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AQA A Level Physics > 8. Nuclear Physics > Flashcards

8. Nuclear Physics Flashcards

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1
Q

What did the alpha particle scattering experiment enable?

A

The calculation of the size of the nucleus

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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
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3
Q

What was the screen in the scattering experiment made out of?

A

Zinc sulphide

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4
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°
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5
Q

What did the results from the alpha scattering experiment show?

A

The atom must contain a small concentrated positive charge with mass

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6
Q

What charge do alpha particles in the scattering experiment have?

A

Positive

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7
Q

In nuclear physics, what can Coulomb’s law be used to calculate?

A

The distance between two particles when they have an electrostatic force

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8
Q

In the scattering experiment, at what point will an alpha particle scatter back?

A

When its kinetic energy equals its electric potential energy

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9
Q

What law can be used to find the distance between two charged particles?

A

Coulomb’s

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10
Q

What does 1u mean?

A

One atomic mass unit

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11
Q

Does the strong force only affect adjacent nucleons?

A

Yes

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12
Q

Approximately, how many times bigger is the diameter of a uranium atom than its nucleus?

A

23,000 x

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13
Q

Approximately, how many times bigger is the diameter of a hydrogen atom than its nucleus?

A

145,000 x

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14
Q

What does it mean, that radioactive decay is spontaneous?

A

The rate cannot be changed by heating/cooling, dissolving in acid etc.

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15
Q

What will NOT change the rate of radioactive decay?

A
  • heating/cooling
  • dissolving in acid
  • applying pressure
  • applying a magnetic or electric field
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16
Q

Is radioactive decay continuous?

A

No

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17
Q

What happens in alpha decay?

A

A nuclei decays into a new nuclei and emits an alpha particle

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18
Q

What happens in beta decay?

A

A nuclei decays into a new nuclei by changing a neutron into a proton and electron

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19
Q

What happens in gamma decay?

A

After alpha or beta decay, surplus energy is sometimes emitted

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20
Q

Is the atom changed when it emits gamma?

A

No

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21
Q

What are the properties of gamma radiation?

A

High frequency, short wavelength

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22
Q

What is the most ionising type of radiation?

A

Alpha

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23
Q

Why can alpha only travel a few cm in air?

A

It is highly ionising

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24
Q

Why do alpha particles from the same source all travel the same distance in air?

A

They have the same energy if they are from the source, so they travel the same distance before they have lost all their energy

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25
Q

Why do alpha particles ionise air?

A

To gain the electrons they need to become a helium atom

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26
Q

What can alpha radiation be blocked by?

A

A sheet of paper

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27
Q

What can beta radiation be blocked by?

A

A few mm of aluminium

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28
Q

What can gamma radiation be blocked by?

A

A few cm of lead

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29
Q

Why does each beta particle travel a different distance?

A

It has a range of energies

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30
Q

Why can gamma rays travel large distances?

A

They barely interact with air molecules

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31
Q

Why does gamma radiation intensity decrease?

A

They spread out

intensity ↓ as beam area ↑

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32
Q

What equation shows how the intensity of gamma rays varies with distance?

A

I = k / x2

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33
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
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34
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)
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35
Q

How should sources of radiation be stored?

A

In a lead box

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36
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
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37
Q

What are alpha particles used in?

A

Smoke alarms

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38
Q

Why are alpha particles used in smoke alarms?

A

Allow current in air to flow, but don’t travel very far

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39
Q

How do smoke alarms work?

A
  • 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
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40
Q

What is beta radiation used in?

A

Control thickness of sheets of material e.g. paper, Al foil or steel

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41
Q

What is gamma radiation used in?

A
  • radioactive tracers - help diagnose patients without need for surgery
  • treatment of cancerous tumours
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42
Q

What law does gamma follow?

A

Inverse square

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43
Q

What is the activity of a source?

A

The average number of undecayed nuclei which decay per second

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44
Q

If a source has one nucleus decay per second, what is its activity?

A

1 Bq

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45
Q

What is the unit for activity?

A

Bq = Becquerels

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46
Q

What is the symbol for activity?

A

A

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47
Q

What is the decay constant?

A

The probability of a given nucleus decaying in the next second

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48
Q

What is the symbol for the decay constant?

A

λ

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49
Q

What is the equation for activity?

A

A=λN

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50
Q

What does A stand for in A=λN?

A

Activity (Bq)

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51
Q

What does λ stand for in A=λN?

A

Decay constant (s-1)

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52
Q

What does N stand for in A=λN?

A

Number of undecayed nuclei

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53
Q

What is half life?

A

The time taken for half of the radioactive nuclei to decay into other nuclei

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54
Q

Graphically, what does radioactive decay look like?

A

An exponential curve

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55
Q

What are the radioactive decay equations?

A

N = N₀e^-λt

A = A₀e^-λt

56
Q

What does N₀ mean in N = N₀e^-λt?

A

Initial number of radioactive nuclei present

57
Q

What does N mean in N = N₀e^-λt?

A

Number of radioactive nuclei remaining at time t

58
Q

What does A₀ mean in A = A₀e^-λt?

A

The initial activity of the sample

59
Q

What does A mean in A = A₀e^-λt?

A

The activity at time t

60
Q

What is Avogadro’s constant used for?

A

To calculate the number of atoms/nuclei that are present in a known mass of an element

61
Q

What is the equation using Avogadro’s constant?

A

N = mNₐ / M

62
Q

How is the equation for half life (T₁/₂ = ln2/λ)?

A

A = A₀e^-λt

at half life, A = A₀/2

A₀/2 = A₀e^-λt₁/₂

1/2 = e^-λt₁/₂

take natural logs: ln2 = λt₁/₂

63
Q

How does carbon dating work?

A
  • 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
64
Q

What does C-14 decay to in B- decay?

A

N-14, electron and an anti-neutrino

65
Q

What does C-14 decay to in B+ decay?

A

N-16, positron and a neutrino

66
Q

Equation for electron capture of C-14?

A

C-14 + e- → B-14 + neutrino

67
Q

What counts as ‘light’ isotopes?

A

With proton number from 0-20

68
Q

For light isotopes on the N-Z (neutron-proton) graph, what pattern do they follow?

A

Follow the straight line N=Z

69
Q

What happens to stable nuclei as Z number increases beyond about 20?

A

They have more neutrons than protons

70
Q

Why do larger nuclei have more neutrons than protons?

A

Extra neutrons help to bind nucleons together without introducing the repulsive electrostatic forces than protons would

71
Q

What type of nuclei are often alpha emitters?

A

With proton number beyond about 60 (but most with > 80p and 120n)

72
Q

Why are very large nuclei, with more neutrons than protons, often unstable?

A

Strong nuclear force between nucleons is unable to overcome the electrostatic force of repulsion between the protons

73
Q

In a N-Z graph, where do B- emitters lie?

A

To the left of the stability belt

74
Q

Why, on an N-Z graph, do B- emitters lie to the left of the stability belt?

A

These isotopes are neutron rich

75
Q

How do the nuclei to the left of the stability belt on an N-Z graph become more stable?

A

Neutron rich, so they convert a neutron to a proton and electron

76
Q

In a N-Z graph, where do B+ emitters lie?

A

To the right of the stability belt

77
Q

Why, on an N-Z graph, do B+ emitters lie to the left of the stability belt?

A

These isotopes are proton rich

78
Q

How do the nuclei to the right of the stability belt on an N-Z graph become more stable?

A

Proton rich, so they convert a proton to a neutron and positron

79
Q

On an N-Z graph, which region does electron capture take place in?

A

To the right of the stability belt

80
Q

On an N-Z graph, what does electron capture lie in the same region as?

A

B+ emission

81
Q

On an N-Z graph, where does a nucleus that emits an alpha particle move to?

A

Moves diagonally downwards to the left

82
Q

On an N-Z graph, where does a nucleus that emits a B+ particle move to?

A

Moves diagonally upwards, left

83
Q

On an N-Z graph, where does a nucleus that captures an electron move to?

A

Moves diagonally upwards

84
Q

On an N-Z graph, where does a nucleus that emits a B- particle move to?

A

Moves diagonally downwards, right

85
Q

What is the technetium generator used for?

A

In hospitals, to produce a source which emits gamma radiation only

86
Q

What is a metastable state?

A

A long-lived excited state in radioactive nuclei

87
Q

In the technetium generator, how is Tc-99 formed?

A
  • Tc-99 forms in an excited state after alpha/beta emission
  • it stays in the excited state long enough to be separated from its parent isotope
  • decays to ground state by gamma emission
88
Q

What is the half life of Technetium-99?

A

6h

89
Q

Is the Tc-99 used in technetium generators in the ground state?

A

No

90
Q

What state is the Tc-99 used in technetium generators?

A

Metastable

91
Q

What are the uses of Tc-99 in the metastable state?

A

Diagnosis;

  • monitoring blood flow
  • gamma camera - image internal organs and bones
92
Q

When might an unstable nucleus emit gamma radiation?

A

When the ‘daughter’ nuclei is formed in an excited state after it emits an alpha or beta particle or undergoes electron capture

93
Q

What is binding energy?

A

The energy required to separate an atom into its constituent parts

94
Q

How can it be shown that Ca has binding energy?

A
  • 20p, 20e, 20n
  • total mass of protons, neutrons and electrons is 40.34u
  • however Ca has a mass of 39.96u
  • use E=mc² to calculate binding energy
95
Q

How would you calculate the binding energy of an atom?

A
  • 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
96
Q

What does a graph of binding energy per nucleon against nucleon number reveal?

A

The stability of the elements

97
Q

On a graph of binding energy per nucleon against nucleon number, which are the most stable elements?

A

Those with a nucleon number around 56

98
Q

Which type of elements release energy from fusion versus fission?

A
  • smaller nucleon number - fusion

* high nucleon number - fission

99
Q

Why do we know energy is released in fusion?

A

Binding energy per nucleon increases. Mass defect is greater. Energy has been released

100
Q

Why do we know energy is released in fission?

A

As a heavy nucleus split binding energy of each fragment is greater. Mass defect is greater therefore energy has been released

101
Q

What is fusion?

A

The process by which light nuclei join together forming heavier nuclei

102
Q

Where does fusion happen?

A

In stars

103
Q

What temperatures are required for fusion?

A

Above 8 million K

104
Q

At 8 million K for fusion, how will positive nuclei be?

A

In a plasma, moving at very high speeds

105
Q

When, in fusion, will nuclei fuse?

A

When they overcome the electrostatic repulsion

106
Q

What happens, in fusion, after nuclei overcome the electrostatic repulsion?

A

The strong nuclear force holds them together

107
Q

What is induced nuclear fission?

A

The process by which energy is released when a radioactive isotope is forced to split

108
Q

What is used in induced nuclear fission and why?

A

Uranium 235 - long half life and abundance mean it is found in large quantities

109
Q

How is nuclear fission undergone?

A

The radioactive nucleus absorbs a slow neutron, causing it to become unstable and split

110
Q

Why is energy released in induced nuclear fission?

A

Due to change in mass

111
Q

What does the chain reaction that is nuclear fission consist of?

A
  • when a nucleus is split, more neutrons are released
  • these can then split other uranium nuclei
  • the process keeps going
112
Q

In induced nuclear fission, why do neutrons need to be slowed down?

A

Or they will bounce off the (uranium) nuclei

113
Q

In induced nuclear fission, what are neutrons slowed down using?

A

A moderator

114
Q

In induced nuclear fission, why do extra neutrons need to be absorbed?

A

So the reaction stays at a constant rate

115
Q

In induced nuclear fission, how are extra neutrons absorbed?

A

Using control rods

116
Q

What is the critical mass of a fuel?

A

The minimum mass required to establish a self-sustaining chain reaction

117
Q

What does the reactor core contain?

A
  • fuel rods
  • control rods
  • coolant
118
Q

What is the coolant in a nuclear reactor?

A

Water at high pressure

119
Q

What is the reactor core connected to in a nuclear reactor?

A

A heat exchanger, via steel pipes

120
Q

What is function of the control rods?

A

To absorb neutrons

121
Q

What does the depth of the control rods control?

A

The number of neutrons in the core

122
Q

What happens if the control rods are pushed in further?

A

They absorb more neutrons so that the number of fission events per second is reduced

123
Q

What condition must be true, in a nuclear reactor, for a chain reaction to occur?

A

The mass of the fissile material (e.g. U-235) must be greater than a minimum mass (the critical mass)

124
Q

Why does the mass of the fissile material need to be greater than the critical mass for a chain reaction to occur?

A
  • 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
125
Q

What are the safety features of nuclear reactors?

A
  • 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
126
Q

How is the reactor core being a thick steel vessel a safety feature?

A
  • to withstand high pressure and temperatures in the core

* absorbs beta emission and some gamma radiation and neutrons from the core

127
Q

How is the reactor core being in a building with thick concrete walls a safety feature?

A

Absorb neutrons and gamma radiation that escape from the reactor vessel

128
Q

How is every reactor having an emergency shut-down system a safety feature?

A

Control rods are inserted completely into the core to stop fission when needs be

129
Q

How is radioactive waste categorised?

A

High, intermediate or low level, depending on its activity

130
Q

Example of high level radioactive waste?

A

Spent fuel rods

131
Q

How are spend fuel rods stored (high level waste)?

A
  • stored underwater in cooling ponds for a year as they continue to release heat
  • then stored in sealed containers in deep trenches in Sellafield
132
Q

How is intermediate level radioactive waste stored?

A

Sealed in drums that are encased in concrete then stored in special buildings with walls of reinforced concrete

133
Q

How is low level radioactive waste stored?

A

Sealed in metal drums and buried in large trenches

134
Q

Examples of low level radioactive waste?

A

Lab equipment and protective clothing

135
Q

What does 1 u equal in MeV?

A

931.5 MeV

AQA A Level Physics (10 decks)

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