Radioactivity 1 輻射 Presented by Mr. Y. N. Chan LIU PO SHAN MEMORIAL COLLEGE.

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Presentation transcript:

Radioactivity 1 輻射 Presented by Mr. Y. N. Chan LIU PO SHAN MEMORIAL COLLEGE

History 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 ? 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 Radiation   -rays   -rays   -rays There are THREE types of radiation:  Helium Nucleus  Fast-moving Electrons  Light waves with very short wavelength

Unstable Nucleus Revision Alpha Particle Gamma Ray Beta Particle More Stable Nucleus

Properties of Radiation Charge and Mass Deflection in Electric Field Deflection in Magnetic Field Ionization Power Penetration Power Range in air

Charge and Mass Electron Light Wave (short )  -particle  -particle  -ray Charge Mass +2 Nil 4 Negligible Nil Helium Nucleus

Deflection of Electric Field +ve -ve   

Deflection of Magnetic Field  -ray  -ray  -ray Magnetic Field What is the direction of the magnetic field ?

What are the penetrating power of ,  and  ray ?  - rays  - rays  - rays

The penetrating power of ,  and  radiation (1)  - rays  - rays  - rays Paper

The penetrating power of ,  and  radiation (2)  - rays  - rays  - rays Paper5mmAl

The penetrating power of ,  and  radiation (3)  - rays  - rays  - rays Paper5mmAlLead 100%50% 2 cm

Ionization Power (1) It is the ability of the radiation to ionize the air molecule. Ionization Power:  >  > 

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 of ,  and  radiation  - rays  - rays  - rays 5 cm about 5 m Over 100 m

Detectors of Radiation Photographic Film G.M. tube Cloud Chamber

Tracks in Cloud Chamber (1)  -particles  -particles  -rays

Tracks in Cloud Chamber (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 CharacteristicsReason

Tracks in Cloud Chamber (3) Zig-zag path diffuse Mass is smaller Range is longer  -particles CharacteristicsReason

Background 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)  immediate death  cause cancer  genetic changes Large Dose Small Dose Very small Dose

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 1 END