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Physics > 3.8 Nuclear Physics > Flashcards

3.8 Nuclear Physics Flashcards

1
Q

What is meant by activity?

A
  • The rate of decay of the radioactive nuclei in a given isotope
  • It is proportional to the total number of undecayed nuclei in the sample
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2
Q

What is activity measured in?

A

Becquerels

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3
Q

Alpha decay:
What is it?
How ionising?
Stopped by?

A
  • The emission of an alpha particle (2 protons and 2 neutrons) from an unstable nucleus (usually one with too many nucleons) to make it more stable.
  • Alpha radiation is strongly ionising and is stopped by a few centimetres of air or a sheet of paper.
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4
Q

Beta decay:
What is it?
How ionising?
Stopped by?

A
  • The emission of a beta particle when a proton turns into a neutron (or vice versa) in an unstable nucleus.
  • Beta minus radiation is weakly ionising.
  • stopped by 3mm of aluminium foil but beta plus radiation is immediately annihilated by an electron
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5
Q

What is binding energy?

A

The amount of energy required to split a nucleus into all its separate constituent nucleons. It is equivalent to the mass defect.

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6
Q

What is a chain reaction?

A

​The process by which neutrons released by a fission reaction induce further fissile nuclei to undergo fission.

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7
Q

What can closest approach be used for?

A

Estimating a nuclear radius (by firing a alpha particle at it)

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8
Q

What is contamination?

A

​The introduction of radioactive material to another object. The object is consequently radioactive.

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9
Q

What is meant by the critical mass?

A

The minimum mass of fissile material required in a fission reactor for a chain reaction to be sustained.

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10
Q

What is electron capture?

A

A process that occurs in proton-heavy nuclei, in which an electron is drawn into the nucleus, causing a proton to transition into a neutron. An electron neutrino is also produced.

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11
Q

Describe the process of fission

A
  1. An unstable nucleus absorbs a thermal neutron
  2. The nucleus of fuel splits into two smaller daughter nuclei.
    Releasing two or three fast moving neutrons, and a lot of energy.
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12
Q

What is meant by fusion?

A

The joining of two smaller nuclei to form a larger nucleus and to release energy.

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13
Q

Gamma radiation:
What is it?
How ionising?
Stopped by?

A
  • The emission of Gamma rays from an unstable nucleus that has too much energy.
  • Gamma radiation is very weakly ionising but requires several centimetres of lead to be stopped.
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14
Q

Define half-life

A

The average time it takes for the number of radioactive nuclei in a sample to halve

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15
Q

What is irradiation?

A

​The exposure of an object to radiation. The exposed object does not become radioactive.

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16
Q

What is the mass defect?

A

The difference in mass between a nucleus and the sum of the masses of its constituent nucleons.

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17
Q

What is radioactive dating? What isotope is commonly used?

A
  • The use of radioactive isotopes with known half-lives to date objects
  • The isotope that is usually used is Carbon-14
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18
Q

What does Rutherford scattering show?

A
  • The existence and nature of the nucleus.
  • That the nucleus is small, dense and positively charged
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19
Q

3 observations and conclusions from Rutherford scattering experiment

A

Most alpha particles passed straight through the foil with no deflection​
- ​atom is mostly empty space​ (and not a uniform density)

A small amount of particles were deflected by a large angle
- centre of the atom is positively charged

Very few particles were deflected back by more than 90 degrees
- centre of the atom was very dense - charge is concentrated in centre

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20
Q

Applications of beta radiation

A

Thickness measurements of paper and aluminium foil

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21
Q

Which radiation follows inverse square law?

A

Gamma radiation

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22
Q

How to verify inverse square law of gamma radiation?

A
  • Measure the count rate of a gamma source at different distances from the GM tube​ (making sure to adjust for the background radiation)
  • then plot a graph of corrected count against 1/x^2 , which will form a straight line verifying the above equation.
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23
Q

Danger of radiation if gets in body

A

Can cause mutations and damage to cells (which can lead to cancer etc.)

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24
Q

3 sources of background radiation

A

● Radon gas​ - which is released from rocks
● Cosmic rays​ ​- which enter the Earth’s atmosphere from space
● Artificial sources​ - caused by ​nuclear weapons testing​ and ​nuclear meltdowns

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25
Q

How can waste with a long half-life be stored?

A

Underground in steel casks (to prevent damage to environment and people in future)

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26
Q

Decay that happens if nucleus has too many neutrons

A

Beta minus decay (so that a neutron changes into a proton)

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27
Q

Decay that happens if nucleus has too many protons

A

Beta-plus emission or electron capture

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28
Q

Decay that happens if nucleus has too many nucleons

A

Alpha emission

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29
Q

What happens if nucleus has too much energy?

A

Gamma emission (usually occurs after a different type of decay, as nucleus becomes excited)

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30
Q

4 reasons technetium-99m is useful in medical diagnosis

A
  • It only emits gamma rays (which are weakly ionising)
  • Half-life (6hrs) short enough to not remain in body for too long after the medical examination
  • Half-life (6hrs) long enough to complete the diagnosis
  • Can be made near to the hospital
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31
Q

How to estimate nuclear radius using closest approach

A
  • All kinetic energy is converted into electric potential energy
  • (gives an overestimate - doesn’t include distance between alpha particle and nucleus, which won’t touch)
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32
Q

2 ways to calculate nuclear radius

A
  • Distance of closest approach
  • Electron diffraction
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33
Q

Why is energy released during nuclear fission?

A

The smaller daughter nuclei have a higher binding energy per nucleon than the original unstable nucleus

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34
Q

Why is energy released during nuclear fusion?

A

The larger nucleus has a much higher binding energy per nucleon

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35
Q

After what element do elements begin to undergo fission as opposed to fusion? Why?

A

Iron - it has the highest binding energy per nucleon​

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36
Q

How to calculate energy released during fission?

A

Mass defect = Mass before - Mass after
Mass defect * 931.5MeV (or use E = mc^2)

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37
Q

How do you induce fission in uranium?

A

By firing a thermal neutron into the uranium nucleus, making the nucleus extremely unstable

38
Q

What is a thermal neutron?

A

A slow moving neutron, which low energy

39
Q

What is the purpose of moderators?

A

To slow neutrons released in fission reaction to thermal speeds, using elastic collisions, so that the neutrons are absorbed by the uranium

40
Q

What is the purpose of control rods?

A

To absorb neutrons in the reactor in order to control chain reactions

41
Q

What is the purpose of coolant (in a nuclear reactor)?

A
  • To absorb the heat released during fission reactions in the core of the reactor
  • This heat then makes steam which powers electricity-generating turbines
42
Q

Example materials for moderators, control rods and coolants

A
  • Moderator - Water (inexpensive, not very reactive, similar in mass to neutron), graphite
  • Control rods - Boron and Cadmium
  • Coolant - Water (high SHC), helium
43
Q

What happens in an emergency shut-down?

A

The control rods are dropped into the reactor core entirely​ in order to stop fission reactions from occurring as soon as possible by absorbing all the free neutrons in the core

44
Q

How to safely insert fuel rods into reactor?

A

Remotely​ to limit the worker’s exposure to radiation

45
Q

What is shielding (in a nuclear reactor)?

A

​Very thick concrete shielding around the nuclear reactor​, which ​blocks radiation from escaping from the reactor and affecting the workers in the power station

46
Q

3 safety features at a nuclear reactor

A
  • Remote handling of fuel
  • Shielding
  • Emergency shut-down
47
Q

What is the source of the most dangerous waste?

A

Fission fragments from the fission of spent uranium-235 fuel rods

48
Q

How is radioactive waste treated?

A
  • Must be placed in cooling ponds for up to five years
  • Uranium/plutonium is separated to be recycled
  • High level waste is vitrified (made solid) and then placed in thick steel casks deep underground
49
Q

Describe the Rutherford scattering experiment

A
  • Beam of alpha particles is directed at a thin gold foil
  • Occurs in a vacuum so that no collisions between air particles and alpha particles can occur
  • Experiment was done in order to determine structure of atom
50
Q

How is electron diffraction used to determine the diameter of an atom?

A
  • electron beam fired at thin sheet of desired atom (only several atoms thick)
  • diffraction pattern produced on screen behind
  • using the angle of the first minimum we can use equations to calculate the diameter
51
Q

Why is there a mass defect?

A

Binding energy is released when a nucleus is formed. When a nucleus is formed, some of its mass is converted into energy, hence atomic mass is less than mass of constituent particles

51
Q

cm of air to stop alpha particles

A

4cm

52
Q

cm of air to stop beta particle

A

40cm

53
Q

How far can gamma rays travel in air?

A

About 1km

54
Q

Define atomic mass unit

A

1/12 mass of atom of Carbon 12-6

55
Q

Why a high temperature for nuclear fusion?

A
  • Nuclei need to be close together for strong nuclear force to be involved
  • But the electrostatic force is repulsive and tries to prevent this
  • If temperature is high, then nuclei have high kinetic energy to overcome this repulsion
55
Q

Why a high temperature for nuclear fusion?

A
  • Nuclei need to be close together for strong nuclear force to be involved
  • But the electrostatic force is repulsive and tries to prevent this
  • If temperature is high, then nuclei have high kinetic energy to overcome this repulsion
56
Q

How do you reduce the power output of nuclear reactor?

A

Insert control rods further into the nuclear reactor, so more neutrons are absorbed, reducing further fission reactions

57
Q

All nuclei have approximately the same density. State two conclusions about nucleons in the nucleus that can be deduced from this fact

A
  • Nucleons have a constant separation
  • Neutrons and protons have similar masses
58
Q

What happens to U-235 and U-238 in nuclear reactors when they absorb neutrons?

A
  • U-235 absorbs thermal neutrons, and splits into two smaller nuclei, giving out neutrons
  • U-238 absorbs a neutron, but doesn’t undergo fission
59
Q

4 advantages of nuclear power

A
  • Greater energy output per unit mass of fuel (than fossil fuels)
  • Little green house gas emissions (so less global warming effects)
  • Some (but not all) nuclear power stations can adjust their output quickly
  • Nuclear power can be produced continuously (whereas renewables are dependent on sunlight/wind etc)
60
Q

3 safety procedures when dealing with radioactive sources in schools in colleges

A
  • Using​ long handled tongs​ to move the source (to maximise distance between source and person)
  • Stand behind a lead absorber (for shielding)
  • Storing the source in a ​lead-lined container​ when not in use (to limit exposure time)
61
Q

Why is carbon dating unsuitable if a sample is less than 200 years old?

A

It is difficult to measure accurately the small drop in activity

62
Q

Why is carbon dating unsuitable if a sample is more than 60,000 years old?

A

The activity would be very small/comparable to the background

63
Q

Define the Avogadro constant

A

The number of atoms in 12g of carbon-12 (or the number of molecules in one mole of substance)

64
Q

What is the dominant force between two protons?

A

Coulomb repulsion

65
Q

4 problems with dealing with dangerous waste and how they are dealt with

A
  • Waste is initially very hot and radioactive → placed in cooling ponds
  • High level waste may leak in liquid form → vitrified and placed in steel barrels
  • the waste will be radioactive for thousands of year → stored somewhere geologically stable (deep underground)
  • transporting waste presents a danger to public → processed onsite or nearby
66
Q

How does carbon dating work for fossils?

A
  • All living organisms have the same amount of carbon-14 atoms as a percentage of all carbon isotopes
  • Once the organism dies, it no longer absorbs carbon from the atmosphere.
  • Carbon-14 is radioactive and so will decay over a known half-life.
  • The older a fossil is, the fewer carbon-14 isotopes it will contain and the less radiation it emits.
67
Q

What happens when U-238 interacts with neutrons in a nuclear reactor?

A

U-238 absorbs/scatters neutrons

68
Q

Why are spent fuel rods more dangerous than unused fuel rods?

A
  • Unused fuel rods only emit alpha radiation (which is weakly penetrating)
  • Spent fuel rods contain fission fragments with a larger proportion of neutrons than nuclei of a similar atomic number
  • Making them unstable, and emitters of beta and gamma radiation, which are strongly penetrating
69
Q

Which method of estimating the nuclear radius is more accurate?

A
  • Electron diffraction
  • Electrons are leptons, so don’t interact with the strong nuclear force (unlike alpha particles)
70
Q

Where does the first minimum occur in electron diffraction?

A

sin θ = 1.22λ / 2R
(where R is the radius of the nucleus the electron has been scattered by)

71
Q

What is θ in the equation sin θ = 1.22λ / 2R?

A

The angle at which the first minimum occurs in electron diffraction.

72
Q

What does the graph of intensity against angle for electron diffraction look like?

A

Like the single source pattern (centre peak twice the width and much more intense that subsidiary maxima unless a log graph has been plotted)

73
Q
  1. Radius of a typical atom
  2. Radius of a typical nucleus
A
  1. 0.05nm (5E-11m)
  2. 1fm (1E-15m)
74
Q
  1. Radius of a typical atom
  2. Radius of a typical nucleus
A
  1. 0.05nm (5E-11m)
  2. 1fm (1E-15m)
75
Q

What word best describes the decay of an individual radioactive nucleus?

A

Random

76
Q

Which of alpha, beta, gamma radiation are affected by magnetic fields?

A

Alpha and beta

77
Q

What is likely to happen after an alpha/beta/electron capture decay?

A

A gamma ray is emitted, as the nucleus often has excess energy
(gamma ray always emitted in electron capture)

78
Q

What do you need to draw for rutherford scattering

A
  • Alpha source
  • Thin gold foil
  • Circular detector
  • labelled as a vacuum
79
Q

What is the decay constant?

A

The probability a given nucleus will decay per second

80
Q

3 desired features of materials for a moderator in a fission reactor

A
  • small/light nuclei
  • should not become radioactive when bombarded with electrons
  • available in large amounts (for low cost)
81
Q

Applications of gamma radiation

A
  • Thickness measurements of steel
  • Medical radioactive tracers
82
Q

Describe the location of the line of stability for an N/Z graph

A

Straight line N=Z until N=20, then curves upwards and becomes steeper

83
Q

If writing an equation for electron capture, what two things should you not forget on the RHS?

A
  • The electron neutrino
  • The gamma ray photon released
84
Q

What were Geiger and Marsden expecting to see in the alpha particle scattering experiment?

A
  • The flashes all occurring within a small angle of the beam
  • Because the positively-charged alpha particles would be deflected a small amount by the electrons (in the plum pudding model)
85
Q

Why must the coolant in a nuclear reactor be a gas or liquid?

A

So it can be pumped around the reactor (e.g. water!)

86
Q

Why must the coolant in a nuclear reactor have a high specific heat capacity?

A

So it can absorb more heat energy for a small increase in temperature

87
Q

What is the likely initial mode of decay for fission fragments and why?

A
  • Fission fragments have are neutron rich
  • So beta minus decay
88
Q

True or False: RMS speed = most probable speed

A

False, RMS speed is greater than most probable speed (peak of distribution)

Physics (11 decks)

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