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 .

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, γ

Radioactive DetectorsRadioactive 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

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

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.

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LESSON 2Analysing radioactive decay

(4) Geiger-Muller tube (GM tube)

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:

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

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.

The characteristics of radioactive emissions

(1) Natural characteristics- particles : Helium nucleus or

- particles : Fast moving electrons or

-rays : Electromagnetic waves (2) Charge

- particles : + 2e- particles : -e -rays : No charge

(3) Speed

- particles : Up to 10% of speed of light- particles : Up to 99% of speed of light -rays : Speed of light

(4) Ionising power

- particles : Strong- particles : Medium-rays : Very weak

(5) Penetrating power

- particles : Low- particles : Average-rays : High

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Actual reading = Reading recorded – Background Reading.

(6) Range in air- particles : Several centimetres- particles : Several metres-rays : Several hundred metres

(7) Effect of electric field- particles : Small deflection towards

negatively charged plate- particles : Large deflection towards

positively charged plate-rays : No deflection

The size of deflection of - particles < - particles because the mass of - particles > - particles.

(8) Effect of electric field

- particles : Small deflection- particles : Large deflection in opposite

direction of the - particles -rays : No deflection

The size of deflection of - particles < - particles because the mass of - particles > - particles.The direction of deflection is determined by using Fleming’s left-hand rule.

Radioactive decayRadioactive 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:

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 ;

→ +

(2) Beta decay The general equation of alpha decay is:

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 ;

→ + (3) Gamma emission

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High frequency electromagnetic radiation coming from the nuclei of decaying atom is call gamma radiation. The general equation of alpha decay is:

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

For example ;

→ +

Example 1Balance the following equations:

(a) → +

(b) → + +

Example 2How many alpha particles and beta particles are emitted when decays into ?

A decay seriesRadioactive 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 can be represented as follows:

Example 3

The diagram shows part of a radioactive decay series.

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

Decay curve

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

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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 4The 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.

Example 5The 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?

Example 7The 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 1 of its initial value? 16

Example 8The diagram shows the graph activity against time for radioactive material.

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

1 Radioactivity is a process of

A releasing the radio waves B combination of small atoms into a larger

atom .C emission of the emission of energetic

particles from an unstable D splitting of a nucleus with big mass into two

or more smaller nuclides

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TUTORIAL 2

2 Which of the following is not true about radioactivity process?

A produce radioactive raysB produce more stable nucleus C spontaneous and randomlyD increasing the mass of the nucleus

3 Which of the following is not produced by a radioactive source?

A X - ray B - particleC - particle D - ray

4 Which of the following is not a radioactive detector?

A Spark counterB Cloud chamberC Maltese cross tubeD Geiger – Muller tube

5 Which of the following device can detect all the three types of radioactive radiation - particles , - particles and - rays ?

A Spark counter B Cloud chamberC Gold leaf electroscope

6 Most of the radioactive detectors make use of

A the energy of particlesB the penetration powerC the ionisation process D condensation process

7 Which of the following device cannot detect all the three types of radioactive radiation - particles , - particles and - rays ?

A Cloud chamberB Photographic film C Geiger - Muller tubeD Gold leaf electroscope

8 The diagram shows a source of alpha particles which is brought near a positively- charged gold leaf electroscope.

What happens to the gold leaf?

A No change to te gold leafB The deflection decreasesC The deflection increases

9 Which the following rays can be detected by a spark counter?

A Alpha - particleB Gamma -rayC Alpha particle and Beta- particleD Gamma -ray and Beta- particle

10 Which of the following detector cannot detect the gamma - rays ?

A Cloud chamberB Photographic filmC Geiger - Muller tubeD Gold leaf electroscope

11 Tracks produced in a cloud chamber caused by _____________________

A the fusion process of the dry ice B the reflection of light by alcohol droplet. C the condensation process of alcohol

vapour on ions D the penetrating power of the radioactive

radiations

12 Which of the following about alpha particles , beta particles and gamma rays is correct?

Alpha Beta Gammaparticles particles rays

13

A Helium Fast Electro-nucleus moving magnetic

electrons waves

B Electro- Fast Heliummagnetic moving nucleuswaves electrons

C Helium Electro- Fastnucleus magnetic moving

waves electrons

D Fast Helium Electro-moving nucleus magneticelectrons waves

13 Which of the following statements is a characteristic of alpha - particle

A Can deflected by an electromagnet fieldB Can penetrates a sheet of paperC Fast moving electrons

14 Which of following shows an - particle has charge ?

A Darken the photo filmB Ionises the air molecules C Deflected by a magnetic fieldD Stopped by a thin sheet of paper

15 Which of the following shows the ionising power of - particle , - particle and - ray in ascending order?

A > > B > > C > > D > >

16 Which comparison is correct about the speeds of , and ?

A > > B > > C > > D > >

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Based on the diagram above , which of the following is true about - particle , - particle and - ray in ascending order?

A Size of deflection in an magnetic fieldB Penetration powerC Ionising power

18 Which of the following has the highest penetrating power?

A X - ray B - particleC - particle D - ray

19 Which of the following statements is correct for - ray?

A deflected by electric field B has low penetration powerC is an electromagnetic waves D its speed up to 10% of speed of light

20 Diagram shows three types of radioactive rays, X , Y and Z directed towards a sheet of paper, a sheet of aluminium and sheet of lead.

Which of the following rays are represented by X, Y and Z?

X Y ZA Alpha Gamma BetaB Beta Alpha GammaC Gamma Alpha BetaD Gamma Beta Alpha

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21 Which of the following statements is a characteristic of gamma radiation?

A It has a positive chargeB It is an electromagnetic waveC It has a higher ionizing power than alpha

radiationD It has a lower penetrating power than beta

radiation

22 Which of the following about alpha particles and beta particles is correct?

A Have electrical chargesB Have the same massesC Travel with the speed of lightD Have the low power of ionisation

23 A group of radioactive rays are forced to pass through a magnetic field. Which diagram is correct to show the track of the rays?

24 Why is the size of deflection of the beta - particles greater than the alpha - particles in an electric field?

A The mass of alpha - particles greater than beta - particles

B The ionisation power of alpha – particles greater than beta - particles

C The penetration power of alpha – particles greater than beta - particles

25 Which of the following has a similar characteristic to - particles?

A ProtonB ElectronC Helium nucleusD Electromagnetic wave

26 The diagram shows an absorber is placed between a radioactive source and a G tube. The absorbers are made from different materials.

The table shows the result obtained from the counter for different absorbers.

Absorber Reading of counter / counts per minute

Without absorber 500Thin paper 220Aluminium 50Lead block 48

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27 What is the type of radiations emitted by the source?A - particlesB - particles and - particles C - particles and - raysD - particles , - particles and - rays

28 In which type of nucleur reaction are the nuclei heavier after the reaction than they were before?

A - decay C - decayC - emission D Nuclear fusion

29 A radioactive nucleus emits a - particle. What is the change in its nucleon numbers?

A It decreases by 2B It decreases by 1C It increases by 1 D It does not change

30 The following equation represents the decay of a Polonium nucleus.

210 Po 206 Pb + x 84 82

What is particle or ray represented by X ?

A Gamma ray B Beta - particleC Alpha - particle D Proton

31 The following equation represents the decay of a Kalium nucleus. 40 K 40Ca + X 19 20

A Proton B Alpha- particleC Gamma - ray D Electron

32 The following equation represents the decay of a Plumbum nucleus.

214 Pb 214 Bi + 0 X + Y 82 83 -1

What are X and Y represented?X Y

A B C

D

33 216 Po nucleus decays and emits an alpha - 84

particle. The new nucleus decays again and emits a beta - particle. The new nucleus is ____

A 213Pb B 212 Hg 82 80

C 212Bi D 213 TI 83 81

34 208 nucleus decays and emits two alpha - 86

particles and a beta- particle. The new nucleus decays again and emits a gamma- ray and a beta – particle.K nucleus is represented by ________________

A 200K B 200 K 84 86

C 202K D 204 K 86 84

35 Both alpha and beta particles are produced when decays to . How many alpha and beta particles are produced.

Alpha particle Beta particleA 4 6B 4 8C 4 10D 10 4

36 A nuclide 232 V decays to nuclide 220 W 92 90

and emits alpha- particles and beta- particles. .How many alpha and beta particles are produced.

A 4 and 3

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B 3 and 4C 4 and 4D 3 and 3

37 The diagram shows a series of radioactive decays for the nucleus of uranium-238 to that of radium-226.

What is the number of the alpha particles and beta particles emitted during this process?

The number of The number ofalpha particles beta particles

A 2 3B 3 2C 4 1D 1 1

38 The half-life of a radioactive material is

A the taken for its activity to be halvedB the time for its volume to be halvedC the time for its nucleon numbers to be

halvedD the time for its proton numbers to be

halved

39 Which of the following graph number of atoms, against time for a radioactive material is true?

40 In an experiment to find the half-life of

radioactive isotope, the following results were obtained.

Activity/ count per minute

410 290 200 140 95 69

Time / minute

0 2 4 6 8 10

What is the approximate half-life of the isotope?

A 2 minutes B 4 minutesC 6 minutes D 8 minutesE 10 minutes

41 The table shows the activity of a radioactive source is measured in 160 s .

Time / s Activity / number per second

0 32040 16380 82120 x160 20

What is the value of x?

A 60 B 51C 45 D 39E 31

42 The initial mass of a radioactive element is 10 g and its half-life is 6 days.Which of the following statements about the mass of the radioactive element is true?

A 6 days earlier , the mass is 20 gB 12 days earlier, the mass is 80 gC 6 days later, the mass is 2.5 g

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D 12 days later, the mass is 5.0 g

43 A recorder records 160 pulses per second for a radioactive material. The half-life of this source is 5 days. What is the reading after 15 days?

A 20 B 24C 32 D 60E 120

44 The diagram shows a graph activity against time for a radioactive element.

What is its activity after 12 hours?

A 4 counts per secondB 8 counts per secondC 16 counts per secondD 64 counts per second

45 A radioactive source has a half-life 8 hours. What is the percentage of atoms to decay after 24 hours?

A 12.5 % B 25.0%C 50. 0% D 87.5 %

46 A recorder records 60 pulses per second for a radioactive material. The half-life of this source is 24 seconds. What is the reading of the recorder in 24 seconds before?

A 15 B 30C 120 D 240E 480

47 The half-life of carbon is estimated 6 400 years . A artifact has been discovered. The activity of carbon in this artifact is ¼ of its initial activity.

How old is the artifact?

A 3 200 years B 6 400 yearsC 8 000 years D 12 800 yearsE 19 200 years

48 A radioactive source has a half-life of 12 hours How long does it take for 75% of the atoms in the source to decay?

A 6 hours B 12 hoursC 24 hours D 30 hoursE 48 hours

49 The background reading of a GM tube is 40 counts per minutes . A radioactive source is the placed near the tube , the reading is 280 counts per minutes . After 3 hours , its reading drops to 70 counts per minutes . What is the half-life of the radioactive source??

A 1 hour B 3 hoursC 6 hours D 9 hourE 12 hours

50 The table shows the half-lives and its initial masses of two radioactive substances P and .

Radioactive substance

P Q

Half-life / day 10 5 Initial mass / g 1 2

How long P and Q take time to become the same mass?

A 5 days B 10 daysC 15 days D 20 daysE 25 days

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51 Three different rays P,Q and R are emitted from a radioactive substance.The following table shows the characteristics of the rays.

Type of rays P Q RDeflection of the gold leaf of an electroscope.

unchanged unchanged decreases

Effect on Spark Counter

no spark no sparks produce sparks

Tracks in cloud chamber

Penetrating power

A few centimetres of lead can absorb a significant amount of it

Stopped by a few milimetres of aluminium

Stopped by a sheet of paper

Based on the graph above,(a) State the nature characteristic of rays

(i) P…………………………………………

(ii) Q………………………………………….

(b) Which of the rays has the strongest penetrating power?

…………………………………………………(c) Explain why

(i) rays P can produce sparks in a spark counter?

…………………………………………

…………………………………………(ii) rays P and Q cannot be detected by

a gold leaf electroscope?

…………………………………………

…………………………………………

(d) Explain why rays R produce thick and straight tracks in cloud chamber?

…………………………………………

52 (a) A radioactive source contains an isotope of thorium.Thorium ( ) decays by -particle emission to radium (Ra).Write an equation to show this decay.

(b) The radium produced is also radioactive. Diagram 1 shows a laboratory experiment to test for the presence of the radioactive emissions from the thorium source, using aradiation detector. In the laboratory there is a background count of 20 counts/minute.

The readings are given in the table.

Position Reading in count / per minute

P 2372Q 361

State and explain(i) which radiation could be causing the

count at Q,...........................................................

. ...........................................................

............................................................(ii) which radiations could be causing the

count at P.

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............................................................

............................................................(c) All three types of radioactive emission

cause some ionisation of gases.(i) Explain what is meant by the term

ionisation of gases.

............................................................

............................................................

............................................................

............................................................(ii) Suggest a reason why -radiation

produces very little ionisation.

............................................................

............................................................

............................................................

............................................................

53 The diagram shows part of 238 U decay series.

Based on the diagram above;

238 U 234 Th 234 Pa 234 U 92 90 91 92

230 Th 226 Ra 90 88

(a) Name the rays or particles emitted when 234 Th decays to 234 Pa 90 91

..................................................................(b) State a pair of an isotope.

..................................................................(c) The half life of 234 Pa is 6.7 hours and

91Initially has a mass 32 g.(i) What is meant by ‘ half-life’?

............................................................

............................................................(ii) What is the mass of 234 Pa after

91

33.5 hours?

54 Diagram 1 and Diagram 2 show the rate of decay of radioactive substances, flourine-22 and sodium-24.

(a) Based on the decay curves of flourine-22 and sodium-24,(i) state a common characteristic

for the change in activity in

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relation to time for the decay of flourine-22 and sodium-24.

…………………………………….

…………………………………….

…………………………………….

………………………………........(ii) complete the table below

Activity/Count per minute

2000 1000 500 250

Decay time of fluorine-22/ s

0 4 8

Decay time of sodium-24/ s

0 1 2

(b) What happens to the activity of(i) flourine-22 after every 4 s?

…………………………................

……………………………............(ii) sodium-24 after every 1 s?

…………………………................

……………………………............ (c) Based on the answers in (b)(i) and

(b)(ii), draw a conclusion from theactivity change with time for these twosources to form a physics concept. …………………………………….............

…………………………………….............

…………………………………….............

…………………………………….............(d) When flourine-22 decays, it produces a

beta particle and neon-22 (Ne).

(i) What is a beta particle?…………………………………….

…………………………………….(ii) Complete the following equation

for the decay of fluorine-22.

........ Ne Fl .......

.......22

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55 A radioactive source is placed at a distance 5 cm in front of a G-M tube. The G-M tube is connected to a counter.The following table shows the result obtained from the counter.

Time/minute

Reading of the counter

Count per minute for every minutes

0 01 4571 45712 7903 33323 10157 22544 11860 17035 13117 12576 14075 9587 14726 6518 15158 4329 15408 25010 15597 189

(a) (i) Plot a graph count per minute against time.

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(ii) From the graph, determine the half-life of the radioactive source.

(b) Name the radioactive radiations emitted by the radioactive source.

…………………………………………………

(c) To get the actual reading from the radioactive source the background reading should be recorded.(i) Give one source that produced the

background reading.

…………………………………………..

…………………………………………..(ii) How an adjustment must be made to

the calculation to get the actual reading from the radioactive source.

…………………………………………..

…………………………………………..

…………………………………………..

…………………………………………..

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