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

Unstable Nucleus

RevisionRevision

Alpha Particle

Gamma Ray

Beta Particle

More StableNucleus

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 Electric FieldDeflection of Electric Field

+ve

-ve

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

Detectors of RadiationDetectors of Radiation

• Photographic Film

• G.M. tube

• Cloud Chamber

Tracks in Cloud Chamber Tracks in Cloud Chamber (1)(1)

-particles -particles -rays

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

Radioactivity 1Radioactivity 1

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