radioactivity

1
radioactivity
2
what you need to know

• name the common detectors for alpha-particles,
  beta-particles and gamma-rays
• show understanding that radioactive emissions
  occur randomly over space and time
• distinguish between the three kinds of emissions
  in terms of (i) their nature (ii) their relative
  ionising effects (iii) their relative penetrating
  powers
• explain what is meant by radioactive decay, using
  equations (involving symbols) to represent
  changes in the composition of the nucleus when
  particles are emitted
• explain what is meant by the term half-life
• apply understanding of half-life to solve simple
  problems
• describe how radioactive materials are handled,
  used and stored in a safe way

3
types of radiation

• (a) alpha particles
• Consist of two protons and two neutrons
• Helium nucleus
• Is considered massive!

4
types of radiation

• (b) beta particles
• Are high energy electrons
• Small and light

5
types of radiation

• (c) gamma particles
• Are electromagnetic waves
• no weight or size!

6
properties

• Ionization
• Ability to make ions wherever they go!

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properties

• Ionization
• Ability to make ions wherever they go!

?
?
?
Mostionizing
Leastionizing
Moderatelyionizing
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properties

• Penetrating effect
• Ability to travel how far

9
properties

• Penetrating effect
• Ability to travel how far

?
?
?
Most penetrating
Least penetrating
Moderately penetrating

• can penetrate a few cm of air
• can be stopped by skin/ paper

• can penetrate through a few metres of air
• can be stopped by 5 mm thick aluminium sheet

• can penetrate through a few hundred metres of air
• can be stopped by thick lead or a few meters of
  concrete

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properties

• charge

?
?
?
2 protons 2 neutrons
Electromagnetic wave
1 electron
-
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properties

• deflection in an electric field

-
- - - - - - - - - - - - - - - -
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properties

• deflection in an electric field

?
?
?
Not deflected
Deflectedlike positively-charged particles
Deflected like negative-charged particles
13
properties

• deflection in a magnetic field

X X X X X X X X X
X X X X X X X X X
X X X X X X X X X
X X X X X X X X X
-
X X X X X X X X X
X X X X X X X X X
X X X X X X X X X
X X X X X X X X X
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properties

• deflection in a magnetic field

?
?
?
Deflected like positively-charged
particles (Imagine direction of conventional
current)
Deflected like negative-charged
particles(Imagine direction of electron current)
Not deflected
15
detection

• Photographic detectors
• All three types of radiation cause photographic
  film to blacken

16
detection

• (b) Gold leaf electroscope
• Radiation causes ionization
• ? negative and positive ions are formed
• ? ions will be attracted to the cap of charged
  electroscope and collapse the leaves

?
?
?
?







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detection

• (c) Geiger müller tube
• Radiation enters the tube ionizes the argon
  atoms
• argon ions electrons formed and attracted to
  the anode and cathode
• form pulses of current
• pulses are amplified counted over time period

?
?


To rate meter
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detection

• (c) Diffusion cloud chamber
• Radiation causes ionization of the air
• Chamber is saturated with water alcohol vapour
• ? supersaturated because of cold temperature
• ? particles, ions passing through causes trails
  of water droplets as the alcohol and water
  condense on the ions produced

Felt pad soaked with alcohol
Radioactive source
Solid CO2
Metal base
Sponge
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detection

• (c) Diffusion cloud chamber

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properties

• radioactive decay
• radioactive nuclei are unstable
• become more stable by emitting alpha-particles or
  beta-particles

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properties

• radioactive decay

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properties

• radioactive decay
• radioactive decay is spontaneous
• cannot be controlled
• unaffected by temperature, pressure other
  physical conditions
• impossible to predict which one will decay
• random process

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nuclear processes

• (a) alpha decay

10 P 10 N
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nuclear processes

• (a) alpha decay

A Z
A - 4 Z - 2
4 2
X ? Y He
Parent nuclide
Daughter nuclide
Helium nucleus ?-particle
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nuclear processes

• (b) beta decay

10 P 10 N
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nuclear processes

• (b) beta decay

A Z
A Z 1
0 - 1
X ? Y e
Parent nuclide
Daughter nuclide
Electron ?-particle
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nuclear processes

• (c) gamma emission

( X) ? X ?
A Z
A Z
Excited state
Gamma Radiation Energy
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nuclear processes

• example
• A nucleus contains 92 protons and 144 neutrons.
  What will the new nucleus be like after emitting
• (a) an alpha-particle?
• (b) a beta-particle?

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nuclear energy

• In nuclear reactions
• E mc2
• This equation relates mass and energy!

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fission

• Process in which a heavy nucleus of an atom is
  split into 2 or more fragments
• Usually neutrons and energy is released

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fusion

• Process in which two light nuclei combine to form
  a heavy nucleus
• Energy is usually released because ofmass defect

32
half-life

• definition
• Time taken for half the number of atoms in any
  sample of a radioactive element to decay

2 h
2 h
2 h
2 h
33
half-life

• Different radioactive elements decay at different
  rates
• Proportional to the number of radioactive atoms
  present in the sample at that time

80
70
60
50
counts per second
40
30
20
10
t/ s
0
40
80
120
160
34
background radiation

• radiation due to
• radioactive substances present in the earths
  crust and atmosphere
• cosmic radiation from space

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background radiation

• Cosmic radiation is made up from charged
  particles such as protons and Helium zooming
  through space that originate from the sun and the
  wider universe. When these particles enter the
  earth atmosphere they disrupt atoms producing
  radiation.

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background radiation

• Radiation is always present on the earth, about
  17 of it is from cosmic radiation, about 50
  from radon gas, about 20 from minerals in the
  soil and about 13 from our bodies (from food and
  water).

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Background radiation ________ GM Reading
________
Background radiation 30 cts/ s GM Reading
30 cts/ s
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Background radiation ________ Radioactive
source ________ GM Reading ________
Background radiation 30 cts/ s Radioactive
source 70 cts/ s GM Reading
Background radiation 30 cts/ s Radioactive
source 70 cts/ s GM Reading 100 cts/s
Background radiation 30 cts/ s Radioactive
source GM Reading
39
Background radiation ________ Radioactive
source ________ GM Reading ________
Background radiation 30 cts/ s Radioactive
source GM Reading 50 cts/ s
Background radiation Radioactive source 20
cts/ s GM Reading
40
half-life

• technique 1
• Use arrows

2 h
2 h
2 h
2 h
41
half-life

• technique 2
• Use tables

42
half-life

• example
• A radioactive material has a half-life of 3
  hours. How much of the material will be left
  after 9 hours if the original amount is 24 g?

43
half-life

• example
• A radioactive substance has an initial count of
  480 counts per second. After an hour, the count
  rate has dropped to 60 counts per second. What is
  the half-life of the substance?

44
half-life

• example
• A radioactive substance has a half life of 30
  minutes. Its initial count rate was 100 cts/ s.
  After 1.5 h, the count rate was found to be30
  ct/ s. What is the count rate of the background
  radiation?

45
storage and handling

• Exposure is harmful to health!
• causes burns, bleeding, cataracts, cancer and
  death
• even mutations!

46
storage and handling

• Storage
• Lead boxes
• Display radiation symbol

47
storage and handling

• Handling
• use forceps
• use remote controls and robots
• wear dosimeters (film badges)