Transcript 4.2 Radioactivity

1
Origin and nature of nuclear
radiation
 particles
Nature
Charge
Speed
 particles
helium nuclei Fast-moving
electrons
+2
1
 rays
EM waves
similar to X-ray
no charge
up to 10% of up to 90% of speed of light
speed of light speed of light
Properties of ,  and  radiations (2)
Mass
(nucleon
unit)
4
1/1850
Effect of
Fields
Ionizing
power
Very small
deflection
Strong
Large
deflection
Weak
(10% of )
0
No deflection
Very weak
(0.01% of )
Properties of ,  and  radiations (3)
Range in air
Penetrating
power
~5 cm
~5 m
Stopped by a Stopped by
sheet of paper 5 mm of
aluminium
~500 m
Never fully
absorbed :
reduced to
half by 25
mm of lead
Properties of ,  and  radiations (4)
Detectors
Radioactive
transmutation
Photographic
Photographic
Photographic
film
Ionization
chamber
Cloud chamber
Spark counter
Thin window
GM tube
film
Cloud
chamber
GM tube
film
Cloud chamber
GM tube
A
Z
X  ZA42Y  24He
A
Z
X Z A1Y  10e
No
transmutation
5
Deflection in electric & magnetic field
a In electric field
electric field

radioactive
source

+
mass of  >> mass of 
 deflection:  << 
 (+)
 (–)
5
Deflection in electric & magnetic field
F
b In magnetic field
B

I

radioactive
source
B-field (into paper)

Flow of  particles = Flow of positive charges
= Direction of Current
Flow of  particles = Flow of negative charges
= Opposite direction of current
b In magnetic field
 radiation tracks in
a very strong B-field
(Photo credit: Lord Blackett’s Estate)
Radiation Detectors

Photographic Film


Spark counter


To detect  -particles
Cloud Chamber


To detect -particles
Ionization Chamber


To detect ,  and  radiations
To detect  and  particles
Geiger-Müller Tube

To detect ,  and  radiations
Photographic Film

The photographic film has been blackened by
radioactivity except in the shadow of the key.
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 α-radiation.
Earthed grid
To the positive terminal
of the EHT supply
 source
Metal
cylinder
(+)
2 kV d.c
Radioactive
source

Brass rod
electrometer


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.
Note


1. When the applied voltage (V) is
increased, the ionization current is
larger since more ions and electrons
Ionizing current (I)
can reach the electrodes. Until a
certain voltage, all ion-pairs produced
 source
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
 source
chamber is suitable for detecting particles and -particles since their
ionizing powers are relatively strong.
But the ionizing power of -rays is
V
very weak; it cannot be measured by
the ionization chamber.
Cloud Chamber (1)

The diagrams below show a diffusion cloud
chamber and its structure.
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
Cloud Chamber Tracks (1)
 radiation
Having a strong ionizing power,
the heavy  particles give
straight and thick tracks of
about the same length.
 radiation
They are twisted because the
particles are small in mass
and bounce off from air
molecules on collision.
Tracks of  rays can hardly be seen.
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.

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.
GM tube
Aluminium tube
Central wire as
anode (+)
radiation
400 V d.c
Mica end-window
Argon gas at low
pressure

counter
A GM tube is filled with argon gas and a high voltage (~
400 V) is applied to the central wire.
GM tube
Aluminium tube
Central wire as
anode (+)
radiation
400 V d.c
Mica end-window
Argon gas at low
pressure




counter
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.
1
Three types of decay
Alpha decay
AX
Z
A4
Z  2Y
2p
2n
+ 42He
Beta decay
AX
Z
A
Z + 1Y
+10 e
Gamma emission
A X*
Z
A
ZX
+ 
energy
Example 1
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

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.
no. of undecayed nuclei
2250
2000
1750
1500
1250
1000
750
500
250
0
Always decreasing.
Decrease rapidly in the
beginning.
Decreases gently
finally.
Becomes zero after a
long time.
Time / s
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Activity of a radioactive isotope (1)

Let N(t) be the number of radioactive nuclei in a
sample at time t.
dN (t )
Decay rate (Activity) = 
dt
The `-’ sign indicates that N(t) decreases with time
The SI unit of activity is the becquerel (Bq).
The
decay rate is directly proportional to N(t).
dN (t )

 kN (t )
dt
The constant k is called the decay constant. A large value
of k corresponds to rapid decay.
Activity of a radioactive isotope (2)
From 

dN
 kN,
dt
dN

k  dt
N
k can be interpreted as the probability per unit
time that any individual nucleus will decay.
no. of undecayed nuclei
2250
2000
1750
1500
1250
1000
750
500
250
0
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
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Half life t1/2
Half-life t½
Activity A
A0
½A0
¼A0
⅛A0
Half-life (1)

The graph shows the number of remaining nuclei N(t)
as a function of time.
Half-life (2)

The half-life t1/2 is the time required for the
number of radioactive nuclei to decrease to onehalf the original number No.
At t = t1/2, N(t) = No/2, obtaining e
Taking logarithms to base e, gives
t1 / 2
ln 2 0.693


k
k
kt1 / 2
1

2
Cloud Chamber Tracks (2)
Rate of decay
undecayed nucleus
decayed nucleus
dN (t )

 N (t )
dt
dN(t)
= -kN(t)
dt
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

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%
Hazards due to sealed and unsealed
sources (1)

Hazards due to sealed sources
 α-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 α-emitters
from getting into the body.
 β-particles never constitute a whole-body external
radiation hazard due to their short range in tissue.
 γ-rays have very high penetrating power and
require greater care to avoid receiving excess
dosage.
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.

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


Effective dose
= Absorbed dose x Radiation weighting
factor x tissue weighting factor
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.
Uses of radioisotopes

Medical uses
Treatment of body cancer
 Investigation of Thyroid Gland (甲狀腺)



Radon-222 ( emitted)
Iodine-131 ( emitted)
Industrial uses
Application
Type of
radiation
Half life
Thickness gauge
//
Long / short
Checking oil
leakage
//
Long / short
Smoke detector
//
Long / short
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 ( 147 N01n146 C11H ) 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 ( 146C 147 N  10 e ), the age
of carbon containing material (e.g. wood, linen, charcoal)
can be estimated.

End of this chapter
Alpha-Scattering Experiment (1)

A beam of -particles was directed at a thin sheet of goldfoil and the scattered -particles were detected using a
small zinc sulphide screen viewed through a microscope in
a vacuum chamber.
Side
Zinc sulphide
screen
Gold foil
view
-source
microscope
Evacuated metal box
To vacuum pump
Alpha-scattering Experiment (2)

From the experiment it was found that
most
a
of the -particles passed through the foil unaffected,
few were deflected
at very large angles,
some were nearly
reflected back in the
direction from which
they had come.
Rutherford’s atomic model

Rutherford’s assumptions:
All the atom’s positive charge is concentrated
in a relatively small volume, called the nucleus
10-15 m
of the atom
The electrons surround
the nucleus at relatively
large distance.
Most of the atom’s
mass is concentrated in
10-10 m
its nucleus.

Difficulties of Rutherford’s 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 unstable—
electrons should quickly spiral
into the nucleus.

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 (Δm).
 Δm = Zmp +Nmn- Mnucleus
 The mass defect is small compared with the total
mass of the atom.

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.

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.
Binding Energy (2)
From Einstein’s 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 = Δmc2
where Δm is the mass defect of the nucleus.
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
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.

Binding Energy Curve (1)

The graph shows the variation of the binding
energy per nucleon among the elements.
Fission
Fusion
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.

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.

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:

89
1
+ energy released
U  01n144
Ba

Kr

3
56
36
0n
235
92
94
1
+ energy released
U  01n140
Xe

Sr

2
54
38
0n
235
92
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.

Chain Reactions
http://www.smartown.com/sp2000/energy_planet/en/trad/fission.html#

Fission of uranium nucleus, triggered by neutron
bombardment, released other neutrons that can
trigger more fission. Chain reaction is said to occur.
Nuclear Power Plant

A power plant with cooling tower
Nuclear Reactor (1)
http://www.ae4rv.com/games/nuke.htm

The schematic diagram of a nuclear reactor is shown
below:
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.
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.
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 selfsustaining 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.

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.

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.

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.
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.

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.
Example of Nuclear Fusion

An example of nuclear fusion can be seen in the
Deuterium-Tritium Fusion Reaction.
2
1
H  13H  24 He  01n  17.6MeV
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 DeuteriumTritium 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.
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.
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

Nuclear Waste
Some waste is stored on asphalt pads in drums.
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.
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.
Nuclear Stability (2)
Nuclear Stability (3)
Deflection of α, β and γ rays in
electric and magnetic fields (1)
α
γ
β
+
Deflection in electric field
Deflection in magnetic field
Deflection of α, β and γ rays in
electric and magnetic fields (2)

Under the effect of electric field or magnetic field, (in
the direction of going into the paper);
 α-ray shows small deflection in an upward direction;
 β-ray shows a larger deflection than that of alpha ray,
and in a downward direction;
 γ-ray shows no deflection.
Penetrating Power

The diagram below shows the apparatus used to
deduce the penetrating abilities of α, β and γ
radiations.
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.
mM
v
u
M m
2m
V
u
M m
Control rods

reactor core at the
bottom of a 5 m deep
tank of very pure water

Reactor core glowing at
full licensed power
Heating of Plasma (1)

Ohmic Heating and Current Drive


Neutral Beam Heating


Currents up to 7 million amperes (7MA) flow in the
plasma and deposit a few mega-watts of heating
power.
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.
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.
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.