Radioactivity I Content Radioactive substance Three types of

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 = Helium particle Electron § 2. Beta β = § 3. Gamma γ = He++ 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 electron gas molecule Free electron Radiation passes through gas Remove electron The gas molecule becomes a positive ion The free electron attaches to other molecule to form a negative ion Free charge carriers conduct electricity

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

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

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

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

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

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

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

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

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

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

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

Summarize the properties of the three types of radiation Alpha particle Beta particle γ wave charge +2 -1 no charge speed < 1/10 speed of light < 9/10 speed of light ionizing ability strong weak very week Penetrating power Stopped by a sheet of paper stopped by 5 mm of aluminium reduced to half by 25 mm of lead effect of fields small deflection large deflection no deflection detectors 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 l l radiation damages the cells of living organisms by ionizing the molecules inside increase the probability of getting cancer Damages the genes and affect the heath of next generation § (2) Background radiation (if excessive) l l (i) cosmic radiation (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 uses l l l Radiotherapy (kill cancer cells) Sterilization of equipments Tracers (e. g. brain scan) Answer γ γ β or γ § Industrial use l l l Thickness gauge Smoke detector Sterilization of foods Radioactive lightning conductor Tracer for underground water pipes β α γ

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