Hein Physics 11 3E Enhanced SB - ©¦¦¥

978 1 4425 5458 0978 1 4425 5458 0

VCE Units 3 and 4

Rob ChapmanKeith BurrowsCarmel FryDoug BailAlex Mazzolini Jacinta Devlin Henry Gersh

physics 12heinemann

enhanced3rd edition

978 1 4425 5405 4978 1 4425 5405 4

Carmel FryKeith BurrowsRob ChapmanDoug Bail

physics 11heinemann

enhanced3rd edition

VCE Units 1 and 2Heinemann Physics 11 Third Edition Enhanced is the most up-to-date and complete package for VCE Physics.

This enhanced edition has been updated to support the VCE Physics Study Design, which has been extended to the end of 2014. Key features of the third edition have been retained and these, together with enhanced digital support via Pearson Reader, make this package even easier to use.

Pearson Reader is an interactive online version of the student book with access to student and teacher resources, such as interactive lessons, quizzes, practice exams, practical activities and risk assessments.

Pearson Reader is available online at Pearson Places.

www.pearsonplaces.com.au

3rd edition

physics 11heinemann

enhanced

physics 11heinem

ann3rd edition


physics 11heinemann

enhanced3rd edition

VCE Units 1 and 2

enhancedCarm

el Fry Keith Burrows

Rob Chapman Doug Bail

ii

Pearson Australia(a division of Pearson Australia Group Pty Ltd)20 Thackray Road, Port Melbourne, Victoria 3207PO Box 460, Port Melbourne, Victoria 3207www.pearson.com.au

Copyright Doug Bail, Keith Burrows, Robert Chapman, Carmel Fry, Geoff Millar 2011 First published 2011 by Pearson Australia2015 2014 2013 201210 9 8 7 6 5 4 3 2 1

Reproduction and communication for educational purposesThe Australian Copyright Act 1968 (the Act) allows a maximum of one chapter or 10% of the pages of this work, whichever is the greater, to be reproduced and/or communicated by any educational institution for its educational purposes provided that that educational institution (or the body that administers it) has given a remuneration notice to Copyright Agency Limited (CAL) under the Act. For details of the CAL licence for educational institutions contact Copyright Agency Limited (www.copyright.com.au).

Reproduction and communication for other purposesExcept as permitted under the Act (for example any fair dealing for the purposes of study, research, criticism or review), no part of this book may be reproduced, stored in a retrieval system, communicated or transmitted in any form or by any means without prior written permission. All enquiries should be made to the publisher at the address above.

This book is not to be treated as a blackline master; that is, any photocopying beyond fair dealing requires prior written permission.

Publisher: Ross LamanProject Editor: Suzy FreemanEditors: Marta Veroni and Tim CarruthersDesigners: Nina Heryanto and Kim FergusonCopyright & Pictures Editors: Megan Cassar and Katy MurenuTypesetters: Sunset Publishing Services Pty Ltd and Jan UrbanicCover art: ShutterstockIllustrators: Guy Holt, Margaret Hastie, Brent Hagan, Chris Hurley, Pat Kermode, Cynthia Nge, Wendy Gorton and Bruce RankinPrinted in China

National Library of Australia Cataloguing-in Publication entry

Heinemann physics 11 enhanced VCE units 1& 2 / Carmel Fry ... [et al.].Edition: 3rd ed. enhancedISBN: 9781442554054 (pbk.) Target Audience: For secondary school age.Subjects: Physics--Textbooks.Physics -- Problems, exercise, etc.Victorian Certifi cate of Education examination.Other Authors/Contributors: Fry, Carmel.Dewey Number: 530

Pearson Australia Group Pty Ltd ABN 40 004 245 943

AcknowledgementsThe publishers would like to thank the team at Cider House Tech and PASCO Scientific for creating SPARKlab pracs for Heinemann Physics 11 3E Enhanced Pearson Reader.

We would also like to thank the following for permission to reproduce copyright material. The following abbreviations are used in this list: t = top, b = bottom, l = left, r = right, c = centre.

AAP: pp. 144, 189, 242, 355t, 460; Philippe Halsman, p. 444.Airbus S.A.S. 2011: pp. 470, 474.Alamy: pp. 135, 142, 184r, 210tl, 445r, 536; Phil Degginger, p. 90; Franz Marc Frei, p. 255. ANSTO: p. 453. Astro Photography: p. 338t. Atlantis Resources Corporation: p. 508t. Australian Science Media Centre: Daniel Mendelbaum, p. 158. Coo-ee Picture Library: p. 267l. Corbis Australia: pp. 24t, 111b, 143l, 198c, 201b, 336b, 365l, 385, 440c, 449; Paul Souders, p. 36; Koji Aoki, p. 127bl; Julian Calder, p. 432; Alfredo Escobar, p. 117; John Martin, p. 336t; Bryan Smith, p. 194;

William Whitehurst, p. 281. David Malin Images: pp. 338b, 346, 394.DK Images: p. 245l.Dreamstime: pp. 1, 111t.European Space Agency: pp. 393, 416, 424(a), 424(b).Fergus Photography: Mark Fergus, pp. 258t, 312.Getty Images: pp. 4, 110, 152, 211, 357b, 435; Mark Dadswell, p. 201t; Lucas Dawson, p. 179t; Stuart Hannagan, p. 127tl; Chris Hyde, p. 215; Tony Quinn, p. 165; Cameron Spencer, p. 113; Tobias Titz, p. 183.Imaginova Corporation/Starrynight.com: p. 347 (all).iStockphoto: pp. 93, 95, 131, 143br, 143tr, 156, 161, 179b, 191c, 198b, 207, 210cl, 210br, 225, 237, 291l, 401, 404l.Keith Burrows: pp. 52, 57, 59, 61 (all), 69, 98, 99, 384, 397.Malcolm Cross: pp. 303, 375.Meade Instruments: p. 373 (all).Melbourne Marathon: p. 112.NASA: pp. 151b, 334, 337l, 337c, 376 (all), 383, 386, 390, 407, 409r, 418, 419, 420, 421, 424(c), 424(d), 428, 431, 433, 439, 465, 476.News Limited Images (Newspix): p. 132r; Jon Hargest, 132l.PASCO Scientific: pp. 119r; Doug Ball, p. 512.Photolibrary: pp. 8t, 13t, 26, 30, 31, 37, 41, 101, 118, 154, 206, 234, 245tr, 254, 262, 286, 287, 306, 324, 335, 337r, 340l, 357t, 361, 362, 365r, 366t, 369, 370, 377l, 381, 382r, 398, 402, 403, 404, 405b, 406, 410c, 423, 425, 429, 440t, 440b, 445l, 454, 466, 472, 511, 530b, 532, 535t, 542bl, 542br, 544, 545, 546t, 546b, 553, 554, 555, 559; John Banagan, p. 499; Mark Burnett, p. 53; Professor Harold Edgerton, p. 119l; Professor Peter Fowler, p. 8b; Edward Kinsman, p. 146; Ton Koene, p. 458t; Patrick Landmann, p. 458b; Lawrence Lawry, p. 280; David MD, p. 535b; David Nunuk, pp. 73, 510; David Parker, pp. 6, 13b; Gavin Parsons, p. 92; Alfred Pasieka, p. 2; E. Schrempp, p. 245br; Dr. Gary Settles, p. 226r; Gianni Tortoli, p. 23.Quasar Publishing: p. 355b.Retrospect Photography: Dale Mann, p. 47 (all).RMIT Publishing: p. 475; Craig Mills, p. 259 (all).Shutterstock: cover, pp. 18, 24b, 36, 56, 70, 77, 80, 143c, 151r, 162, 164, 173, 188, 191t, 192, 198t, 199, 210bl, 223, 224, 226l, 229t, 244, 249, 258b, 272, 278t, 317, 333, 343, 399, 409l, 492, 505, 506, 508b, 515, 523, 528, 530, 533, 542t, 548r, 549, 566; Neale Cousland, p. 181; Evgeniya Moroz, p. 178; Derek Yegan, p. 109.Snowy Hydro Ltd: p. 507.Sport the Library: p. 126.State Library of South Australia: Mountford-Sheard Collection, p. 340r.Taoolunga: p. 350.Thinkstock: pp. 191b, 292, 299, 447, 534, 547. Track & Field News: p. 127r.University of Michigan News Service: p. 410t.Yerkes Observatory: p. 366b.

Every effort has been made to trace and acknowledge copyright. However, should any infringement have occurred, the publishers tender their apologies and invite copyright owners to contact them

DisclaimerThe selection of internet addresses (URLs) provided for this book was valid at the time of publication and was chosen as being appropriate for use as a secondary education research tool. However, due to the dynamic nature of the internet, some addresses may have changed, may have ceased to exist since publication, or may inadvertently link to sites with content that could be considered offensive or inappropriate. While the authors and publisher regret any inconvenience this may cause readers, no responsibility for any such changes or unforeseeable errors can be accepted by either the authors or the publisher.

HOW TO USE THIS BOOK vi

ABOUT THE AUTHORS viii

Chapter 1 Nuclear physics and radioactivity 21.1 Atoms, isotopes and radioisotopes 31.2 Radioactivity and how it is detected 81.3 Properties of alpha, beta

and gamma radiation 151.4 Half-life and activity of radioisotopes 201.5 Radiation dose and its effect on humans 26

Chapter review 32

Area of study reviewNuclear physics and radioactivity 34

Chapter 2 Concepts in electricity 372.1 Electric charge 382.2 Electrical forces and fi elds 452.3 Electric current, EMF and electrical potential 512.4 Resistance, ohmic and

non-ohmic conductors 592.5 Electrical energy and power 67

Chapter review 75

Chapter 3 Electric circuits 773.1 Simple electric circuits 783.2 Circuit elements in parallel 843.3 Cells, batteries and other sources of EMF 893.4 Household electricity 97

Chapter review 102

Area of study reviewElectricity 104

1UNITArea of Study 1NUCLEAR PHYSICS AND RADIOACTIVITY 1

1UNITArea of Study 2ELECTRICITY 36

Chapter 4 Aspects of motion 1104.1 Describing motion in a straight line 1114.2 Graphing motion: position, velocity

and acceleration 1224.3 Equations of motion 1304.4 Vertical motion under gravity 135

Chapter review 140

Chapter 5 Newtons laws 1425.1 Force as a vector 1435.2 Newtons fi rst law of motion 1505.3 Newtons second law of motion 1565.4 Newtons third law of motion 164

Chapter review 175

Chapter 6 Momentum, energy, work and power 1786.1 The relationship between momentum

and force 1796.2 Conservation of momentum 1876.3 Work 1916.4 Mechanical energy 1986.5 Energy transformation and power 209

Chapter review 217

Area of study reviewMotion 219

2UNITArea of Study 1MOTION 109

CONTENTS

Chapter 7 The nature of waves 2247.1 Introducing waves 2257.2 Representing wave features 2327.3 Waves and wave interactions 240

Chapter review 247

Chapter 8 Models for light 2498.1 Modelling simple light properties 2508.2 Refraction of light 2588.3 Critical angle, TIR and EMR 2708.4 Dispersion and polarisation of light waves 280

Chapter review 285

Chapter 9 Mirrors, lenses and optical systems 2869.1 Geometrical optics and plane mirrors 2879.2 Applications of curved mirrors:

concave mirrors 2919.3 Convex mirrors 2999.4 Refraction and lenses 3069.5 Concave lenses 3129.6 Optical systems 317

Chapter review 327

Area of study reviewWave-like properties of light 329

Chapter 10 Astronomy 334The story continues ... 335

10.1 Motion in the heavens 33710.2 The Sun, the Moon and the planets 34710.3 Understanding our world 35710.4 The telescope: from Galileo to Hubble 36910.5 New ways of seeing 379

Chapter review 388

2UNITArea of Study 2WAVE-LIKE PROPERTIES OF LIGHT 223

1&2UNITS

Area of Study 3DETAILED STUDIES 333

Chapter 11 Astrophysics 39011.1 The starshow far, how bright? 39111.2 Our favourite star 40111.3 We know the stars by their light 40911.4 Whole new worlds 42311.5 The expanding universe 431

Chapter review 437

Chapter 12 Energy from the nucleus 43912.1 Splitting the atom nuclear fi ssion 44012.2 Aspects of fi ssion 44712.3 Nuclear fi ssion reactors 45312.4 Nuclear fusion 463

Chapter review 468

Chapter 13 Investigations: fl ight 47013.1 The four forces of fl ight 47113.2 Modelling forces in fl ight 48113.3 Investigating fl ight 48613.4 Investigation starting points 489

Chapter 14 Investigations: sustainable energy sources 49214.1 Energy transformations 49314.2 Renewable or sustainable

the key to our future 49814.3 Investigating alternative energy sources 49914.4 Investigation starting points 503

Chapter 15 Medical physics 51515.1 Ultrasound and how it is made 51615.2 Ultrasound interactions 52315.3 Scanning techniques 52815.4 Diagnostic X-rays 53715.5 Radiotherapy, radioisotopes

in medicine and PET 550

Chapter review 558

Appendix AVector skills 560

Appendix BSI units 563

Appendix CUnderstanding measurement 565

Solutions 576

Glossary 599

Index 608

CONTENTS

The most relevant, comprehensive and easy-to-use package for VCE Physics Units 1&2This Enhanced third edition has been updated to support the VCE Physics Study Design which has been extended to the end of 2014. Key features of the third edition have been retained, and together with the enhanced digital support via Pearson Reader, this VCE Physics package is even easier to use.

Student BookKey features retained: lesson-sized, self-contained sections extension and enrichment material clearly indicated wide range of well-graded end-of-section questions and

chapterreviews.

Enhancements include: up-to-date content with the very latest developments and

applications of physics simpler design for easier navigation and access to content all questions have been reviewed and updated as appropriate.

Pearson Reader Pearson Reader is an interactive online version of your Student Book. With links to a range of resources, such as interactive lessons, quizzes and more, it is designed to save teachers time andto present content in the way students like to learn.

We have retained in one location all your favourite learning and teaching support including: detailed answers and worked solutions to all questions in the

Student Book extensive range of short and long practical activities, all with

teacher notes and suggested outcomes and answers sample assessment tasks with marking guidelines teacher work programs.

And brand new content: di erentiated independent student study programs interactive lessons, including videos and animations for each

chapter of the student book quizzes exam advice and two practice exams suitable pracs presented as SPARKlabs

risk assessments and safety notes for pracs.

Pearson Reader has the ability to add and share links with students and teachers to create an online community and enrich the learning experience.

Pearson Reader is available online at Pearson Places.

Pearson Places is the gateway to digital learning material for teachers and students across Australia. Sample the range of resources and register for free at www.pearsonplaces.com.au.

We believe in learning.All kinds of learning for all kinds of people,delivered in a personal style.Because wherever learning fl ourishes, so do people.


physics 11heinemann

enhanced3rd edition

VCE Units 1 and 2

+

Key features of the market-leading third edition have been retained and updated including: exact match to structure and sequence of the

study design

chapters divided into student-friendly sections

clear explanations and development of concepts consistent with the intent and scope of the study design

exam-style questions

extensive glossary.

The text supports students learning in physics while making the subject interesting, enjoyable and meaningful. Clear and concise language is used. All concepts have been fully explored, fi rst in general and then illustrated in context. Illustrative material is relevant, varied and appealing to a wide range of students.

Range of well-graded questions At the end of each

section is a set of homework-style questions that are designed to reinforce the main points. More demanding questions are included at the end of the chapter.

At the end of each Area of Study is a set of exam-style questions. These can be used for revision. The large number of questions is designed to assess students understanding of basic concepts, help with revision and provide problem-solving practice.

Answers are given at the end of the Student Book.

Extended answers and fully worked solutions are available on Pearson Reader. You will see this icon.

Worked Solutions

Physics fi les and Physics in action These features enhance students understanding of concepts and context. These features are clearly delineated from the body text yet are carefully integrated into the general fl ow of information.

Optional contentThe text follows the sequence, structure and scope of the VCE Physics Study Design. Material outside the scope of the VCE Physics Study Design is clearly marked as OPTIONAL. This includes sections and subsections. This material has been included for a number of reasons, including as important background to core concepts, as important physics in its own right and as extension material for more able students.

Detailed studiesAll detailed studies are included in the Student Book. Chapters 1015 are the detailed studies. Students will undertake one detailed study in each unit. The detailed study chosen for Unit 1 must be different from the detailed study chosen for Unit 2.

vi

HOW TO USE THIS BOOK

Heinemann Physics 11 third edition Enhanced has been updated to support the VCE Physics Study Design, which has been extended to the end of 2014.

Motion126

Graphing acceleration

Acc

eler

atio

n (m

s2

)

1

2

0

1

2

Time (s)7 8 9

1

Figure 4.21 The accelerationtime graph for the toy car travelling across the driveway. It was drawn by taking account of the gradient values of the velocitytime graph. The change in the cars velocity is given by the area under the graph.

The area underis a measure of dunits on the axes arfinding the area, a disresults. From Figure 4

area units = m s1

i.e. a displacementThe gradient of a

graph is the acceleratWhen finding the graddivided. From Figure 4

gradient units = mi.e. an acceleration

area = d

gradient = acceleration

v (m

sv

1)

t (s)t

v (m

s

vv1

)

t (s)t

run

rise

(b)

(a)

Figure 4.20 (a) The units on the axes of a vt graph confirm that the area under the graph represents a displacement. (b) The gradient of the line is the acceleration.

Physics file

Until 1964, all timing of events at th, grecorded by handheld stopwatches. Thejudges meant an uncertainty of 0.2 s foAn electronic quartz timing system intrimproved accuracy to 0.01 s, but in clostill had to wait for a photograph of thecould announce the placings.

Currently the timing system used iscanning video system (VLSV). Introducompletely automatic electronic timing pistol triggers a computer to begin timihigh-speed video camera records the imindicates the time at which the chest ofline. This system enables the times of arace to be precisely measured to one-th

Another feature of this system is thrunner breaks at the start of the race

Timing and false starts in athleticsPhysics in action

Figure 4.22 At the 1960 Rome Olympic Games, the judges used handheld stopwatches to measure the times of swimmers and athletes.

Electricity62

Non-ohmic conductorsA light bulb is a common example of a non-ohmic conductor. Typically, a car headlamp bulb may draw about 1 A at 1 V, but as the voltage increases,the current will not increase in proportion, as you can see in Figure 2.29. At12 V the current might be 4 A; so while the resistance at 1 V is 1 , at 12 V the resistance has increased to 3 . While it may sometimes be useful to know the resistance of the bulb at its operating voltage of 12 V, it cannot be used to calculate the current flowing at other voltages. The bulb does not obey Ohms law.

To quote the resistance of the diode in Figure 2.29 would be almostess: it decreases very rapidly once the voltage reaches about

to know about the diode is that once the voltagees, apparently without limit. In

wer dissipated ut.hanges

ctors in ure.

Worked example 2.4B What is the

abc

Solution Resistance is given by R = V/I at any point on the graph. Note that the current is given in mA (100 mA = 0.1 A).a At 24 V

R = 24/0.10 = 240

b At 120 V R = 120/0.20 = 600

c At 240 V R = 240/0.25 = 960

Resistance increases as the filament becomes hotter. Notice that we cannot use the inverse slope of the graph; resistance is simply the ratio V/I at a particular voltage.

Resistance and resistivity nductor? Given

that the resistance of a piece of metal wire is a measure of the ability of the wire to somehow impede the flow of electrons along its length, it is reasonable to expect that:1 If the wire is made longer there will be a greater resistance as there is

more to impede the flow of the electrons.

100 200

200

100

I (mA)

V (V)

Prac 9 SPARKlab

Chapter review

1that come into your mind?

2 Even though we know that there is really no huge celestial sphere rotating around the Earth, astronomers still speak of one. Why is this?

3 The stars are said to have a diurnal and an annual rotation. Whatis the difference between these two expressions and what is thereason for the difference?

4 What is the altitude of the celestial equator above the north horizon in Melbourne? Would it be different from Brisbane orHobart? If so, in what way?

5 Where does the celestial equator meet the horizon as seen fromMelbourne? Would it be different from Brisbane or Hobart? If so, in what way?

6 At the South Pole no stars are visible in the middle of summer. Why not? If there was a sudden eclipse of the Sun and the starsdid become visible, how would the sky differ, or not differ, fromthat seen in the middle of winter?

7 If you observed the stars from a point on the equator at midnighton 21 March and then looked again at midnight on 21 Septemberhow would the two views differ?

8 B

9

a i ii iii iv v vi

b

10

abcd

11 What are the coordinates of the following stars?

a Siriusb Achernarc Vegad Rigel

12 Is the Sun due north at midday each day?

13 After one full sidereal day, compared with the previous day, a star on the celestial equator will:

A be in exactly the same positionB be a little eastC be a little westD have set.

14 In Melbourne, the Sun has a maximum altitude of 75 at the summer solstice and 29 at the winter solstice. What is the maximum altitude at the equinoxes and why is it different insummer and winter?

15 How is the ecliptic related to the celestial sphere? Is it fixed inplace on the celestial sphere or does it move?

16 You are watching the sunset with a crescent moon still in the sky. Which of these pictures best represents the Moon as you will see

A

B

C

D bove.

Astronomy

Area of Study 3

DETAILEDSTUDIES

Chapters 1015 are1015 are15 aree the detathe detathe detailed studiled studiled ies. s. iesesesessssYou will YouYou will undertakeundertakender one detaone detaone detdetailed studileed y in each unit. The The The detailed detaileddetailed study chostudstudy chohosen sens n n for Unit fffor Unit for Unitnit n 1 must be1 must be1 st differendifferend fferfe ent from tht ft from e detailed ddetadetaile study chosen for Unitni 2.2Chapter 100 AstronomyAstronomyonomymyChapter 1Chaha 1 AstrophysicsChapter 1Chapter 1er 1222 Energy frEnergy frEnergne om the nuom the nuom them the n cleuscleuseuChapter 1CChChChapter 133 Investigations: flightChapter 1Chapter hapter 14444 InvestigaInvestigast ations: sutions: tions: sutions: sutions: suons: ns: sustainablestainastss ablee

energy sourcesChapter 155 Medical pMedical pMedical physicshysicshysics

outcome

1UNITS &2

that w a part

of a con

ONALResista cWhat are the factors that determ

apparently w

wire is a mea

ower dissipnd burn outsistance cha

as detectmperatur

bulb. W

cu

To qmeaningless: it decr0.5 V. The important thing to know abouexceeds a certds a certain level theain leve t i ses aexceeds a certexceeds a certain level the ain level th current increases, apparentlypractice therepractice therehere will be a limm will be a limw it to it to the current because the pow

h the diohe diode wde willth become too hot andncludclude dede devicesnc whose resi

e partpartiartie p cularly usefulcu ainnin light lht levels ligh or tem

6060 WW light b

thththahahat hat at at thththehe c

ResiResResiRes ss as theses as the s as the he ffilafila es es es hhhothothothottettettererr. Notice tinverse slope nverse slope verse slopese slopeinverse slope inverse sloinve of the gof the graphof the graph; of the graph; of the graph; he graph; ph; resistanceesistancestance stance nce nceresistanresistaresisres y ty ty thththe he he he rararatatioio V/VV I at II

Resistance and resiststtivity ivity vity vitvitytyyyty yivitythat determine the resistance of

OPTIONA

ament becomes hotter. Notice that we catance is simply the ratio V/I at a particular

istivity ermine the resistance of a conducof metal wire is a measure of the

e the flow of electrons along its l

ere will be a greater resistanlectrons.

OPTIONAL

for aroducose finishe finish be

is a vertical lineduced in 1991, this i

system. The starting ming. At the finish line, aimage of each athlete and f each one crosses the all the athletes in the housandth of a second.hat it indicates when a e. Each starting block

Figuhandhe

gdrawn by taking account of cars velocity is given by the

vvv t

t of

the Ohe OThe

ming and n action

Time (s)

toy ctoy car travelling ar travelling g across the driacross t veway. It was yy g ys of s of the vethe vevel ilocitytime grlocitytime grlocitytime gry g aph. The changaph. The changhangaph. The chaaph. Thp e in the e

the are e grahe graphhe graph. graph. aph. p

e Olympicmpic Games ames waswas e Olympic Gameames wawashe reaction ton times of s of thethe

or any measursuremenent. uced in 1964 64

ishes the judjudges es before theyhey

ine-his is a a

ing ne, a

nd

falalslsee se st st stastastastartartartrten

igure 4.22 22 2 At the At the 19At the 196the 1960the 1960 Re 1960 R1960 R1960 R960 R60 RR At th A ome Oome Oome Oome omo he judgehe judgehe judgejudgegess us uses useds used s used s used used ed d ndheld stopwwatches to atches to mees to meass to measto measmeasmeaseassatches ure the times ure the timese timesure the times ure the times ure the times ure thure of swiof swimmeof swimmersof swimmers aof swimmers anof swimmers animmers anmers aners anandd atd athd athled athleteathletes.hletes.

e line is the acceleration.

4, all timing of events at the Oly4, all timing of events at the Olyy handheld stopwatches. The rnt an uncertainty of 0.2 s fo

quartz timing system iny to 0.01 s, but

Timing and faPhysics in action

isp4.2

s nta velocityti

ion of the odient, the unit4.20b:

m s1/s = m s2,n.

displacement

Acc

eler

1

2

1 22 3 4Area = 12 m s

= v

elerationtimet of the g

Tim5 6 7 8 9

m s1

v

time graph for the toy cyg pthe gradient vat values of lues of th

e area under nder ththe grathe grahe gra

er a veloclocf displac

are mdisp

sics file

014

t

o wn. n.

ange nge inin ring the first 6 s can be determined from titionontimtim mm ss11. Thishis c cTh king at the ve n WoWorkorked examexampleample 44 mm s1, a changeange ofge of

(m

erat

ion

(m ss

22))

0

1

22

1 22 3 4 5

locitytime time glocit glacemcement. Whnt. Whhen ten te multipmultiplied whewhen en isplacementment unit it4.20a:

s = m

ytime he object. t.

nits are re

lee

5 6 7 8 9

The area under a velocitytime is a measure of displacement. Wunits on the axes are multipliedfinding the area, a displacementresults. From Figure 4.20a:

area units = m s1 s = mi.e. a displacement

gradient of a v

Physics file

8 Because of the Earths atmosphere the Sun rises a little earlier and sets a little later thanthan it would otheit would oth rwise. Assuming that the

ch of thit?

A

f the ab

Detailed studdiesies388

and sets a little later thane later than it would othe it would otherwise. Assuminrwi g that the atmosphere isosphere is is uniform, 100 kuniform, 100 km thick and haham thick and s a refractives a refractive index index of 1.00of 1.003, use003, use se Snells law toSnells law to determine thedetermine the amount of ref amount of ref a ractionraction on

unrise andat sunrise aat su and sunset and sunset and, hence, the extrence, the extra time that thi h ha t Se Sun is n is visible. (Uisible. (Use vis Earths radiusarths radius == 6400 km.) How 6400 km.) closely does youransanswer agree wagree with the actual extra time?

9999 From Mrom Melbourne, latitude 38, some of the stars are alwayys in thethe sky (A), some spend part of the day in the sky (P) and somome never appear (N).

a Classify each of the following stars as either A, P or N. i a star within 20 of the SCP ii a star within 20 of the NCP iii the Southern Cross iv Orion v a star 50 north of the celestial equator

vi a star 50 south of the celestial equatorb For those stars you classified as P, give a rough estimate of

the time they will spend in the sky.

10 Use a starfinder or chart to find the stars closest to the positionsns given by the following celestial coordinates:

a RA 14 h 13 min, dec. +19b RA 5 h 50 min,RA dec. +7c RA 14 hA 14 h 40 min, dec. 60ddd RA 4 h 30 min,30 min, dec. +15

BBB

CC

D None of t

rs m

ht er

er he ex on is ur

summer and winter?

15 How is the ecliptic related to the cplace on the celestial sphere or doe

16 You are watching the sunset with a cWhich of these pictures best represit?

A

We have simplifi ed the package so that all teaching and learning support can be found at one locationPearson Reader.

Pearson Reader is an interactive, online version of your Student Book with links to a range of resources such as worked solutions and interactive lessons.

When you see this icon, it refers to a teaching and learning resource on Pearson Reader.

All your favourite learning and teaching support has been updated and included:

detailed answers and worked solutions to all questions in the Student Book

extensive range of short and long practical activities, all with teacher notes and suggested outcomes and answers

sample assessment tasks with marking guidelines

teacher work programs.

And brand new resources have been added:

differentiated independent student study programs

interactive lessons, including videos and animations for each chapter of the Student Book

quizzes

exam advice and two practice exams

suitable pracs presented as SPARKlabs

risk assessments and safety notes for pracs

the ability to add and share links with students and teachers to create an online community that enriches the learning experience.

Pearson Reader is available online at www.pearsonplaces.com.au

.

vii

An example of an interactive lesson available on Pearson Reader

viiiviii

ABOUT THE AUTHORS

DOUG BAILIs an experienced physics educator and writer with a particular interest in the development and integration of new technologies into science teaching. He has previously been a Head of Science and senior physics teacher, and maintains a passion for making physics relevant, stimulating and accessible to all students. Doug now runs his own company developing and distributing products for physics education throughout Australasia.He led the development of the practical activities that form part of Pearson Reader. These activities were extensively trialled throughout Australia and include a range of activities from teacher demonstrations to discovery-based investigations, suiting a range of learning styles and needs. This includes many short activities, for when time is limited!

KEITH BURROWSHas been teaching senior physics in Victorian schools for many years. He is a member of the Australian Institute of Physics Victorian Education Committee and was actively involved with the VCAA in the design of the new course. Keith was a VCAA representative involved in the introduction of the new VCE course to physics teachers in Victoria and in running the workshop sessions for teachers. He is particularly keen to portray the Big Picture of physics to students. Keith would like to acknowledge Maurizio Toscano of the University of Melbourne who provided invaluable help and advice in the preparation of the Astronomy and Astrophysics detailed studies.

ROB CHAPMANFrom the time Rob started teaching physics, the Earth has completed around 30 orbits of the Sun and mobile phones have shrunk from brick-size to wafer-thin. Rob has been enthusiastic in exploring the possibilities presented by changing technologies over the years. He has been Science Coordinator at St Columbas College in Essendon, where he was instrumental in introducing the use of datalogging technology to junior science and senior physics classes. Rob is currently teaching Senior Physics at PEGS (Penleigh and Essendon Grammar School). He has written a wide variety of curriculum support materials, including physics units for the CSFII. Rob has also produced a physics study guide and trial examination papers.

CARMEL FRYHas 22 years involvement in development of text, CD and on-line curriculum materials for VCE Physics and Science. She is Head of Science at Ivanhoe Grammar School, where she continues her interest in providing high-quality curriculum resources and learning experiences for students. Carmel is the author of numerous texts, multimedia resources and teacher-resource materials developed for senior physics. These materials are currently in use in many parts of Australia and overseas. Carmel is particularly passionate about providing physics curriculum materials that involve a variety of approaches to learning, and that support independent learning through stimulating and appealing contexts and activities. Carmel would like to acknowledge the on-going support of her husband and children over her many years of publishing.

REVIEW PANELThe publisher and authors would like to acknowledge and thank the following people for their contribution to the text: the expert review panel consisting of experienced VCE teachers and educatorsLuke Bohni, Mike Davies, Barry Homewood, Chris Hourigan, John Joosten, Terry Trevena, Steve Treadwell, Lyndon Webb and Chris Ward.

ACKNOWLEDGMENTSThe publisher would like to acknowledge and thank the author team for their ongoing commitment and passion for this project. It is a huge and complex task and the demands, including short timelines, are great. Carmel, Keith, Rob and Doug, it has been a pleasure and privilege to work with you.

Area of Study 1

NUCLEAR PHYSICSAND RADIOACTIVITY

1UNIT

On completion of this area of study, you should be able to explain and model relevant physics ideas to describe the sources and uses of nuclear reactions and radioactivity and their effects on living things, the environment and in industry.

outcome

Nuclear physics and radioactivityMany people think that they never come into contact with radioactive materials or the radiation that such materials produce. They are wrong to think this way. Human beings have always been exposed to radiation from a variety of natural sources. The ground that we walk on is radioactive. Every time we inhale, we take minute quantities of radioactive radon into our lungs. Even the food we eat and the water we drink contain trace amounts of radioactive isotopes. It is now accepted that exposure to higher than normal levels of high-energy radiation leads to the development of cancerous tumours and leukaemia. However, radiation and radioactive elements can also be used in a variety of applications that are of real benefit to people in industry and in medicine, for example:

Radioactive substances are used in the diagnosis and treatment of cancer. The photograph shows an image taken with a gamma ray camera. Technetium-99m (a radioactive isotope) was injected into the bloodstream of a patient. This allows the blood-flow patterns within the brain to be studied.

Smoke detectors usually contain a small sample of the radioactive element americium-241.

Geologists and archaeologists determine the age of rocks, artefacts and fossils by analysing the radioactive elements in them.

In this chapter, we will examine radioactivity and discuss the associated dangers and benefits of its many applications. An understanding of this topic will help you to develop an informed opinion on this important issue.

1CHAPTER

you will have covered material from the study of nuclear physics and radioactivity including:

the origin, nature and properties of , and radiation

the detection of , and radiation stable, unstable, natural and artifi cial isotopes

production of artifi cial radioisotopes

the half-life of a radioactive isotope

radiation doses from internal and external sources

effects of , and radiation on humans and other organisms

nuclear transformations and decay series.

by the end of this chapter

Chapter 1 Nuclear physics and radioactivity 3

Atoms, isotopes and radioisotopes1.1

AtomsIn order to understand radioactivity, it is necessary to be familiar with the structure of the atom. The central part of an atom, the nucleus, consists of particles known as protons and neutrons. Collectively, these particles are called nucleons and are almost identical in mass and size. However, they have very different electrical properties. Protons have a positive charge, whereas neutrons are electrically neutral and have no charge. The nucleus contains nearly all of the mass of the atom, but accounts for less than a million-millionth (1012) of its volume. Most of the atom is empty space that is only occupied by negatively charged particles called electrons. These are much smaller and lighter than protons or neutrons and they have various amounts of energy.

The simplest atom is hydrogen. It consists of just a single proton with a single electron. Compare this with a uranium-238 atom. Its nucleus contains 92 protons and 146 neutrons. Its 92 electrons occupy the region around the nucleus. Uranium-238 is the heaviest atom found in the Earths crust.

A particular atom can be identified by using the following format:

mass number

A

Z X element symbolatomic number

The atomic number defines the element. Atoms with the same number of protons will all belong to the same element. For example, if an atom has six protons in its nucleus (i.e. Z = 6) then it is the element carbon. Any atom containing six protons is the element carbon, regardless of the number of neutrons.

In an electrically neutral atom, the number of electrons is equal to the number of protons. Any neutral atom of uranium (Z = 92) has 92 protons and 92 electrons.

The complete list of elements is shown in the periodic table in Figure 1.6.

IsotopesAll atoms of a particular element will have the same number of protons, but may have a different number of neutrons. For example, lithium exists naturally in two different forms. One type of lithium atom has three protons and three neutrons. The other type has three protons and four neutrons. These different forms of lithium are isotopes of lithium. Isotopes are chemically identical to each other. They react and bond with other atoms in precisely the same way. The number of neutrons in the nucleus does not influence the way in which an atom interacts with other atoms. The

Two important terms that are used to describe the nucleus of an atom are its: ATOMIC NUMBR (Z)the number of protons in the nucleus of an atom. MASS NUMBR (A)the total number of protons and neutrons in the nucleus.

i

To gain an idea of the emptiness of atoms and matter, consider this example. If the nucleus of an atom was the size of a pea and this was placed in the centre of the MCG, the electrons would exist in a three-dimensional space that would extend into the grandstands.

Physics file

Figure 1.2 (a) Hydrogen is the simplest atom. It consists of just one proton and one electron. (b) Uranium-238 is the heaviest naturally occurring atom. Its nucleus contains 238 nucleons92 protons and 146 neutrons.

Figure 1.1 The nucleus of an atom occupies about 1012 of the volume of the atom, yet it contains more than 99% of its mass. Atoms are mostly empty space!

(a)

(b)

nucleus consisting

protons ( )of neutrons ( ) and

cloud of electrons

Nuclear physics and radioactivity4

difference between isotopes lies in their physical properties. More neutrons in the nucleus will mean that these atoms have a higher density.

When referring to a particular nucleus, we talk about a nuclide. In this case, we ignore the presence of the electrons. For example, the nuclide lithium-6 has three protons and three neutrons. Stable isotopes can be found for most of the elements and, in all, there are about 270 stable isotopes in nature. Tin (Z = 50) has ten stable isotopes, while aluminium (Z = 13) has just one.

RadioisotopesMost of the atoms that make up the world around us are stable. Their nuclei have not altered in the billions of years since they were formed and, on their own, they will not change in the years to come.

However, there are also naturally occurring isotopes that are unstable. An unstable nucleus may spontaneously lose energy by emitting a particle and so change into a different element or isotope. Unstable atoms are radioactive and an individual radioactive isotope is known as a radioisotope. By way of illustration, carbon has two stable isotopes, carbon-12 and carbon-13, and one isotope in nature that is not stable. This is carbon-14. The nucleus of a radioactive carbon-14 atom may spontaneously decay, emitting high-energy particles that can be dangerous. If you look at the periodic table in Figure 1.6, you will see that every isotope of every element with atomic mass greater than that of bismuth (Z = 83) is radioactive.

Most of the elements found on Earth have naturally occurring radioisotopes; there are about 200 of these in all. As well as these, about 2000 radioisotopes have been manufactured. During the 20th century, an enormous number of radioisotopes were produced in a process known as artificial transmutation.

Artificial transmutation: how radioisotopes are manufacturedNatural radioisotopes were used in the early days of research into radiation. Today, most of the radioisotopes that are used in industrial and medical applications are synthesised by artificial transmutation. There are now more than 2000 such artificial radioisotopes. In the periodic table, every element with an atomic number greater than 92 (i.e. past uranium) is radioactive and is produced in this way.

One of the ways that artificial radioisotopes are manufactured is by neutron absorption. (In Australia, this is done at the Lucas Heights reactor near Sydney.) In this method, a sample of a stable isotope is placed inside a nuclear reactor and bombarded with neutrons. When one of the bombarding, or irradiating, neutrons collides with a nucleus of the stable isotope, the neutron is absorbed into the nucleus. This creates an unstable isotope of the sample element.

ISOTOPS are atoms that have the same number of protons but different numbers of neutrons. Isotopes have the same chemical properties but different physical properties.

i

Figure 1.3 Isotopes of lithium. (a) The nucleus of a lithium-6 atom contains three protons and three neutrons. (b) The nucleus of a lithium-7 atom contains three protons and four neutrons.

Figure 1.4 This symbol is used to label and identify a radioactive source.

Figure 1.5 Artificial radioisotopes for medical and industrial uses are manufactured in the core of the Lucas Heights reactor in Sydney. This is Australias only nuclear reactor facility and has been operating since 1958. The original reactor was replaced by the OPAL (Open Pool Australian Light-water) reactor in 2007.

(a)

(b)

5Chapter 1 Nuclear physics and radioactivity

This is how the radioisotope cobalt-60 (widely used for cancer treatment) is manufactured. A sample of the naturally occurring and stable isotope cobalt-59 is irradiated with neutrons. Some of the cobalt-59 nuclei absorb neutrons and this results in a quantity of cobalt-60 being produced: 10n +

5927Co

6027Co. This nuclear transformation is illustrated in Figure 1.7.

Figure 1.6 The periodic table of elements.

Figure 1.7 The artificial radioisotope cobalt-60 is used extensively in the treatment of cancer. It is produced by bombarding a sample of cobalt-59 with neutrons.

Group

Period 1

Group

2

3

4

5

6

7

Lanthanides

Actinides

Every isotope of theseelements is radioactive

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 1 2

H He 1.01 4.00 3 4 5 6 7 8 9 10

Li Be B C N O F Ne 6.94 9.01 10.81 12.01 14.01 16.00 19.00 20.18 11 12 13 14 15 16 17 18

Na Mg Al Si P S Cl Ar 22.99 24.31 26.98 28.09 30.97 32.06 35.45 39.95 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 39.10 40.08 44.96 47.90 50.94 52.00 54.94 55.85 58.93 58.71 63.54 65.37 69.72 72.59 74.92 78.96 79.91 83.80 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe 85.47 87.62 88.91 91.22 92.91 95.94 (99) 101.07 102.91 106.4 107.87 112.40 114.82 118.69 121.75 127.60 126.90 131.30 55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86

Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn 132.91 137.34 138.91 178.49 180.95 183.85 186.2 190.2 192.2 195.09 196.97 200.59 204.37 207.19 208.98 (210) (210) (222) 87 88 89 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118

Fr Ra Ac Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Uuq Uup Uuh Uus Uuo (223) (226) (227) (261) (262) (263) (264) (277) (268) (271) (272) (277) (289) (289) (292) (293)

58 59 60 61 62 63 64 65 66 67 68 69 70 71

Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 140.12 140.91 144.24 (145) 150.35 151.96 157.25 158.92 162.50 164.93 167.26 168.93 173.04 174.97

90 91 92 93 94 95 96 97 98 99 100 101 102 103

Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr 232.04 (231) 238.03 (237) (242) (243) (247) (247) (249) (254) (253) (256) (254) (257)

cobalt-59: stable cobalt-60: radioactive

The heaviest stable isotope in the universe is 209

83Bi. Every isotope of every element with more than 83 protons, i.e. beyond bismuth in the periodic table, is radioactive. For example, every isotope of uranium (Z = 92) is radioactive. Technetium (Z = 43) and promethium (Z = 61) are the only elements with an atomic number below bismuth (Z = 83) that do not have any stable isotopes. Uranium is the heaviest element that occurs naturally on Earth. All the elements with atomic numbers greater than 92 have been artificially manufactured.

Physics file


Worked example 1.1A Use the periodic table in Figure 1.6 to determine:a the symbol for element 95

42X

b the number of protons, nucleons and neutrons in this isotope.

Solution a From the periodic table, the element with an atomic number of 42 is Mo, molybdenum.b The lower number is the atomic number, so this isotope has 42 protons. The upper

number is the mass number. This indicates the number of particles in the nucleus, i.e. the number of nucleons, so this atom has 95 nucleons. The number of neutrons can be found by subtracting 42 from the mass number. This isotope has 53 neutrons.

Our understanding of the atom has changed greatly in the past 100 years. It was once thought that atoms were like miniature billiard balls: solid and indivisible. The word atom comes from the Greek atomos meaning indivisible. That idea was changed forever when the first subatomic particlesthe electron, the proton and then the neutronwere discovered in the period from 1897 to 1932.

Since World War II, further research has uncovered about 300 other subatomic particles! Examples of these include pi-mesons, mu-mesons, kaons, tau leptons and neutrinos. For many years, physicists found it difficult to make sense of this array of subatomic particles. It was known that one family of particles called the leptons had six members: electron, electron-neutrino, muon, muon-neutrino, tau and tau-neutrino.

Then in 1964 Murray Gell-Mann put forward a simple theory. He suggested that most subatomic particles were themselves composed of a number of more fundamental particles called quarks. Currently, it is accepted that there are six different quarks, each with different properties (and strange names!): up, down, charmed, strange, top and bottom. The latest quark to be identified was the top quark, whose existence was confirmed in 1995. The proton consists of two up quarks and one down quark, while neutrons consist of one up quark and two down quarks. Subatomic particles that consist of quarks are known as hadrons. Leptons are indivisible point particles; they are not composed of quarks.

A significant amount of effort and money has been directed to testing Gell-Manns theoryboth theoretically and experimentally. This has involved the construction of larger and larger particle accelerators such as Fermilab in Chicago and CERN in Geneva. Australia built its own particle acceleratora synchrotronnext to Monash University. This began operating in 2007.

While the current theory suggests that quarks and leptons are the ultimate fundamental particles that cannot be further divided, the nature of scientific theories and models is such that they can change as new experimental data are obtained. Are quarks and leptons made of smaller particles again? Time will tell!

Quarks and other subatomic particles!Physics in action

Figure 1.8 This particle accelerator is at the CERN European centre for high-energy physics. It accelerates protons from rest to 99.99995% of the speed of light in under 20 seconds!


The nucleus of an atom consists of positively charged protons and neutral neutrons. Collectively, protons and neutrons are known as nucleons. Negatively charged electrons surround the nucleus.

The atomic number, Z, is the number of protons in the nucleus. The mass number, A, is the number of nucleons in the nucleus, i.e. the combined number of protons and neutrons.

Isotopes of an element have the same number of protons but a different number of neutrons. Isotopes

of an element are chemically identical to each other, but have different physical properties.

An unstable isotopea radioisotopemay spon tan-eously decay by emitting a particle from the nucleus.

Artificial radioisotopes are manufactured in a process called artificial transmutation. This commonly takes place as a result of neutron bombardment in the core of a nuclear reactor.

Atoms, isotopes and radioisotopes1.1 summary

1 1 How many protons, neutrons and nucleons are in the following nuclides? a 4520Cab 19779Auc 23592Ud 23090Th

2 2 How many protons and neutrons are in these atoms? Use the periodic table to answer this question.a cobalt-60b plutonium-239c carbon-14

3 3 What is the difference between a stable isotope and a radioisotope? Give three examples of stable isotopes.

4 4 Can a natural isotope be radioactive? If so, give an example of such an isotope.

5 5 Which of these atoms are definitely radioactive?

2412Mg, 5927Co,

19578Pt,

21084Po,

23892U

Explain how you made your choice.

6 Radium was discovered by Marie and Pierre Curie in 1898. Two isotopes of radium are Ra-226 and Ra-228. Use the periodic table to answer this question.a How many protons, neutrons and nucleons are in

a nucleus of radium-226?b How many protons, neutrons and nucleons are in

a nucleus of radium-228?

c A geologist is examining a small lump of radium-226 and a small lump of radium-228. The samples contain exactly the same numbers of atoms as each other. Is it possible for the geologist to determine which isotope is which? Explain.

7 7 The nucleus of a gold atom has a radius of 6.2 1015 m while the atom itself has a radius of 1.3 1010 m. Given that the volume of a sphere is V = 43 r3, determine the value of the fraction:

volume of nucleus volume of atom

8 8 As part of a science project, a student wanted to make a scale model of a gold atom using a marble of radius 1.0 cm as the nucleus. Calculate the radius of the sphere to be occupied by the electrons in this model. Use the information in Question 7 to assist your calculations.

9 9 Krypton-84 is stable but krypton-89 is radioactive. a Discuss any differences in how these atoms would

interact chemically with other atoms.b Describe the difference in the composition of these

two atoms.

10 10 A particular artificial radioisotope is manufactured by bombarding the stable isotope 27Al with neutrons. The radioisotope is produced when each atom of 27Al absorbs one neutron into the nucleus. Identify the radioisotope that is produced as a result of this process.

Atoms, isotopes and radioisotopes1.1 questions

Worked Solutions


Radioactivity and how it is detected1.2

Through the Middle Ages, alchemists tried without success to change lead into gold. They thought that it would be possible to devise a chemical process that would change one element into another. We now know that it is extremely difficult to change one element into another and that chemistry is not the way to do it. About 100 years ago, Ernest Rutherford and Paul Villard discovered that there were three different types of emission from radioactive substances. They named these alpha, beta and gamma radiation. Further experiments showed that the alpha and beta emissions were actually particles expelled from the nucleus. Gamma radiation was found to be high-energy electromagnetic radiation, also emanating from the nucleus. When these radioactive decays occur, the original atom spontaneously changes into an atom of a different element. Nature was already doing what the alchemists had so fruitlessly tried to do!

Scientists such as Ernest Rutherford quickly made use of these new particles to investigate the nature of matter. Later on, scientists also used them to create new isotopes.

Alpha decay 42

When a heavy nucleus undergoes radioactive decay, it may eject an alpha particle. An alpha particle is a positively charged chunk of matter. It consists of two protons and two neutrons that have been ejected from the nucleus of a radioactive atom. An alpha particle is identical to a helium nucleus and can also be written as 42He

2+, 2+, 42 or simply .

Uranium-238 is radioactive and may decay by emitting an alpha particle from its nucleus. This can be represented in a nuclear equation in which the changes occurring in the nuclei can be seen. Electrons are not considered in these equationsonly nucleons. The equation for the alpha decay of uranium-238 is:

23892U

23490Th +

42 + energy

or 23892U

23490ThIn the decay process, the parent nucleus 23892U has spontaneously emitted

an alpha particle () and has changed into a completely different element, 234

90Th. Thorium-234 is called the daughter nucleus. The energy released is mostly kinetic energy carried by the fast-moving alpha particle.

When an atom changes into a different element, it is said to undergo a nuclear transmutation. In nuclear transmutations, electric charge is conservedseen as a conservation of atomic number. In the above example 92 = 90 + 2. The number of nucleons is also conserved: 238 = 234 + 4.

Figure 1.9 Marie Curie performed pioneering work on radioactive materials. In fact, Marie Curie coined the term radioactivity and is one of only four scientists to have been awarded two Nobel prizes. She received one for chemistry and one for physics.

Figure 1.10 Ernest Rutherford was born in New Zealand and is considered to be one of the greatest experimental physicists who ever lived. He used the newly discovered alpha particles to investigate the nature of matter. His discoveries form the foundation of nuclear physics.

Figure 1.11 When the nucleus of uranium-238 decays, it will spontaneously eject a high-speed alpha particle that consists of two protons and two neutrons. The remaining nucleus is thorium-234. Kinetic energy, carried by the thorium-234 and alpha particles, is released as a result of this decay.

alpha particle

thorium-234uranium-238: unstable


Beta decay 1

0Beta particles are electrons, but they are electrons that have originated from the nucleus of a radioactive atom, not from the electron cloud. A beta particle can be written as 1

0e, , or 10. The atomic number of 1 indicates that it has a single negative charge, and the mass number of zero indicates that its mass is far less than that of a proton or a neutron.

Beta decay occurs in nuclei in which there is an imbalance of neutrons to protons. Typically, if a light nucleus has too many neutrons to be stable, a neutron will spontaneously change into a proton, and an electron and an uncharged massless particle called an antineutrino ( ) are ejected to restore the nucleus to a more stable state.

Consider the isotopes of carbon: 126C, 13

6C and 14

6C. Carbon-12 and carbon-13 are both stable, but carbon-14 is unstable. It has more neutrons and so undergoes a beta decay to become stable. In this process one of the neutrons changes into a proton. As a result, the proton number increases to seven, and so the product is not carbon. Nitrogen-14 is formed and energy is released.

The nuclear equation for this decay is: 14

6C 14

7N + 10 + + energy

The transformation taking place inside the nucleus is:10n

11p + 1

0e +

Once again, notice that in all these equations the atomic and mass numbers are conserved. (The antineutrino has no charge and has so little mass that both its atomic and mass numbers are zero.)

Gamma decay Generally, after a radioisotope has emitted an alpha or beta particle, the daughter nucleus holds an excess of energy. The protons and neutrons in the daughter nucleus then rearrange slightly and off-load this excess energy by releasing gamma radiation (high-frequency electromagnetic radiation). Gamma rayslike all lighthave no mass and are uncharged and so their symbol is 00. Being a form of light, gamma rays travel at the speed of light.

A common example of a gamma ray emitter is iodine-131. Iodine-131 decays by beta and gamma emission to form xenon-131 as shown in Figure 1.13.

In any nuclear reaction, including radioactive decay, atomic and mass numbers are conserved. Energy is released during these decays.

i

Figure 1.12 The nucleus of carbon-14 is unstable. In order to achieve stability, one neutron transforms into a proton, and an electron and antineutrino are emitted in the process. The emitted electron is a beta particle, and it travels at nearly the speed of light.

carbon-14:unstable

nitrogen-14:stable

beta particle 10

antineutrino A different form of beta decay occurs in atoms that have too many protons. An example of this is the radioactive decay of unstable nitrogen-12. There are seven protons and five neutrons in the nucleus, and a proton may spontaneously change into a neutron and emit a neutrino () and a positively charged beta particle. This is known as a + (beta-positive) decay and the positively charged beta particle is called a positron.

The equation for this decay is:

127N

126C + +1

0e + + energy

Positrons, +10, have the same

properties as electrons, but their electrical charge is positive rather than negative. Positrons are an example of antimatter.

Physics file


The equation for this decay is:

13153I

13154Xe + 1

0e + or

13153I

, 13154Xe

Since gamma rays carry no charge and have almost no mass, they have no effect when balancing the atomic or mass numbers in a nuclear equation.

The chart in Figure 1.14 identifies the 272 stable nuclides, as well as some radionuclides and decay modes.

Worked example 1.2A Strontium-90 decays by radioactive emission to form yttrium-90. The equation is:

9038

Sr 90

39Y

+ X

Determine the atomic and mass numbers for X and identify the type of radiation that is emitted during this decay.

Solution By balancing the equation, it is found that X has a mass number of zero and an atomic number of 1. X is an electron and so this must be beta decay. The full equation is 9038

Sr 9039

Y + 1

0e.

Worked example 1.2B Iodine-131, a radioisotope that is used in the treatment of thyroid cancer, is produced in a two-stage process. First, tellurium-130 (130

52Te) is bombarded with neutrons inside the core

of a nuclear reactor. This results in the formation of the very unstable tellurium-131 and a gamma ray.a Write down the balanced nuclear equation for this process.b Tellurium-131 decays by beta emission to produce a daughter nuclide and an

antineutrino. Identify the daughter nuclide.

Solution a 130

52 Te + 1

0n 131

52 Te +

b Both the atomic and mass number of the antineutrino are zero. The beta particle has a mass number of zero and an atomic number of 1.

13152

Te 13153

X + 1

0 + Balancing the nuclear equation gives the unknown element an atomic number of 53 and

a mass number of 131. The periodic table reveals the daughter nuclide to be iodine-131.

Figure 1.13 In the beta decay of iodine-131, a high-energy gamma ray photon is also emitted. This high-energy electromagnetic radiation has no electric chargejust energy. The beta particle and xenon nucleus also carry energy.

iodine-131:unstable

xenon-131gamma ray 00G

beta particle 10B

Gamma decay alone occurs when a nucleus is left in an energised or excited state following an alpha or beta decay. This excited state is known as a metastable state and it usually only lasts for a short time. An example of this is the radioactive decay of iodine-131, usually a two-stage process.

First, a beta particle is emitted and the excited nuclide xenon-131m is formed. Then, the nucleus undergoes a second decay by emitting a gamma ray:

The m denotes an unstable or metastable state. Cobalt-60 and technetium-99 also exist in metastable states.

I 13153

Xe + 131m54

e 01

Xe 131m54

Xe + 13154

Physics file


Figure 1.14 From this table of stable isotopes and radioisotopes, it is evident that for larger nuclei there is a distinct imbalance between the number of protons and neutrons. The line of stability of the stable nuclides can be seen as a line that curves away from the N = Z line. Notice that every element, up to and including bismuth, has stable isotopes, except for technetium and promethium. Also notice that every isotope of every element beyond bismuth is radioactive.

+ + +

+ + + + +

+ + + + + + + +

+ + + + + +

+ + +

+ + + + + + + + + + + + + + + + + + + + + + +

+ + + + + + + + + + + + + + + + + +

+

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + + +

+ + + + + + + + +

+ + + + + + +

+ + +

+ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + +

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

+ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + +

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + +

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + + + + + + + + + + ++ + + + + + +

+ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + +

+ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + +

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + +

+ + + + + +

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + +

+ + + + + ++ + + + ++ + + + + + + + + + + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + +

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + + ++ + + + + + + + +

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + + ++ + + + + + + + +

+ +

+ + +

+ + + + + + + + + + + +

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

Line of stability

+ + ++ + ++ + ++

N = Z

+ + + + + + + +

+ + + + + + + +

+ + + + ++ + + ++

++ + + + + + ++ + + ++

+ + + +

+ + + + + + + + ++++

+ + + + + + +

+ + + + + +

+ + + + + + + + +

+ + +

+

++++

+

L

++++++++

++++

+

i

++++++

+++

+

+

++

+++

+

++++++++++

++++++

+

++++++++++

+

+

+ + + + ++ + + + + + + +

+ + + + + + +

+++++

+

++

+

++

+

140

130

120

110

100

90

80

70

60

50

40

30

20

10

0

Num

ber

of n

eutr

ons

(N)

0 10 20 30 40 50 60 70 80 90 100

Atomic number (Z)

Bismuth, Z = 83

Promethium, Z = 61

Technetium, Z = 43

stable nuclide emitter+ + emitter emitter

Key


Why radioactive nuclei are unstableInside the nucleus there are two completely different forces acting. The first is an electric force of repulsion between the protons. On its own, this would blow the nucleus apart, so clearly a second force must act to bind the nucleus together. This is the nuclear force, a strong force of attraction between nucleons, which acts only over a very short range.

In a stable nucleus, there is a delicate balance between the repulsive electric force and the attractive nuclear force. For example, bismuth-209, the heaviest stable isotope, has 83 protons and 126 neutrons, and the forces between the nucleons balance to make the nucleus stable. Compare this with bismuth-211. It has two extra neutrons and this upsets the balance between forces. The nucleus of 211Bi is unstable and it ejects an alpha particle in an attempt to attain nuclear stability.

Figure 1.14 shows all the stable nuclei with their proton and neutron numbers. It is evident that there is a line of stability along which the nuclei tend to cluster. Nuclei away from this line are radioactive. For small nuclei with atomic numbers up to about 20, the ratio of neutrons to protons is close to one. However, as the nuclei become bigger, so too does the ratio of neutrons to protons. Zirconium (Z = 40) has a neutron to proton ratio of about 1.25, while for mercury (Z = 80) the ratio is close to 1.66. This indicates that for higher numbers of protons, nuclei must have even more neutrons to remain stable. These neutrons dilute the repelling forces that act between the extra protons. Elements with more protons than bismuth (Z = 83) simply have too much repulsive charge and additional neutrons are unable to stabilise their nuclei. All of these atoms are radioactive.

How radiation is detectedOur bodies cannot detect alpha, beta or gamma radiation. Therefore a number of devices have been developed to detect and measure radiation.

A common detector is the Geiger counter. These are used: by geologists searching for radioactive minerals such as uranium to monitor radiation levels in mines to measure the level of radiation after a nuclear accident such as the

accident at Fukushima, Japan, in 2011 to check the safety of nuclear reactors to monitor radiation levels in hospitals and factories.

A Geiger counter consists of a GeigerMuller tube filled with argon gas as shown in Figure 1.15. A voltage of about 400 V is maintained between the positively charged central electrode and the negatively charged aluminium tube. When radiation enters the tube through the thin mica window, the argon gas becomes ionised and releases electrons. These electrons are attracted towards the central electrode and ionise more argon atoms along the way. For an instant, the gas between the electrodes becomes ionised enough to conduct a pulse of current between the electrodes. This pulse is registered as a count. The counter is often connected to a small loudspeaker so that the count is heard as a click.

People who work where there is a risk of continuing exposure to low-level radiation usually pin a small radiation-monitoring device to their clothing.

Neutrinos are particles with the lowest mass in nature, and they permeate the universe. Neutrinos have no charge and their mass has only recently been discovered to be about one-billionth that of a proton, i.e. about 1036 kg. While you have been reading these sentences, billions of neutrinos have passed right through your body, and continued on to pass right through the Earth! Fortunately neutrinos interact with matter very rarely and so are harmless. It has been estimated that if neutrinos passed through a piece of lead 8 light-years thick, they would still have only a 50% chance of being absorbed!

Physics file

Figure 1.15 A radioactive emission that enters the tube in a Geiger counter will ionise the argon gas and cause a pulse of electrons to flow between the electrodes. This pulse registers as a count on a meter.

+

thin micawindow

positively chargedelectrode

argongas

negatively chargedaluminium tube

Interactive


This could be either a film badge or a TLD (thermoluminescent dosimeter). These devices are used by personnel in nuclear power plants, hospitals, airports, dental laboratories and uranium mines to check their daily exposure to radiation. When astronauts go on space missions, they wear monitoring badges to check their exposure to damaging cosmic rays.

Film badges contain photographic film in a lightproof holder. The holder contains several filters of varying thickness and materials covering a piece of film. After being worn for a few weeks, the film is developed. Analysis of the film enables the type and amount of radiation to which the person has been exposed to be determined.

Thermoluminescent dosimeters are more commonly used than film badges. TLDs contain a disk of lithium fluoride encased in plastic. Lithium fluoride can detect beta and gamma radiation as well as X-rays and neutrons. Thermoluminescent dosimeters are a cheap and reliable method for measuring radiation doses.

Figure 1.16 Thermoluminescent dosimeters are used by doctors, radiologists, dentists and technicians who work with radiation, to monitor their exposure levels.

Technetium-99m is the most widely used radioisotope in nuclear medicine. It is used for diagnosing and treating cancer. However, this radioisotope decays relatively quickly and so usually needs to be produced close to where it is to be used. Technetium-99m is produced in small nuclear generators that are located in hospitals around the country. In this process, the radioisotope molybdenum-99, obtained from Lucas Heights, is used as the parent nuclide. Molybdenum-99 decays by beta emission to form a relatively stable (or metastable) isotope of technetium, technetium-99m, as shown below:

9942Mo

99m43Tc + 1

0 +

Technetium-99m is flushed from the generator using a saline solution. The radioisotope is then diluted and attached to an appropriate chemical compound before being administered to the patient as a tracer. Technetium-99m is purely a gamma emitter. This makes it very useful as a diagnostic tool for locating and treating cancer. Its decay equation is:

99m43Tc

9943Tc +

How technetium is producedPhysics in action

Figure 1.17 Technetium generators are used in hospitals that require radioisotopes. The generator has a thick lead shield that absorbs the beta and gamma radiation.

SPARKlab Risk AssessmentPrac 1


Radioactive isotopes may decay, emitting alpha, beta and gamma radiation from their nuclei.

An alpha particle, , consists of two protons and two neutrons. It is identical to a helium nucleus and can be written as 42,

2+ or 42He. A beta particle, , is an electron, 10e, that has been

emit ted from the nucleus of a radioactive atom as a result of a neutron transmuting into a proton.

A gamma ray, , is high-energy electromagnetic

radiation that is emitted from the nuclei of radioactive atoms. Gamma rays usually accompany an alpha or beta emission.

In any nuclear reaction, both atomic and mass numbers are conserved.

Radiation can be detected using a device such as a Geiger counter. People can monitor their exposure to radiation with film badges and thermoluminescent dosimeters.

Radioactivity and how it is detected1.2 summary

1 From which part of a radioisotope, the nucleus or the electron cloud, are the following particles emitted?a alpha particles b beta particles c gamma rays

2 2 Discuss the physical differences between , and radiation.

3 3 Identify each of these particles.a 1

0A b 11B c 42C d

10D

4 4 Determine the atomic and mass numbers for the unknown elements, X, in these decay equations, then use the periodic table to identify the elements.a 21884Po X + + b 23592U

, X

c 22888Ra X + + d 19879Au , X

5 5 Determine the mode of radioactive decay for each of the following transmutations.a 21886Rn

21484Po + X + b 23491Pa

x, 23492U

c 21482Pb 214

83Bi + X + d 23994Pu x, 23592U

e 60m27 Co 6027Co + X

6 6 When the stable isotope boron-10 is bombarded with neutrons, it transmutes by neutron capture into a different element X and emits alpha particles. The equation for this reaction is:

105B +

10n X +

42He.

Identify the final element formed.

7 7 Identify the unknown particles in these nuclear transmutations.a 147N +

178O + X b

2713Al + X

2712Mg +

11H

c 147N + X 14

6C + 11p d

2311Na + X

2612Mg +

11H

8 8 Carbon-14 decays by beta emission to form nitrogen-14. The equation for this is 146C

147 N + 1

0e + . It can be seen that the carbon nucleus initially has six protons and eight neutrons.

a List the particles that comprise the decay side of this equation.

b Analyse the particles and determine which particle from the parent nucleus has decayed.

c Write an equation that describes the nature of this decay.

d Energy is released during this decay. In what form does this energy exist?

9 9 Use the chart in Figure 1.14 to answer these questions.a List all the stable nuclides of calcium, Z = 20.b How many stable nuclides does niobium, Z = 41,

have?c 4819K has a large imbalance of neutrons over protons

and so is radioactive. Find potassium-48 on the chart and determine whether it is an alpha or beta emitter.

d Write the decay equation for potassium-48 and determine whether the daughter nucleus is itself stable or radioactive.

e Calculate the ratio of neutrons to protons for each of potassium-48 and its daughter nucleus.

f 21787Fr is a radioisotope. Is it an alpha or beta emitter?

g Determine the decay processes that francium-217 undergoes before it becomes a stable nuclide; identify this nuclide.

10 10 Gold has only one naturally occurring isotope, 197Au. If a piece of gold foil is irradiated with neutrons, neutron capture will occur and a radioactive isotope of gold will be produced. This radioisotope is a beta emitter. Write an equation that describes the:a neutron absorption processb decay process.

Radioactivity and how it is detected1.2 questions

Worked Solutions

Chapter 1 Nuclear physics and radioactivity 15

Properties of alpha, beta and gamma radiation1.3

Alpha particles, beta particles and gamma rays all originate from the same placethe nucleus of a radioisotope. Each type of radiation has enough energy to dislodge electrons from the atoms and molecules that they smash into. This property is what makes radiation dangerous, but it also enables it to be detected. The properties of alpha, beta and gamma radiation are distinctly different from each other. During early investigations of radioactivity, the emissions from a sample of radium were directed through a magnetic field. As shown in Figure 1.18, the emissions followed three distinct paths, suggesting that there were three different forms of radiation being emitted.

Alpha particlesAlpha particles, , consist of two protons and two neutrons. Because an alpha particle contains four nucleons, it is relatively heavy and slow moving. It is emitted from the nucleus at speeds of up to 20 000 km s1 (2.0 107 m s1), just less than 10% of the speed of light.

Alpha particles have a double positive charge. This, combined with their relatively slow speed, makes them very easy to stop. They only travel a few centimetres in air before losing their energy, and will be completely absorbed by thin card. They have a poor penetrating ability.

Beta particlesBeta particles, , are fast-moving electrons, created when a neutron decays into three partsa proton, an electron (the beta particle) and an antineutrino. Beta particles are much lighter than alpha particles, and so they leave the nucleus with far higher speedsup to 90% of the speed of light.

Figure 1.18 When radiation from radium passes through a magnetic field, the radiation splits up and takes three different paths. One path is undeflected. The other two paths deviate in opposite directions and to different extents. This suggests that there are three different forms of radiation being emitted from radium.

Figure 1.20 Gamma rays can pass through human tissue and sheets of aluminium quite readily. A 5 cm thick sheet of lead is needed to stop 97% of the gamma rays in a beam. By comparison, alpha particles are not capable of penetrating through a sheet of paper or beyond the skin of a person.

Figure 1.19 The relative speeds of alpha, beta and gamma radiation. (a) Alpha particles are the slowest of the radioactive emissions. Typically they are emitted from the nucleus at up to 10% of the speed of light. (b) Beta particles are emitted from the nucleus at speeds up to 90% of the speed of light. (c) Gamma radiation, being high-energy light, travels at the speed of light (3.0 108 m s1).

N

SRamagnet

aluminium lead

c

~0.1c

~0.9c

42

10

00 ray

(a)

(b)

(c)

Interactive Risk AssessmentPrac 1


Beta particles are more penetrating than alpha particles, being faster and with a smaller charge. They will travel a few metres through air but, typically, a sheet of aluminium about 1 mm thick will stop them.

Gamma raysGamma rays, , being electromagnetic radiation with a very high frequency, have no rest mass and travel at the speed of light3.0 108 m s1 or 300 000 km s1. They have no electric charge. Their high energy and uncharged nature make them a very penetrating form of radiation. Gamma rays can travel an almost unlimited distance through air and even a few centimetres of lead or a metre of concrete would not completely absorb a beam of gamma rays.

The ionising abilities of alpha, beta and gamma radiationWhen an alpha particle travels through air, its slow speed and double positive charge cause it to interact with just about every atom that it encounters. The alpha particle dislodges electrons from many thousands of these atoms, turning them into ions. Each interaction slows it down a little, and eventually it will be able to pick up some loose electrons to become a helium atom. This takes place within a centimetre or two in air. As a consequence, the air becomes quite ionised, and the alpha particles are said to have a high ionising ability. Since the alpha particles dont get very far in the air, they have a poor penetrating ability.

Beta particles have a negative charge and are repelled by the electron clouds of the atoms they interact with. This means that when a beta particle travels through matter, it experiences a large number of glancing collisions and loses less energy per collision than an alpha particle. As a result, beta particles do not ionise as readily and will be more penetrating.

Gamma rays have no charge and move at the speed of light, and so are the most highly penetrating form of radiation. Gamma rays interact with matter infrequently, when they collide directly with a nucleus or electron. The low density of an atom makes this a relatively unlikely occurrence. Gamma rays pass through matter very easilythey have a very poor ionising ability but a high penetrating ability.

The energy of alpha, beta and gamma radiationThe energy of moving objects such as cars and tennis balls is measured in joules. However, alpha, beta and gamma radiation have such small amounts of energy that the joule is inappropriate. The energy of radioactive emissions is usually expressed in electronvolts (eV). An electronvolt is the energy that an electron would gain if it were accelerated by a voltage of 1 volt.

One LCTRONVOLT is an extremely small quantity of energy equal to 1.6 1019 J, i.e. 1 eV = 1.6 1019 J.

i

Some types of radiation, such as radio waves, are harmless. Other types, however, are dangerous to humans. Known as ionising radiation, these interact with atoms, having enough energy to remove outer-shell electrons and create ions. Alpha particles, beta particles and gamma rays are all ionising. So too is electromagnetic radiation with a frequency above 2 1016 Hz. Thus, X-rays and ultraviolet-B radiation are ionising. When ionising radiation interacts with human tissue, it is the ions it produces that are harmful and that lead to the development of cancerous tumours.

Lower-energy electromagnetic radiation such as radio waves, microwaves, infrared, visible light and ultraviolet-A are non-ionising. We are exposed to significant amounts of such radiation each day with no serious consequences. Non-ionising radiation does not have enough energy to change the chemistry of the atoms and molecules that make up our body cells.

Physics file

X-rays and gamma rays are ionising radiation. They are both high-energy forms of electromagnetic radiation (released as high-energy photons), but gamma rays usually have higher energies. This means that gamma rays are usually more highly penetrating than X-rays. The defining distinction between X-rays and gamma rays is the method of production.

X-rays are created from electron transitions within the electron cloud, whereas gamma rays are emitted from the nuclei of radioactive atoms. Gamma rays and X-rays have similar properties, but X-rays are not the result of radioactive decay.

Physics file

Prac 3 Risk Assessment


Alpha and beta particles are ejected from unstable nuclei with a wide range of energies. Alpha particles typically have energies of 510 million electronvolts (510 MeV). This corresponds to speeds of about 16 000 km s1, about 510% of the speed of light.

Beta particles are usually ejected with energies up to a few million electronvolts. For example, sodium-24 emits beta particles with a maximum energy of 1.4 MeV. This is equivalent to 2.24 1013 J. These particles are travelling at speeds quite close to the speed of light.

Gamma rays normally have less than a million electronvolts of energy. They may even have energy as low as 100 000 electronvolts. For example, the gamma rays emitted by the radioactive isotope gold-198 have a maximum energy of 412 000 eV (412 keV) or 6.6 1014 J. Increasing the energy of a gamma ray does not increase its speed; it increases the frequency of the radiation.

Table 1.1 The properties of alpha, beta and gamma radiation

Property particle particle rayMass heavy light none

Charge +2 1 none

Typical energy 5 MeV 1 MeV 0.1 MeV

Range in air a few cm 1 or 2 m many metres

Penetration in matter 102 mm a few mm high

Ionising ability high reasonable poor

Worked example 1.3A Uranium-238 emits alpha particles with a maximum energy of 4.2 MeV. a Explain why a sample of this radioisotope encased in plastic is quite safe to handle yet,

if inhaled as dust, would be considered very dangerous. b Calculate the energy of one of these alpha particles in joules.

Solution a The alpha particles have a poor penetrating ability and so would be unable to pass

through the plastic casing. However, if the radioactive uranium was on a dust particle and was inhaled, the alpha-emitting nuclei would be in direct contact with lung tissue and the alpha particles would damage this tissue.

b 4.2 MeV = 4.2 106 eV = 4.2 106 1.6 1019 J = 6.7 1013 J

The energy released during any nuclear reaction (including radioactive decay) is many times greater than that released in a typical chemical reaction. For example the chemical reaction of a sodium ion capturing an electron releases about 1 eV of energy.

Na+ + e Na + 1 eV

Nuclear reactions such as alpha, beta and gamma decays typically release energies of the order of megaelectronvolts, MeV, i.e. nuclear reactions release about a million times more energy than chemical reactions.

Physics file


Each year, dozens of people in Australia die as a result of do

Documents

Hein Physics 11 3E Enhanced SB - ©¦¦¥