Radioactivity Radiation stream of particleswaves Radioactive Materials material

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. 1 a) – 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 -12 m

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 5 mm Aluminium • Gamma ( ) – very weak ( no charge) – never fully absorbed, 25 mm 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 ( high penetrating power) • Inside body: alpha ray is most dangerous ( 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