Radiation and Reason

By: Wade Allison

Radiation andReason

The Impact ofScience on a

Culture of FearWade Allison

PublicationInformation

Published by Wade Allison Publishing Wade Allison2009 All rights reserved. No part of this publication maybe reproduced, stored on a retrieval system, ortransmitted, in any form or by any means, withoutexplicit permission from the author. Enquiries should besent to w.allison@physics.ox.ac.ukPublished in paperback (October 2009) ISBN 978-0-9562756-1-5and in e-book and Kindle editions (July 2011) ISBN 978-0-9562756-2-2 and 978-0-9562756-3-9Website http://www.radiationandreason.comHardcopy printed and distributed by York PublishingServices Ltd 64 Hallfield Road, Layerthorpe, York, UKYO31 7ZQLast saved 3 May 2012

mailto:w.allison@physics.ox.ac.ukhttp://www.radiationandreason.com

DedicationFor Alfie, Alice, Joss, Minnie, Edward, George

and those who come after,may they understand one day.

About the AuthorProfessor Wade Allison, MA DPhil, is a Fellow of KebleCollege and a Professor Emeritus at the University ofOxford where he has studied and taught physics forover 40 years. His earlier research work was inelementary particle physics, in particular the radiationfield of relativistic particles, but his interests andexpert is e h a v e s p re a d m u c h wid e r. H e recentlypublis hed Fundamental Ph ysi cs f o r Pro b i n g andImaging, an advanced textbook for his course at Oxfordon medical physics, including radiation and it s u s e inclinical medicine a n d t h e wider environment. Of thesafety of radiation and nuclear technology, he says

I have no axe to grind, I have no links with theindustry, I just want to see the truth out there. Somany people have been under a misapprehensionfor so long.

Of the conclusions of this book he saysIt brings good news - but are the people of theworld ready t o re-examine past assumptions inthe light of current science? It is important thatthey do, because, without nuclear energy, thefuture for mankind looks bleak.

ContentsChapter 1 - PerceptionsA mistake - Personal risk and knowledge - Individualand collective opinions - Confidence and decisions -Science and safetyChapter 2 - Atmospheric EnvironmentSize and composition of the atmosphere - Atmosphericchange - Energy and agricultureChapter 3 - The Atomic NucleusPowerful and beneficial - Size scales - Atoms andelectrons - The nuclear atom - The quiescent nucleus -Energy for the SunChapter 4 - Ionising RadiationThe spectrum of radiation - Damage from radiation -Nuclear stability - Measuring radiation - NaturalenvironmentChapter 5 - Safety and DamageProportionate effects - Balancing risks - Protection ofman - Damage and stress - Time to repair - Collectivedose - Safety margins - Multiple causes - Beneficial andadaptive effects - Surprise at Chernobyl

Chapter 6 - A Single Dose of RadiationWhat happens to molecules - What happens to cells -Evidence at high dose - Repair mechanisms - Low andintermediate doses - Survivors of Hiroshima andNagasaki - Radiation-induced cancers - Medicaldiagnostic scans - Nuclear medicine - People irradiatedat Chernobyl - Thyroid cancer - Other cancers atChernobylChapter 7 - Multiple Doses of RadiationDistributed doses - Cancer therapy - Fractionation -Doses in the environment - Radon and lung cancer-Radiation workers and dial painters - Biological defencein depthChapter 8 - Nuclear EnergyRealising nuclear energy - Explosive devices - Civilpower from fission - Energy without weapons - WasteChapter 9 - Radiation and SocietyPerceiving radiation - Public concern - Testing andfallout - Deterrence and reassurance - Judging radiationsafetyChapter 10 - Action for SurvivalRelaxed regulations - New power stations - Fuel andpolitics - Waste strategy - Decommissioning -Proliferation and terrorism - Fusion power - Costs and

the economy - Fresh water and food - Education andunderstandingChapter 11 - Summary of ConclusionsEpilogue - FukushimaInstability and self destruction - Explanation orappeasementFurther Reading and References

Preface

Preface to the e-book edition

What happened at Fukushima has not changed what isknown of the benefits and dangers of nuclear radiation.However, it has highlighted many of the arguments andI have added an epilogue that discusses these;otherwise the text is largely unaltered from the first printedition. As was the case for earlier accidents somereactors at Fukushima were destroyed but the impact ofthe released radiation on the population has beenoverstated with significant consequences for all thoseaffected. Initial reactions around the world toFukushima and its implications for nuclear technologyhave varied from one nation to another, depending inpart on its historical experience. Nuclear technology cando much for our lives and our view of it should bebased on science -- and that is the same in everycountry. Political and geological instabilities affectmany aspects o f a nation's life, and nuclear questions

should not be exceptional.It is natural that when there is an accident the questionshould be asked 'who is to blame?' but this questionmay have no answer even when many must pay for theconsequences. I hope that this book with it s epiloguewill provide a welcome and accessible account of thescience and a basis for understanding, mutual trust andoptimism for the future.I have taken the opportunity to clarify the section'Doses in the environment' in chapter 7.

Wade Allison, Oxford, June 2011

Preface to the firstedition

The human race is in a dilemma; it is threatened byeconomic instability on one hand and by climate changeon the other. Either o f these could lead t o widespreadunrest and political turmoil, if the right choices are notmade now. In particular, prosperity without carbonemission implies a comprehensive switch in our sourcesof energy. With luck, the activity generated by theprocess of switching will also contribute to prosperityin the short and medium term. There are many solutions

- wind, tidal, solar, improved efficiency - but the mostpowerful and reliable source is nuclear. However, it iswidely supposed that this presents a major problem ofsafety. Is this long-held concern about radiation andnuclear technology fully justified? Straightforwardquestions should have simple answers, and the simplestanswer is No. Explaining and exploring the question andthis answer in accessible terms is the subject of thisbook.Over the years I have taught and studied many areas ofphysics, including nuclear physics and medical physics,although I have never had a close link with the nuclearindustry. While it always seemed clear to me thatradiation safety was somewhat alarmist and unbalanced,in earlier decades the apparent freedom to opt for fossilfuel as the primary source of energy meant that therewas no special reason to confront public perceptions ofthe issue. But now the situation has changed, and it istime to address the whole question.But how, and with what voice? A discussion in popularterms that would appeal to the native common sense ofthe reader is too easily dismissed b y the science. Butscientific answers are impenetrable to many readers, andso fall on deaf ears. A way forward is to vary the tone,sometimes scientific but still accessible, and sometimeswith illustrations and examples that appeal to general

experience. Nevertheless, I shall probably tax eachreader's tolerance in places, one way or the other, andfor that I apologise. While ways of avoiding the use ofequations have been found except in some footnotes,use is made of the scientific notation for very large andsmall numbers (thus 106 means one million, 1 followedby six noughts and similarly 10- 6 means one millionthpart.) Finding passages that seem either trivial orimpenetrable, the reader is encouraged to skip forwardto rejoin further on. The passages that discuss recentscientific results are supported with references labelledin square brackets in the text and listed in full at theback. Most references may be found on the Web at theaddress given - but the text is self-contained and doesnot suppose that these are consulted. Also at the back,there is a short list of books and papers, headedFurther Reading.The story starts with the physical science, much ofwhich has been es tablis hed f o r d e c a d e s - theatmosphere, the atomic nucleus and radiation. And thenit moves on to the effect of radiation in biology, most ofwhich was not so well known 30 years ago. Often,popular science is written to amaze and inspire - andthat is important. But here the target is more prosaic andpractical, namely a clear understanding o f the scientificbackground to some of the threats and decisions that

are likely to determine our environment and thence oursurvival. The central question i s this : h o w significanta r e t h e h ea lth ris ks d u e to radiation and nucleartechnology? In Chapters 6 and 7 the current evidence isshown with the relevant ideas in modern biology. Notall questions can be answered completely yet, but theycan be answered quite well enough. The conclusionsa r e rather surprising, a n d d o n o t ma t c h we l l withcu rren t ly enforced radiation safety levels. Thischallenge by modern radio-biology to radiation safetyregulation is well aired in scientific papers, but has notbeen explained to the community at large, who have asignificant interest in the matter. The cos ts o f nucleartechnology a r e v e ry h ig h , i n p a r t becaus e o f theexceptional radiation s afety provis ion t h a t i s made.Scaling back such provision by a large factor wouldhave a major beneficial effect on the financial viability ofan extensive nuclear power programme.Thes e scientific findings d o n o t depend o n climatechange, although that is what makes the questionimportant at this time. But why, in the past, did most ofthe human race come to hold extreme views about thedangers o f radiation and nuclear technology? The lastpart of the book describes what nuclear technologynow offers, a large-scale supply o f carbon-free electricpower, with further options for the supply of food and

fresh water.E M Forster wrote

I suggest that the only books that influence usare those for which we are ready, and which havegone a little farther down our particular paththan we have yet gone ourselves.

I hope that for some readers the message of this book istimely.To keep the discussion focussed on a few main points,many important topics have been omitted or just notedin passing - in particular, the subject o f micro-dosimetryis treated rather briefly, in spite of its importance forfuture understanding. No doubt mistakes have beenmade too, and credit not given where it was due. Suchchoices, mistakes and lapses are mine, and I apologisefor them.I have benefited from conversations with manycolleagues during the writing of this book. It has been aprivilege to have had the opportunity for quietreflection and study, undisturbed by the pursuit ofgrant funding that dis torts s o much academic studytoday. T h is wo rk wo u ld n o t h a v e reached fruitionwithout the contributions of many people. Formerstudents and members of their families, members of myown family too, have spent long hours, reading andproviding feedback on my efforts to produce an

accessible account. In particular, I should like to thankMartin Lyons, M a rk Germain, Ja me s Hollow, GeoffHollow, Paul Neate, Rachel Allen, John Mulvey andJohn Priestland for their reading of the text andimportant comments. Chris Gibson and Jack Simmonshave provided me with invaluable comment andinformation. Throughout, I have relied heavily o n theencouragement of Elizabeth Jackson and my wife, Kate -their advice and persistence were essential. I thank Kateand all members of my family for their love and toleranceover the past three years while I have been ratherabsorbed.Finally I would like to thank Paul Simpson of LynkITand Cathi Poole of YPS for their enthusiastic ideas andcan do reaction to the task of printing and promotingthis book and its message.

Wade Allison, Oxford, September 2009

Chapter 1

PerceptionsScience is the great antidote to the poison ofenthusiasm and superstition.

Adam Smith, economist (1723-1790)

A mistakeRadiation is seen as a cause for exceptional concern andalarm, though few people have any personal experienceof its dangers. Is this view justified, and how did it cometo be held?Prior to the Second World War there was a degree ofrelaxed public acceptance o f radiat ion , principallybecause few knew anything to suggest otherwise. Thatchanged with the arrival of the Nuclear Age.The destruction of the Japanese cities of Hiroshima andNagasaki by nuclear bombs in 1945 was a military andpolitical success that avoided a land invasion of Japan,which would have been immensely costly in lives forboth sides. As a technical scientific enterprise, it was atriumph - n o project depending on fundamentally new

physical developments on such a scale had ever beenattempted before.As an exercise in the public understanding of science, itwas a disaster whose consequences still persist. Themessage that came through was very clear - whathappened was both extraordinarily dangerous, andincomprehensible to all but a few. The extremeapprehension generated i n t h e population w a s self-sustaining. Sources of fear inhibit free enquiry, and fewin the population ever questioned the extent of thedanger. In the decades of the Cold War that followed,this fear was a useful weapon in international politics,and its basis was not doubted, even by those in aposition to do so. And then there was Chernobyl - afurther failure of public understanding. In the publicmind the fear of nuclear war had infected views on civilnuclear power. Most people simply wanted t o distancethemselves from anything nuclear.More questions should have been asked, althoughsome of the answers could not have been given inearlier decades. There are three concentric concerns,related like the layers of an onion, as it were. The firstand innermost is to understand the effect of radiationo n h u ma n life . T h is i s a s cientific ques t ion , notdependent on the other two. The second task is toeducate public opinion and formulate safety regimes in

the light of the solution to the scientific question. Thefinal problem i s to discourage nation states andterrorists from exploiting radiation as a source of fear bythreatening and posturing. This depends critically onthe second task, establishing robust public opinion anda regulation regime that can face up to international armtwisting.In the last 50 years these problems have been confused.During the Cold War era, international politics exploitedpublic fear and ignorance o f radiation, wh ile onlyrecently has the scientific evidence and understandingbecome established to answer the prior scientificquestion. In the absence of a clear picture of thebiology and of adequate human-based evidence,radiation safety guidelines and legislation becameestablished on a reactive basis. Public concerns werehandled by imposing draconian regulation on radiationand nuclear technology, in the expectation that thiswould provide the necessary reassurance. But the veryseverity of the restraints only increased public alarmand people were not reassured.But now in the new century there have been twochanges. Firstly, the scientific answers that were lackingprevious ly a r e now largely available. Secondly, newnuclear power plants are urgently needed so that theuse of fossil fuel can be reduced - this does not change

the safety of radiation but it does affect the importanceof setting matters right as soon as possible. So thepurpose of this book is to explain the science in fairlyaccessible terms, together with some of the evidence,and to offer a rough but justified estimate of the level ofnew safety regulation. Consequences for public policyand international diplomacy may then follow.

Personal risk andknowledge

Making decis ions t o red u ce t h e r i s k o f accidentsinvolves everybody in society, what they believe to bethe level of risk, as well as what is actually the level ofrisk. People may be alarmed, when they do not need tobe - they may be fearless, when they should be morecautious.What level of risk is tolerable in exceptionalcircumstances? We should n o t s a y zero - a risk-freesociety is utopian and unachievable. Although personalfear may feel absolute and unquantifiable, i t should becontrolled - any risks involved should be compared withthose of alternative courses of action. Even the durationof life on Earth will have its term, hopefully not caused

by early escalating climate change. But for u s asindividuals, the end is closer and more certain, forfinally we all die - life expectancy may be 70 to 80 years,depending on standard of living, health and diet. Sowhat is the average effect on a life of an accident thatcarries a 1% risk of death? For an average age of 40, thatmeans a life expectancy reduced by an average of 0.4years, or 5 months. If the lifelong risk is 0.1%, thereduction in life expectancy is just 2 weeks. This is atthe same level as many risks and choices that peopleincur a s they go about their daily lives. Many peoplewould, willingly, give up 2 weeks of life for the benefitof their children or grandchildren if that would reallybenefit the large-scale prospect for the planet. Well,wouldn't they? So , thinking s traight, a lifetime ris k ofdeath at the level of one in a thousand is sensible - ifundertaken for good reason, of course. As we shall see,the evidence shows that only under quite exceptionalconditions is any nuclear risk at such a high level.In general, those who make decisions need to be surethat they themselves understand the relevant situation.If their information is picked up from others on the basisof a collective idea that everybody knows, there is achance that wrong decisions will be made. The greaterthe number of people relying on the opinion of others,the longer it takes for them to realise if something is

wrong. So, the bigger a blind spot in understanding, thegreater the chance that basic questions go unasked andunanswered. At a practical level, a hard question maybe beyond the immediate field of an individual - and sobe passed to an expert for a specialised opinion,perhaps without reference to other problems. In thisway the true picture may become distorted in the formof a collection of separate narrowly defined opinions.An example was the flow of intelligence and decision-making in the conduct of the First World War. Aconsequence was the extreme loss of life, for example,on the Somme in July 1916. Decisions on the course ofaction were taken by commanders, who did not know orappreciate the actual situation in the field. And those onthe battlefield were not permitted to use their ownintelligence to modify the plan. It was assumed that theheaviest possible artillery bombardment would destroythe barbed wire and overcome the machine-gun posts -but it did not. The commanders did not find out, and themen on the ground were required to obey instructions.The result was an avoidable massacre.A more recent example was the effect on the stability ofthe world financial system of various trading andinsurance practices employed in the first few years ofthe 21st century. Financial regulators and seniormanagers of corporations, who, in the years leading up

to 2008, encouraged their dealers to negotiate andexchange contracts of risk for money, were not able tograsp the instability of the structures that they werebuilding. These were described a s co mp l ex andsophisticated - words t h a t should themselves be awarning. Used to impress, they invite acceptancewit h o u t ques t ion . W h e n t h e financia l structurescollapsed, nobody was able to determine the ownershipand the worth of their holdings. The absence of anyonewith the ability to see the consequences of what washappening was as serious as on the Somme in 1916. Thefinancial dislocation, which played a dramatic part in thecollapse of 2008, was foreseen eight years earlier byWilmott [1], who wrote as follows.

The once 'gentlemanly' business of finance hasbecome a game for 'players'. These players areincreasingly technically sophisticated, typicallyhaving PhDs in a numerate discipline.Unfortunately, as the mathematics of financereaches higher levels so the level of commonsense seems to drop. ... I t i s clear tha t a majorrethink is desperately required if the world is toavoid a mathematician-led market meltdown.

When decisions are scientific, the availability ofadequate firsthand unders tanding c a n b e a majorhurdle, because such understanding is sparse in the

population. This is especially true for decisionsinvolving nuclear radiation. To the general populationand those who make decisions for society, the wordsand ideas that describe the science do not have familiarmeanings. Apprehension of anything nuclear, orconcerned with radiation, is deeply engrained in popularculture, a n d few scientists have pursued the broaderinter-disciplinary field.For reasonable decision-making, i t i s essential that thetruth underlying t h e fe a rs o f nuclear material andradiation are properly exposed and that the science ismore widely understood. This is more urgent nowbecause new dangers affect the survival of theenvironment as a whole, not just the lives ofindividuals.

Individual andcollective opinions

S h o u ld d ec is io n s o n m a j o r d a n g e rs b e madein d iv id u a lly or collectively? Many creaturesconcentrate on collective survival at the expense of theindividual - the herd or the swarm comes first. But manis different - he places special value o n the importance

of individual rights, as well as the collective agreementst h a t a r e es sential t o s ociety a n d i t s survival. Thisdynamic relationship between individuals and society iswhat being human i s about. Bu t wh a t happens i f acollective understanding takes a wrong turn, leading toa consensus that threatens survival? Then the problemneeds to be re-examined, which is most difficult if it islargely scientific.What people understand of the world depends on theirprevious experience, including education andupbringing. Even what they think that they see isshaped and filtered by their background. Through thecharacter of the Professor in his children's book, TheLion, the Witch and the Wardrobe, C S Lewis advisesthat we should listen to evidence from others, assesstheir reliability and s an ity, a n d t h e n a wa it furtherdevelopments. Recent scientific reports [2] relate how,even today, the experience of our own bodies can bedistorted alarmingly by suggestion and supposition, ina wa y dating b ack t o ancient witchcraft. In modernphysics , too , there a re s erious ques tions concerningreality in its different manifestations [3].So reality is tricky. It is not just an academic matter forphilosophers, but a practical matter that is the source ofeveryday disagreements. If differing views arereconciled, plans of action can then be agreed and

decisions taken that lead to success and increasedconfidence. So, decisions need an acceptable collectivepicture o f reality, a n d this only becomes establishedthrough repeated observations at different times and bydifferent people, and is confirmed when expectationsbased on it turn out to be correct. This is most crediblewhen scientific observations are found to confirmprecise mathematical predictions - although we cannotacco u n t f o r t h e unreas onable relevance o f suchmathematics in the world. There is no logic that requiresthat, when I wake tomorrow morning, the world as Iknow it will still be there. A chicken, accustomed tobeing fed by the farmer each morning, is unprepared forh is d a y o f s laughter, although that was the ultimatepurpose of each morning feed.1 So we becomeaccustomed to the continuity and predictability o f ourexperience. But could it be otherwise? We need alwaysto be alert to the possibility that our collectiveunderstanding is quite wrong. It is the task of thefollowing chapters to try to unpick the dangers ofradiation and nuclear technology and to explain how wewere previously mistaken.Philosophers and physicists may mull over evidence forthe existence o f parallel streams of reality. Some mayfollow the ideas of Descartes by looking at whichproperties of the Universe are necessary, s imply to

allow us to be here now asking questions. This is calledt h e Anthropic Principle and it turns out to havesignificant consequences, if you accept its premise. Butour task is different, though related in a practical andlocal sense. We are re-opening our attitude to radiationa n d nuclear technology in order to help answer thelarger question: what kind of world and choice of lifestyle will permit the possibility that mankind will be herein the future to ask questions? This local anthropicprospective is also restrictive. If no solution is found,human life on Earth as we know it will die out.

Confidence anddecisions

Consider an example. In earlier centuries exploration andthe transport of people and goods depended on theconfidence and safety of navigation. Observations andsightings had t o be agreed, a ship's course calculatedand steered - the arrival of the ship at its destinationwas the demonstration that these decisions were notjust matters of opinion. The calculation of the positiono f t h e ship relied on measurements and the knownapparent orbits of the Sun, Moon and stars, the

magnetic field of the Earth, the tides and otherquantities. With every improvement in navigation camean uplift in world communication; better accuracy gaveimproved confidence, leading to more ambitiousvoyages and better trading. Conversely, wheneverconfidence in the natural world fails, human activitygets choked off and prosperity declines.I f t h e r e i s dis agreement, t h e obs ervat ions andpreconceptions have t o b e talked through to reach aconsensus. Bu t i t i s an important concession to thevariety of human experience that individuals have theright o f choice. Except when i t is unavoidable, we donot exercise choice on behalf of others - and then onlywith a degree of caution that we would not exercise forourselves. And so it is in matters of safety, especiallywhere apprehension is high.The dangers of radiation and nuclear technology havebeen a matter of vocal public concern for half a century,mainly among the currently middle-aged and elderlywho remain confused and apprehensive. The youngergeneration never experienced the Cold War and aremore relaxed. In the past many scientists kept away fromthe long-running debate o f nuclear politics. Meanwhile,rad iat ion s a fe t y re ma in s s u b je c t t o exceptionallystringent conditions, although few people appreciatethe related expense and no one seems to feel safer as a

result. In the 21st century the agenda has changed anddecisions are needed for the future of the environmentwhere the choice of primary energy source is betweennuclear power with the dangers of its waste and thecombustion of fossil fuel with its waste.

Science and safetyThe astronomer who first predicted an eclipse andannounced it to the political masters of his daydiscovered the influence that scientific knowledge canbring. His ability was held in awe by all around him.Today physics and astronomy have given the humanrace control over much of the natural world. In earliertimes and in the absence of scientific interpretation,darkness, fog, thunder, lightning and other variations innature tended to generate superstition and thoughts ofdivine intervention, even punishment. Such feelingssuppress confidence and discourage initiative andenterprise.The scientific enlightenment from the 17th to mid 20thcenturies showed man how to overcome fear of theunknown by empirical study. Through universaleducation, training and communication this encouragedprosperity a n d better s tandards o f living and health.However, misapprehension of the natural world is still

the background of life for many.Scientists, too, suffer misapprehensions, but these areovercome by continual re-measurement, re-thinking andre-calculation, like the helmsman steering the wrongcourse who, by making further observations, discoversand corrects his error. If this is not done, confidencemay fail and unguided imagination and panic fill itsplace. Then careful observation and calm thought are atrisk, and the opportunity to correct errors is reduced.This is particularly true for those dangers that cannotbe sensed. The prospect of a threat, unseen but lethal,makes people worry, even panic. Trivially, in the dark,when sources of danger cannot be seen, people can befrightened until the light is turned on. This case isinstructive - to give people confidence they need to seefor themselves using a basic instrument, like a flashlightor torch. Just telling them that they should not befrightened is not effective. Equally, consulting peoplefor their opinion about safety, when they d o no t knowo r understand, may simply accelerate an implosion ofconfidence - decisions taken in everybody's bestinterest cannot emerge in this way. Regulation and legalrestraint do not give people confidence either. Only realeducation of a sizeable fraction of the community canreassure, and this should be based on an objectiveunderstanding of the issues.

For the confidence of those on board, the ship shouldbe on the right course, and be known to be on the rightcourse. The two aspects of safety - actual and apparent- are different, though equally important. Once actualsafety has been established, apparent safety becomes amatter for education, communication and information. Ifan appearance of safety is given priority over actualsafety, real danger can follow, as reassured passengerson board the Titanic learned to their cost.

1 - Remarked by Bertrand Russell.

Chapter 2

AtmosphericEnvironment

Size andcomposition of theatmosphere

The environment comprises the Earth's crust, theoceans and the atmosphere. The depth of the crust thataffects us on a regular basis is between a few hundredand perhaps a thousand metres, and the oceans have asimilar mass. But the atmosphere is much smaller -although i t reaches t o a n effective height o f about10,000 metres, its density is a thousand times less thanwater. So it is equivalent to a layer of water on the Earthjust 10 metres thick - less than 1% of the mass of theoceans or the Earth's crust. So it is easily polluted and,

being composed of gas, any pollution is quicklydispersed into the whole.The composition of the atmosphere today is 78%nitrogen, 20% oxygen and 1% argon with smalleramounts of carbon dioxide and water vapour. Oxygenand water are fiercely reactive but nitrogen, carbondioxide and argon are less reactive or totally unreactive.Until two and a half billion years ago there was littleatmospheric oxygen. Its concentration was increased byphotosynthesis in early plant life powered b y the Sun.This break-up of carbon dioxide into free oxygen andcarboniferous plant life 'charged the battery' fo r all lifeprocesses . Oxygen remains a powerful chemical, notonly when this battery is discharged in the burning ofplant matter, fossilised or not, but also in relatedoxidative processes in living cells. These may bebenign, as in the oxidation of sugars that provides theenergy for living creatures; they may also be malign, aswhen oxidation disrupts cellular processes and leads tocancer. Fortunately life has evolved ways in which toprotect itself against such oxygen damage that areeffective most of the time. Coincidentally, these sameprotection mechanisms turn out to b e equally effectiveagainst the damage caused by radiation, as we shall seelater.

Atmosphericchange

The average surface temperature of the Earth is criticallydependent on the composition of the atmosphere, and asmall release o f pollution c a n have a relatively largeeffect on the climate. The reason for this is explored inChapter 4 in terms of the spectrum of thermal radiationabsorbed and emitted by the Earth. Pollution releasedin t o t h e oceans wo u ld also have an environmentaleffect, bu t a much diluted o n e that would not impactdirectly on the temperature. In the case of the Earth'scrust dangerous materials - suitably buried - c a n stayp u t for many millions of years. So care of theenvironment is concerned first and foremost with theatmosphere.Since man s t art ed t o employ fire a n d organisedagriculture to raise his standard of living, he hasreleased an increasing mass of contaminants into theatmosphere, although only recently has the extent oftheir effect been appreciated.

Figure 1 - The concentration of carbon dioxide in theatmosphere for three separate epochs.

Left: prehistoric variation (measured from Antarctic icecores).

Centre: historic data (also from ice cores). Right: modern measurements (direct from the atmosphere).

For example, the growth in the concentration of carbondioxide in the atmosphere is shown in Figure 1. The leftpart of the diagram shows the concentration for most ofthe past 160,000 years, going up and down within therange 200-280 parts per million (ppm) a n d spanningvarious states of the world's ice sheets. The central partof the plot shows that it was fairly constant at 280 ppmfrom 1000 A D until t h e industrial revolution, with itsrapid increase in population and pollution. Since then ithas risen remorselessly as shown on the right - thelatest data say that it has risen b y 40 ppm in 25 yearsand currently stands at 360 ppm. Note the large changein timescale for the three parts of the plot.A plot for methane would show a similar rapid increase.These effects come from the increased burning of fossilfuels and the destruction of forests, exacerbated by therising world population of humans and animals. Thesegases are called greenhouse gases because they havethe effect of causing a rise in the average world

temperature, as explained in Chapter 4. The temperaturechange i s expected t o b e self-reinforcing f o r severalreasons whose relative importance is still uncertain.Firstly, the water vapour in the atmosphere naturallyincreases as the air gets warmer, and, since watervapour is also a greenhouse gas (as explained later inchapter 4), it is expected to contribute a further rise intemperature.Secondly, as the temperature rises the extent of thepolar ice caps is reduced, and, without the reflection ofthe snow and ice, the surface of the Earth becomesdarker to sunlight. The extra solar absorption in polarregions is responsible for another increase in thesurface temperature.Thirdly, as the temperature rises, plant material that waspreviously preserved and locked in the 'deep freeze' ofthe permafrost s tarts t o ro t a n d decompose, emittingfurther greenhouse gases, specifically methane.Any increased incidence o f fores t fires accompanyingthe temperature rise releases yet more gases. As livingplant life absorbs carbon dioxide and releases oxygen,any reduction in forestation is harmful on both counts.T h e re-abs orption of carbon dioxide from theatmosphere by sea water and through plant growth isslow. In fact, on average, it takes about a hundred yearsfor any carbon dioxide, once released, to be re-

absorbed. So, e v e n i f a l l emis s ions we r e stoppedimmediately, climate change wo u ld con t inue f o r acentury o r s o before possibly stabilising. I f emissionscontinue, t h e climate will continue to change. Thepopulation that the world can support may be reducedan d , a s des erts expand, la rg e migrations o f peopletowards more temperate regions may b e expected. Toreduce greenhouse g a s emis s ion , o t h e r w a y s ofproviding sufficient energy and food for the worldpopulation must be found, and all available solutionspursued simultaneously.Much energy can be saved with care and by investmentin new technology, for example efficient power suppliesa n d LEDs (light-emitting diodes). For the energyproduction itself, wind, tidal, solar, geothermal andhydroelectric sources provide electric power withoutgas emission. Each is appropriate to a particular kind oflocality. Some are intermittent, some are expensive andmany are limited in scale. Intermittent sources need tobe coupled with energy storage, but there are no easyoptions there. Energy for transport also needs storage,but battery technology and hydrogen storage havesignificant limitations.

Energy and

agricultureIncreased populations with rising standards of livingexpect more fresh water and food. The shift from abasic, mainly vegetarian, diet to a regular meat-eatinglifestyle requires more water. But the extra waterconsumption of ruminants and their added gas releasesare both significant. Meanwhile many parts of the worldsuffer increased desertification and depletion of groundwater supplies. Unlimited clean water can be obtainedfrom sea water by the process o f desalination bu t thisrequires significant amounts of energy.Much food goes t o waste though traditionally itsdeterioration may be reduced by refrigeration, but thisalso requires energy, both to power the refrigerationand to transport the refrigeration units. Alternativelyfood ma y b e preserved b y irradiation, a method thatrequires no ongoing energy supply but is little used.Food waste and an affluent diet increase the demand formore agricultural land, which leads in turn to furtherdeforestation.T h e s e obs ervations mo t iv ate a re-examination ofsociety's attitude towards radiation and the nuclearoption, as the major source of energy for almost allpurposes.

The word energy is used frequently in the followingchapters and it might be helpful to explain what itmeans. Energy is measured in joules, and 100 joules ofenergy is what it takes to power a 100 watt light bulb for1 second. Energy is conserved - that means it does notget lost - and it is inter-convertible between differentforms, to some extent. Forms of energy include heat,sunlight, chemical, nuclear, electrical, hydro and manyothers.In a waterfall the same quantity of energy may becarried by a large mass of water that drops a smallheight, or a smaller mass of water that drops through alarger height. But the difference can be important. Thereis a similar distinction between nuclear and fossil fuelenergy sources. The same total energy may come from asmall number of atoms each releasing a large energy, ora large number of atoms (or molecules) releasing a smallenergy. The former is what happens in a nuclear powerstation and the latter in a fossil fuel one. Usually in thefollowing chapters the word energy will refer t o theenergy per atom. I t should be understood that many,many atoms may deliver much energy, but the amountof fuel required and the waste generated for each jouleproduced increases if the energy per atom is small.This energy per atom is five million times smaller forfossil fuel than for nuclear, as explained in footnote 6 at

the end of chapter 3. So, for the same amount ofelectricity, the amount of fossil fuel required (with itswaste) is five million times the amount of nuclear fuel(with its waste). This is the crux of the story.

Chapter 3

The Atomic NucleusHis enormous head bristled with red hair;between his shoulders was an enormous hump...The feet were huge; the hands monstrous. Yetwith all that deformity was a certain fearsomeappearance of vigour, agility and courage... 'It'sQuasimodo, the bell ringer. Let all pregnantwomen beware!' cried the students. '...Oh thathideous ape! ... As wicked as he is ugly ...it's thedevil.' The women hid their faces.

Victor Hugo, writer (1802-1885)

Powerful andbeneficial

I n h is novel, The Hunchback of Notre Dame, VictorHugo introduces the central figure with these words.While the citizens of mediaeval Paris are repelled by hisugliness and afraid of his strength, no one cares tod is cover his true nature. As the story unfolds,

Quasimodo reveals a natural gentleness and kindnesstowards Esmeralda, the beautiful gypsy girl, who iscondemned to death on the gallows. The people's fearprevents them from appreciating him until he uses hisstrength in the attempt to save Esmeralda's life.Such is the public image of radiation. Like Quasimodo, itis seen as ugly, strong and dangerous. Like him itengenders an almost universal reaction of fear andrejection. Many do not want to be near anything to dowith radiation or even to understand such things. Thisis unfortunate, because the human race has survivedthrough the power of thought and understanding. Thesuspension of that power is not good news for thefuture.The following descriptive but scientifically robustaccount shows how radiation and the atomic nucleus fitinto the natural physical world.

Size scalesT h e s t a g e on which the science of radiation,radioactivity and fundamental life processes is setrequires a broad range of scales - very small distancesas well as larger ones, and very small energies and muchla rg e r o n e s t o o . De s p it e t h e i r differences thesed is t ances a n d en erg ies a r e inter-related through

fundamental science.

Figure 2 - The scales of the different structures relevant tothe interaction of radiation with life, from man through

cells, molecules and atoms to nuclei.

Figure 2 gives a n idea of these spatial scales, startingfrom a human on the scale of a metre, Figure 2a.Roughly speaking the biological structure of eachhuman is realised in the form of a large ensemble ofcells, each on a scale of about 10 -5 metres, Figure 2b,although some cells are very much smaller and somelarger. This means that some are just about visible with

the naked eye but for many a microscope is needed.Cells vary as much in function as in size. Each iscomposed of about 70% water and a large number ofbiological molecules.Figure 2c is a sketch of a section of a biologicalmolecule - typically these form long chains that fold upwithin cells. Such are the working proteins and thedouble-helical DNA that holds t h e genetic records.Each molecule is a particular sequence of chemicalatoms. Simple diatomic molecules, like the oxygen andnitrogen in the atmosphere, have just two atoms. Thepolyatomic ones, like carbon dioxide, methane andwater, have three or more, so that they can stretch, turnand wriggle about - which gives them their greenhousegas properties. Big biological molecules are composedof hundreds of thousands of atoms.Whereas there is a multitude of different molecules,there are only a small number of different types of atom.The information and variety of molecules lies in thearrangement of these atoms and t h e i r chemicalconnections . Biological molecules are composed ofhydrogen, carbon, nitrogen and oxygen atoms only,with special additional roles fo r calcium, phosphorus,sodium and potassium. Within less than a factor two allatoms have the same size, about 10-10 metres across. Inother words, each atom is as about 100,000 times smaller

than a typical cell, roughly the same factor by which acell is smaller than a man.Figure 2d shows an atom as made of a tiny nucleussurrounded by a cloud of electrons. The number ofelectrons in this cloud is known as the atomic number Zand this alone determines the atom's chemical behaviour- the nucleus with a balancing charge Ze makes the atomelectrically neutral overall but takes no part in the'social' behaviour between atoms. This is because thescale of the nucleus i s 100,000 times smaller than theatom itself, coincidentally the same ratio as an averagecell is to a man and as an atom to an average cell. Alltypes of nuclei are of a similar size, about 10-15 metresacross.What do we know about these atoms and nuclei, andhow were they discovered?

Atoms andelectrons

To the eye and to the touch most materials are smoothand continuous. A few are grainy but the grains varyand are not fundamental in any sense. Only theoccurrence of crystals with their highly regular facets

gives a clue of hidden structure. But that was no t whythe Greeks, Leucippus and Democritus, suggested thatmatter is composed of atoms. Their arguments were notreally based on observation at all, it seems. They weresimply unhappy in principle that matter s hou ld beindefinitely divisible. Based on such a vague argument,perhaps it is not surprising that the atomic idea fell intodisfavour in classical times, not least because Aristotlewas not impressed by it.Only a t t h e s tart o f t h e 19th century d id t h e atomictheory reappear, this time to account for observations.These were that the proportions o f p u re chemicalstaking part in reactions, burning for example, are relatedby simple whole numbers. Altogether there are over 90different types o f atom - the elements. These atomsthemselves do not change - changes to their mutualgroupings are all that is needed for a simplified accountof chemistry. As these patterns change, the storedenergy may increase or decrease, and when this energyis released, the material becomes hotter.Some chemical changes do not happen unless theatoms are hot in the first place. This can lead to arunaway process, in that, the hotter the materialbecomes, the more heat is released. This is the unstableprocess that we all know as fire, a chain reaction that isoften highly destructive. It was an early and crucial

s tage in human development when man learnt how tocontrol fire, to use it for warmth and to cook with it. Hecame to accept its risks in exchange for the better lifethat it brought. Even today much expense is incurred inprotecting against its dangers and many still die everyyear through fire accidents. I n s p it e o f t h i s nocivilisation has banned fire on safety grounds - it is toovaluable a technology to lose.But there are lessons that early man did not learn aboutfire. The waste products a r e s o lid a s h a n d gas,predominantly carbon dioxide and water vapour,released into the atmosphere. Once in the atmosphere, ifthe temperature is sufficiently low, the water condensesout in a few hours or days in the form of rain, but thecarbon dioxide persists. Only now has mankind startedto appreciate the danger o f releasing the was te o f thischain reaction. Unfortunately he did not understandthis when he first started making use of fire inprehistoric times.But it w a s discovered that there is more to thebehaviour of atoms than simply rearranging them tomake different molecules. Towards the end of the 19thcentury with advances in glass and vacuum technologyi t became possible t o make sealed glass tubes of lowpressure gas through which electric currents could bepassed between metal electrodes if one of these was

heated. These currents emit light and this technology isthe basis of the sodium and mercury lights commonlyused in street lighting, the neon tube used in signs, andenergy-saving fluorescent tubes. If fully evacuated,such a tube is called a cathode ray tube - familiar todayas an old TV tube, now largely replaced by flat paneldisplays. In the science laboratory two earlyfundamental physics discoveries were made with suchtubes.Firstly, the current, as it passes through a cathode raytube, is composed of a stream of charged particles ofvery small mass. Remarkably thes e electrical particlesare o f the same type, whatever the atomic compositionof the electrodes or gas. These particles, present in allatoms, are electrons, a new universal particle discoveredin 1897 by J J Thompson. In a TV tube the picture is'painted' by a beam of these electrons striking the insideof the front face of the tube and lighting up the differentcoloured phosphors there.Secondly, if these electrons are given enough energyand then strike a me t a l p la t e , invis ible unchargedradiation i s emitted. These X-rays were found t o beelectrically neutral a n d highly penetrating, unlike theparent electron beam. This discovery was made byRontgen i n 1895. Ve r y q u ickly t h e v a lu e o f thepenetrating power of this radiation was appreciated for

medical imaging and therapy. The relationship betweenelectrons, atoms and the electrically charged ions, asthey appear in the workings of electric cells andbatteries, was explained - ions are formed when anuncharged atom gains or loses one or more electrons.However, knocking such small parts off an atom - anelectron forms less than one thousandth of the weightof an atom - did not reveal much about the compositionor structure of the rest of the atom. There was more tobe discovered, deeper within.

The nuclear atomThe firs t evidence o f activity ins ide the atom, beyondthe addition o r lo s s o f electrons , c a me w i t h thediscovery of radioactivity in 1896 b y Henri Becquerel,whose work was followed later by the discoveries madeby Pierre and Marie Curie.3 They found that all chemicalsalts o f certain heavy elements emitted radiation andthat this energy was not dependent on the ionised stateof these elements, or on their physical and chemicalstate. Evidently the energy was coming from the deeperinner a t o m a n d n o t fro m t h e surrounding electrons.Through careful work the Curies showed that chemicalelements were being transformed, s o that new ones

appeared whenever an atomic nucleus emitted radiationconnected with its radioactivity. Three types of thisradiation were identified, alpha, beta and gamma - quiteoften in physics discoveries are given such enigmaticnames, because, initially at least, not enough is knownto name them in terms of their true characteristics. Laterit was shown that alpha, beta and gamma radiation arein fact streams of helium ions, electrons andelectromagnetic radiation4, respectively.Radioactive atoms are very unusual and heavy - theimplications for the structure o f ordinary elements thata re n o t radioactive were quite unclear initially. Someyears later Ernest Rutherford showed b y experimentthat, fo r every atom, a ll o f t h e mass (except for theelectrons) and all the balancing positive charge areconcentrated in a tiny volume at the centre of the atom -the nucleus. With its surrounding atomic electrons, thisis the atom as we understand it today. The arrangementhas been compared with the Sun and its solar system ofplanets. But this is deceptive - the proportions arewrong. The Sun is a thousand times smaller than thesolar system while the nucleus is a hundred thousandtimes smaller than the atom. Seen from the edge of theatom, the nucleus would be far too small to be seen withthe naked eye - if, for a moment, you can imagineyourself on such a scale. The rest of the atom is quite

empty apart from the thin cloud of electrons.Since the 1920s quantum mechanics, the radical shift inour understanding o f the physical world, has explainedin full precisely why molecules, atoms and nuclei havethe structure and behaviour that they do. Recently, ascomputers have become faster, it has been possible toext e n d s u c h explanations and predictions to theproperties of larger and larger chemical and biologicalmolecules.For reasons explained below, nuclear change is veryenergetic compared with chemical change and it powersthe Sun upon which life depends. Earlier in the historyof the Universe all the chemical elements were formedb y nuclear change from the primordial hydrogen andhelium that remained after the Big Bang. However, sincethe Earth was formed roughly six thousand million yearsago, only one nucleus in a million has undergone anychange at all. Within a small range all materials are99.975% nuclear by weight - electrons only account for0.025%. S o n u clear material i s v e r y co mmo n butsubstantial nuclear change is quite remarkably rare.In the early 1930s it was shown that every nucleus iscomposed of a certain number of protons and neutrons.The proton is the positively charged nucleus of simplehydrogen, and the neutron is its uncharged counterpart.The proton and neutron are almost identical in size and

weight, and their properties differ only on account ofelectrical charge. Elements are characterised by theirchemistry - that is by the number of surroundingelectrons. This is the same as the characteristic numberof protons t o ensure electrical neutrality. However, agiven element may exist in several forms called isotopes- the only difference between these is the number ofneutrons each contains. Apart from the variation inmass, different isotopes behave identically, except onthe rare occasions when nuclear change is involved.They are named by their element and then their atomicmass number A - this is just the total number o f protonsand neutrons that each contains. Examples are uranium-235, lead-208 and oxygen-16.Whereas the number of neutrons that an atom containshas little influence on its external behaviour, the internalstructure and stability o f t h e a t o mic n u cleu s aresignificantly affected, including whether it rotates . Infact each element has only a small number of isotopes,of which only a few are stable. Most unstable oneshave decayed away long ago and are no longer to befound in nature. If a nucleus rotates, it behaves like atiny magnet.5 In a large magnetic field these rotatingnuclei tend to line up like iron filings or compassneedles. Their alignment can be controlled andmeasured using radio-waves without invoking any

nuclear change. This is called nuclear magneticresonance (NMR) and is the basis of magneticresonance imaging (MRI). In clinical use the descriptionnuclear has been dropped from the name, in deferenceto the risk of worry that this label might cause! In factthe magnetic energy of a nucleus in MRI is about onemillionth of a typical chemical energy.On the other hand the typical energy o f a proton orneutron inside a nucleus is large - about a million timeslarger than the energy of an electron inside an atom,that is normal chemical energy. The reason for this is auniversal b as ic feature of quantum mechanics. Thesimple two-line calculation given in footnote 6 at theend of this chapter gives a factor of about five million.This ratio does not change much if calculated moreprecisely and sets the scale of the enhancement ofnuclear energy over chemical en erg y. S o roughlyspeaking, a nuclear power station gives a million timesas much energy per kilogram of fuel, and per kilogram ofwaste, as a fossil fuel power station delivering the sameelectrical energy.

The quiescentnucleus

Each nucleus remains really rather isolated at the centreof its atom. Other than flipping its spin under theinfluence of radiowaves, as in MRI, it can do nothing. Itmay be moved passively within its deep shield ofsurrounding electrons, but only as an inert mass. Whatprevents it taking a more active role? There are severalreasons for this remoteness.Like the electrons the behaviour of a nucleus within anatom is described by quantum mechanics - in bothcases there are only a certain number of states that canbe occupied. For the electrons these states are not farseparated in energy. As a result atoms changeelectronic state frequently, emitting or absorbing lightand generally taking part in chemical or electricalactivity. But nuclei c an n o t d o this because theseparation of their states is typically five million timesgreater.6 So nuclei are effectively 'frozen' into theirlowest state unless extreme energy is available.The second reason for the remoteness of the nucleus isthat the electrons ignore it, except through the electricalattraction. As a result the electrons on the one handand the protons and neutrons on the other keep theirown company and the nuclear core remains separated atthe atomic centre.Even if nuclei do not interact with electrons much, whydo they not interact with each other? Actually th is is

n o t possible because, being all highly positivelycharged, they are kept well apart by their strong mutualelectrical repulsion. This acts like a powerful springbetween them and it takes enormous energy to drivethem close enough together to touch, effectively.Nuclei are not excited when illuminated by beams ofradiation either, unless the energy of the radiation isquite exceptionally high or the radiation is composed ofneutrons. If the radiation is a beam of protons or alphaparticles, these are repelled before they ever get close toany nucleus . A beam o f electrons or electromagneticradiation is equally ineffective because these do notreact with nuclei except electrically, as just discussed.The only way in which the outside world can effect anychange in a nucleus is through collision with a neutron.Having no electrical charge, neutrons are not repelledand can enter a nucleus with ease. But free neutrons areunstable, decaying with a half-life 7 of 15 minutes, andso their presence in the natural environment isexceptional.If the influence of the environment on a nucleus is veryrare, how about the other way around? When do nucleiaffect the environment? On its own, all that a nucleuscan do is decay, if it is unstable, thereby releasing acertain energy in t o the environment. M o s t naturallyoccurring nuclei are stable and cannot do this. For the

handful of naturally occurring nuclei that do decay, theprocess is very slow and rare, and this is why unstablenuclei were not discovered until 1896. Most varieties ofnuclei that could decay, already did so within a fewmillion years of being formed, more than six thousandmillion years ago.When a nucleus decays, the total energy of the emittedradiation and the residual nucleus (including its mass)must equal the total energy of the initial nucleus(including its mass). This is because in a decay noenergy is lost - and no electric charge is lost either.The same is true for the atomic mass number A, the sumof the number of neutrons plus protons, that is N+Z.The total A present before the decay equals thecombined number afterwards.8 Table 1 explains how thealpha, beta and gamma decays first studied by theCuries match with these rules.Table 1 The usual types of natural radioactivity, alpha,

beta and gamma, where N and Z are the numbers ofneutrons and protons in the initial nucleus.

TypeResidual nucleus

Neutrons Protons Charge

Alpha N-2 Z-2 Z-2 helium nucleusBeta N-1 Z+1 Z+1 electronGamma N Z Z electromagnetic

In alpha decay both N and Z of the residual nucleusdecrease by two and an alpha particle, a helium ioncomposed of the four nucleons, is emitted. In betadecay a neutron becomes a proton with emission of anelectron to balance electric charge. There is a secondtype of beta decay in which a proton is changed into aneutron and a positive anti-electron (or positron) - suchdecays are of great importance in nuclear medicine.Actually in all types of beta decay another particle isemitted too. It is called the neutrino. But we are notinterested in neutrino radiation in this context because iteffectively disappears, without depositing any energyor doing any damage.9

In fission decay the nucleus splits into two fairly equalhalves with the emission o f a few extra neutrons. Suchdecays are exceedingly ra re ' i n t h e wild ', e v e n forradioactive isotopes, which a r e thems elves rare.However, i n t h e artificial circumstance in which anucleus has just absorbed a neutron, fission can occurefficiently and quickly. This induced fission process

requires a flux of such neutrons - for instance inside afission reactor. Each fission releases further neutronsthat may then be absorbed by other nuclei, thusbuilding up a neutroninduced chain reaction. This is likea chemical fire which is stimulated b y i t s o wn heatproduction. A difference i n the nuclear case is thatremarkably few materials are 'combustible', as it were,and the 'fire' is very difficult to ignite.

Energy for the SunThe Sun provides the energy that drives all aspects oflife and the environment. Without the energy of theSun, the Earth's surface would cool towards minus270C, the temperature of inter-stellar space. Only thedull heat of the radioactive energy released within theEarth would raise the temperature at all.Viewed o v e r geological periods , fos s il fu e ls a c t aschemical batteries that absorb the Sun's energy in onegeological period and then give it back in another. Theproblem for mankind is that these batteries , chargedover millions o f years , a r e being discharged on thetimescale of a century.It is no surprise that the Sun was worshipped as a godin ancient times as the source of heat and light - a rathersensible choice of deity. An important ques tion is,

where does the Sun gets its energy? If this came from achemical fire, it would have run out of fuel after about5,000 years, but it has been shining fo r a million timeslonger than that already. The Sun is made of hydrogenand a small quantity of the element helium.10 However,there is no air or oxygen with which to support chemicalcombustion of the hydrogen.The source of the Sun's energy is nuclear - it is a largereactor in which hydrogen 'burns' by fusion to formhelium. The increase in energy relative to a chemical firewill enable the Sun to shine for many more thousandmillion years yet. This fusion reaction can only happenin the centre o f the Sun where the temperature reachesseveral million degrees. Just as a chemical fire has to bestarted by a source of heat to get the reaction going, soto ignite a nuclear fusion fire, the hydrogen atoms mustbe given enough energy that, when they collide head-on, the nuclei can fuse together. A t lower temperaturesthey simply bounce off one another without touchingbecause of their mutual electrical repulsion. The visiblesurface of the Sun is at 5,800C but the temperature riseswith depth, and towards the centre it gets hot enoughfo r th is fus ion t o occur, a proces s t h a t i s n o w wellunderstood. The energy released near the centre thenfinds its way slowly outwards towards the solar surface.The Sun burns 600 million tons of hydrogen every

second and yields 596 million tons of helium in its place.This curious loss of mass is balanced b y the energythat streams out in all directions, such as towards theEarth. The rate at which the Sun loses energy E isrelated to the rate at which it loses mass m, the fourmillion tons per second, by the equation E = mc2 wherec is the velocity of light. It is sometimes suggested thatnuclear physics has a special connection with Einstein'sTheory of Relativity, but this is not true - energy of allkinds is connected to mass in this way. One kilogram ofmass i s equivalent to 9x1016 Joules, or about 2x1010kilowatt-hours. This is so large that i t i s on ly i n thenuclear case that the mass change is noticeable. In thecase of hydrogen fusing to helium it i s just under 1%.This vast solar energy flux spreads as it radiates away.By the time it reaches the radius of the Earth's orbit it isa pleasantly warm 1.3 kilowatts per square metre.Pleasant, maybe, but such a nuclear energy sourcedeserves to be respected by modern man, as it was bythe ancients. It is unwise to lie out in its radiation forlong periods. However, the majority of people take asensible attitude, enjoying a modest exposure withoutexpecting that the risk of sunburn or skin cancer can becompletely eliminated. By applying ultraviolet blockingcream and by limiting the duration of exposure, thewarmth of sunshine at longer wavelengths may be

enjoyed. No one seeks absolute safety from the Sun'srays - otherwise summer vacations taken in totaldarkness deep under ground would be keenly soughtafter! As with fire, mankind has learnt to live with theSun, enjoying its benefits and avoiding its hazards. Inboth cases straightforward education and simple rulesplay their part. A similar measured attitude to otherkinds of radiation would be beneficial.

3 - The discoveries of 1895, 1896 and 1897 were sounexpected and came in such short succession that lesscareful experimenters felt encouraged to come forwardwith claims based on fantasy. I n particular the magicalpowers attributed to so-called N rays were only shownto be false after much publicity.

4 - Described in Chapter 4.

5 - Every rotating charge behaves as a magnet - this is auniversal relationship. Conversely, all magnets are dueto rotating or circulating charge.

6 - In quantum mechanics a proton or electron of massm contained in a region of size L must have a momentumP of about h/L, where h is Planck's constant. In simplemechanics, for a mass m with velocity v, there is arelation between the momentum P = mv and the kinetic

energy E = 1/2mv2. So that E = P2/2m = h2/(2mL2).Using this formula we can compare the energy of anelectron in an atom with that of a proton or neutron in anucleus. The ratio of the size of the region L is 100,000;the ratio o f mass m is 1/2000. The formula tells u s thattypical nuclear energies are larger than atomic (that ischemical) energies by the ratio of mL2, that is about 5million. The rest, as they say, is history!

7 - In a group o f neutrons the decay o f any particularone occurs quite randomly in time - except that each candecay only once. So the number that remain fallsnaturally with time, and consequently so does the rateat which these decay. If the time for half of the nuclei todecay is T (called the halflife), then the number left isreduced by a further factor of a half with everysuccessive time interval T. This is called an exponentialdecay and describes any unstable atom and nucleus.

8 - The rules for charge and energy conservation aredeeply embedded in the principles of physics, althoughthe rule for A is empirical.

9 - Neutrinos interact so seldom that they can pass rightthrough the Sun or the Earth, although after 50 years ofexperiments they are now well understood.

10 - The name, helium, comes from the Greek name forthe Sun. It is scarce on Earth but abundant in the Sunwhere its presence was first discovered. It is a very lightgas that escapes upwards in the atmosphere on Earth -indeed it is used to fill fairground and party balloons forthat reason. Fortunately, plentiful supplies of helium forthese and other uses come from the emission of alpharadiation in the decay of naturally radioactive atoms inthe rocks of the Earth's crust.

Chapter 4

Ionising Radiation

The spectrum ofradiation

So what exactly is radiation? The simplest answer is thatit is energy on the move - and there are many kinds.Sunshine, music and waves on the surface of water areexamples. At low levels many are quite harmless andeven beneficial to life. Extreme levels can cause damagein almost every case - very loud music can damagehearing, and too much sun causes sunburn. However, alittle sunshine is positively good for the skin bypromoting the production of important vitamins.Similarly music that is not too loud may be positive anduplifting.There i s a n important point here. I t i s n o t that gentlemusic causes only a little damage, but that it causes nodamage to hearing whatever. When compared with thedamage due to excessively loud sounds, t h e effect is

n o t proportionate. Technically such a relationship istermed non-linear and this will be an important idea insubsequent chapters. In the case of music and damageto hearing the non-linearity may be obvious, but forother forms of radiation the distinction betweenproportionate and non-proportionate response will needto be looked at using both experimental data a n d anunderstanding of what is happening.Most of the radiation from the Sun comes in the form ofelectromagnetic waves - this includes light and otherparts of a wide spectrum. Each such wave involvesentwined electric and magnetic fields. It has a frequencyand an intensity just as a sound wave has a pitch and avolume. Ou r understanding of electromagnetic wavesdates from the work o f James ClerkMaxwell in the 19thcentury, who built on the work of Michael Faraday andothers. As for any wave, the speed at which it moves isequal to the frequency times the wavelength. Since thespeed is essentially constant, the wave may be labelledby its wavelength instead of its frequency, but eitherwill do. On a radio receiver, for example, some stationsare labelled by their frequency in MHz (mega-hertz,millions of waves per second), while for others thewavelength in metres is used. The product of the two isthe speed of radio-waves, 300 million metres per second,the same as that of light.

Figure 3 - The frequency spectrum of electromagneticwaves.

How a wave is received is determined largely by thefrequency not the intensity. For example, a radioreceiver selects a station by choosing its frequencyrather than its loudness. In the same way that for soundthere are frequencies that cannot be heard by the ear, sofor light there are frequencies that are invisible to theeye. In fact only a tiny range of frequencies ofelectromagnetic waves is visible. The whole spectrum is

represented in Figure 3 with a logarithmic frequencyscale running u p the page and covering more than 15powers of 10, as shown in the second column inoscillations per second (Hz). The first column gives thecorres ponding wavelength . Vis ib le l i g h t w i t h itscharacteristic spectrum of rainbow colours is the narrowcrosshatched band half way up the diagram. The pointis that there really is no fundamental difference betweenthese waves, from radio through light to X-rays, exceptthe frequency. At the highest frequencies (and shortestwavelengths) the powers of 10 become harder t o copewith and a third scale based on the electron volt (eV) isoften used.11 This is shown on the right of Figure 3with the usual prefixes for powers of 10.12

Much benefit has been brought to everyday lifethrough enabling mankind effectively to see using theseother frequencies [4]. Lower in the diagram are radio-waves u p t o 109 Hz, used for example in MRI to seeinside the human body and in radar to see ships andplanes in fog and darkness. Slightly higher is thermalimaging, used to see warm bodies accidentally buried orconcealed. Just below the visible frequencies is a regioncalled the infrared absorption band, shown as shadedin the diagram. At these frequencies many materials areopaque because the rotation and vibration of molecules

are in tune and resonate with electromagnetic waves.Above the visible there is another band, the ultravioletabsorption band. He re i t i s t h e mo re nimble atomicelectrons that are in tune and the cause of theabsorption. So here too materials are opaque, as markedby the shading.Heavier elements with their more tightly boundelectrons have an ultraviolet absorption b a n d thatextends to much higher frequencies than light elements.This is the frequency range of the X-rays. Here, metalsl ike co p p er a n d calcium absorb radiation whereascarbon, hydrogen and oxygen are transparent. Medicalima g e s o f a pat ien t 's t e e t h o r b o n e s (calcium)illuminated w i t h s u c h radiat ion s h o w c lea rly anyfracture or disease becaus e t h e enveloping tissue(carbon, hydrogen and oxygen) is transparent.Above about 100 keV atomic electrons, even those thatare most tightly bound in the heavier elements, cannotmove fast enough to follow t h e oscillating wave.13Consequently there is no resonance and all materials arelargely transparent. This region is called the gamma rayregion. Historically the distinction between X-rays andgamma rays depended on the source - electrons andnuclei, respectively. This distinction is deceptivebecause their effect does not depend on the source,only on their energy (or frequency). Today this switch

of name is usually made at about 100 keV, but thedistinction is really only a convention. Gamma rays arevery penetrating, being only weakly absorbed, which iswhy they are used in radiotherapy to target energy intoa cancer tumour, deep within a patient's body. Thisenergy may then be absorbed in the tumour withsufficient intensity that its cells are killed and it ceasesto function. There are practical difficulties in doing this,as discussed later in Chapter 7.

Damage fromradiation

So understanding light, and then learning to see withradiation in other parts of the spectrum, is really useful.But what of the risks? Th e spectrum c a n b e dividedroughly in t o t wo halves separated a t ab o u t 10 eV.Radiation o f greater frequency or energy i s calledionising radiation, that below, non-ionising radiation.The distinction is that ionising radiation can ionise andbreak molecules apart - this is the radiation with whichthis book is primarily concerned.Public concern about weak levels of non-ionisingradiation, for instance from overhead power lines or

mobile phones, is misplaced. The only known way inwhich such radiation can cause damage is by heating.14Put briefly, these radiation sources are safe if heat is notsensed - even then, benefits may dominate over anyreasonable risk. Warmth from sunshine or a domesticfire is brought by the same kind o f radiation as that in amicrowave oven. While the radiation levels in such anoven can certainly be dangerous, the heat radiated by aglowing fire on a cold winter's day is a quite acceptablesource of radiation hazard for most people - in spite ofthe fact that its heat level can be sensed, indeedbecause of it.B u t non-ion is ing rad ia t io n s t i l l h a s a crucialenvironmental impact. On the right hand side of Figure 3are two boxes labelled sunshine and earthshine. Veryhot materials like the Sun emit light in the visible region,but cooler materials also emit, though predominantly inthe infrared frequency range. The sunshine boxindicates the range of frequencies that the Sun emits.Because this is centred on the visible region for whichthe atmosphere is largely transparent, much of thisradiation reaches the surface of the Earth for the benefitof all, including plant life. (Actually the spectrum of theSun extends a bit into the infrared and ultraviolet, too -the infrared part provides warmth, the ultraviolet causessunburn, if not filtered by barrier cream and the small

concentrat ion o f o z o n e p re s e n t i n t h e upperatmosphere.) The earthshine box indicates thefrequency band of radiation that the surface of the Earthemits with it s lower temperature - but not all of thisradiation succeeds in getting out of the atmospherebecause of infrared absorption by polyatomic gases15,in particular carbon dioxide, water vapour and methane.With an atmosphere containing more of these the Earthis not able to cool itself nearly as effectively as it is ableto absorb the sunshine. S o energy i s trapped i n theatmosphere and the temperature increases. Crudely, thisi s h o w t h e Greenhouse Effect works. If theconcentration of these gases rises, the Earth gets hotterand the climate changes. An extraordinary example isclose at hand - Venus has a surface temperature of460C, thanks in part to an atmosphere with 97% carbondioxide.Like electromagnetic waves, beams of charged particlessuch as alpha and beta radiation can also damagemolecules, so that they are classified as ionisingradiation - and beams of neutrons and other ions too,although these are less common in t h e naturalenvironment.

Nuclear stability

But what makes a nucleus decay? Or rather, what holdsit together in the first place? The mutual electricalrepulsion of the protons ma ke s la rg e n u c le i moreunstable than small ones. Stability only comes from thenuclear force that attracts neighbouring protons andneutrons together. This nuclear force overwhelms theelectrical repulsion, but only at short distances withinabout 10-15 metres. As a result it favours small nucleifor which the protons and neutrons can huddle closetogether. The result is a balance between thepreferences for nuclei to be not too large and no t toosmall, which gives ris e t o t h e nuclear stability curve,Figure 4. The most stable atoms are those with nuclei att h e highest point o n t h e curve, t h e tightes t averagebinding. These are in the region of iron, A = 56.While quantum mechanics prefers nuclei with roughlyequal numbers of protons and neutrons, the disruptiveelectrical force makes nuclei with too many protonsunstable. The result is that all stable nuclei, except thelargest, have roughly equal numbers of protons andneutrons, so that iron (Z = 26) has 30 neutrons. Asshown in Figure 4, for smaller values of A the bindingeffect of the nuclear force is reduced; at larger values ofA the disruptive influence of the electrical effect isincreased - either way the binding is less. Above ironthe compromise favours nuclei with more neutrons than

protons because the disruption only acts on theprotons. So for example, the most abundant isotope oflead, lead-208, has 82 protons but 126 neutrons. Thereare no naturally occurring elements above uranium (Z =92) - thos e above actinium (Z = 89) are collectivelyreferred to as the actinides.

Figure 4 - The average binding energy per proton orneutron as it depends on the atomic mass number, A.

The curve shows that in principle nuclei with small Acould fuse together to release energy due to the nuclearforce, as shown by the arrow on the left. This is nuclearfusion and the source of stellar energy, including that ofthe Sun. In addition, nuclei with large A can in principlerelease energy by splitting apart and moving towardsgreater stability as shown by the arrow on the right.This is nuclear fission.16 Because, like lead, the parentnucleus has more extra neutrons than its stable fissionproducts, there are excess free neutrons emitted in thefission process. The liberation of these extra neutrons iscrucial to the nuclear chain reaction mechanism.In practice fission is very rare. Alpha decay in which aheavy nucleus splits into helium and a smaller nucleusi s more common. This is the source of much of thenatural radioactive energy i n t h e Earth 's crus t - theenergy source o f natural geothermal power, in fact. Inalpha decay nuclear energy is released by moving to theleft along the curve in steps of four units in A. As Areduces, the excess proportion of neutrons has also tobe reduced, and this occurs by beta decay in which aneutron in the nucleus decays emitting an electron andleaving behind an extra proton within the nucleus.

Table 2 - The four distinct primordial radioactive serieswith their head members and half-lives (T1/2), and alsoend members. T is given in G-year, a thousand million

years.

4nseries

4n+1series

4n+2series

4nseries

Head thorium-232neptunium-237

uranium-238

uranium-235

T1/214.1 G-year

0.002 G-year

4.5 G-year

0.70 G-year

End lead-208bismuth-209

lead-206

lead-207

The natural radioactivity of heavy nuclei consists of asequence of alpha and beta decays in which energy isreleased a s the nucleus moves to lower A along thestability curve (Figure 4). There are four distinct seriesof nuclei, depending on whether A is o f t h e fo rm 4n,4n+1, 4n+2, or 4n+3, where n is a whole number. Within

each series nuclei may decay, one into another, byalpha or beta decay. Each series has a long-livedprimordial head member and an end member which iseffectively stable - these are given in Table 2. The 4n+1neptunium series has already died out, but the otherthree are still active in the natural environment. Thesuccessive members of the 4n+ 2 series, with theirdecays and half-lives, are shown in Table 3, as anexample.

Table 3 - Members of the uranium-238 series (the A = 4n+2series). Some half-lives are measured in thousands of years

(k-year).

Element-A Z N Decay Half-lifeuranium-238 92 146 alpha 4.5 G-yearthorium-234 90 144 beta 24.1 dayproactinium-234

91 143 beta 1.17 minute

uranium-234 92 142 alpha 240 k-yearthorium-230 90 142 alpha 77 k-year

radium-226 88 138 alpha 1.6 k-yearradon-222 86 136 alpha 3.82 daypolonium-218 84 134 alpha 3.05 minute

lead-214 82 132 beta 26.8 minutebismuth-214 83 131 beta 19.8 minutepolonium-214 84 130 alpha

164microsecond

lead-210 82 128 beta 22.3 yearbismuth-210 83 127 beta 5.01 daypolonium-210 84 126 alpha 138.4 day

lead-206 82 124 metastable

Measuring

radiationTo speak usefully of the effect on human life of differentdoses of ionising radiation, these must be measured,somehow. But how exactly?The first step in quantifying a radiation exposure is tomeasure how much energy is absorbed per kilogram ofliving tissue during the exposure. This energy maycause chemical damage by breaking molecules apartthat leads to biological (cellular) damage and finally toclinical damage, such as cancer or other disease. Suchclinical damage turns out to be more difficult to relate tothe exposure, especially as it may manifest itself indifferent ways, and on long or short timescales, fromdays to years.In earlier decades knowledge of cell biology was tooprimitive to provide confident understanding, andadequate evidence of the effect of radiation on humanswas not available to corroborate any particular view. Intheir absence, and for lack of anything better, theknowledge gap was bridged by a rule of thumb - amodel in science-speak. This is the Linear No-Threshold model, abbreviated LNT. This assumes thatclinical damage is in simple proportion to the initialradiation energy dose. No justification was given for it,

but it was a reasonable working hypothesis at the time.Despite the poor state of knowledge, a start had to bemade somewhere.However, g iv e n mo d e rn biological knowledge andextensive records of human data, this model is nowredundant and many of its more cautious implicationscan be ignored. The details are for discussion in laterchapters. First, we return to the questions of thequantification o f radioactivity a n d abs orp t ion ofradiation energy in materials.The rate a t which energy i s emitted b y a radioactivesource depends on the number of radioactive nuclei N,the energy of the decay, a n d t h e half-life T o f thenucleus. The value o f N is reduced by half with everysuccessive time interval T and the average activity isproportional to N/T. Activity is measured in decays pers e c o n d , c a l l e d b e c q u e re l a n d ab b rev ia t ed Bq.Sometimes the activity may be measured in a cubicmetre of material, thus Bq m-3.So what does this mean in practice? Contamination byradioactive nuclei with a short half-life results in highactivity for a short time; the same contamination with alo n g er half-life results in a lower activity, but itcontinues for longer. Half-life values vary between asmall fraction of a second and many times the age of theEarth. So sources of radioactive contamination with

short half-lives fade away while others with longerhalflives continue on. This is in contrast to mostchemical pollutants, such as heavy metals like mercuryor arsenic, that remain hazardous indefinitely. A slightlydifferent situation arises when a dose of ionisingradiation energy comes from an external beam producedby an accelerator (such as an X-ray machine) or from anexternal radioactive source.Either way the important question is, how far does theradiation travel in material before being absorbed? Someradiation is so strongly absorbed i n air, o r a n y thinmaterial, that it never reaches human tissue unless thesource is on the skin or inside the body. Other radiationis weakly absorbed and can pass through the body. Sowhat is important is not the intensity of the radiation,b u t t h e amount that i s absorbed,17 fo r ins tance, perkilogram of tissue. The extent to which it is absorbeddepends on the kind of radiation and its energy (orfrequency).Alpha radiation is stopped even by air, and so thedecay energy is deposited very close to the s ite o f theradioactive contamination itself, with no dose at all onlya little further away. An example i s t h e energetic, butshort range, alpha radiation emitted by the decay of theradioactive isotope polonium-210. A large internal doseof this was used allegedly by Russian agents to kill

Alexander Litvinenko i n London i n 2006. N o energyescaped the immediate location of the poison but therethe tissue received the full radiation energy dose.Beta decay produces electrons that travel further inmaterial and, therefore, the deposited energy dose ismore diffusely distributed around the radioactivesource. Gamma rays go further still. So for a radioactivesource in rock, for example, any alpha and most betaradiation is absorbed internally within the rock, andonly the gamma radiation escapes to give an externalenergy deposition. In general a deposited energy doseis quantified as the number of joules of energyabsorbed per kilogram o f material, s u c h as patienttis sue. On e joule p e r kilogram i s called a Gray (Gy).Typically d o s e s a r e meas ured i n milligray, w i t h amilligray (mGy) being one thousandth part of a Gray.The clinical damage caused to living tissue by thisdeposited radiation develops as a result of a number ofsteps.

1. The immediate molecular mayhem left by theradiation.

2. Biological damage in which living cells are put out ofaction - this changes with time as the tissueresponds to the radiation dose.

3. The incidence of cancer (and other possible delayed

or heritable effects) related to the exposure, perhapsdecades later.

4. The reduction in life expectancy as a result of suchcancers (this effect on life expectancy is called theradiation detriment of the exposure).

5. The chance of death shortly after exposure due toacute radiation sickness brought on by cell deathand the shutdown of the normal biological cycle inone or more vital organs.

The two lasting consequences for life are described bythe sequences 1-2-3-4 and 1-2-5, and later we willdiscuss how each of these outcomes relates to theinitial radiation energy dose.There are other causes of cancer, unrelated to radiation.Some causes - we shall refer to them generally asstresses - are natural, others are imposed by choice oflifestyle. Following decades of study much is knownabout how these stresses are related to the occurrenceof cancer - to the detriment in fact. An importantquestion is how the outcome is influenced when there ismore than one stress. These stresses may be quiteindependent, as in smoking and radiation, but the resultmay not be. There remain some unanswered questions.But the point is that the range of residual uncertainty istoo small to prevent mankind from taking decisions nowabout how radiation can be used with effective safety.

For a single acute dose the damage is related to the sizeof the dose and the type of radiation. The effects of X-rays, gamma rays and electrons are found to be roughlythe same for the same energy dose in milligray.However, for other types of ionising radiation thebiological damage is different. Quantitatively, themeasured ratio of damage relative to X-rays is called therelative biological effectiveness (RBE). So the RBE of aradiation dose indicates how much more clinical damageit causes than is caused by the same number of milligrayof energetic gamma rays. Essentially these RBE factorsare measured quantities.RBE factors vary with the clinical end point - that is withthe cancer or dis ease concerned. Timing effects areimportant and we look at these later. The variation withradiation type is particularly interesting although nottoo large. For most practical applications of radiationsafety, which we are thinking about in this discussion,we need to watch the factors of ten, a hundred and athousand. RBE factors close to one are less important.Only in radiotherapy are t h e effects o f radiation veryfinely balanced - but in that case gamma rays areusually used and so RBE i s 1.0 anyway. S o fo r thissimplified discussion i t is sensible to ignore the RBEfactor in the first instance.Nev erth e les s t h e In ternat ional Co mmis s io n for

Radiological Protection (ICRP) has felt it necessary toinclude RBE in some way. In their radiation safetystandards they multiply each energy dose in Gray by aweighting factor, wR, which plays the role of a broad-brush averaged RBE. [They define wR for protons to betwo; for alpha, fission fragments and other heavy ionsto be 20; for neutrons it depends on the energy; forelectrons and photons it is just one, by definition.] Theresult they define to be the equivalent dose, measuredin units of Sievert (Sv) - or millisievert (mSv). In ignoringRBE initially we treat doses measured in milligray andmillisievert as equivalent, and come back later t o thedistinction when a variation in the type of radiation hassomething special t o s ay about how radiation damageoccurs.These measures of energy