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.

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• 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.

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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.

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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.

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no. of undecayed nuclei

• Always decreasing.
• Decrease rapidly in the beginning.
• Decreases gently finally.
• Becomes zero after a long time.

Time / s
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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.

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Activity of a radioactive isotope (2)
From
,

• k can be interpreted as the probability per unit
  time that any individual nucleus will decay.

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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
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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.

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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

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• 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
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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.

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• 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.

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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.

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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.

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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
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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Nuclear Stability (2)
79
Nuclear Stability (3)
80
Deflection of a, ß and ? rays in electric and
magnetic fields (1)
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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.

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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.