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Background radiation There is a small amount of radiation around us all the

time because of radioactive materials in theenvironment. This is called background radiation. Itmainly comes from natural resources such as soil,rocks, air, building materials, food, drink and evenspace. Much of the radiation we are exposed to iscosmic radiation streaming down through theatmosphere.

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Characteristics of three kinds of

radiation

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-particles -particles -particles

Made up of heliumnucleus

Made up of electrons Electromagnetic waves

Positive charge Negative charge No charge

Higher mass Lower than alpha Very light

0.1 times the speed of

light

0.9 times the speed of

light

Same as the speed of

light

Strong ionising effect weak ionising effect Very weak ionising effect

Not very penetrating.

By a few centimetre ofair. Thick sheet ofpaper can stop it.

Penetrating, but stopped

a few centimetres ofaluminium or othermetal.

Very penetrating; never

completely stopped, leador thick concrete willreduce its intensity.

Deflected by electricand magnetic fields

Deflected by electric andmagnetic fields

No effect.

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Radiation detectionA Geiger counter detects incoming radiation by itsionizing effect on enclosed gas in the tube.

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Geiger-Muller (GM) tube

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This can be used to detect alpha, beta, and gammaradiation. The window at the end is thin enough foralpha particles to pass through. If an alpha particleenters the tube, it ionizes the argon gas inside. This

sets off a high-voltage spark across the gas and a pulseof current in the circuit. A beta particle or burst ofgamma radiation has the same effect. The G.M.counter can detect the radiations but it cannot

differentiate between the different types of radiations.

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Cloud Chamber It is a chamber in which the track of an ionising

particle can be made visible. The chamber contains asupersaturated vapour (alcohol or water) in a dust-freeatmosphere. The vapour is made to be very cold whenthe ionising particle passes through the chamber. Thevapour then condenses on the ions formed revealingthe track of the particle as a trail.

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The tracks formed in the cloud chamber aredistinguished as follows:

(a) Tracks of alpha particles are straight (because theyare massive) and are thick (because they have a strongionising power).

(b) Tracks of beta particles are much fainter (because

they have a low ionising power). Their lines are notstraight (because they can be easily deflected bycollisions as their mass is very small).

(c) Tracks of gamma rays are very short and faint; they

are produced due to indirect ionisation caused bygamma rays.

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The range of radiations in air The range of alphas in air is only few centimetres,

The range of betas in air is usually one metre or more,

The range of gammas in air is very large.

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The absorption of radiations by different

materialsAlpha particles are completely absorbed by a sheet of

paper

Beta particles are completely absorbed by a fewmillimetres of aluminium.

Gamma rays are very penetrating and cannot becompletely absorbed even by very thick sheets of lead.

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Deflection of alpha, beta and gamma by

magnetic fields

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The alpha beam is a flow of positively (+) chargedparticles, so it is equivalent to an electric current. It isdeflected in a direction given by Flemings left-handrule

The beta particles are much lighter than the alphaparticles and have a negative (-) charge, so they aredeflected more, and in the opposite direction.

Being uncharged, the gamma rays are not deflected by

the field.

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Radioactive decay The particles and waves radiate from the nucleus, so

they are sometimes called nuclear radiation.

Materials which emit nuclear radiation are known as

radioactive materials. The disintegration of a nucleusis called radioactive decay.

If an isotope is radioactive, it has an unstablearrangement of neutrons and protons in its nuclei. The

emission of an alpha or beta particle makes thenucleus more stable, but alters the numbers of protonsand neutrons in it. So it becomes the nucleus of adifferent element.

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Radioactive decay happens spontaneously (all byitself) and at random.

When a nucleus emits an alpha or a beta particle, a

different element is formed. The changing of one element to another is called

transmutation.

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When an alpha particle is ejected, atomic numberdecreases by 2 and mass number reduces by 4.

88Ra226

86Rn222

+

2He4

When a beta particle is ejected, atomic number

increases by 1 and mass number remains the same.6C14

7N14 +

-1e0

Gamma emission has no effect on the mass numberor the atomic number.

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Radio active Half - life Radioactive isotopes decay at different rates. The

radioactive decay rate is measured in terms of acharacteristic time, the half-life.

The half-life of a radioactive material is the timeneeded for half of the radioactive atoms to decay.

Radium-226, for example, has a half-life of 1620 years.

This means that half of any given specimen of Ra-226will have undergone radioactive decay by the end of1620 years.