Nuclear Flashcards
Nature of the atom, discovery of the nucleus, Nuclear forces, the liquid drop model, Radioactivity, Decay chains, rate equations, Nuclear reactions, Cross Sections, Nuclear reactors and bombs. (135 cards)
1
Q
Who discovered the electron?
A
JJ Thomson
2
Q
How did JJ Thomson discover the electron?
A
Using cathode rays which he proposed were made up of particles
3
Q
What model of the atom did JJ Thomson propose? Describe it?
A
The Plum Pudding model. It consisted of electrons within positively charged matter
4
Q
What was Ernest Rutherford’s experiment? Describe it?
A
The gold foil experiment. A beam of alpha particles was aimed at a piece of gold foil. Most of the particles passed through the foil but some were scattered backwards.
5
Q
What result in Rutherfords experiment would be rare using Thomson’s model?
A
A significant fraction of the particles were scattered at high angles as if encountering something dense.
6
Q
What model did Rutherford propose? Describe it.
A
The nuclear atomic model. Positive charge in the nucleus, electrons orbit nuclear forming electronic cloud and most of the atoms mass is in the nucleus.
7
Q
What does the differential cross-section measure?
A
The relative intensity of particles being scattered in a given direction
8
Q
What does the total cross-section measure?
A
The total scattering rate
9
Q
What do you need to use to find the size of the nucleus?
A
Particles whose quantum mechanical wavelengths are comparable to the nuclear dimensions.
10
Q
What relation must you use to connect a particles momentum to it’s quantum mechanical wavelength
A
The De Broglie relation
11
Q
In particles how does energy relative to d and what does this mean?
A
Energy increases as d decreases so more energy is needed to see smaller scales
12
Q
What are the probing particles considered when K»mc^2?
A
ultra-relativistic
13
Q
What is the correction to rutherford’s result and why is it there?
A
The form factor because of the finite size of the nucleus. It can be shown that the charge distribution is related to the form factor
14
Q
In the nucleus how does charge distribution vary?
A
It is approximately constant for most of the range but decays near the edge of the nucleus.
15
Q
Who made the discovery of protons and how did they make that discovery?
A
Rutherford. Showed that alpha particles colliding with nitrogen nuclei would sometimes produce hydrogen nuclei, the hydrogen nuclei were protons.
16
Q
Who discovered the neutron and how did they do it?
A
Chadwick. Using alpha particles which collided with certain elements to generate a new kind of radiation, this radiation was actually composed of electrically neutral particles of almost the same mass as protons.
17
Q
What is Z and what does it determine?
A
The atomic number, number of protons. Determines the atomic species.
18
Q
What is A and what does it detrmine?
A
The mass number, the sum of the number of protons and neutrons. Determines the element’s isotope.
19
Q
What are protons and neutrons kept together with in the nucleus?
A
The nuclear force
20
Q
Describe the nuclear force?
A
Small range force which is attractive for large distances and repulsive for short distances resulting in a stable equilibrium point. It is independent of electric charge.
21
Q
What is the mass of a nucleus determined by?
A
Its total energy. The sum of the rest- mass energies of protons and neutrons and the binding energy.
22
Q
What two features does the nucleus have that are found in incompressible fluids?
A
- A constant mass density
- A constant binding energy per particle
23
Q
What does the liquid drop model account for?
A
Nuclear masses
24
Q
Why is the coulomb term used in the liquid drop model?
A
Due to the nucleus having charge which increases the energy and so decreases the binding energy.
25
What is the problem with the formula without the asymmetry term?
If the total number of nucleons is kept constant it predicts that the maximum binding energy is achieved by Z->0, corresponding to a nucleus comprised of only neutrons. This is not found in real atoms.
26
What does the classical liquid drop model not take into account?
That protons and neutrons are fermions and cannot occupy the same state, they are subject to the Pauli exclusion principle.
27
How does asymmetry in protons and neutrons effect the energy?
It increases energy
28
What would happen to energy if neutrons could be turned into protons?
The energy would decrease. So for a fixed nucleon number asymmetry in the nucleons increases energy.
29
What effect does the paring term account for?
The nuclear force binds protons together stronger than to a neutron (the same goes for neutrons) This causes protons to pair up and neutrons to also pair up.
30
What happens to the pairing term when A is odd?
There is an unpaired nucleon and so the pairing term disappears
31
What happens to the pairing term when N and Z are even?
The energy gain is positive
32
WHat happens to the pairing term when N and Z are odd?
The energy gain is negative due to there being one proton-neutron pair.
33
How does the volumetric term in the semi-empirical mass formula depend on A?
It increases linearly with A
34
What would happen if only the volumetric and surface terms were present in the semi-empirical mass formula?
Arbitrarily large nuclei would be energetically favourable, and there would be no upper limit to the size of the nuclei.
35
In the semi-empirical mass formula which term is dependent on both A and Z?
The coulomb term
36
How does the coulomb term depend on A?
The coulomb term grows with A as A^5/3, this increases the modulus more quickly than the volumetric term and will eventually overtake it for A large enough, destabilizing the nucleus.
37
What is the dominance of the coulomb term for a large enough A the result of?
Long range of the electric force contrasted with the short range of the nuclear force.
38
What term in the semi-empirical mass formula works against the coulomb term?
The Asymmetry term
39
How does the asymmetry term depend on A in the semi-empirical mass formula?
Assuming a constant proportion of the number of protons and the neutrons the asymmetry term is proportional to A, and grows like the volumetric term with atomic mass
40
How does the pairing term in the semi-empirical mass formula depend on A?
It decreases with A and so is not important for heavier atoms. It causes small-scale oscillations in the binding energy as the number of protons and neutrons change from even to odd and vice versa.
41
In terms of binding energy when is the most stable state?
When the binding energy is maximised
42
What are the three types of radiation?
Alpha
Beta
Gamma
43
Describe alpha radiation?
The least penetrating of all. Rutherford discovered that it consisted of positively charged high-velocity particles, later revealed to be helium nuclei consisting of two protons and two neutrons.
44
Describe beta radiation?
Medium penetration. Made of high energy electrons
45
Describe gamma radiation?
The most penetrating of all. Consists of very high-energy photons, with frequencies above x-rays
46
Describe an alpha decay?
An atom changes into a different element. (Z,A)->(Z-2,A-4)+(2,4)
47
Describe a nuclear beta decay?
A neutron within the nucleus decays into a proton and a neutron and a neutrino and the atomic number increases by 1.
(Z,a)->(Z+1,A)+e+v
48
What is a neutrino
An electrically neutral particle of very small mass with 1/2 spin. The spin saves the angular momentum conservation in the beta decay.
49
When can a proton undergo the Beta + decay?
When in isolation
50
Describe a Beta + decay?
Process that can happen within the nucleus since the interaction between the nucleons affect the energy balance in the nucleus as a whole.
(Z,A)->(Z-1,A)+e++v
(e+ is a positron)
51
Describe what happens in an electron capture beta decay?
A proton in the nucleus can capture an electron from the electronic cloud and turn it into a neutron
(Z,A)+e->(Z-1,A)+v
52
Describe a gamma decay
Gamma radiation is the emission of high energy photons by the nucleus. Nucleus is initially in an excited state and then decays to a lower energy state.
(Z,A*->(Z,A)^0+ gamma
53
Consider a decay when is the decay energetically possible?
If the mass of the products in smaller than the mass of the original nucleus. The mass difference between the products and the initial nucleus is transformed into kinetic energy of the product particles.
54
Which decay processes try to maximize their binding energy?
Beta decay and Beta + decay
55
In the beta decay valley which masses of an even-even nuclei or and odd-odd nuclei lies on a lower parabola?
Even-even nuclei
56
Describe the correlation using the half-live of an alpha decay?
The lower the energy the longer the decay.
57
What causes the nucleus to zig-zag around the stability line and when does it stop?
Alpha decay can take a nucleus outside the beta stability alley and a Beta decay then take the nucleus back to stability line. It stops when it is stable in both beta and alpha decays
58
What is a radioactive series?
A sequence of alpha and beta decays
59
What are the 4 main radioactive series?
1. 4n series with A=4n
2. 4n+1 series with A=4n+1
3. 4n+2 series with A=4n+2
4. 4n+3 series with A=4n+3
60
What kind of process is the decay of individual atoms in a radioactive sample?
A quantum-mechanical random process whose precise time cannot be predicted
61
What is the intensity of radioactivity?
The number of decays per second in a radioactive sample
62
How is radioactivity measured?
The Geiger counter works by using an inert gas like helium or argon to separate two electrodes kept at high potential difference. Radioactive particles are ionizing, they can knock off atoms because of their high energy. The electrodes detect small currents generated in the gas when an ionization event takes place.
63
What is a multimodal decay?
When a radioactive species decays into two or more species.
64
What happens to the rate equations when an unstable nucleus is being continually produced?
A production term is introduced
65
What happens when the production of the nuclide becomes the same as its rate of decay?
The system reaches an equilibrium
66
What explains why we can find elements with half lives short relative to earths age?
Dur=e to a decay chain having one element decay much slower than the other element.
67
What is a nuclear reaction?
Considering a collision between two nuclei a nuclear reaction is said to occur if there is a rearrangement of the nuclear constituents as a result of the collision
68
What is elestic scattering?
Particles interact for a short while and then part ways without changing their identity
69
What is inelastic scattering?
The incident particle does not change, but the target particle is altered in the scattering process, either by going to an excited state, or breaking up.
70
What is radioactive decay?
The nucleus spontaneously undergoes a transformation, without the need of an external agency.
71
Whats the typical set up for a nuclear reaction?
A nucleus is at rest and is hit by a particle.
72
In nuclear reactions what is the Q value?
The difference in the rest-mass energy before and after a nuclear reaction
73
How do you find the correct condition for a nuclear reaction to be energetically possible?
Must go to the center of mass reference frame where the total momentum is 0.
74
What kind of reactions are those with Q<0
Endothermic
75
What kind of reactions are those with Q>0
Exothermic
76
Why might a nuclear reaction not happen even if it is energetically possible?
Because the particles have to overcome an energy barrier (e.g. Electrostatic repulsion), not a problem for neutrons
77
What is a compound nucleus
The intermediate nucleus formed during a nuclear reaction which then decays into the products
78
What do multiple decay channels mean for a nuclear reaction
A compound nucleus may decay in a number of different ways, each way being called a channel
79
What does Bohr's independence hypothesis state?
That in a nuclear reaction during the existence of the compound nucleus all the neutrons and protons are mixed up randomly.
80
What is the consequence of Bohr's independence hypothesis
The decay from the compound nucleus is independent on its preparation or its initial conditions
81
When are nuclear processes scattering
When the products are the same as the initial nuclei
82
When are nuclear processes radioactivity
If the compound state lasts a long time so that the unstable compound nucleus is easily observable
83
When are nuclear processes a nuclear reaction?
If the compound nucleus is short-lived and not directly observable
84
What is neutron flux?
The number of neutrons per unit area per second
85
What is the mean free path?
The average distance the neutron travels before reacting
86
What kind of reactions can happen for low-energy neutrons?
Exothermic reactions
87
Where does the total cross section have resonance peaks?
Energies where compound nuclei are formed
88
What is the width of resonance peaks determined by?
The average lifetime
89
What needs to happen for the independence hypothesis to be valid?
The lifetime of the compound nucleus must be much longer than the time it would take the incident nucleus to cross the length of the target nucleus
90
What are reactions which violate the independence hypothesis?
Direct relations
91
What do direct relations not have and what is their product distribution like?
Direct reactions whose products are not distributed isotropically as in the compound nucleus reactions. Instead they are concentrated near angles near the direction of the incident particle
92
What is spontaneous fission?
When it is energetically favourable for heavier nuclei to split into two lighter nuclei
93
Describe the energy balance if a spherical nucleus is deformed into an ellipsoidal shape.
It's area increases increasing surface-tension energy. The electrostatic energy decreases because the protons are farther apart from each other compared to the spherical configuration
94
For small A (A<144) describe the energy balance in spontaneous fission
The increase in the surface area dominates and the overall energy increases with deformation. This means that the process is energetically unfavourable and the nucleus is stable to spontaneous fission.
95
Can nuclei with A<144 undergo spontaneous fission?
Yes, through quantum mechanical tunneling through the deformation energy barrier
95
Describe the energy balance in spontaneous fission for A>144
The decrease in electrostatic energy dominates and the energy decreases with the deformation. These nuclei are unstable to spontaneous fission
96
Describe what induced fission is?
A neutron capture by a heavy nucleus usually generates a compound nucleus in an excited state. If the extra energy in the compound state is above the energy required to overcome the deformation energy barrier, nuclear fission becomes possible
97
What does the absorption of a neutron do to an even-odd nucleus and what does this cause?
Turns it into an even-even nucleus. Because of the pairing term this results in a nucleus with a substantially lower energy (greater binding energy) than the original nucleus
98
What are emitted as a by-product in nuclear fission?
Neutrons
99
What will a nuclei do instead of fission after absorbing a neutron?
A nuclei will emit gamma a gamma photon and decay to a lower energy state. Other neutrons will escape through the surface of the material.
100
What does fission efficiency depend on?
The size of the fissile material
101
What is a moderator and how is it used?
A material which has a very low neutron capture cross-section and low A, so that neutrons scatter many times off moderator nuclei and lose energy quickly without being captured. 'Cools down neutrons'
102
What are examples of good moderators?
Graphite (carbon) and heavy water (D2O)
103
How is a nuclear reaction controlled?
q is controlled by inserting into the rods of a substance with high cross section for neutron absorption (control rods). By controlling how much of the rods are inside the reactor we can tune q.
104
What are good control rod materials for a nuclear reactor?
Boron and Cadmium
105
What is a by product of a nuclear reactor?
Plutonium
106
Describe what a fast reactor is?
A fast reactor uses fast fissile materials such as plutonium as a fuel with no moderator. The lack of moderator makes them smaller with higher power density
107
What are some advantages of fast reactors?
- Fast neutron reactors burn all heavy transuranic elements and so reduce total radiotoxicity of nuclear waste.
- They can use all or most of the fuel in the waste.
- They permit nuclear fuels to be bred from almost all heavy elements.
108
Disadvantages of fast reactors
- Fast reactors are costly to build and operate
Due to the low cross sections of most materials at high neutron energies , critical mass is much higher than a thermal reactor.
- Sodium is used as a coolant in fast reactors because it does not moderate neutron speeds much and has a high heat capacity, however it burns and foams in air.
109
What is the source of energy in stars?
Fusion
110
How does nuclear fusion occur in stars?
The high temperatures needed to start fusion are generated when a gas nebula undergoes gravitational collapse, and part of the gravitational potential energy is transformed into heat by friction.
After the reaction starts, it is sustained by the it generates and the gas is kept together by the gravitational force.
111
What does the proton proton chain rely on?
Weak interaction and huge pressures and temperatures
112
What are two principal designs for nuclear reactors and the disadvantage?
1. Inertial : Lasers are used to heat and compress a fuel pellet to the high temperatures required
2. Magnetic: A plasma is maintained at a high temperature in a magnetic field
Disadvantage: Neutron damage to container walls causes relatively large amounts of low level waste.
113
What is a fissile nucleus?
An even- odd nucleus which absorbs a neutron becoming an even-even nucleus. The resultant nuclei has a lower binding energy and so a even neutrons with small energies can induce fission in fissile nuclei. They emit a large quantity of energy in the process and the neutrons they emit causes a chain reaction.
114
Why is it not possible to make a nuclear bomb with natural uranium?
Natural uranium consists of 99% U-238 so neutrons are more likely to interact with U-238. Only a very small fraction of these interactions result in a fission reaction. Most of the interactions lead to inelastic scattering with the neutron losing part of its energy. After the neutron energy drops below 1 MeV neutrons cannot induce a decay in U-238. This results in a very small fraction of neutrons inducing fission in U-238. So for a chain reaction to occur U-235 would need to be enriched.
115
What is a moderator and how does it maintain a reaction?
A moderator is used in a nuclear reactor to prevent the neutrons from being absorbed, maintaining the chain reaction. The moderator has a very low neutron capture cross section and a low A. This means that the neutrons can scatter several times off the moderator without losing energy and being absorbed. If a large enough number of collisions take place then the neutrons will be cooled down to thermal energies.
116
Why is a moderator not needed in a nucler bomb?
Because the chain reaction occurs so quickly the chain reaction does not need to maintained for a long period of time.
117
Why is nuclear fusion only triggered at high temperatures whereas nuclear fission can be triggered at any temperature?
Nuclear fusion requires high temperatures and pressures because the nuclei require enough kinetic energy to overcome the electrostatic repulsion between the nuclei allowing the to fuse. Fission occurs using low energy neutrons and so does not require thermal energy. The energy produced in fission comes from the change in the binding energies for the daughter nuclei.
118
What are delayed neutrons and why do they play a key role in nuclear reactors and not nuclear bombs?
They are neutrons that have been remitted after being absorbed by an isotope such as U-238. They contribute to the fission reaction the nuclear reactor. Because the time the chain reaction takes place is so small for the nuclear bomb the chain reaction is over before any remission can take place.
119
Why is nuclear fusion only observed for light nuclei and fission for heavy nuclei?
Lighter nuclei can undergo fusion due to having a stronger short-range nuclear force in comparison to electrostatic repulsion. Heavier nuclei undergo fusion because they have a weaker strong nuclear force holding them together. So when bombarded with a nucleus the electrostatic force pushes them apart.
120
What feature lead scientists to develop the liquid drop model?
- A constant mass density
- A constant binding energy per particle
121
Why is the coulomb term important? And stable nuclei have more neutrons than protons.
The coulomb term accounts for the electrostatic force in the nucleus. Protons increase the electrostatic force in the nucleus. By having more neutrons the strong nuclear force can be increased without increasing the electrostatic repulsion leading to a more stable nucleus.
122
Why is the semi-empirical mass formula not a good approximation for light nuclei?
There are more prominent quantum effects we need to take into account for light nuclei. Light nuclei can't be considered to have a constant mass density per particle and constant binding energy.
123
How was the existence of the neutrino deduced and discovered?
By energy conservation. For a beta decay if a proton and a neutron are the products the emitted electron would have the same energy. However the energy measurement of the emitted electron showed a continuous spectrum.
124
What two principle observations lead to the discovery of the neutrino?
- Energy conservation: The emitted electron showed a continuous spectrum.
- Angular momentum conservation: Some beta decays seemed to violate the conservation of angular momentum.
125
Discuss the behaviour of alpha radiation?
It is the least penetrating. Rutherford discovered that it consisted of positively charged particles high-velocity particles, later revealed to be helium nuclei consisting of two protons and two neutrons. After an alpha decay an atom changes into a different element.
126
Describe the behavior of beta radiation
Medium penetration. It turned of to be made of high energy electrons. After a beta decay a neutron within the nucleus decays into proton, neutron and neutrino.
127
Describe the behavior of gamma radiation
Highest penetration. It consists of high- energy photons, with frequencies about x-rays. After a gamma decay, neither Z or A changes but the nucleus decays to a lower energy state.
128
Why can a neutron decay into a proton but not the other way around. When can this be possible?
Because the neutron is heavier then the proton. A proton can decay into a neutron during electron capture when a nucleus traps an electron.
129
Why will we have to move away from nuclear reactors and U-235 in the future?
Eventually the supply of U-235 will run out as it is rare. Fast breeder reactors will be used because they use fast fissile materials as a fuel source.
130
What advantage does using heavy water as a moderator have?
Heavy water contains deuterium which works as a more efficient moderator ensuring less neutrons are lost during the moderation process.
131
What is the drift current
The movement of charge carriers within a semiconductor because of an electric field
132
What is diffusion current?
The diffusion of charge carriers within a semiconductor
133
What is carrier mobility?
The drift velocity of the carriers per unit applied electric field
134
