Radioactivity I §Content: §Radioactive substance §Three types of radiation §Properties of radiation §To investigate the radiation by apparatus §To summarize the properties of the three types of radiation §Radiation hazards §Which type of radiation is suitable for each of the following applications?

Radioactive Substance §Some material emits radiation §To blacken photographic film §they are penetrating and dangerous.

Three types of radiation  1.Alpha α  3.Gamma γ  2.Beta β Helium particle He ++ Electron e - Electromagnetic wave = = =

Properties of radiation §I.Ionizing power §II.Penetrating power §III.Deflection in electric field §IV.Deflection in magnetic field

I. Ionizing effect of radiation Radiation passes through gas Free electron Remove electron The gas molecule becomes a positive ion gas molecule The free electron attaches to other molecule to form a negative ion Free charge carriers conduct electricity electron

paper aluminium (5 mm) lead (25 mm) II.Penetrating power α source γ source β source

III.Deflection in electric field α source γ source β source _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ α γ β

× × × × × × × Magnetic field (Points into the paper) IV.Deflection in magnetic field α source γ source β source α γ β current magnetic force no magnetic force current magnetic force

Investigate the radiation by apparatus §I.Spark counter §II.Geiger Muller tube (GM counter) §III.Cloud chamber §The working principle of them is based on ionizing power

EHT - + Sparks are observed Very high voltage When α radiation passes through the metallic gauze Ionization of gas molecules occur Free ions are accelerated between the gauze and the wire I. Spark counter metallic gauze conducting wire

II. Geiger Muller counter Scaler or ratemeter 400V d.c. thin mica window Aluminium tube (-ve ) central wire (+ ve ) Argon gas at low pressure an electric pulse arise Radiation enter the tube Ionization of argon gas molecules Conduction of electricity A count is recorded Free charge carriers move to respective electrodes no conduction now current flow

III. Diffusion cloud chamber invisible alcohol vapour diffuses from the ring foam dry ice felt ring soaked in alcohol plastic lid insulation weak source dry ice cools down the chamber alcohol vapour becomes saturated radiation ionizes the alcohol vapour along its path alcohol vapour condenses around the ions -droplets formed -tracks are observed with strong illumination radiation passes through the vapour alcohol vapour becomes supersaturated in the cooler region strong illumination

Cloud chamber photographs α source The tracks of α source - straight - thick - all tracks about the same length (the tracks are not coplanar) Explanation - the greatest mass and charges among the 3 types of radiation - higher ionizing power

β source Cloud chamber photographs The tracks of β source - thin - twisted or zigzag Explanation - smaller mass and charge of β particle - low ionizing power - bounce off from the gas molecules - continuous range of K.E.

γ source Cloud chamber photographs The tracks of γ source - similar to that of β - the tracks are much shorter and thinner Explanation - electromagnetic γ wave remove the electrons from the gas molecules - these electrons behave like β particles

Right-angled fork tracks α source The occurrence of right-angled fork tracks - collision between two equal mass particles only - an evidence shows that α particle is helium particle filled with helium gas - rare photograph

Summarize the properties of the three types of radiation Alpha particle Beta particle charge speed ionizing ability Penetrating power effect of fields detectors γ wave +2 no charge < 1/10 speed of light < 9/10 speed of light speed of light strong weak very week Stopped by a sheet of paper stopped by 5mm of aluminium reduced to half by 25 mm of lead small deflection large deflection no deflection photographic film cloud chamber spark counter thin window GM tube photographic film cloud chamber GM tube Photographic film Cloud chamber GM tube

Radiation hazards §(1) Biological hazards l radiation damages the cells of living organisms by ionizing the molecules inside l increase the probability of getting cancer l Damages the genes and affect the heath of next generation §(2)Background radiation (if excessive) l (i) cosmic radiation l (ii) radiation from environment e.g.rocks, concrete, X-ray tube and TV

Which type of radiation is suitable for each of the following applications? §Medical usesAnswer l Radiotherapy (kill cancer cells) l Sterilization of equipments l Tracers (e.g. brain scan) §Industrial use l Thickness gauge l Smoke detector l Sterilization of foods l Radioactive lightning conductor l Tracer for underground water pipes γ γ β or γ β α α γ γ

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