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a2 physics > 3.8.1 - Radioactivity > Flashcards

3.8.1 - Radioactivity Flashcards

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1
Q

What happens in the Rutherford scattering experiment?

A

A stream of alpha particles from a radioactive source are fired at a very thin gold foil. When alpha particles from a radioactive source strike a fluorescent screen a tiny visible flash of light is produced.

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2
Q

What was expected to happen in the Rutherford scattering experiment?

A

The alphas would all be slightly deflected.

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3
Q

What happened in reality in the Rutherford scattering experiment?

A

Majority of alphas passed through. 1 in 2000 were deflected. 1 in 10 000 were deflected at an angle greater than 90.

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4
Q

What were the conclusions drawn from the Rutherford scattering experiment?

A

The atom is mainly empty space. The nucleus must have a large positive charge as it defects positively charged alphas. The nucleus must be tiny as very few alphas are deflected by an angle greater than 90. Most of the mass must be in the nucleus since the alpha particles with high momentum are deflected by the nucleus.

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5
Q

Describe the equipment in the Rutherford scattering experiment.

A

Source of alpha particles aimed at thin gold foil - all in vacuum. A detector which moves around the sphere of vacuum which flashes when alpha hits.

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6
Q

At closest approach what does the electrical potential energy of the alpha equal?

A

The KE it has to begin.

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7
Q

What is the equation for measuring nuclear radius by closest approach?

A

Initial KE = Electric potential energy = Q nucleus x q alpha / 4 pi e 0 r

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8
Q

Why can electron diffraction be used to measure nuclear radius?

A

Electrons are leptons and so don’t interact with the strong nuclear force.

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9
Q

What is the equation for finding nuclear radius using electron diffraction?

A

sin x = 1.22 x λ / 2R

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10
Q

What is the typical value for nuclear radius?

A

Usually around x10^-15

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11
Q

What is the relationship between nucleon number and radius?

A

R is proportional to A^1/3

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12
Q

Using experimental data what is the equation for the radius of a nucleus?

A

R = R0 x A^1/3

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13
Q

What is r0 between?

A

1.1-1.4fm

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14
Q

What is the equation for nuclear density?

A

3M / 4π x r0^3

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15
Q

What should the value of nuclear density be around?

A

x10^17 kgm^-3

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16
Q

What are the 4 types of nuclear radiation?

A

Alpha, beta minus, beta plus, gamma

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17
Q

What is an alpha?

A

2p, 2n, helium nucleus.

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18
Q

What is a beta minus?

A

Fast moving electron.

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19
Q

What is a beta plus?

A

Fast moving positron.

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20
Q

What is a gamma?

A

High energy, high frequency, short wavelength wave.

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21
Q

What is the mass in u of alpha?

A

4

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22
Q

What is the mass in u of beta?

A

Negligible

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23
Q

What is the mass of gamma in u?

A

0

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24
Q

What is the penetrating power of alpha?

A

Stopped by a few cm of air, paper or skin.

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25
Q

What is the penetrating power of beta minus?

A

Stopped by a few sheets of paper or thin aluminium.

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26
Q

What is the penetrating power of beta plus?

A

Will annihilate with electrons so not very penetrating.

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27
Q

What is the penetrating power of gamma?

A

Stopped by thick lead, typically a few cm.

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28
Q

Which is the most ionising radiation?

A

Alpha

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29
Q

Which is the least ionising radiation?

A

Gamma

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30
Q

What is affect by B field?

A

Alpha, beta.

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31
Q

How can you use a magnetic field to identify types of radiation?

A

Charged particles moving perpendicular to a uniform magnetic field are deflected in a circular path. The direction in which they are deflected depends on the charge.

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32
Q

What feature of alpha particles makes it good for smoke alarms?

A

They are strongly positive and so easily ionise atoms. The alpha particle quickly ionises many atoms and loses all its energy. Moreover they allow current to flow, but have a short range.

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33
Q

What is beta used for?

A

Creating sheets of material, such as paper, aluminium foil or steel as they are used to control the thickness.

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34
Q

Why can gamma be used in medicine?

A

It is weakly ionising so does little damage to the body tissue.

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35
Q

What is gamma used for in medicine?

A

Used in radioactive tracers to help diagnose patients without treatment. Also in the treatment of cancerous tumours.

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36
Q

What are the problems of gamma in medicine?

A

It can damage healthy cells leading to side effects such as reddening or soreness of the skin as well as infertility.

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37
Q

How can you measure background radiation?

A

Take 3 reading of the count rate using a Geiger counter without a radioactive source being present.

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38
Q

What are the main sources of background radiation?

A

Radioactive radon gas released from rocks, cosmic radiation due to cosmic rays colliding with particles in the upper atmosphere, living things containing carbon-14.

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39
Q

What is the relationship between the intensity of gamma radiation and the distance from the source?

A

I = k / x^2

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40
Q

What is intensity of radiation?

A

The amount of radiation per unit area.

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41
Q

What is intensity measured in?

A

Wm^-2

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42
Q

What is k?

A

A constant measured in W

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43
Q

When investigating the inverse square law what values do you need to record?

A

3 readings of the background radiation. 3 reading for distances of d, 2d, 3d and 4d away from the source. Then find the average for each distance and minus the average background radiation.

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44
Q

What graph do you plot when investigating the inverse square law?

A

Count rate against distance.

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45
Q

What are examples of how to stay safe when handling radioactive sources?

A

Use long handling tongs to minimise the radiation absorbed in the body. Sources should be always stored in lead box when not being used. Make sure sources are only out for shortest period of time possible.

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46
Q

What is decay?

A

Completely random and unpredictable.

47
Q

What is the activity of a sample?

A

The number of nuclei that decay each second.

48
Q

What is the activity of the sample proportional to?

A

The number of unstable nuclei in the sample.

49
Q

What is the decay constant?

A

The probability of a specific nucleus decaying per unit time, and is a measure of how quickly an isotope will decay.

50
Q

What does a bigger decay constant mean?

A

The faster the rate of decay.

51
Q

What does activity = ?

A

λN

52
Q

What is activity measured in?

A

Bq

53
Q

What is the unit of the decay constant?

A

s^-1

54
Q

What does activity also = ?

A

-ΔN / Δt

55
Q

What does ΔN / Δt =

A

-λN

56
Q

Number of atoms in a sample? (N)

A

Number of moles x Avogadro’s number

57
Q

How can you find -λ from a graph?

A

Use the graph of the natural log of the number of unstable nuclei against time and find the gradient.

58
Q

What is the number of unstable nuclei decaying per second proportional to?

A

The number of nuclei remaining. A = A0 e^ -λt

59
Q

What is the half life of an isotope?

A

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

60
Q

What are the applications of radioactive isotopes?

A

Radioactive dating, medical diagnosis, storage of radioactive waste.

61
Q

How does radioactive dating work?

A

When living plants take in CO2 they also take in carbon-14. When they die the activity of carbon-14 starts to fall. Therefore you can test a material to see its age.

62
Q

What is the issue with radioactive dating and its reliability?

A

The object may be contaminated by other radioactive sources, or may be too high background radiation. Or source may be too small to get a reliable count.

63
Q

What is used in medical diagnosis?

A

Gamma ray emitters such as technetium-99.

64
Q

When is a nucleus unstable?

A

Too many neutron, too few neutrons, too many nucleons, too much energy.

65
Q

On the curve of stability what does above the curve mean?

A

Neutron rich.

66
Q

On the curve of stability what does below the curve mean?

A

Neutron poor.

67
Q

What does alpha decay occur in?

A

Very heavy atoms.

68
Q

What happens in alpha decay?

A

Proton number decreases by 2, and nucleon by 4.

69
Q

What happens in beta minus emission?

A

Emission of an electron with an antineutrino.

70
Q

In what does beta minus emission occur?

A

Happens in isotopes which are neutron rich.

71
Q

In what does beta plus emission occur?

A

Isotopes which are proton rich.

72
Q

What happens in beta plus emission?

A

A proton changes into a neutron - so proton number decreases by 1 and the nucleon number stays the same..

73
Q

What is also produced in beta plus emission?

A

A neutrino.

74
Q

When does gamma emission occur?

A

After an alpha or beta decay the nucleus often has excess energy - its is in an excited state.

75
Q

What happens in gamma emission?

A

There is no change to nuclear constituents - the nucleus just loses excess energy.

76
Q

When else is gamma radiation produced?

A

Electron capture - when a nucleus captures and absorbs one of its own orbiting electrons, which causes a proton to change into a neutron.

77
Q

What is the mass defect?

A

The difference between the mass of the nucleus and the mass of its constituent nucleons.

78
Q

How is energy related to the mass defect?

A

The lost mass is converted into energy and released. The amount of energy released is equivalent to the mass defect.

79
Q

What is the binding energy?

A

The energy needed to separate of the nucleons in a nucleus.

80
Q

Average binding energy per nucleon =

A

Binding energy / nucleon number

81
Q

What does higher binding energy per nucleon mean?

A

More energy is needed to remove nucleons from the nucleus, and therefore the more stable the nucleus.

82
Q

Which is the most stable element?

A

Fe-56

83
Q

Describe the binding energy curve?

A

Peaks at He and Fe (shaped like an r)

84
Q

What are the axis on the binding energy curve?

A

Y - Binding energy per nucleon (MeV)

X - Mass number

85
Q

What is nuclear fission?

A

When large unstable nuclei are split into smaller daughter nuclei.

86
Q

Why is energy released in nuclear fission?

A

The smaller nuclei have a higher binding energy per nucleon.

87
Q

How can fission be induced?

A

By making a neutron enter a U-235 nucleus, causing it to become very unstable.

88
Q

What is fusion?

A

When two light nuclei can combine to create a larger nucleus.

89
Q

Why is a lot of energy released during nuclear fusion?

A

The new heavier nuclei have a much higher average binding energy per nucleon.

90
Q

What is the equation for fusion in the sun?

A

2/1H + 1/1H -> 3/2He + energy

91
Q

On the binding energy curve which elements release energy by fission?

A

Elements to the right of Fe-56

92
Q

On the binding energy curve which elements release energy by fusion?

A

Elements to the left of Fe-56

93
Q

Why can only certain elements on the binding curve release energy by fission/fusion?

A

Energy is only released when the average binding energy per nucleon increases.

94
Q

What are the rods made of in a nuclear reactor?

A

Uranium-235.

95
Q

Why is a nuclear reactor an example of a chain reaction?

A

The fission reactions produce more neutrons which then induce other nuclei to fission.

96
Q

When will neutrons cause a chain reaction?

A

When they are slowed down, allowing them to be captured. They are called thermal neutrons.

97
Q

What does the moderator do?

A

Slows down the neutrons so that they can be absorbed by the uranium nuclei.

98
Q

How does the moderator slow neutrons down?

A

Through elastic collisions with nuclei of the moderator material.

99
Q

How do you choose a moderator?

A

One with a similar mass to the neutrons

100
Q

What is the critical mass?

A

The amount of fuel you need to continue the fission reaction at a steady rate.

101
Q

What is used to control the rate of fission?

A

Control rods.

102
Q

How do control rods work?

A

They limit the number of neutrons in the reactor by absorbing neutrons. They can be inserted any varying amounts to control the rate.

103
Q

How do you choose the material for the control rods?

A

Has to be a material which absorbs neutrons (e.g. boron)

104
Q

What does coolant do?

A

Removes the heat produced by fission.

105
Q

What should the coolant be?

A

A liquid or gas at room temperature, and be efficient at transferring heat.

106
Q

What is the coolant often?

A

The same water which is used as the moderator.

107
Q

Benefits of nuclear power?

A
  • Enough fuel to keep generating power for centuries.
  • No greenhouse gases emitted.
  • Efficient.
108
Q

Risks of nuclear power?

A
  • Have to be designed and build extremely carefully to minimise the danger of a nuclear disaster.
  • Waste has to be dealt with as very harmful.
  • Fission doesn’t produce greenhouse gases, but parts of the process do such as transporting the uranium rods to a power station.
109
Q

What is the reactor shielding?

A

A thick concrete case.

110
Q

How does the emergency shut down work?

A

The reactor can be shut down automatically in an emergency by the release of the control rods into the reactor. They are fully lowered into the reactor which slows down the reaction as quickly as possible.

111
Q

What happens with the uranium rods?

A

Unused rods only emit alpha so are easily contained. Spent rods are more dangerous as they have a larger proportion of neutrons than nuclei of a similar atomic number, therefore they are unstable and radioactive.

112
Q

Waste?

A

Emit beta and gamma radiation which is strongly penetrating.

113
Q

What happens to waste when it initially removed from the reactor?

A

It is very hot so is placed in cooling ponds until the temperature falls to a safe level. Radioactive waste is then stored in sealed containers until its activity has fallen sufficiently.

a2 physics (25 decks)

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