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Radioactivity I
 Content:
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Radioactive substance
Three types of radiation
Properties of radiation
To investigate the radiation by apparatus
To summarize the properties of the three types of
radiation
 Radiation hazards
 Which type of radiation is suitable for each of the
following applications?
Radioactive Substance
 Some material emits radiation
 To blacken photographic film
 they are penetrating and dangerous.
Three types of radiation
 1. Alpha
α
=
Helium particle
Electron
 2. Beta
β
=
 3. Gamma
γ
=
He++
e-
Electromagnetic
wave
Properties of radiation
 I. Ionizing power
 II. Penetrating power
 III. Deflection in electric field
 IV. Deflection in magnetic field
I. Ionizing effect of radiation
electron
gas
molecule
Free
electron
Radiation passes through gas
Remove electron
The gas molecule becomes a positive ion
The free electron attaches to other molecule to form a
negative ion
Free charge carriers conduct electricity
II. Penetrating power
α source
β source
γ source
paper
aluminium
(5 mm)
lead
(25 mm)
III. Deflection in electric field
+ + + + + + + + + + + + + + +
α source
β
α
γ source
γ
β source
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _
IV. Deflection in magnetic field
×
×
magnetic force
α source
×
×
×
×
×
×
×
×
α
×
×
×
×
×
×
×
×
×
current
×
γ source
β source
×
no magnetic force
×
×
×
×
×
×
γ×
×
×
×
×
×
×
×
current
×
Magnetic field
×
×
×
×
× β ×
×
×
(Points into the paper)
×
×
×
×
magnetic force
×
Investigate the radiation by
apparatus
 I. Spark counter
 II. Geiger Muller tube (GM counter)
 III. Cloud chamber
 The working principle of them is based on
ionizing power
I. Spark counter
Whenα radiation passes through the metallic gauze
Ionization of gas molecules occur
Free ions are accelerated between the gauze and the wire
Sparks are observed
Very high voltage
metallic
gauze
conducting
wire
II. Geiger Muller counter
central wire (+ ve )
Argon gas
at low pressure
no conduction
current flownow
400V d.c.
thin mica
window
Radiation
enter the tube
Aluminium tube (-ve )
Ionization of
argon gas molecules
A count
is recorded
Scaler
or
an
electric
pulse
arise
ratemeter
Free charge carriers move to
respective electrodes
Conduction
of electricity
III. Diffusion cloud chamber
plastic lid
felt ring soaked in alcohol
alcohol
vapour
invisible
alcohol
vapour
becomes
saturated
diffuses
from
the ring
dry ice cools down
the chamber
weak source
-droplets
formed
alcohol
vapour
condenses the
radiation
radiationionizes
passes
alcohol vapour becomes
supersaturated
-tracks
arethe
observed
around
ions
alcohol
vapour
through
the along
vapourits path
in the cooler region
with strong illumination
strong
illumination
dry ice
insulation
foam
Cloud chamber photographs
The tracks of α source
- straight
- thick
- all tracks about the
same length
(the tracks are not coplanar)
Explanation
- the greatest mass and charges
among the 3 types of radiation
α source
- higher ionizing power
Cloud chamber photographs
The tracks of β source
- thin
- twisted or zigzag
Explanation
- smaller mass and charge
of β particle
- continuous range of K.E.
- low ionizing power
β source
- bounce off from the
gas molecules
Cloud chamber photographs
The tracks of γ source
- similar to that of β
- the tracks are much
shorter and thinner
Explanation
- electromagnetic γwave
remove the electrons from
the gas molecules
γ source
- these electrons behave like
βparticles
Right-angled fork tracks
filled with helium gas
The occurrence of right-angled
fork tracks
- rare photograph
- collision between two equal
mass particles only
- an evidence shows that
α particle is helium particle
α source
Summarize the properties of the three
types of radiation
Alpha particle
Beta particle
γ wave
charge
+2
-1
no charge
speed
< 1/10 speed of light
< 9/10 speed of light
speed of light
ionizing
ability
strong
weak
very week
Penetrating
power
Stopped by
a sheet of paper
stopped by
5mm of aluminium
reduced to half by
25 mm of lead
effect of
fields
small deflection
large deflection
no deflection
detectors
photographic film
cloud chamber
spark counter
thin window GM tube
photographic film
cloud chamber
GM tube
Photographic film
Cloud chamber
GM tube
Radiation hazards
 (1) Biological hazards
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radiation damages the cells of living organisms by ionizing
the molecules inside
increase the probability of getting cancer
Damages the genes and affect the heath of next generation
 (2) Background radiation (if excessive)
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(i) cosmic radiation
(ii) radiation from environment
• e.g.
rocks, concrete, X-ray tube and TV
Which type of radiation is suitable for each of
the following applications?
 Medical uses
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Radiotherapy (kill cancer cells)
Sterilization of equipments
Tracers (e.g. brain scan)
Answer
γ
γ
β or γ
 Industrial use
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Thickness gauge
Smoke detector
Sterilization of foods
Radioactive lightning conductor
Tracer for underground water pipes
β
α
γ
α
γ