Radioactivity Discovery of radioactivity Discovery of radioactivity (1896) : Henri Becquerel Next Slide Exposure of film by X-ray Discovery of radioactive material (1898) : Marie Curie and her husband Photo Extract radium from uranium ore Photo Introduction 1

Radioactivity Different kinds of radiation Three different kinds of radiation : Next Slide alpha particle (  ), beta particle (  ), gamma ray (  ) Common sources in school laboratory : i. americium source for alpha radiation ii. strontium source for beta radiation iii. cobalt source for gamma radiation Introduction 2

Radioactivity Nature of different kinds of radiation Next Slide Emission of radiation from a nucleus Penetrating power and range in air Ionizing effect Effects of electric field and magnetic field on the paths of radiation Diagram Nature of radiation

Radioactivity Different methods of detecting radiation Photographic method : exposure of film Next Slide Scintillation counters Spark Counter Geiger-Muller (GM) tube Cloud chamber Diagram Detection of radiation

Radioactivity Radiation Hazards Safety precautions Next Slide Background radiation Explanation Radiation hazards

END of Radioactivity

Radioactivity Henri Becquerel Click Back to Back to Introduction 1

Radioactivity Click Back to Back to Maria Curie Introduction 1

Radioactivity Click Back to Nature of different kinds of radiation Back to Gamma (  )Alpha particle (  )Beta particle (  ) EM waves with wavelength Helium nucleus Fast moving electron none 2 protons + 2 neutrons 1 electron none Mass ratio : 7200 Mass ratio : 1 none Charge : +2e Charge :  1e < 0.1 c< 0.9 cc Nature of radiation

Radioactivity Click Back to Emission of radiation from the nucleus Back to protonneutron Gamma ray Beta particle Alpha particle Nature of radiation

Radioactivity Next Slide Alpha particle : Range in air : 5 cm Beta particle : Weak penetrating power (stopped by aluminum foil of thickness 5 mm) Range in air : 5 m Gamma ray : Strong penetrating power (cannot be completely absorbed by several cm of lead) Range in air : 100 m Very weak penetrating power (stopped by a piece of paper) Nature of radiation

Radioactivity Click Back to The penetrating power of different kinds of radiation is summarized in the following diagram. Back to a sheet of paper aluminium (several mm) lead (several cm) Nature of radiation

Radioactivity Next Slide As radiation passes through a gas, some gas molecules would be ionized as shown in the following figure. radiation positive ion electron negative ion gas molecule A molecule loses an electron and becomes a positive ion. Another molecule gains the electron and becomes negative ion. Nature of radiation

Radioactivity Click Back to Alpha particle : Strong ionizing effect (large amount of ions are produced as it passes through the air) Beta particle : Weak ionizing effect (small amount of ions are produced as it passes through the air) Gamma ray : Very weak ionizing effect (very small amount of ions are produced as it passes through the air) Back to Nature of radiation

Radioactivity Next Slide The effect of electric field on the paths of different kinds of radiation is shown below :    lead box source Nature of radiation

Radioactivity Next Slide Gamma ray : No effect since gamma ray poses no charge Alpha particle : Deflected to the negative charged plated since an alpha particle poses positive charge Beta particle : Deflected to the positive charged plated since a beta particle poses negative charge The experiment must be done in vacuum since the range of alpha particles in air is small. Since an alpha particle has a larger mass than a beta particle, its deflection is smaller than that of the beta particle. Nature of radiation

Radioactivity Next Slide The effect of magnetic field on the paths of different kinds of radiation is shown below :    lead box source magnetic field lines going into the screen Nature of radiation

Radioactivity Click Back to Gamma ray : No effect since gamma ray poses no charge Alpha particle and beta particle : Treat the radiation as flows of +ve charge (current) and -ve charge respectively. We can find the directions of the force acting on the particles and hence the directions of deflection. The experiment must be done in vacuum since the range of alpha particles in air is small. Since an alpha particle has a larger mass than a beta particle, its deflection is smaller than that of the beta particle. Back to Nature of radiation

Radioactivity Click Back to A scintillation counter is shown below : Back to Detection of radiation

Radioactivity Click Back to A spark counter is shown below : Back to Detection of radiation

Radioactivity Next Slide A Geiger-Muller (GM) tube is shown below : Detection of radiation

Radioactivity Next Slide A Geiger-Muller (GM) tube with scalar is shown below : Detection of radiation

Radioactivity Click Back to The ions produced inside the tube would be accelerated to the electrodes and produce more ions on their paths. Back to radiation electrodes 400 V d.c. scalar or ratemeter The electrical pulses produced are detected by the scalar and ratemeter. Detection of radiation

Radioactivity Next Slide Cloud chamber: By courtesy of Prof. S. Y. Mak Cloud chamber with tracks Detection of radiation

Radioactivity Next Slide Structure of a cloud chamber is shown below : Detection of radiation

Radioactivity Next Slide Alcohol vapour condenses around the ions produced by the radiation and forms white tracks. source radiation dry ice layer foam full of alcohol vapour transparent plastic chamber Different kinds of radiation produce different amount of ions and hence different white tracks. Detection of radiation

Radioactivity Click Back to Alpha particles : thick, short and clear tracks Back to Beta particles : thin and twisted tracks Gamma ray : very unclear and diffuse tracks Detection of radiation

Radioactivity Click Back to Lead container should be used to store and transport all radioactive sources. Back to Do not use bare hand to touch or handle the sources. Forceps should be used instead. The sources should be at least one arm’s length away from your body and other people’s bodies. Never point the sources towards the eyes. Radiation hazards

Radioactivity Click Back to Sources of background radiation (natural radiation) Back to i.cosmic rays ii.natural radioactive materials on earth iii.medical diagnosis (X-ray) iv.industrial and scientific nuclear sources and wastes v.radon and thoron (radioactive gases) from rocks and building materials (concrete) Radiation hazards