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Radioactivity

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Radioactivity
• Radiation:
– stream of particles/waves
• Radioactive Materials:
– material that emit penetrating/dangerous
radiation
– radiation comes from nucleus of atom
Discovery of Radioactivity
• Becquerel (1896)
– blackening of photographic plate in a
drawer with uranium salt
• Today
– Placing a radioactive source over sealed
film (fig.27.1a)
– Shadow of key: radiation can penetrate
through plastic but not metal
Ionizing Effect
• Nuclear radiation ionizes gas molecules as
they move along
• Positive and negative ions are produced n
their path of the radiation
• Radiations with high charge and lower
speed produce more ions
Detecting Radiation
•
•
•
•
Photographic Plate
Diffusion Cloud Chamber
Spark Counter
Geiger-Muller Tube
Detector: Photographic Plate
• Radiation causes blackening of
photographic plate
• Level of blackening indicates the level of
radiation
• Radiation worker wears a film badge to
check their exposure to radiation
Detector: Cloud Chamber
• Cold chamber filled with alcohol vapour
• Radiation cause alcohol vapour to
condenses
• The condensed droplet shows white tracks
in the cloud chamber
• More ionization, thicker track
Detector: Spark Counter
• High voltage set up between gauze and wire
• Radiation can produce sparks between the
wire and the gauze
• More radiation will produce more sparks
Detector: GM-Counter
• A GM-tube can detect every radiation
comes in
• A counter/ratemeter records the total
number of radiation
• GM-Tube + Counter/ratemeter
= GM-Counter
Three Types of Radiation
• Alpha ()
– helium nucleus : 2 proton+ 2 neutron
– charge = +2, heavy
• Beta ()
– fast moving electron
– charge = -2, light
• Gamma ()
– short wavelength electromagnetic wave
– no charge, wavelength 10-12m
Ionizing Ability
• Alpha ()
– strong ( big charge, slow)
• Beta ()
– weak ( small charge, fast)
• Gamma ()
– very weak ( no charge)
Penetrating Power
• Alpha ()
– weak ( big charge, slow)
– stopped by a sheet of paper
• Beta ()
– weak ( small charge, fast)
– stopped by 5mm Aluminium
• Gamma ()
– very weak ( no charge)
– never fully absorbed, 25mm lead absorb half
Deflection in Magnetic field
• Alpha ()
– few deflection ( heavy)
• Beta ()
– large deflection ( light)
• Gamma ()
– no deflection ( no charge)
Track in Cloud Chamber
• Alpha ()
– straight thick track ( big charge, heavy)
• Beta ()
– thin twisted track
–  small charge, mass: bounce off on collision
• Gamma ()
– very weak track
–  no charge, they sometimes ionize air
molecules
Radiation Hazard
•
•
•
•
Destroy or damage living cells
lead to cancer
Genetic effect (e.g. abnormal fetus)
Outside body: gamma ray is most
dangerous (пЃ‘high penetrating power)
• Inside body: alpha ray is most dangerous
(пЃ‘high ionizing power)
Safety Precaution
•
•
•
•
•
•
Use sealed, weak source in school
store in lead container in remoted area
handle source with forceps
never point the source to human
keep the source at arm’s length
warn others by a warning sign
Background Radiation
• There are small amount a radiation everywhere.
• Sources of Background radiation:
– cosmic ray
– radioactive materials in rock, soil, food, ...
– radioactive gas
– medical diagnosis
– other: nuclear bomb, nuclear reactors, nuclear
waste…..
Radiation dose
•
•
•
•
Effective dose is measured in Sv
< 0.1 Sv : no health effect
0.1 to 2 Sv : cancer
> 10 Sv : fatal
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