Transcript 1 - Island Physics

Atomic Structure
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A Helium Atom
Neutron
{
Electron
Nucleus
Proton
Particle
Mass
Proton
1
+1
Neutron
1
none
Electron
1/1840th
Charge
-1
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12
6
Nucleon
number
A
Proton
number
Z
1
C
1
X
H
Symbol
of the
element
7
3
Li
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1
1
H
2
1
H
3
1
H
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4
2
He
3
2
He
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12
6
C
14
6
C
Now lets have a closer look at the
nuclei of these isotopes.
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12
6
C
14
6
7
C
N
Carbon-12 is stable but Carbon-14 is
unstable (a radio-isotope).
Carbon-14 emits a beta particle and
decays to become nitrogen
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Calculate the number of protons, electrons and neutrons
shown below -
12
13
C
6
14
C
6
C
6
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Notes from Syllabus:



An atom has a small central nucleus made from
protons and neutrons surrounded by electrons.
All atoms in an element have the same number
of protons.
An atom can have different isotopes (different
number of neutrons).
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Half Life
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How about with Real Atoms

Look at the generated graph.
How long does it take for ½ of the atoms to decay?
How long for 3/4?
How long for 7/8?
How long for 15/16

Decay




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Half Life
Half-life is the time it takes for half of the atoms of a
sample to decay.
For example:
• A student was testing a sample of 8 grams of radioactive
protactinium. Protactinium has a a half life of 1 minute and decays
into actinium.
• After 1 minute there would be 4 g of protactinium (and 4 g of
actinium).
• After 2 minutes there would be 2 g of protactinium remaining (and
now 6g of actinium).
• After 3 minutes there would be 1 g of protactinium remaining (and
now 7g of actinium)
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Dating materials using half-lives
Question: Uranium decays into lead. The half life of uranium is
4,000,000 years. A sample of radioactive rock contains 7 times as
much lead as it does uranium. Calculate the age of the sample.
Answer: The sample was originally completely uranium…
1 half life
later…
1 half life
later…
1 half life
later…
8
8
4
8
2
8
1
…of the
sample was
uranium
Now only 4/8 of
the uranium
remains – the
other 4/8 is lead
Now only 2/8 of
uranium remains
– the other 6/8
is lead
Now only 1/8 of
uranium remains
– the other 7/8
is lead
8
So it must have taken 3 half lives for the sample to decay until only 1/8
remained (which means that there is 7 times as much lead). Each half
life is 4,000,000 years so the sample is 12,000,000 years old.
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An exam question…
Potassium decays into argon. The half life of potassium is
1.3 billion years. A sample of rock from Mars is found to
contain three argon atoms for every atom of potassium.
How old is the rock?
(3 marks)
The rock must be 2 half lives old – 2.6 billion years
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Notes from Syllabus:


Radioactive substances emit radiation from the
nuclei of their atoms all the time.
The half-life of a radioactive isotope is
 Either the time it takes for the number of
nuclei of the isotope in a sample to halve
 or the time it takes for the count rate from a
sample containing the isotope to fall to half
its initial level.
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Types of Radiation
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Types of Radiation [Use all of the other side of the paper]
Alpha
Beta
Gamma
Description
Electric Charge
Relative Atomic
Mass
Penetrating Power
Ionising Effect
Effect of
Magnetic /
Electric Field
Uses
Dangers
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What is radioactive decay?
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How do materials affect radiation?
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How do magnetic fields effect radiation?
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Using the information on the
following slides to fill in your table
Movie
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What is alpha (α) radiation?
Description
2 neutrons, 2 protons
Note:– An alpha particle is
the same as a helium nucleus
Electric charge
+2
Relative atomic mass
4
Penetrating power
Stopped by paper or a few
centimetres of air
Ionizing effect
Strongly ionizing
Effect of magnetic/
electric field
Weakly deflected
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What is beta (β) radiation?
Description
High energy electron
Electric charge
-1
Relative atomic mass
1/1860
Penetrating power
Stopped by a few
millimetres of aluminium
Ionizing effect
Weakly ionizing
Effect of magnetic/
electric field
Strongly deflected
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Gamma () radiation
Description
High energy electromagnetic
radiation
Electric charge
0
Relative atomic mass
0
Penetrating power
Stopped by several centimetres of
lead or several metres of concrete
Ionizing effect
Very weakly ionizing
Effect of magnetic/
electric field
Not deflected
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Types of radiation and penetrating power
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Types of radiation and range in air
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Uses
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What is radiation used for?
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How can radiation detect a fire?
Smoke alarms contain a weak source of alpha radiation.
The alpha particles ionize the air.
If there is smoke present, it
interacts with the ions produced
by the alpha particles and
ionization is reduced.
smoke
particle
α
This means that less current is
flowing through the air, which
causes the alarm to sound.
α
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How is radiation used in making paper?
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How can radiation find leaks in pipes?
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How can radiation detect cracks?
Gamma rays can also be used to
detect cracks after an object has
been welded.
Gamma rays are like X-rays.
welded
metal pipe
welding flaws
If a gamma source is placed on
one side of the welded metal,
and a photographic film on the
other side, any flaws will show up
on the film like an X-ray.
photographic film
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Microbes can be
killed using gamma
radiation
High
Level
nuclear
waste
Increasing
dose
healthy
brain
tissue
tumour
view through
the head
Gamma rays can be used
to treat brain tumours
skull
Uses of radiation – activity
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Dangers
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Dangers of ionizing radiations
Fill in the last part of your table
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Radiation safety
The three types of radiation differ in their effects and
physical nature.
All radioactive sources must be handled safely.
The hazard symbol for radiation is shown below:
As well as the normal laboratory safety rules you follow,
are there any extra rules concerning radioactivity?
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How are radioactive sources used safely?
Radioactive materials could be very dangerous to
handle if no safety precautions were taken.
This is because people and their clothing could
become contaminated.
The safety precautions are:
Write down on first side of the paper
 keep exposure times as short as possible
 monitor exposure with a film dose badge
 label radioactive sources clearly
 store radioactive sources in
shielded containers
 wear protective clothing
 use tongs or a robotic arm to
handle radioactive materials.
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Background radiation
Background radiation is the radiation all around us.
Most of the radioactivity you are exposed to is from
natural sources.
How many different sources of background radiation can
you think of?
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Calculating background radiation
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