Radioactivity IRadioactivity IContent:

Radioactive substanceThree types of radiationProperties of radiationTo investigate the radiation by apparatusTo summarize the properties of the three types of radiationRadiation hazardsWhich type of radiation is suitable for each of the followin

g 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 powerα 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 counterII. Geiger Muller tube (GM counter)III. Cloud chamber

The working principle of them is based on ionizing power

EHT- +

Sparks are observedVery high voltage

Whenα radiation passes through the metallic gauzeIonization of gas molecules occur

Free ions are accelerated between the gauze and the wire

I. Spark counter

metallicgauze

conductingwire

II. Geiger Muller counter

Scaler orratemeter

400V d.c.

thin mica window

Aluminium tube (-ve )

central wire (+ ve )

Argon gasat low pressure

an electricpulse arise

Radiationenter the tube

Ionization of argon gas molecules

Conduction of electricity

A countis recorded

Free charge carriers move to respective electrodes

no conduction nowcurrent flow

III. Diffusion cloud chamber

invisible alcohol vapourdiffuses from the ring

foam

dry ice

felt ring soaked in alcoholplastic lid

insulation

weak sourcedry ice cools down

the chamber

alcohol vapour becomes saturated

radiation ionizes the alcohol vapour along its path

alcohol vapour condensesaround the ions

-droplets formed-tracks are observed with strong illumination

radiation passes through the vapour

alcohol vapour becomes supersaturatedin 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 γwave remove the electrons from the gas molecules

- these electrons behave like βparticles

Right-angled fork tracks

α source

The occurrence of right-angledfork 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

ionizingability

Penetratingpower

effect offields

detectors

γ wave

+2 -1 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 by25 mm of lead

small deflection large deflection no deflection

photographic filmcloud chamberspark counter

thin window GM tube

photographic filmcloud chamber

GM tube

Photographic filmCloud chamber

GM tube

Radiation hazards

(1) Biological hazards 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) (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 Answer Radiotherapy (kill cancer cells) Sterilization of equipments Tracers (e.g. brain scan)

Industrial use Thickness gauge Smoke detector Sterilization of foods Radioactive lightning conductor Tracer for underground water pipes

γγ

β or γ

βα

αγ

γ

Tea time