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A LEVEL - Physics > Section 12 - Nuclear Physics > Flashcards

Section 12 - Nuclear Physics Flashcards

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1
Q

Describe how ideas atoms have changed over time.

A
  • 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
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2
Q

What was the original model for atom structure?

A

Plum pudding model

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3
Q

Describe the plum pudding model.

A

Atoms are made of positive charge with electrons stuck in them like plum pudding.

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4
Q

Who suggested an alternative to the plum pudding model?

A

Rutherford (and Marsden)

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5
Q

Which experiment showed the existence of a nucleus in atoms?

A

Rutherford scattering

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6
Q

Describe the Rutherford scattering experiment.

A
  • Beam of alpha particles is fired at thin gold foil
  • Circular 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°)
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7
Q

If the plum pudding model of atomic structure were true, what would you expect to see in the Rutherford scattering experiment?

A

The alpha particles would be deflected by a small amount by the electrons.

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8
Q

Describe the main conclusions of the Rutherford scattering experiment.

A

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
• Most of the mass must be in the nucleus, since positively-charged alpha particles were repelled and deflected by a large angle

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9
Q

What does the Rutherford scattering experiment tell us about the empty space in the atom?

A

Most of the atom must be empty space, since most of the alpha particles passed straight through the foil

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10
Q

What does the Rutherford scattering experiment tell us about the charge of the nucleus?

A

Nucleus must have a large positive charge, since positively-charged alpha particles were repelled and deflected by a large angle

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11
Q

What does the Rutherford scattering experiment tell us about the size of the nucleus?

A

The nucleus is small, since most of the alpha particles passed straight through the foil

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12
Q

What does the Rutherford scattering experiment tell us about the distribution of mass in the atom?

A

Most of the mass must be in the nucleus, since positively-charged alpha particles were repelled and deflected by a large angle

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13
Q

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?

A

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)

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14
Q

Describe how you can estimate the closest approach of a scattered particle to a nucleus, given the initial kinetic energy.

A
  • Equate the initial kinetic energy that the particle was fired with with the potential energy of the particle at the turning point
  • Initial kinetic energy = Electric potential energy
  • Ek = Qgold x Qalpha / 4πε₀r
  • Calculate r
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15
Q

Give the equation used to find the closest approach of an alpha particle to the a gold nucleus.

A

Ek = Qgold x Qalpha / 4πε₀r

Where:
• Ek = Kinetic energy (J)
• Qgold = Charge of the gold nucleus (C)
• Qalpha = Charge of the alpha particle (C)
• ε₀ = 8.85 x 10^-12 F/m
• r = Distance from centre of nucleus (m)

(NOTE: Not given in exam)

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16
Q

What is the charge of a nucleus?

A

+Ze

Where:
• Z = Proton number
• e = Size of charge of an electron

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17
Q

How can the radius of a nucleus be estimated using scattered particles?

A
  • Calculate an estimate for the closest approach of an alpha particle to the nucleus
  • This is the maximum possible radius
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18
Q

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.

A
  • 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.
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19
Q

What are the two methods of estimating nuclear radius and which is better?

A
  • Closest approach of scattered particle
  • Electron diffraction

Electron diffraction gives more accurate values.

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20
Q

Why are electrons used to estimate nuclear radius?

A

They are leptons, so they do not interact with the strong nuclear force.

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21
Q

Why can electron beams be diffracted?

A

They show wave-particle duality and have a de Broglie wavelength.

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22
Q

What is the equation for the de Broglie wavelength of electrons AT HIGH SPEEDS?

A

λ = hc / E

Where:
• λ = de Broglie wavelength (m)
• h = Planck constant = 6.63 x 10^-34
• c = Speed of light in a vacuum (m/s)
• E = Electron energy (J)

(Note: Not given in exam, but can be derived!)

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23
Q

Derive the equation for the de Broglie wavelength of electrons at high speeds.

A
  • The speed of high-energy electrons is almost the speed of light, c.
  • So λ = h / mv = h / mc
  • Since E = mc²:
  • λ = hc / E
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24
Q

In order to use electron diffraction to determine nuclear radius, what must the electrons’ energy be and why?

A

High, because the wavelength must be very small in order for diffraction to be observed due to the tiny nucleus.

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25
Q

In order to use electron diffraction to determine nuclear radius, of what order must the electrons’ wavelength be?

A

10^-15

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26
Q

When a beam of high-energy electrons is directed onto a thin film of material, what is seen?

A

A diffraction pattern on a screen behind it.

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27
Q

What is the equation for the first minimum on the diffraction pattern caused by high-energy electron diffraction?

A

sinθ = 1.22λ / 2R

Where:
• θ = Angle from normal (°)
• 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!)

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28
Q

Describe how electron diffraction can be used to estimate nuclear radius.

A
  • 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
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29
Q

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.

A
  • 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
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30
Q

Describe the diffraction pattern for a beam of high-energy electrons directed at a thin foil.

A

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

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31
Q

Remember to practise drawing out the graph for relative intensity against the angle of diffraction for electron diffraction.

A

Pg 156 of revision guide

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32
Q

What is the approximate radius of an atom?

A

0.05nm

5 x 10^-11 m

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33
Q

What is the radius of the smallest nucleus?

A

1fm

1 x 10^-15 m

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34
Q

What are nucleons?

A

Protons and neutrons

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35
Q

What is the symbol for nucleon number?

A

A

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36
Q

Describe the graph of radius of nucleus against nucleon number.

A
  • Starts at origin
  • Curve, starting with strep gradient and then becoming shallower

(See diagram pg 157 of revision guide)

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37
Q

What equation relates nucleon number to atomic radius?

A

R = R₀A^1/3

Where:
• R = Radius of nucleus
• R₀ = Constant = 1.4fm
• A = Nucleon number

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38
Q

How can the relationship between radius of nucleus and nucleon number be demonstrated?

A
  • Plot R against A^-1/3
  • This gives a straight line
  • So R ∝ A^-1/3
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39
Q

Describe the graph of R (radius of nucleus) against A^1/3 (nucleon number).

A
  • Straight line with positive gradient
  • Goes through origin

(See diagram pg 157 of revision guide)

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40
Q

In R = R₀A^1/3, what is the value of R₀?

A

About 1.4fm

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41
Q

Relatively speaking, what is the density of the nucleus like?

A

Huge

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42
Q

How does the volume of protons and neutrons compare?

A

It is about the same.

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43
Q

Do different nuclei have the same density?

A

Yes

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44
Q

Derive the equation for the density of a nucleus.

A
  • p = mass / volume
  • p = A x m(nucleon) / (4/3 x πR³)
  • p = A x m(nucleon) / (4/3 x (R₀A^1/3)³)
  • p = 3m(nucleon) / 4πR₀³ = Constant
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45
Q

What is the equation for the density of a nucleus?

A

p = 3m(nucleon) / 4πR₀³ = Constant

Where:
• p = Density (kg/m³)
• m(nucleon) = Mass of a nucleon
• R₀ = Constant = 1.4fm

(Note: Not given in exam!)

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46
Q

What is the value of R₀?

A

1.4fm

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47
Q

What is the value for nuclear density?

A

1.45 x 10^17 kg/m³

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48
Q

What type of nuclei are radioactive?

A

Unstable nuclei

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49
Q

What things can cause a nucleus to be unstable?

A
  • Too many neutrons
  • Not enough neutrons
  • Too many nucleons altogether
  • Too much energy
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50
Q

What is radioactive decay?

A

When an unstable nucleus releases energy and/or particles until it reaches a stable form.

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51
Q

Why are radioactive emissions also known as ionising radiation?

A

When a radioactive particle hits an atom, it can knock off electrons, creating an ion.

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52
Q

Is radioactive predictable?

A

No, it is random.

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53
Q

What are the 4 types of radioactive decay?

A
  • Alpha
  • Beta minus
  • Beta plus
  • Gamma
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54
Q

What makes up alpha radiation?

A

2 protons and 2 neutrons (helium nucleus)

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55
Q

What makes up beta-minus radiation?

A

Electron

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56
Q

What makes up beta-plus radiation?

A

Positron

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57
Q

What makes up gamma radiation?

A

Short-wavelength, high-frequency EM waves

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58
Q

What is the charge on an alpha particle?

A

+2

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59
Q

What is the charge on a beta-minus particle?

A

-1

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60
Q

What is the charge on a beta-plus particle?

A

+1

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61
Q

What is the charge on gamma radiation?

A

0

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62
Q

What is the mass of an alpha particle (in atomic mass units)?

A

4

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63
Q

What is the mass of an beta-minus particle (in atomic mass units)?

A

Negligible

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64
Q

What is the mass of an beta-plus particle (in atomic mass units)?

A

Negligible

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65
Q

What is the mass of an gamma radiation (in atomic mass units)?

A

0

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66
Q

What stops alpha radiation?

A

Paper

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67
Q

What stops beta-minus radiation?

A

3mm aluminium

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68
Q

What stops gamma radiation?

A
  • Many cm of lead

* Several m of concrete

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69
Q

Describe how you can investigate the penetrating power of different radiation types.

A

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) Look at when the count rate significantly decreased. From this, work out what kind of radiation is emitted.

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70
Q

For an alpha particle, describe the ionising power, speed, penetrating power and whether it is affected by a magnetic field.

A
  • Ionising power = Strong
  • Speed = Slow
  • Penetrating power = Absorbed by paper or a few cm of air
  • Affected by magnetic field
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71
Q

For a beta-minus particle, describe the ionising power, speed, penetrating power and whether it is affected by a magnetic field.

A
  • Ionising power = Weak
  • Speed = Fast
  • Penetrating power = Absorbed by 3mm of aluminium
  • Affected by magnetic field
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72
Q

For a beta-plus particle, describe the ionising power, speed, penetrating power and whether it is affected by a magnetic field.

A

Annihilated by electron - so virtually 0 range.

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73
Q

For a gamma ray, describe the ionising power, speed, penetrating power and whether it is affected by a magnetic field.

A
  • 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
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74
Q

How can material thickness by controlled using radiation?

A
  • 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)
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75
Q

Give a use of alpha particles.

A

Smoke alarms

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76
Q

Why do alpha particles not travel very far?

A

They quickly ionise many atoms and lose their energy.

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77
Q

Why are alpha particles suitable for use in smoke alarms?

A

They allow current to flow, but have a short range.

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78
Q

When are alpha particles dangerous?

A

When they are ingested, because they cannot penetrate skin, but quickly ionise body tissues, causing damage.

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79
Q

Give a use of beta radiation.

A

Controlling the thickness of a material in production.

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80
Q

Compare the speed of alpha and beta particles.

A

Beta particles are faster

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81
Q

Compare the number of ionisations per mm in air for alpha and beta particles.

A
  • Alpha - 10,000 ionisations per mm

* Beta - 100 ionisations per nm

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82
Q

What are some uses of gamma rays?

A
  • Radioactive tracers

* Treatment of cancerous tumours

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83
Q

How can gamma rays be used as a tracer in medicine?

A
  • Radioactive source with a short half-life is injected or eaten by patient
  • Detector is then used to detect emitted gamma rays
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84
Q

How can gamma rays be used to treat cancerous tumours in medicine?

A
  • Rotating beam of gamma rays is used to kill tumour cells

* This lessens the effect of the radiation on healthy cells

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85
Q

What are some short and long term effects of exposure to gamma radiation?

A 
SHORT:
• Tiredness
• Reddening of skin
• Soreness of skin
LONG:
• Infertility
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86
Q

In experiments, how is background radiation accounted for?

A

Measure background radiation separately and subtract it from your measurements.

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87
Q

What are some sources of background radiation?

A

1) The air
2) Ground and buildings
3) Cosmic radiation
4) Living things
5) Man-made radiation

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88
Q

Why is the air a source of background radiation?

A
  • It contains radon gas released from rocks

* Radon is an alpha emitter

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89
Q

Why is the ground and buildings a source of background radiation?

A

All rock contains radioactive isotopes

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90
Q

Why is cosmic radiation a source of background radiation?

A
  • Cosmic rays are particles from space

* When they collide with the upper atmosphere, they produce nuclear radiation

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91
Q

Why are living things a source of background radiation?

A
  • All plants and animals may contain C14

* They also contain other radioactive materials

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92
Q

Why is man-made radiation a source of background radiation?

A

Medical and industrial sources give off some radiation.

93
Q

Why type of radiation does radon gas emit?

A

Alpha particles

94
Q

What are cosmic rays?

A

Particles (mostly high-energy protons) from space

95
Q

How does the intensity of gamma radiation change with distance from the source?

A
  • It decreases by the square of the distance from the source

* I = k / x²

96
Q

What is the equation for the intensity of gamma radiation at a given distance from the source?

A

I = k / x²

Where:
• I = Intensity (counts/sec)
• k = Constant
• x = Distance from the source (m)

97
Q

What sort of equation is the equation that relates the intensity of gamma radiation at a given distance from the source?

A

Inverse square law

98
Q

Does the inverse square law apply for all radioactive sources?

A

Yes

99
Q

How can the inverse square law for radioactive source be applied to safety?

A

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.

100
Q

How can you investigate the inverse square law for radioactive sources?

A

1) Set up a Geiger counter at the end of a metre rule.
2) Turn on the Geiger counter and take a reading of the background radiation count rate (in counts/sec). Do this 3 times and take an average.
3) Place the radioactive source at a distance d from the Geiger tube.
4) Record the count rate at that distance. Do this 3 times and take an average.
5) Repeat this at distances 2d, 3d, 4d, etc.
6) Put away the source immediately afterwards.
7) Correct each reading for background radiation. Plot a graph of corrected count rate against distance of the counter from the source. You should see that as the distance doubles, the corrected count rate drops to a quarter.

101
Q

When investigating the inverse square law for a radioactive source, what is it important to remember?

A

Correct each reading for background radiation.

102
Q

What does the graph for corrected count rate from a radioactive source against distance look like?

A

1/x² graph

103
Q

Do different isotopes decay at different rates?

A

Yes

104
Q

Will different samples of a particular isotope decay at different rates?

A

No, the same proportion of atomic nuclei will decay in a given time.

105
Q

What is the activity of a radioactive sample?

A

The number of nuclei that decay per second.

106
Q

Does the size of a radioactive sample affect its activity?

A

Yes - the activity is proportional to the size of the sample.

107
Q

What is the difference between the rate of decay and the activity of a sample?

A
  • Rate of decay - Proportion of atomic nuclei that decay in a given time
  • Activity - Number of nuclei that decay each second
108
Q

What is the unit for radioactive activity?

A

Becquerels (Bq)

109
Q

What is 1 becquerel?

A

1 decay per second

110
Q

What is the decay constant?

A

The probability of a given nucleus decaying per second.

111
Q

What are the units for decay constant?

A

s^-1

112
Q

A large value for the decay constant shows a … rate of decay.

A

Fast

113
Q

In decay equations, what is N?

A

The number of unstable nuclei.

114
Q

In decay equations, what is λ?

A

The decay constant

115
Q

In decay equations, what is A?

A

The activity of the sample

116
Q

What equation relates the activity of a sample to the number of nuclei?

A

A = λN

Where:
• A = Activity (Bq)
• λ = Decay constant (s^-1)
• N = Number of unstable nuclei

117
Q

What is the equation for the rate of change of the number of unstable nuclei?

A

ΔN/Δt = -λN

Where:
• ΔN/Δt = Rate of change of the number of unstable nuclei (s^-1)
• λ = Decay constant (s^-1)
• N = Number of unstable nuclei

118
Q

Derive ΔN/Δt = -λN.

A
  • A = λN
  • A is the rate of change of N, so:
  • ΔN/Δt = -λN
  • There is a negative sign because the number of atoms left is always decreasing.
119
Q

Define the half-life of an isotope.

A

The average time it takes for the number of unstable nuclei to halve.

120
Q

What is the symbol for the half-life of an isotope?

A

T(1/2)

Where 1/2 is subscript

121
Q

Put simply, how is the half-life of an isotope measured?

A

Measuring the time for the activity to halve.

122
Q

How does an isotope’s half-life relate to how long it is radioactive for?

A

The longer the half-life, the longer it stays radioactive.

123
Q

Describe the graph for N against t (for a radioactive source).

A
  • Starts at a positive y-intercept
  • The gradient becomes gradually less negative
  • The x-axis is an asymptote
  • Exponential decay, so the time to halve N is always the same

(See graph pg 162 of revision guide)

124
Q

How can you show that a graph is exponential decay?

A
  • Find the time for the y value to halve.
  • Do this at multiple points.
  • If they are the same, then this is exponential decay.
125
Q

Instead of a N against t graph for radioactive decay, what are you more likely to see and why?

A
  • A against t, which is the same graph.

* This is because A is easier to record than N.

126
Q

How can the graph of N against t for radioactive decay be made linear?

A
  • Plot ln(N) against t.

* This should be a straight line of negative gradient.

127
Q

How can the half-life of a radioactive isotope be found from the N against t graph (or A against t)?

A
  • Read of the count rate when t = 0
  • Go to half the original value and draw a horizontal line to the curve then down to the x-axis
  • Read off the t value at this point
  • Repeat these steps for a quarter of the original value
128
Q

How can the half-life of a radioactive isotope be found from the ln(N) against t graph (or ln(A) against t)?

A

Gradient = -λ

129
Q

On an N against t graph for radioactive decay, what is the y intercept?

A

N₀

130
Q

On an ln(N) against t graph for radioactive decay, what is the gradient equal to?

A

-λ

131
Q

Remember to practise drawing out the graphs for:
• N against t
• ln(N) against t

A

Pg 162 of revision guide

133
Q

What is the equation for the half-life of a radioactive sample?

A

T(1/2) = ln2 / λ (= 0.693/λ)

Where:
• T(1/2) = Half-life (s)
• λ = Decay constant (s^-1)

134
Q

What is the equation for the number of unstable nuclei remaining in a radioactive sample?

A

N = N₀e^(-λt)

Where:
• N = Number of unstable nuclei remaining
• N₀ = Original number of unstable nuclei
• λ = Decay constant (s^-1)
• t = Time (s)

135
Q

What is the equation for the activity remaining in a radioactive sample over time?

A

A = A₀e^(-λt)

Where:
• A = Activity remaining (Bq)
• A₀ = Original activity (Bq)
• λ = Decay constant (s^-1)
• t = Time (s)
136
Q

Give some uses of radioactive substances.

A
  • Dating organic material
  • Diagnosing medical problems
  • Sterilising food
  • Smoke alarms
137
Q

What isotope is used in the radioactive dating of objects?

A

Carbon-14

138
Q

How does radioactive dating of objects work?

A
  • Living plants take in carbon dioxide for photosynthesis, including the radioactive isotope carbon-14
  • When they die, the activity of the C-14 starts to fall, with a half life of 5730 years
  • Materials that were once living can be tested to find the current amount of C-14 in them, and date them
139
Q

What is the half-life of carbon-14?

A

5730 years

140
Q

What type of radiation is best for use in radioactive tracers?

A

Gamma

141
Q

Give an example of a radioactive tracer and why it is used.

A
  • Technetium-99m

* It is a gamma emitter, has a half-life of 6 hrs and decays to a much more stable isotope

142
Q

What is the problem with a long half-life?

A

It can be dangerous, because the isotope stays radioactive for a long time.

143
Q

Describe how standard notation of elements works.

A
  • Symbol for the element is written in large text
  • Mass number is in the top left
  • Atomic number is in the bottom left
144
Q

What is the symbol for mass number?

A

A

145
Q

What are the two main forces acting on a nucleus? What does each do?

A
  • Strong nuclear force - Holds the nucleus together

* Electromagnetic force - Pushing protons apart

146
Q

What is plotted on a the axis of a stability graph for isotopes?

A

Number of neutrons (N) against number of protons (Z)

147
Q

When will a nucleus be unstable?

A

If it has:

1) Too many neutrons
2) Too few neutrons
3) Too many nucleons altogether
4) Too much energy

148
Q

Describe the stability graph for nuclei.

A
  • 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.
149
Q

What is the line of stability on a stability graph?

A

The line (and surrounding region), in which stable nuclei may be found.

150
Q

Remember to practise drawing out the stability graph for nuclei.

A

Pg 164 of revision guide

151
Q

What is the symbol for atomic number?

A

Z

152
Q

In what nuclei does alpha emission happen and why?

A
  • Very heavy nuclei

* These nuclei are too massive to be stable, so losing nucleons makes them more stable

153
Q

When an alpha particle is emitted, what happens to the nucleon number and proton number?

A
  • Nucleon number -> Decreases by 4

* Proton number -> Decreases by 2

154
Q

In what nuclei does β⁻-emission happen and why?

A
  • Neutron rich nuclei

* This converts a neutron into a proton, so the nucleus becomes more stable

155
Q

When an beta-minus particle is emitted, what happens to the nucleon number and proton number?

A
  • Nucleon number -> Stays the same

* Proton number -> Increases by 1

156
Q

What happens in β⁻ decay?

A

A proton is changed into a neutron, while an electron and antineutrino are emitted from the nucleus.

157
Q

How is a β⁻ particle symbolised using standard notation?

A

0
β
-1

158
Q

In what nuclei does gamma-emission happen and why?

A
  • Nuclei with too much energy

* Losing a gamma ray helps lower the energy, making the nucleus more stable

159
Q

When might a nucleus have too much energy so that a gamma ray is released?

A
  • After alpha or beta decay.

* After a nucleus captures one of its own electrons (electron capture).

160
Q

When a gamma ray is emitted, what happens to the nucleon number and proton number?

A

There is no change to either, just a decrease in energy.

161
Q

What is the equation for electron capture?

A

p + e -> n + ve + γ

Note: The gamma ray isn’t always included.

162
Q

Describe when each type of radioactive emission may occur.

A
  • α - Heavy nuclei
  • β⁻ - Neutron-rich nuclei
  • γ - Nuclei with too much energy
163
Q

Describe the energy level diagram for an alpha emission.

A
  • Horizontal line for the unstable isotope
  • Arrow going to the bottom right, with a shallow gradient (labelled alpha)
  • Horizontal lone with the product

(See diagram pg 165 of revision guide)

164
Q

Describe the energy level diagram for a beta emission followed by a gamma emission.

A
  • Horizontal line for the unstable isotope
  • Arrow going to the bottom right, with a steep gradient (labelled beta)
  • Arrow going vertically down (labelled gamma)
  • Horizontal lone with the product

(See diagram pg 165 of revision guide)

165
Q

Remember to practise drawing out energy level diagrams for nuclear reactions.

A

Pg 165 of revision guide

166
Q

What quantities are conserved in nuclear reactions?

A
  • Energy
  • Momentum
  • Charge
  • Nucleon number
167
Q

What is nuclear fission?

A

When a large, unstable nucleus splits into two smaller nuclei and 2/3 neutrons, while releasing energy.

168
Q

What nuclei can undergo nuclear fission?

A

Large nuclei (at least 83 protons)

169
Q

What are the two types of nuclear fission?

A
  • Spontaneous

* Induced

170
Q

How can nuclear fission be induced?

A

Making a low energy neutron enter a U-235 nucleus.

171
Q

What sort of neutron is required in order to induce nuclear fission and why?

A
  • Low energy neutrons (a.k.a. thermal neutrons)

* Only these can be captured

172
Q

What is another name for the low energy neutrons used in inducing nuclear fission?

A

Thermal neutrons

173
Q

Why is energy released in nuclear fission?

A

The new, smaller nuclei have higher binding energy per nucleon.

174
Q

How does a nucleus’ size impact it’s stability?

A

The larger the nucleus, the more unstable it will be.

175
Q

How many protons are needed in a nucleus in order for it to undergo fission?

A

At least 83

176
Q

Which nuclei are most likely to undergo spontaneous nuclear fission and why?

A

Very large ones, because they are unstable.

177
Q

What limits the number of possible elements?

A

Nuclear fission

178
Q

Aside from two smaller nuclei, what is produced in induced nuclear fission?

A

2 or 3 neutrons

179
Q

How can we harness the energy released during nuclear fission?

A

Using a thermal nuclear reactor.

180
Q

Describe the structure of a nuclear reactor.

A
  • Fuel rods in centre
  • Control rods are inserted partly between the fuel rods
  • Moderator (water) surrounds fuel and control rods (closed system)
  • Pump pushes water through pipes in a heat exchanger
  • Cool water is pumped into the heat exchanger and steam is pumped out (to a turbine)
  • Concrete case surrounds everything
181
Q

Name all of the parts of a nuclear reactor.

A
  • Control rods
  • Fuel rods
  • Moderator (water)
  • Pump
  • Heat exchanger
  • Concrete case
182
Q

Remember to practise drawing out a diagram of a nuclear reactor.

A

Pg 166 of revision guide

183
Q

What fuel do nuclear reactors use?

A

Uranium-235 (and some U-238, but this doesn’t undergo fission)

184
Q

How are fuel rods inserted into a nuclear reactor?

A

Remotely, which keep workers as far away from the radiation as possible.

185
Q

How does the chain reaction in a fission reactor work?

A
  • Fission reactions produce more neutrons

* These then induce other nuclei to fission

186
Q

What does the moderator do and why?

A
  • Slows down neutrons -> To allow them to be captured by the uranium nuclei
  • Absorb neutrons -> To control the rate of reactions
187
Q

What is the name for neutrons that have been slowed down by the moderator?

A

Thermal neutrons

188
Q

What is the moderator in nuclear reactors?

A

Water

189
Q

How does a moderator work?

A

Elastic collisions slow down the neutrons.

190
Q

What type of collisions are involved when neutrons collide with the moderator?

A

Elastic (kinetic energy is conserved)

191
Q

Why is water used as a moderator in nuclear reactors?

A
  • Collisions with particles of a similar mass are most efficient at slowing down neutrons
  • Water contains hydrogen
  • So it fits this condition
192
Q

What is the perfect amount of fuel for a steady fission reaction called?

A

Critical mass

193
Q

What mass of fuel do nuclear reactors use?

A
  • Supercritical (more than is needed for a steady reaction)

* Control rods are used to control the rate of fission

194
Q

How do control rods work?

A

They absorb neutrons so that the rate of fission is controlled.

195
Q

Give an example of a material that control rods can be made from.

A

Boron

196
Q

How does an emergency shutdown of a nuclear reactor work?

A

The control rods are released into the reactor, which stops the reaction as quickly as possible.

197
Q

How does a nuclear reactor generate energy?

A

The coolant sent around the reactor removes heat and takes it for powering an electricity-generating turbine.

198
Q

What is the role of the thick concrete case in a nuclear reactor?

A

Prevents radiation escaping

199
Q

Why are the waste products of nuclear fission reactor still unstable and radioactive?

A

They have a larger proportion of neutrons than nuclei of a similar atomic number.

200
Q

Do the radioactive products of nuclear reactors have any uses?

A

The less radioactive ones can be used as tracers in medicine, etc.

201
Q

Describe what happens to radioactive waste from a nuclear reactor.

A
  • Placed in cooling ponds (remotely)

* Stored in sealed containers until the activity has fallen sufficiently

202
Q

What is nuclear fusion?

A

The joining of two light nuclei to give a larger nucleus.

203
Q

Why is energy released during nuclear fusion?

A

The new, heavier nuclei has a much higher binding energy per nucleon.

204
Q

What is the nuclear fusion reaction that happens in the Sun?

A

Hydrogen nuclei fuse in a series of reactions to form helium.

205
Q

Give the chemical equation for nuclear fusion in the Sun.

A

2H1 + 1H1 -> 3He2 + Energy

206
Q

Why is nuclear fusion difficult to achieve?

A

It requires a lot of energy to start it.

207
Q

Why does nuclear fusion require a lot of energy to achieve?

A
  • All nuclei are positively charged, so there is an electrostatic force of repulsion between them.
  • A large amount of energy is required to overcome this repulsion so that the nuclei get close enough for the strong interaction to hold the nuclei together.
208
Q

How much kinetic energy is required to make nuclei fuse together?

A

About 1 MeV

209
Q

How does the mass of a nucleus compare to the mass of its constituent parts?

A

The mass of a nucleus is LESS than the mass of its constituent parts.

210
Q

What is mass defect?

A

The difference between the mass of a nucleus and the mass of its constituent parts.

211
Q

What happens in terms of energy when two small nuclei join?

A

The total mass decreases, so the lost mass is converted to energy and released.

212
Q

Define binding energy.

A

The energy required to separate all of the nucleons in a nucleus.

213
Q

What is the unit for binding energy?

A

MeV

214
Q

How does binding energy compare to the mass defect?

A

Binding energy is the energy equivalent of mass defect.

215
Q

Estimate the binding energy in eV of the nucleus of a lithium atom 6Li3, given that its mass defect is 0.0343 u.

A

1) Convert the mass defect into kg.
• Mass defect = 0.0343 x (1.661 x 10^-27) = 5.697 x 10^-29
2) Use E = mc².
• E = (5.697 x 10^-29) x (3.00 x 10^8)² = 5.127 x 10^-12 J
• E = 32.0 MeV

216
Q

What is u equal to?

A

1 u = 1.661 x 10^-27 kg

217
Q

What is the energy equivalent of 1 u?

A

931.5 MeV

218
Q

What is a useful way of comparing the binding energies of different nuclei?

A

Looking at the average binding energy per nucleon.

219
Q

What is the equation for average binding energy per nucleon?

A

Average binding energy per nucleon = B / A

Where:
• Average binding energy per nucleon is in MeV
• B = Binding energy (MeV)
• A = Nucleon number

(Note: Not given in exam!)

220
Q

What graph is typically plotted with binding energies?

A

Average binding energy per nucleon against nucleon number.

221
Q

What does a high binding energy mean?

A

A large amount of energy is required to remove nucleons from the nucleus.

222
Q

Describe the graph of average binding energy per nucleon against nucleon number.

A
  • Starts at nucleon of 2 (hydrogen) and a very small average binding energy
  • Increases rapidly and then begins to plateau
  • Peak is at Fe-56
  • Gradually slopes off at increasingly negative gradient
223
Q

Where are the most stable nuclei on a graph of average binding energy per nucleon against nucleon number?

A

The maximum point on the graph (at Fe-56).

224
Q

Where is the peak on a graph of binding energy per nucleon against nucleon number?

A

At Fe-56

225
Q

Describe why fusion and fission release energy in terms of binding energy.

A

In both, the average binding energy per nucleon increases, so energy must have been released.

226
Q

Where does fusion happen on a graph of average binding energy per nucleon against nucleon number?

A

To the left of Fe-56.

227
Q

Where does fission happen on a graph of average binding energy per nucleon against nucleon number?

A

To the right of Fe-56.

228
Q

Which usually releases more energy: fusion or fission?

A

Fusion, because there is a greater change in average binding energy per nucleon (steeper gradient on graph).

229
Q

Remember to practise drawing out the graph of average binding energy per nucleon vs nucleon number.

A

Pg 168 + 169 of revision guide

230
Q

Remember to practise doing the binding energy calculations on pg 169 of revision guide.

A

Do it

A LEVEL - Physics (14 decks)

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