1. Rutherford’s scattering experiment
Introduction 1
Atom
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Rutherford’s scattering experiment
Rutherford
Photo
Rutherford’s scattering experiment
Diagram
Atomic model
Diagram

2. Basic terms and definitions
Introduction 2
Atom
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Basic terms and definitions
Atomic number (Z) : no. of protons in nucleus
Mass number (A) : total no. of protons and neutrons in nucleus
Symbol of a nucleus with Z and A :
Nuclide : a kind of atom with a particular A and Z
Radionuclide : nuclide which is radioactive

3. Basic terms and definitions
Radioactive Decay 1
Atom
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Basic terms and definitions
Isotopes : nuclides with same value of Z
Radioactivity : emission of radiation by unstable nuclei
Radioactive decay : the process of emission of radiation by unstable nuclei
Radioactive decay, parent nucleus, daughter nucleus and decay products
Diagram

4. Radioactive decay Radioactive Decay 2 Atom Alpha emission
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Radioactive decay
Alpha emission
Diagram
Beta emission
Diagram
Gamma emission
Diagram
Radioactive decay series
Diagram

5. Random decay Radioactive Decay 3 Atom Random decay : assumptions
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Random decay
Random decay : assumptions
Diagram
Activity of the source : number of disintegrations per second of the source
Half-life of a radionuclide :
Time required for half of the radionuclides in the source to undergo radioactive decay
Diagram

6. Applications Uses of radioisotopes Atom
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Applications
Radiotherapy : killing of cancer cells
Sterilisation : killing of bacteria and viruses
Tracers
Explanations
Thickness gauge
Diagram
Tracing and monitoring flow systems
Diagram
Smoke detector
Photo
Carbon dating
Diagram

7. Nuclear fission and fusion
Atom
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Nuclear fission and fusion
Nuclear fission : Uranium-235
Diagram
Controlled and uncontrolled chain reactions
Diagram
Nuclear fusion : Hydrogen-2 & Hydrogen-3
Diagram
Nuclear debate

8. END of Atom

9. Back to
Radioactivity
Introduction 1
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Rutherford

10. Radioactivity Introduction 1 Next Slide
A piece of thin gold foil is placed in front of a radium source as shown in the following figure.
A zinc sulphide screen is used to detect the path of alpha particles passing through the foil.
movable
detector
gold foil 
source

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Most of the particles pass through the gold foil without any deflection.
Some particles are deflected and few (1 in 8000) is reflected backwards.
movable
detector
gold foil 
source

12. Radioactivity Introduction 1 Next Slide
Atomic model to explain the results :
i. Nucleus is a small point at the centre containing most of the mass of an atom.
ii. There are two kinds of particles (protons & neutrons) in the nucleus. Each has a mass 1800 times that of an electron.
iii. A strong force, which is called nuclear force, holds the protons and neutrons together in the nucleus.
iv. Proton carries a +ve charge of the same magnitude as that of an electron.

13. Radioactivity Introduction 1 Next Slide
v. Electrons orbit around the nucleus at fixed energy levels which are called electronic shells. They constitute the “skin” of an atom. Most of an atom is empty space.
vi. The no. of electrons and protons are the same to form a neutral atom.

14. Radioactivity Introduction 1 Next Slide
Atomic model of a helium atom :
electron
nucleus with 2
protons and 2
neutrons

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Scattering of  particles
gold atoms
alpha particles

16. Back to Radioactivity Radioactive Decay 1 Click Back to
A radionuclide is shown below :
92 p
146n
90p
144n 2p 2n
Uranium-238
Thorium-234
 particle
Parent
nucleus
Daughter
Decay products
Z and A are always conserved on both sides of the equation.

17. Back to Radioactivity Radioactive Decay 1 Click Back to
Alpha emission is shown below :
92 p
146n
90p
144n 2p 2n
Uranium-238
Thorium-234
 particle

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Beta emission is shown below :
90 p
144n
91p
143n
Thorium-234
Protactinium-234
 particle
When an electron is emitted, one neutron is changed to proton.
The mass of an electron is very small compared with an proton or neutron, its mass no. is considered as zero.

19. Back to Radioactivity Radioactive Decay 1 Click Back to
Gamma emission is shown below :
90 p
144n
Thorium-234
(excited state)
 ray
(normal)
Sometimes a nuclide may contain more energy than usual (e.g. after emitting  or  particle). We say that it is in an excited state. The extra energy may be emitted in the form of EM waves ( ray).

20. Radioactivity Radioactive Decay 1 Next Slide
If the decay process repeats again and again for each daughter nuclides until a final stable nuclide is produced, then we have a decay series.
The decay series could be shown graphically.

21. Back to Radioactivity Radioactive Decay 1 Click Back to
Z (atomic no.)
N (neutron no.) 92 88 84 80 76 96
145
143
141
139
137
 decay
 decay

22. Back to Radioactivity Radioactive Decay 3 Click Back to
Radioactive decay is a random uncontrolled process.
No definite answers to questions like :
i. Which nucleus will undergo disintegration next?
ii. When will a specific nucleus decay?
iii. Where does the emitted particle go?

23. Radioactivity Radioactive Decay 3 Next Slide
The decay of a sample of radium-226 to radon-222 is illustrated. The half-life of radium-226 is 1620 year. Originally, we have 80 million radium-226.
radium-226
radon-222
80 million
40 million
20 million
10 million
5 million
0 million
60 million
70 million
75 million
time
0 year
1620 year
3420 year
4860 year
6840 year
half-life

24. Radioactivity Radioactive Decay 3 Next Slide
A graph of no. of undecayed nuclei vs. time is shown below :
Since activity is directly proportional to no. of radionuclides, the graph of activity vs time also has the same half-life:
time
no. of undecayed nuclei
activity

25. Radioactivity Radioactive Decay 3 Next Slide
Therefore, we can measure the activity of a source and hence deduce the half-life of the source. However, under normal situations, we measure the activity as well as the background radiation as shown :
activity
time
Background
radiation

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We subtract the background radiation from the activity to get the actual activity and hence deduce the half-life.
time
activity

27. Back to Radioactivity Uses of radioisotopes Click Back to
We insert a small amount of weak radionuclide into a system. Then we can use a GM tube to detect the radiation as well as the flowing process of the system, e.g. bloodstream or water pipe.

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Paper produced by a factory passes through a strontium-90 (beta source) and G.M. tube as shown.
Since beta radiation is partly absorbed by the paper, the reading detected by the G.M. tube could be used to monitor the thickness of the paper produced.
source

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Gamma source may be used to detect any leakage in the water pipe as shown. If there is any leakage, the radiation may be detected by the G.M. tube.
radiation detected by G.M. tube

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A smoke detector is shown below :

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The ratio of carbon-14 (radionuclide : half-life : 5600 years) to carbon-12 in atmosphere is a constant.
This ratio takes on the same value in animal and plant’s bodies due to respiration.
However, for dead animals or plants, the ratio changes as carbon-14 undergoes radioactive decay.
By measuring the difference between this ratio in a dead body and the normal value, we can deduce the time of death of the animal or plant.

32. Radioactivity Fission and Fusion Next Slide
Uranium-235, which constitutes about 0.7% of natural uranium, can undergo a fission when bombarded by a slow neutron as shown in the following equation.
The total mass of the product is smaller than the parent nuclides. The lost mass has been turned into energy.
The neutrons produced would trigger other uranium-235 to undergo the same reaction. The process repeats again and again and we have a chain reaction.

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If the mass of the uranium is larger than a certain limit, the chain reaction takes place very quickly.

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Uncontrolled chain reaction : atomic bomb
Controlled chain reaction : nuclear reactor to generate electricity

35. Back to Radioactivity Fission and Fusion Click Back to
If two light nuclei are joined to form a heavy nucleus, fusion occurs.
The fusion of two isotopes hydrogen-2 and hydrogen-3 is shown below :
It is again a chain reaction. Now we cannot use controlled fusion to gain energy. An uncontrolled fusion is actually an hydrogen bomb.