Nuclear Physics Flashcards
1
Q
What did John dalton say about particles ?
A
All matter was made up of fundamental particles called atoms. All atoms of a substance would be identical and would be different to the atoms of another substance
2
Q
What did jj Thomson suggest about the atom ?
A
The plum pudding model - lead by the discovery of electrons .
Jj Thomson suggested that the atom was a ball of positive charge with negative electrons embedded in it
3
Q
Who came up with the nuclear model ?
A
Rutherford
4
Q
What did the nuclear model developed by Rutherford portray ?
A
Atoms have a small dense positively charged nucleus with electrons orbiting the nucleus
5
Q
Who suggested that the electrons should be orbiting at specific distances (shells or energy levels ) around the nucleus?
A
Neils Bohr
6
Q
What did James Chadwick discover ?
A
Neutrons
7
Q
What is the plum pudding model ?
A
A ball of positive charge with negatively charged electrons embedded in it
8
Q
What was the conclusion from the Rutherford’s alpha scattering experiment ?
A
An experiment that led to the conclusion that the atoms consisted of small dense positively charged nucleus
9
Q
If you were to draw the alpha scattering experiment you would have :
A
- an alpha source
- a thin sheet of gold foil
- movable detector around the surface
- the are vacuumed
10
Q
What variables had to be controlled during the experiment?
A
- the speed of the alpha particle (different speeds cause different deflections )
- must be experimented within a vacuum chamber
- alpha source should have a long half life
11
Q
What were the observations of the Rutherford experiment?
A
-small / no / large deflection
- few alpha particles had been reflected by more than 90 degrees
12
Q
What did most of the alpha particle passing straight through with no deflection tell us ?
A
That most of the atom is empty space
13
Q
What did some alpha particle deflecting at small angles tell us ?
A
The nucleus had a large positive charge (only some passes through so the nucleus was small )
14
Q
What did a few alpha particles being deflected at angles greater than 90 tell us ?
A
The nucleus was small and that most of the mass and positive charge was concentrated in the nucleus
15
Q
Why should the gold foil sheet be very thin ?
A
- the alpha particle must not be absorbed by the foil and must only be scattered once
16
Q
Why should the air be removed from the rutherfords experiment?
A
To prevent absorption / interaction of the alpha particles by air molecules
17
Q
What is the force responsible for the deflection of alpha particles ?
A
Electrostatic force
18
Q
What is the average radius of an atom ?
A
10 ^-10m
19
Q
What is the radius of the average nucleus ?
A
10^-14 m
20
Q
What are the two ways to estimate the nuclear radius ?
A
Distance of closest approach and electron diffraction
21
Q
What is the distance to closest approach method to work out the radius ?
A
- alpha particles are with an initial energy of Ek are fired at the gold nucleus / sheet
- as they travel closer to the nucleus they feel a force of repulsion which increases converting the Ek to Ep
- when all of the Ek is converted to Ep and comes to rest it is known as the distance to closest approach
22
Q
How do you work out the distance of the closest approach ?
A
Initial Ek = Ep = kQ/r
If you know what the initial kinetic energy is you can sub in the values :
r = k Qq/ Ek
Providing an estimate of the nuclear radius
23
Q
Advantages of the distance of closest approach method to calculate the radius ?
A
- provides a good estimate of the upper limit of the nuclear radius
24
Q
What are disadvantages of the distance of closest approach method to calculate the radius of the nucleus ?
A
- doesn’t provide an accurate value for the radius
- you can increase the Ek and if they get to close to the nucleus they can get affected by the strong nuclear force
- alpha particles must have the same kinetic energy at the start
25
What is the electron diffraction method ?
- electrons are accelerated through high potential difference to provide a high speed, showing wave like properties ( de brogie wavelength )
- they accelerated onto a thin sheet of foil diffracting , due to wave like properties
- circular fringes of bright and dark circles
- sin@ = 1.22 wavelength / d
26
What is the equation to estimate the nuclear radius via electron diffraction ?
Sin@ = 0.61wavelength / r
@ = centre to the first minima
27
Advantages of electron diffraction to estimate the nuclear radius ?
- an accurate method
- electrons are leptons so unaffected by the strong force
28
Disadvantages of electron diffraction to estimate the nuclear radius ?
The Ek need to be high to produce a small debrogie wavelength to similar in size to the nuclear diameter
29
What is the relationship between the nuclear radius and the nucleon number ?
R is directly proportional to the cubed root of the nucleon number
30
What is the gradient produced when r is plotted against cube root of the nucleon number ?
R0 - constant
31
What is the y intercept and the gradient of ln R plotted against lnA ?
Gradient = 1/3
Intercept = Ln R0
32
What is the equation for density (nucleus) ?
3*M(nucleons) / 4*pi * R0 ^3
33
How do you get the equation for the density of a nucleus ?
By p= m(A*nucleon ) / v ( 4/3 pi r(R0 *A^1/3 ) ^3 )
34
How to convert MeV to joules ?
MeV *10^6 = ev
Ev *1.6*10^-19
35
What is an alpha particle ?
- helium nucleus (2 protons and neutrons )
36
What are properties of alpha particles ?
- Most ionising
- highest charge
- absorbed easily so a low range
- Least penetrating
37
What are applications of alpha particles ?
An alpha particles is emitted ionising the air particles - the negatively charged ions are separated
As a result of the electric field electrons move towards the negative plates passing a current
When smoke particles passes through the current decreases triggering the alarm
38
What kind of long life is required for alpha particles within a smoke detector?
Long
39
What are beta minus particles ?
High speed electrons emitted from the nucleus
40
What is a beta minus particle ?
High speed positron emitted from the nucleus
41
What are properties of beta particles?
- moderately ionising and penetrating
- stopped by a thin sheet of foil
42
Applications of beta particles ?
To Fermin the thickness of foil or paper
- the same amount of beta particles emitted must be close to the same amount to have a uniform thickness
43
What is gamma radiation ?
High energy electromagnetic wave - the excess energy released after a decay
44
Properties of gamma radiation ?
Least ionising and most penetrating
- stopped by lead or concrete walls
45
What are applications of gamma radiation ?
Detect leaks within pipes by placing a source within the supply
- sterilising equipment
- tracers to monitor what occurs in the body
- radio therapy
46
What kind of half life would you want gamma radiation to have when used for tracers and radio therapy ?
Short - decays quickly so doesn’t remain in the body for long periods of time
47
How do ionising chambers ?
Cathode is connected to the negative terminal with the anode connected to the positive terminal. When ionizing chambers enter the gas chamber colliding into the atoms of the gas , freeing electron.
When there is an electric field the negative electrons are attracted to the positive anode producing a current. Every ionisation produces an electron measured as the current
48
How do gm tubes work?
Similarly to the ionisation chamber but there would be a anode rod through the centre and the wall of the tube acting as the cathode
49
What is background radiation?
The low level radiation in the environment due to natural artificial sources
50
Examples of background radiation ?
- radon gas
- ground and buildings
- cosmic radiation
- living things
- man made radiation
51
How can you eliminate background radiation from experimental results ?
- measure the background radiation count rate without the source by taking 3 readings and producing an average subtracting it for the experimental results
52
What is the relationship between the distance and the intensity of gamma radiation ?
I= k / r^2
53
What is the equation to work out the intensity at a specific distance given that you have a distance and its intensity?
I1r1^2 =K = I2r2 ^2
54
How can you safely handle radioactive sources ?
-smaller amounts of radioactive source
- limit exposure to radiation
- add a shielding between between the source and you
- keep the source shielded when not in use
- wear goggles / gloves
- use tongs
55
What is radioactive decay ?
The process by which unstable isotopes emits radiation to become more stable.
56
What are radioisotopes ?
The isotopes that undergo radioactive decay
57
What type of process is radioactive decay ?
Random and spontaneous
58
What is meant by random process in terms of decay ?
It is impossible to predict when an atone will decay and what atom will decay
59
What is meant by spontaneous?
The rate can’t be influenced can only be estimated
60
What is activity ?
The number of nuclei that decay per second (measured in becquerel) Bq or s^-1
61
What is half life ?
The time taken for the number of nuclei of the isotopes to decrease to half the initial number. Measures in seconds , minutes , years
62
If the number of unstable nuclei increase what happens to the activity ?
The activity would increase
63
What is the relationship between the activity and number of unstable nuclei ?
A = lambda N
Lambda is the decay constant measured in s^-1
64
What is the decay constant ?
The fraction of the total number of nuclei decaying per unit time
65
What is the exponential decay equation ?
N = N0 * e ^ - ŷt
66
What does N0 stand for in the decay equation?
The initial number of undeclared nuclei
67
What does ŷ stand for in the exponential decay equation ?
Decay constant
68
What does N stand for in the decay equation ?
The number of unstable nuclei
69
What can be substituted instead of the number of nuclei and the initial number of unstable nuclei ?
- The activity and the initial activity
- the mass and the initial mass of the sample
70
What is the molar mass ?
Mass of one mole of a substance (can be found in the mass number )
Radium- 226 means there 266g of a substance in a mole
71
What does avargados constant tell you ?
The number of atoms within a mole which is 6.02*10^23 atoms
72
How can you work out the decay constant given that you have to mass number and the mass of the substance and the activity ?
( Mass of substance / mass number ) - = number of moles
Number of moles * avargados constant = atom number
Activity / number of atoms = decay constant
73
How do you work out the number of nuclei within a sample ( decay ) ?
Mass of substance / mass number * 6.02*10^23
74
How can you determine the age of a decaying substance ?
Using the exponential decay equation with the activity and initial activity values in replace of the number of nuclei
75
What how can you work out the half live using the decay constant ?
Ln2 / ŷ
76
How do you work out the half life using an exponential graph with the number of nuclei against time ?
Find the time at a value of half N
77
How do you work out the half life using a log graph with the LnN against time ?
- take ln of the decay equation
- the gradient will give the decay constant and the y intercept gives you lnN0
- ln2 / decay constant =half life
78
Applications of Tc99
Tracers - short half life
79
What are applications of carbon 14 ?
Date organic objects by measuring the amount of carbon 14 and comparing it to the initial amount
80
What does the N- Z graph tell you about the nuclear stability for light nuclei ?
When Z is less than 20 N= Z
81
As the proton number increases what happens to the number of neutrons ?
The number of neutrons is high in proportion to the proton to provide a strong force to hold the nucleus together
82
Why is a gamma photon produced ?
Following an alpha or beta decay the daughter nucleus may be formed in an exited state . When the daughter nucleus de excites and moves to the ground state energy is released in the form of gamma ray photons
83
What did Einstein say ?
Mass and energy are interchangeable. E= mc^2
If an object loses energy there will be a decrease in the mass
And if the energy increases the mass will also increase
84
What is the units to measure mass difference ?
Atomic unit mass
85
What is the mass of the nucleus lower than the mass of its constituent particles ?
The mass had been converted into energy which is released ( mass defect )
86
What is the binding energy ?
The binding energy is energy released when the nucleons join to form the nucleus as mass is converted into energy
Or the energy required to separate the nucleus into separate nucleons
87
What is the atomic mass unit ( u )
1/12th of the mass of an atom of carbon 12
88
How to convert atomic mass unit to kg ?
Multiply by 1.661*10^-27
89
What is the mass deflect ?
The difference in mass between the mass of a nucleus and its constituent nucleons .
90
How can you calculate the binding energy ?
The mass defect multiplied by the speed of light squared
91
What does a higher binding energy tell us about the stability ?
It is more stable as more energy is required to separate the constituents
92
What is nuclear fission ?
The splitting of large unstable nucleus into smaller daughter nuclei releasing 2 or 3 neutrons and lots of energy
93
What is nuclear fusion ?
When two or more smaller nuclei join to form a larger nucleus releasing lots of energy
94
Why is energy released in fission and fusion?
The daughter nucleus has a higher binding energy per nucleon - energy is released as mass is lost and mass difference is converted to energy
95
What is the most stable include ?
Fe 56
96
What process to nuclides greater than fe 56 undergo ?
Fission
97
What process do nuclides smaller than fe56 undergo ?
Fusion
98
Why does fusion require extremely high temperatures?
Two positively charged objects naturally repel each other so they require enough kinetic energy to overcome the electrostatic repulsion allowing the strong nuclear force to take over
99
Why is the energy released per nucleon from fusion much greater than from fusion ?
Energy is released when the binding energy is increased. The binding energy for fusion per nucleon is greater that fission . Larger gradient at small values of A compared to the gradient at large values of A.
100
How can fission be induced in nuclei such as uranium 235
By firing a thermal neutron at it . This is then captured/ absorbed by the nucleus making it even more unstable causing it to split into smaller nuclei along with a few neutrons.
These neutrons can then further cause fission reactions forming a chain reaction
101
102
What are thermal neutrons ?
Low energy neutrons - must have the right amount of energy to be captured
103
What is the critical mass ?
The minimum mass of fuel required to maintain a steady chain reaction
104
What are fuel rods ?
They contain fuel such as uranium 235
105
What is the purpose of the control rods ?
Control the rate of fission by absorbing excessive neutrons
106
How is the number of neutrons absorbed by the control rods controlled ?
Varying the height :
Lowering - decrease the rate of
Raising - increases the rate of fission
107
What is a moderator ?
To slow down the neutrons released in fission. Through collisions between the moderator and the neutrons ( elastic collisions )
108
Why should the moderator atoms to be a similar size to a neutron ?
The larger the proportion of momentum is transferred slowing down the neutrons
109
What is a typical moderator ?
Water as the hydrogen has a similar size to that of a neutron and is inexpensive
110
What is the purpose of the coolant ?
To absorb and transfer the heat released by the fission reactions to carry them to the water
