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1Origin and nature of nuclear radiation

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1 Origin and nature of nuclear radiation Properties of , and radiations (3) Properties of , and radiations (4) Radiation Detectors Photographic Film ... – PowerPoint PPT presentation

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Title: 1Origin and nature of nuclear radiation


1
1 Origin and nature of nuclear radiation
2
Properties of ?, ? and ? radiations (2)
0
1/1850
4
Mass (nucleon unit)
No deflection
Large deflection
Very small deflection
Effect of Fields
Very weak (0.01 of ?)
Weak (10 of ?)
Strong
Ionizing power
3
Properties of ?, ? and ? radiations (3)
500 m
5 m
5 cm
Range in air
Never fully absorbed reduced to half by 25 mm
of lead
Stopped by 5 mm of aluminium
Stopped by a sheet of paper
Penetrating power
4
Properties of ?, ? and ? radiations (4)
  • Photographic
  • film
  • Cloud chamber
  • GM tube
  • Photographic
  • film
  • Cloud
  • chamber
  • GM tube
  • Photographic
  • film
  • Ionization
  • chamber
  • Cloud chamber
  • Spark counter
  • Thin window
  • GM tube

Detectors
No transmutation
Radioactive transmutation
5
5 Deflection in electric magnetic field
a In electric field
?
electric field
? ()
radioactive source
?

? ()
mass of ? gtgt mass of ?
? deflection ? ltlt ?
6
5 Deflection in electric magnetic field
F
b In magnetic field
B
?
I
?
radioactive source
B-field (into paper)
?
Flow of a particles Flow of positive charges
Direction of Current
Flow of b particles Flow of negative charges
Opposite direction of current
7
b In magnetic field
? radiation tracks in a very strong B-field
(Photo credit Lord Blacketts Estate)
8
Radiation Detectors
  • Photographic Film
  • To detect ?, ? and ? radiations
  • Spark counter
  • To detect ?-particles
  • Ionization Chamber
  • To detect ? -particles
  • Cloud Chamber
  • To detect ? and ? particles
  • Geiger-Müller Tube
  • To detect ?, ? and ? radiations

9
Photographic Film
  • The photographic film has been blackened by
    radioactivity except in the shadow of the key.

10
Spark Counter
  • The spark counter consists of positively charged
    wire mounted under an earthed metal grid.
  • It produces sparks in the presence of ionized
    particles.
  • It can only be used to detect a-radiation.

11
  • When radiation enters the metal can, the gas
    inside is ionized.
  • Under the influence o the electric field,
    electrons move towards the anode while the
    positive ions move towards the cathode.
  • As a result, a small ionization current is
    produced and is recorded by an electrometer.

12
Note
  • 1. When the applied voltage (V) is increased, the
    ionization current is larger since more ions and
    electrons can reach the electrodes. Until a
    certain voltage, all ion-pairs produced reach the
    electrodes and a saturated ionization current is
    obtained.
  • 2. The saturated ionization current is increased
    with the rate of producing ion-pairs. Therefore,
    ionization chamber is suitable for detecting
    a-particles and b-particles since their ionizing
    powers are relatively strong. But the ionizing
    power of g-rays is very weak it cannot be
    measured by the ionization chamber.

13
Cloud Chamber (1)
  • The diagrams below show a diffusion cloud chamber
    and its structure.

14
Cloud Chamber (2)
  • The felt ring round the top of the chamber is
    soaked with alcohol.
  • The cooled chamber is full of alcohol vapour.
  • A weak radioactive source inside the chamber
    emits radiation that produces ions along its
    path.
  • The alcohol vapour which diffuses downwards from
    the top condenses around the ions.
  • The resulting tiny alcohol drops show up as a
    track in the bright light

15
Cloud Chamber Tracks (1)
a radiation Having a strong ionizing power, the
heavy ? particles give straight and thick tracks
of about the same length.
16


Tracks of ? rays can hardly be seen.
b radiation They are twisted because the
particles are small in mass and bounce off from
air molecules on collision.
17
Cloud Chamber Tracks (3)
  • Under diffusion cloud chamber,
  • Alpha source gives thick , straight tracks
  • Beta source produces thin, twisted tracks. They
    are small in mass and so bounce off from air
    molecules on collision.
  • Gamma source gives scattered, thin tracks. Gamma
    rays remove electrons from air molecules. These
    electrons behave like beta particles.

18
GM Counter
  • When ionizing radiation enters the GM tube, ions
    and free electrons are formed.
  • A flow of charge takes place and causes a pulse
    of current.
  • The pulse of current is amplified and counted
    electronically.

19
GM tube
  • A GM tube is filled with argon gas and a high
    voltage ( 400 V) is applied to the central wire.

20
GM tube
  • When radiation enters the tube, it pulls an
    electron from an argon atom and produces an
    ion-pair.
  • The resulting electrons rapidly accelerated
    towards the anode and cause more ions formed as
    they collide with argon gas atoms.
  • In this way, one electron can lead to the release
    of 108 electrons. An avalanche of electrons is
    produced.
  • When the electrons reach the anode, a pulse is
    created and can be counted by the GM counter.

21
1 Three types of decay
Alpha decay
22
Example 1
23
No. of throws No. of dice remaining
0 100
1 89
2 71
3 54
4 46
5 37
6 28
7 24
8 20
9 18
10 16
24
  • The points plotted do not fall exactly on the
    curve. The fluctuations are due to the random
    nature of dice throwing.
  • Radioactive decay is also random in nature
    because, like the dice decay the chance of
    certain nuclei decaying at a particular time is
    random.

25
no. of undecayed nuclei
  • Always decreasing.
  • Decrease rapidly in the beginning.
  • Decreases gently finally.
  • Becomes zero after a long time.

Time / s
26
Activity of a radioactive isotope (1)
  • Let N(t) be the number of radioactive nuclei in a
    sample at time t.

The - sign indicates that N(t) decreases with
time
  • The decay rate is directly proportional to N(t).

The SI unit of activity is the becquerel (Bq).
  • The constant k is called the decay constant. A
    large value
  • of k corresponds to rapid decay.

27
Activity of a radioactive isotope (2)
From
,
  • k can be interpreted as the probability per unit
    time that any individual nucleus will decay.

28
no. of undecayed nuclei
  • 0 4 s 2000 ? 1000
  • 4 8 s 1000 ? 500
  • 8 12 s 500 ? 250
  • 12 16 s 250 ? 125
  • Half life 4 s
  • It takes 4 s for half to decay

Time / s
29
Half life t1/2
Half-life t½
30
Half-life (1)
  • The graph shows the number of remaining nuclei
    N(t) as a function of time.

31
Half-life (2)
  • The half-life t1/2 is the time required for the
    number of radioactive nuclei to decrease to
    one-half the original number No.
  • At t t1/2, N(t) No/2, obtaining
  • Taking logarithms to base e, gives

32
Cloud Chamber Tracks (2)
33
Rate of decay
  • undecayed nucleus

decayed nucleus
34
Radiation hazard
  • Ionizing effect can destroy or damage living
    cells.
  • Radioactive gas and dust cannot be removed once
    taken in.
  • Gamma rays are dangerous due to strong
    penetrating power

35
Background radiation
Cosmic rays 12
Radioactive material in rocks and soil 15
Radioactive gases 40
Living bodies, and food and drinks 15
Medical practice 17
Nuclear discharge 1
36
Hazards due to sealed and unsealed sources (1)
  • Hazards due to sealed sources
  • a-particles usually do not present any external
    radiation hazard because they are unable to
    penetrate to dead layer of skin. But, extremely
    precautions must be taken to prevent a-emitters
    from getting into the body.
  • ß-particles never constitute a whole-body
    external radiation hazard due to their short
    range in tissue.
  • ?-rays have very high penetrating power and
    require greater care to avoid receiving excess
    dosage.

37
Hazards due to sealed and unsealed sources (2)
  • Hazards due to unsealed sources
  • Unsealed sources usually constitute some kind of
    internal hazard. This is the absorption and
    retention of radionuclides into specific organs
    of the body through intake of the materials
    present in air and in water.
  • The radionuclides may be rapidly absorbed by the
    organs causing damage to these organs.

38
Radioactive doses
  • The radiation emitted transfers energy to the
    organs and causes damage.
  • The level of damage depends on
  • 1. energy absorbed by the body
  • 2. type of radiation
  • 3. the parts of human body

39
  • Effective dose
  • Absorbed dose x Radiation weighting factor x
    tissue weighting factor

40
Handling precautions
  • The weak sources used at school should always by
    lifted with forceps.
  • The sources should never by held near the eyes.
  • The source should be kept in their boxes (lead
    container) when not in use.
  • Take great care not to drop the sources when
    handling them.
  • Carefully plan the experiments to minimize the
    time the source is used.

41
Uses of radioisotopes
  • Medical uses
  • Treatment of body cancer
  • Investigation of Thyroid Gland (???)
  • Radon-222 (a emitted)
  • Iodine-131 (g emitted)

42
Industrial uses
Application Type of radiation Half life
Thickness gauge a / b / g Long / short
Checking oil leakage a / b / g Long / short
Smoke detector a / b / g Long / short
43
Archaeological Use (Carbon-14 dating)
  • Carbon-14 exists due to formation by bombardment
    of nitrogen-14 in atmosphere by neutrons ejected
    from nuclei by cosmic rays ( ) and this forms
    radioactive carbon dioxide.
  • Living plants or trees absorb and give out carbon
    dioxide, so the percentage of C-14 in their
    tissue remains unchanged.
  • After death, no fresh CO2 taken in.
  • C-14 starts to decay with a half-life of 5.7 ?
    103 years.
  • By measuring the activity of C-14 ( ), the age
    of carbon containing material (e.g. wood, linen,
    charcoal) can be estimated.

44
  • End of this chapter

45
Alpha-Scattering Experiment (1)
  • A beam of ?-particles was directed at a thin
    sheet of gold-foil and the scattered ?-particles
    were detected using a small zinc sulphide screen
    viewed through a microscope in a vacuum chamber.

46
Alpha-scattering Experiment (2)
  • From the experiment it was found that
  • most of the ?-particles passed through the foil
    unaffected,
  • a few were deflected at very large angles,
  • some were nearly reflected back in the direction
    from which they had come.

47
Rutherfords atomic model
  • Rutherfords assumptions
  • All the atoms positive charge is concentrated in
    a relatively small volume, called the nucleus of
    the atom
  • The electrons surround the nucleus at relatively
    large distance.
  • Most of the atoms mass is concentrated in its
    nucleus.

48
Difficulties of Rutherfords model
  • The Rutherford model was unable to explain why
    atoms emit line spectra. The main difficulties
    are
  • It predicts that light of a continuous range of
    frequencies will be emitted
  • It predicts atoms are unstableelectrons should
    quickly spiral into the nucleus.

49
Mass and Energy
  • The mass-energy relationship
  • Einstein showed that mass and energy are
    equivalent.
  • E mc2
  • Mass defect
  • The difference between the mass of an atom and
    the mass of its particles taken separately is
    called the mass defect (?m).
  • ?m Zmp Nmn- Mnucleus
  • The mass defect is small compared with the total
    mass of the atom.

50
Unified Atomic Mass Unit
  • The unified atomic mass unit (u) is defined as
    one twelfth of the mass of the carbon atom which
    contains six protons, six neutrons and six
    electrons.
  • 1 u 1.660566 10-27 kg
  • Energy equivalence of mass
  • 1 u 931.5 MeV
  • It is a useful quantity to calculate the energy
    change in nuclear transformations.

51
Binding Energy (1)
  • The energy required to just take all the nucleons
    apart so that they are completely separated is
    called the binding energy of the nucleus.

52
Binding Energy (2)
  • From Einsteins mass-energy relation, the total
    mass of all separated nucleons is greater than
    that of the nucleus, in which they are together.
    The difference in mass is a measure of the
    binding energy.
  • According to relativity theory,
  • total binding energy ?mc2
  • where ?m is the mass defect of the nucleus.

53
Binding Energy (3)
  • Binding energy of Helium

?m 4.0330 u - 4.0026 u
0.0304 u
? E 28.3 MeV
Binding energy per nucleon 7.08 MeV per nucleon
54
Binding Energy (4)
  • The values of the binding energy varies from one
    nuclear structure to another.
  • The greater the binding energy per nucleon, the
    more stable the nuclei.

55
Binding Energy Curve (1)
  • The graph shows the variation of the binding
    energy per nucleon among the elements.

Fission
Fusion
56
Binding energy Curve (2)
  • The important features of the binding energy
    curve
  • Maximum binding energy per nucleon is at about
    nucleon number A 50. Maximum binding energy per
    nucleon corresponds to the most stable nuclei.
  • Either side of maximum binding energy per nucleon
    are less stable.

57
Binding Energy Curve (3)
  • When light nuclei are joined together, the
    binding energy per nucleon is also increased. So
    energy is released when light nuclei are fused
    together.
  • When a big nucleus disintegrates, the binding
    energy per nucleon increases and energy is
    released. So fission or radioactive decay both
    lead to an increase of binding energy per nucleon
    and hence to release energy as KE of the product.

58
Principles of Nuclear Fission (1)
  • Nuclear fission is a decay process in which an
    unstable nucleus splits into two fragments of
    comparable mass.
  • Two typical nuclear fission reactions are

59
Principles of Nuclear Fission (2)
  • Further investigations showed that
  • several neutrons are released with the fission
    fragments,
  • many fission products are possible when U-235 is
    bombarded with neutrons,
  • the products themselves are radioactive,
  • slow neutrons are more effective in fissioning
    U-235 than fast neutrons,
  • energy is released on much greater scale than is
    released from chemical reaction.

60
Chain Reactions
http//www.smartown.com/sp2000/energy_planet/en/tr
ad/fission.html
  • Fission of uranium nucleus, triggered by neutron
    bombardment, released other neutrons that can
    trigger more fission. Chain reaction is said to
    occur.

61
Nuclear Power Plant
  • A power plant with cooling tower

62
Nuclear Reactor (1)
http//www.ae4rv.com/games/nuke.htm
  • The schematic diagram of a nuclear reactor is
    shown below

63
Nuclear Reactor (2)
  • Enriched uranium is used as the fuel.
  • The fuel is in the form of rods enclosed in metal
    containers.
  • A moderator is used to slow down fission
    neutrons.
  • Control rods are used to absorb neutrons to
    maintain a steady rate of fissioning.
  • A coolant is pumped through the channels in the
    moderator to remove heat energy to a heat
    exchanger.

64
Processes inside the Nuclear Reactor
  • Each fission of U-235 nucleus produces fission
    fragments including neutrons. The fission
    fragments carry away most of the KE and transfer
    the KE to other atoms that they collide with. So
    the fuel pin get very hot.
  • The fission neutrons enter the moderator and
    collide with moderator atoms, transferring KE to
    these atoms. So the neutrons slow down until the
    average KE of a neutron is about the same as that
    of a moderator atom.
  • Slow neutron re-enter the fuel pins and cause
    further fission of U-235 nuclei.

65
Important features in the design of a nuclear
reactor (1)
  • The critical mass of fuel required
  • The critical mass of fuel is the minimum mass
    capable of producing a self-sustaining chain
    reaction.
  • The fission neutrons could be absorbed by the
    U-238 nuclei without producing further fission.
  • The fission neutron could escape from the
    isolated block of uranium block without causing
    further fission.

66
Important features in the design of a nuclear
reactor (2)
  • The choice of the moderator
  • The atoms of an ideal moderator should have the
    same mass as a neutron. So a neutron colliding
    elastically with a moderator atom would lose
    almost all its KE to the moderator atom.
  • In practice, graphite or heavy water (D2O) is
    chosen as the moderator.
  • The moderator atoms should not absorb neutrons
    but should scatter them instead.

67
Important features in the design of a nuclear
reactor (3)
  • The choice of control rods
  • The control rods absorb rather than scatter
    neutrons.
  • Boron and cadmium are very suitable elements for
    control rods.
  • Control rods are operated automatically.

68
Important features in the design of a nuclear
reactor (4)
  • Coolants should ideally have the following
    properties
  • The coolant must have high heat transfer
    coefficient.
  • The coolant must flow easily.
  • The coolant must not be corrosive.
  • Coolant atoms may become radioactive when they
    pass through the core of the reactor. So the
    coolant must have low induced radioactivity.
  • The coolant must be in a sealed circuit.

69
Important features in the design of a nuclear
reactor (5)
  • The treatment of waste
  • The fuel rods are stored in containers in cooling
    ponds until their activity has decreased and they
    are cooler.
  • The spent fuel is removed from the cans by remote
    control. The fuel is then reprocessed to recover
    unused fuel.
  • the unwanted material is then stored in sealed
    containers for many years until the activity has
    fallen to an insignificant.

70
Nuclear Fusion
  • Fusion is combining the nuclei of light elements
    to form a heavier element. This is a nuclear
    reaction and results in the release of large
    amounts of energy!
  • Energy is released due to the increase in binding
    energy of the product of the reaction.
  • In a fusion reaction, the total mass of the
    resultant nuclei is slightly less than the total
    mass of the original particles.

71
Example of Nuclear Fusion
  • An example of nuclear fusion can be seen in the
    Deuterium-Tritium Fusion Reaction.

72
Conditions for a Fusion Reaction (1)
  • Temperature
  • Fusion reactions occur at a sufficient rate only
    at very high temperature. Over 108 oC is needed
    for the Deuterium-Tritium reaction.
  • Density
  • The density of fuel ions must be sufficiently
    large for fusion reactions to take place at the
    required rate. The fusion power generated is
    reduced if the fuel is diluted by impurity atoms
    or by the accumulation of Helium ash from the
    fusion reaction.
  • As fuel ions are burnt in the fusion process they
    must be replaced by new fuel and the Helium ash
    must be removed.

73
Conditions for a Fusion Reaction (2)
  • Confinement
  • The hot plasma must be well isolated away from
    material surfaces in order to avoid cooling the
    plasma and releasing impurities that would
    contaminate and further cool the plasma.
  • In the Tokamak system, the plasma is isolated by
    magnetic fields.

74
Advantages of Nuclear Fusion
  • Abundant fuel supply
  • No risk of a nuclear accident
  • No air pollution
  • No high-level nuclear waste
  • No generation of weapons material

75
Nuclear Waste
Some waste is stored on asphalt pads in drums.
76
Storage Tanks for Nuclear Waste
  • These storage tanks were constructed to store
    liquid, high-level waste. After construction was
    completed, the earth was replaced to bury the
    tanks underground.

77
Nuclear Stability (1)
  • The Segrè chart below shows neutron number and
    proton number for stable nuclides.
  • For low mass numbers, N?Z.
  • The ratio N/Z increases with A.
  • Points to the right of the stability region
    represents nuclides that have too many protons
    relative to neutrons.
  • To the left of the stability region are nuclides
    with too many neutrons relative to protons.

78
Nuclear Stability (2)
79
Nuclear Stability (3)
80
Deflection of a, ß and ? rays in electric and
magnetic fields (1)
81
Deflection of a, ß and ? rays in electric and
magnetic fields (2)
  • Under the effect of electric field or magnetic
    field, (in the direction of going into the
    paper)
  • a-ray shows small deflection in an upward
    direction
  • ß-ray shows a larger deflection than that of
    alpha ray, and in a downward direction
  • ?-ray shows no deflection.

82
Penetrating Power
  • The diagram below shows the apparatus used to
    deduce the penetrating abilities of a, ß and ?
    radiations.

83
Moderator
http//www.npp.hu/mukodes/lancreakcio-e.htm
  • Use materials that slow the neutrons down to such
    low energies at which the probability of causing
    a fission is significantly higher. These neutron
    slowing down materials are the so called
    moderators.

84
Control rods
  • Reactor core glowing at full licensed power
  • reactor core at the bottom of a 5 m deep tank of
    very pure water

85
Heating of Plasma (1)
  • Ohmic Heating and Current Drive
  • Currents up to 7 million amperes (7MA) flow in
    the plasma and deposit a few mega-watts of
    heating power.
  • Neutral Beam Heating
  • Beams of deuterium or tritium ions, accelerated
    by a potential of 140,000 volts, are injected
    into the plasma.
  • Radio-Frequency Heating
  • The plasma ions and electrons rotate around in
    the magnetic field lines of the tokamak. Energy
    is given to the plasma at the precise location
    where the radio waves resonate with the ion
    rotation.

86
Heating of Plasma (2)
  • Current Driven by Microwaves
  • 10 MW of microwaves at 3.7 GHz accelerate the
    plasma electrons to generate a plasma current of
    up to 3MA.
  • Self Heating of Plasma
  • The helium nuclei (alpha-particles) produced when
    deuterium and tritium fuse remain within the
    plasma's magnetic trap. Their energy continues to
    heat the plasma to keep the fusion reaction
    going.

87
JET Tokamak
  • During operation large forces are produced due to
    interactions between the currents and magnetic
    fields. These forces are constrained by the
    mechanical structure which encloses the central
    components of the machine.
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