核工程專業(yè)英語(yǔ)3-Radioactivity省公開課一等獎(jiǎng)全國(guó)示范課微課金獎(jiǎng)?wù)n件

NUCLEARENERGYPROFESSIONAL(SPECIAL)ENGLISHCOURSECHAPTER3Radioactivity1第1頁(yè)CHAPTER3

Radioactivity（放射性）Manynaturallyoccurringandman-madeisotopeshavethepropertyofradioactivity(放射性),whichisthespontaneous(自發(fā))

disintegration(decay)

(衰變)

ofthenucleuswiththeemissionofaparticle.Theprocesstakesplaceinminerals(礦物質(zhì))

oftheground,infibers(纖維)

ofplants,intissues(組織)

ofanimals,andintheairandwater,allofwhichcontaintraces(痕跡,微量)ofradioactiveelements.2第2頁(yè)3.1RADIOACTIVEDECAY

（放射性衰變）Manyelementsthatarefoundinnatureorman-madeareradioactive(放射性),emittingalphapartic1es(

),betaparticles

(

)

(includingelectronornegatronandpositron),neutrinos(中微子),andgammarays(

).Someexamplesareasfollowing:3第3頁(yè)3.2THEDECAYLAW(衰變規(guī)律)Therateatwhicharadioactivesubstancedisintegrates(andthustherateofreleaseofparticles)dependsontheisotopicspecies(種類，核素),butthereisadefinite(一定)“decaylaw”thatgoverns(管理,支配)

theprocess.Inagiventimeperiod,sayonesecond,eachnucleusofagivenisotopicspecieshasthesamechanceofdecay.Ifwewereabletowatchonenucleus,itmightdecayinthenextinstant(馬上),orafewdayslater,orevenhundredsofyearslater.

4第4頁(yè)Suchstatistical(統(tǒng)計(jì)學(xué))

behaviorisdescribedbyaconstantpropertyoftheatomcalledhalf-life(半衰期).Thistimeinterval(間隔),symbolizedbytHisthetimerequiredforhalfofthenucleitodecay,leavinghalfofthemintact(完整無(wú)缺).

Weshouldliketoknowhowmanynucleiofaradioactivespeciesremainatanytime.IfwestartattimezerowithN0

nuclei,afteralengthoftimetHtherewillbeN0/2;bythetime2tHhaselapsed(流逝),therewillbeN0/4;etc.AgraphofthenumberofnucleiasafunctionoftimeisshowninFig.3.l.5第5頁(yè)6第6頁(yè)Foranytimetonthecurve(曲線圖),theratioofthenumberofnucleipresenttotheinitial(初始)

numberisgivenby

Half-livesrangefromverysmallfractions(小部分,片斷)ofasecondtobillionsof

(數(shù)十億)years,witheachradioactiveisotope

(放射性同位素)

havingadefinitehalf-life.7第7頁(yè)TheformulaforN/N0isnotveryconvenientforcalculationsexceptwhentissomeintegermultipleoftH.Definingthedecayconstant

(lambda),

asthechanceofdecayofagivennucleuseachsecond,anequivalent(同意義)

exponential(指數(shù))

formulafordecayis

Wefindthat

=0.693/tH8第8頁(yè)Thenumberofdisintegrationspersecond(dis/sec)ofaradioisotopeiscalledtheactivity,A.Sincethedecayconstant

isthechanceofdecayeachsecondofonenucleus,forNnucleitheactivityistheproduct(乘積)

A=

NTheunitdis/seciscalledthe

becquerel

(Bq),honoringthescientistwhodiscoveredradioactivity(放射性活度).Anotherolderandcommonlyusedunitofactivityisthe

curie(Ci)

namedaftertheFrenchscientistsPierreandMarieCuriewhostudiedradium(鐳).Thecurieis3.7

1010dis/sec,

whichisanearlymeasuredvalueoftheactivitypergramofradium.9第9頁(yè)3.3MEASUREMENTOFHALF-LIFE

(半衰期測(cè)量)Findingthehalf-lifeofanisotopeprovidespartofitsidentification(判定),neededforbeneficialuseorforprotectionagainstradiationhazard.Letuslookatamethodformeasuringthehalf-lifeofaradioactivesubstance.AsinFig.3.2,adetector

探測(cè)器

(thatcountsthenumberofparticlesstrikingit)isplacednearthesourceofradiation(放射源).10第10頁(yè)11第11頁(yè)Fromthenumberofcountsobservedinaknownshorttimeinterval,thecountingrate

(計(jì)數(shù)率)iscomputed.Itisproportionalto

(成百分比)theratesofemissionofparticlesorraysfromthesampleandthustotheactivityAofthesource.Theprocessisrepeatedafteranelapsedtimefordecay.Theresultingvaluesofactivityareplottedonsemi-loggraphpaper

(半對(duì)數(shù)坐標(biāo)紙)

asinFig.3.3,andastraightlinedrawnthroughtheobservedpoints.Fromanypairsofpointsontheline

andtH=0.693/

canbecalculated(seeExercise3-8).12第12頁(yè)13第13頁(yè)Themeasurementoftheactivityofaradioactivesubstanceiscomplicatedbythepresence(存在)

ofbackgroundradiation(本底輻射),whichisduetocosmicrays

(宇宙射線)fromoutsidetheearthorfromthedecayofmineralsinmaterialsofconstruction(建筑物)

orintheearth.Itisalwaysnecessarytomeasurethebackgroundcounts

(本底計(jì)數(shù))andsubtract(減去)

themfromthoseobservedintheexperiment.

14第14頁(yè)3.4SUMMARY(總結(jié))Manyelementsthatarefoundinnatureorareman-madeareradioactive,emittingalphaparticles,betaparticles,andgammarays.Theprocessisgovernedbyanexponential(指數(shù)）relation,suchthathalfofsampledecaysinatimecalledthehalf-life

tH.ValuesoftHrangefromfractions(小部分,片斷)

ofasecondtobillionsofyearsamongthehundredsofradioisotopes(放射性同位素)

known.Measurementoftheactivity(放射性測(cè)量),asthedisintegrationrateofasample,yieldshalf-lifevalues,ofimportanceinradiationuseandprotection.15第15頁(yè)補(bǔ)充與拓展radioact