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10.2 Radioactivity

What radioactivity is?

Radioactivity is the spontaneous disintegration of an unstable nucleus into a more stable nucleus accompanied by the emission of energetic particles (radioactive rays) or photons .

clip_image002

The process is said to be spontaneous because it is not influenced by any physical factors such as time, pressure, temperature, etc.

The decay occurs randomly because each atom has the same probability of decaying at any moment of time.

Three kinds of radiation

There are three kinds of radiation emitted by radioactive materials :

(1) Alpha particles, α

(2) Beta particles, β

(3) Gamma rays, g

Radioactive Detectors

Radioactive detectors make use of the ionisation process to detect radioactive emission (except for the photographic plate).

The following are the common detectors for radioactive emissions.

(1) Photographic Plate or Film

The photographic film or plate can be used as a special badge or tag to record the dosage of radiation a staff at radiation laboratories is exposed to.

The detector works on the principle that radioactive radiation can cause a chemical change on the plate and produce a dark trace.

The degree of darkening of the photographic film indicates the amount of radiation received.

The photographic film can detect all the three types of radioactive radiation.

(2) Gold Leaf Electroscope

clip_image004

When the charged plate of the electroscope is exposed to the source of radioactive , the gold leaf will collapse slowly.

This is due to the ions produced by radioactive source neutralise the charge in the electroscope.

This method is suitable for detecting alpha particles because alpha particles have high ionizing power.

(3) Spark Counter

clip_image006

When the radioactive source is brought near the spark counter , the sparks are formed.

The radioactive rays will ionise the air molecules.

The sparks are formed due to collision between the ions and air molecules.

The spark counter can only trace alpha particle which have high ionising power.

(4) Geiger-Muller tube (GM tube)

clip_image008

A GM tube is a very versatile , sensitive and useful detector of radiation.

When the radioactive radiations enter the GM tube through the mica window and ionises the argon gas. A pulse current is produced and counted by a scaler or ratemeter .

The actual reading of a GM tube is calculated as follow:

clip_image010

Background reading is produced by radioactive materials from Earth and the surroundings such as stones, sand, soils, etc and also from the cosmic rays in the sunlight.

The GM tube can detect alpha particles, beta particles and gamma rays.

(5) Cloud Chamber

clip_image012

When the radioactive rays enter he upper part , the ionisation of air will occur. The ions allow the saturated alcohol vapour to condense forming tiny alcohol droplets and will cause the formation of misty tracks.

The cloud chamber can detect all the three types of radioactive radiation.

clip_image014

The characteristics of radioactive emissions

(1) Natural characteristics

α- particles : Helium nucleus or clip_image016

β- particles : Fast moving electrons or clip_image018

g-rays : Electromagnetic waves

(2) Charge

α- particles : + 2e

β- particles : -e

g-rays : No charge

(3) Speed

α- particles : Up to 10% of speed of light

β- particles : Up to 99% of speed of light

g-rays : Speed of light

(4) Ionising power

α- particles : Strong

β- particles : Medium

g-rays : Very weak

(5) Penetrating power

α- particles : Low

β- particles : Average

g-rays : High

clip_image020

(6) Range in air

α- particles : Several centimetres

β- particles : Several metres

g-rays : Several hundred metres

(7) Effect of electric field

α- particles : Small deflection towards negatively charged plate

β- particles : Large deflection towards positively charged plate

g-rays : No deflection

The size of deflection of α- particles < β- particles because the mass of a- particles >

g– particles.

clip_image022

(8) Effect of electric field

α- particles : Small deflection

β- particles : Large deflection in opposite direction of the a- particles

g-rays : No deflection

The size of deflection of α- particles < β- particles because the mass of α- particles >

g– particles.

The direction of deflection is determined by using Fleming’s left-hand rule.

clip_image024

Radioactive decay

Radioactive decay is the process of nucleus changing to a more stable nucleus while emitting radiation.

The nucleus before decay is called the parent nuclide and the product of decay is the daughter nuclide.

The radioactive decay results in changes in the number of protons and neutrons in the nuclei.

There are several types of decay:

(1) Alpha decay

The general equation of alpha decay is:

clip_image026

When a nuclide decays by emitting an alpha particle its proton number Z decreases by 2 and its nucleon number, A decreases by 4.

For example ;

clip_image028clip_image030 + clip_image032

(2) Beta decay

The general equation of alpha decay is:

clip_image034

When a nuclide decays by emitting an beta particle its proton number Z increases by 1 and its nucleon number, A does not change.

For example ;

clip_image036clip_image038 + clip_image040

(3) Gamma emission

High frequency electromagnetic radiation coming from the nuclei of decaying atom is call gamma radiation.

The general equation of alpha decay is:

clip_image042

Emitting a gamma does not change the atomic number of the atom; it also has very little effect on the mass.

For example ;

clip_image044clip_image044[1] + g

Example 1

Balance the following equations:

(a) clip_image047clip_image049+ clip_image051

(b) clip_image053clip_image047[1]+ clip_image055g

Solution

Example 2

How many alpha particles and beta particles are emitted when clip_image057 decays into clip_image059?

Solution

A decay series

Radioactive substances often decay several times in a series of steps , emitting radiations and producing a new substance at each step.

A parent substance produces daughter and grand-daughter substances in what is called a decay series.

For example :

The decay series of clip_image061can be represented as follows:

clip_image063

Example 3

The diagram shows part of a radioactive decay series.

clip_image065

Name the particles or radiations are emitted at part I, II and III.

Solution

Decay curve

The number of atoms , mass or activity of a radioactive substance decreases with time.

clip_image067

Half-life

The half-life of a radioactive material is the time taken for half of the unstable atoms to decay.

Or

The half-life of a radioactive material is the time taken for the activity of radioactive fall to half its original activity.

Example 4

The half-life of a radioactive material of mass 40 g is 2 hours. Determine the mass of the radioactive material that has decayed and has not decayed after 6 hours.

Solution

Example 5

The half-life of Sodium-24 is 16 hours. What is the time taken for Sodium-24 to shrink from 0.64 to 0.04 g?

Solution

Example 7

The half-life of Ba-143 is 12 seconds. How long will it take for the activity of a Ba-143 sample to be reduced to clip_image002of its initial value?

 

Solution

Example 8

The diagram shows the graph activity against time for radioactive material.

clip_image069

Based on the graph above , determine the half-life of the radioactive material.

Solution

2 Responses

  1. This blog is great. How did you come up witht he idea? 7 1 7

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