission fragments and why?
* Fission fragments have are neutron rich
* So beta minus decay
88
*True or False:* RMS speed = most probable speed
False, RMS speed is greater than most probable speed (peak of distribution)
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