Radioactivity 1Radioactivity 1輻 射輻 射
Presented byPresented by Mr. Y. N. ChanMr. Y. N. Chan
LIU PO SHAN MEMORIAL COLLEGELIU PO SHAN MEMORIAL COLLEGE
History of RadioactivityHistory of Radioactivity
• In 1896, Becquerel noted that radiation from uranium affected photographic plates.
• In 1899, Curie noted that thorium and radium showed also the same effect on photographic plates.
What is Radioactivity ?What is Radioactivity ?
• Radioactivity are reactions through which the unstable nuclei become more stable.
• Energetic radiations will be emitted during the process.
Unstable Nucleus
More Stable
Nucleus
+ Radiation
Types of RTypes of Radiationadiation
-rays
-rays
-rays
• There are THREE types of radiation:
Helium Nucleus
Fast-moving Electrons
Light waves with very short wavelength
Properties of RadiationProperties of Radiation
• Charge and Mass
• Deflection in Electric Field
• Deflection in Magnetic Field
• Ionization Power
• Penetration Power
• Range in air
Charge and MassCharge and Mass
Electron Light Wave (short )
-particle -particle -ray
Charge
Mass
+2 -1 Nil
4 Negligible Nil
Helium Nucleus
Deflection of Magnetic FieldDeflection of Magnetic Field
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
-ray
-ray
-ray
MagneticField
What is the direction of the magnetic field ?
What are the penetrating power ofWhat are the penetrating power of , , andand ray ?ray ?
- rays
- rays
- rays
The penetrating power ofThe penetrating power of , , andand radiation (1)radiation (1)
- rays
- rays
- rays
Paper
The penetrating power ofThe penetrating power of , , andand radiation (2)radiation (2)
- rays
- rays
- rays
Paper 5mmAl
The penetrating power ofThe penetrating power of , , andand radiation (3)radiation (3)
- rays
- rays
- rays
Paper 5mmAl Lead
100% 50%
2 cm
Ionization Power (1)Ionization Power (1)
• It is the ability of the radiation to ionize the air molecule.
• Ionization Power: > >
Ionization Power (2)Ionization Power (2)
• (1) Its momentum is the greatest
• (2) Its charge is the greatest
• (3) Its speed is the slowest
-ray has the greatest ionization power because
The range in air ofThe range in air of , , andand radiationradiation
- rays
- rays
- rays
5 cm
about 5 m
Over 100 m
Tracks in Cloud Chamber Tracks in Cloud Chamber (2)(2)
• Short• Thick• Straight• Fork track may
be seen
• Range is small• Ionization is great• Momentum is large• Elastic collision with
helium gas atom
-particles
Characteristics Reason
Tracks in Cloud Chamber Tracks in Cloud Chamber (3)(3)
• Zig-zag path• diffuse
• Mass is smaller • Range is longer
-particles
Characteristics Reason
Background RadiationBackground Radiation
• Radiation is detected even when no radioactive substance is present.
• Cosmic Radiation (from outer space)• Radiation coming from the environment
(e.g. rock, concrete, artificial sources)• Radon gas
Sources:
Experimental Result :
Biological Hazards (1)Biological Hazards (1)
immediate death
cause cancer
genetic changes
• Large Dose
• Small Dose
• Very small Dose
Biological Hazards (2)Biological Hazards (2)
-rays is the most dangerous. (penetrating power is highest)
-rays is the most dangerous. (ionization power is highest.)
• Outside the body
• Inside the body
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