核工程专业英语3-Radioactivity省公开课一等奖全国示范课微课金奖课件

NUCLEARENERGYPROFESSIONAL(SPECIAL)ENGLISHCOURSECHAPTER3Radioactivity1第1页CHAPTER3

Radioactivity（放射性）Manynaturallyoccurringandman-madeisotopeshavethepropertyofradioactivity(放射性),whichisthespontaneous(自发)

disintegration(decay)

(衰变)

ofthenucleuswiththeemissionofaparticle.Theprocesstakesplaceinminerals(矿物质)

oftheground,infibers(纤维)

ofplants,intissues(组织)

ofanimals,andintheairandwater,allofwhichcontaintraces(痕迹,微量)ofradioactiveelements.2第2页3.1RADIOACTIVEDECAY

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

),betaparticles

(

)

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

).Someexamplesareasfollowing:3第3页3.2THEDECAYLAW(衰变规律)Therateatwhicharadioactivesubstancedisintegrates(andthustherateofreleaseofparticles)dependsontheisotopicspecies(种类，核素),butthereisadefinite(一定)“decaylaw”thatgoverns(管理,支配)

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

4第4页Suchstatistical(统计学)

behaviorisdescribedbyaconstantpropertyoftheatomcalledhalf-life(半衰期).Thistimeinterval(间隔),symbolizedbytHisthetimerequiredforhalfofthenucleitodecay,leavinghalfofthemintact(完整无缺).

Weshouldliketoknowhowmanynucleiofaradioactivespeciesremainatanytime.IfwestartattimezerowithN0

nuclei,afteralengthoftimetHtherewillbeN0/2;bythetime2tHhaselapsed(流逝),therewillbeN0/4;etc.AgraphofthenumberofnucleiasafunctionoftimeisshowninFig.3.l.5第5页6第6页Foranytimetonthecurve(曲线图),theratioofthenumberofnucleipresenttotheinitial(初始)

numberisgivenby

Half-livesrangefromverysmallfractions(小部分,片断)ofasecondtobillionsof

(数十亿)years,witheachradioactiveisotope

(放射性同位素)

havingadefinitehalf-life.7第7页TheformulaforN/N0isnotveryconvenientforcalculationsexceptwhentissomeintegermultipleoftH.Definingthedecayconstant

(lambda),

asthechanceofdecayofagivennucleuseachsecond,anequivalent(同意义)

exponential(指数)

formulafordecayis

Wefindthat

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

isthechanceofdecayeachsecondofonenucleus,forNnucleitheactivityistheproduct(乘积)

A=

NTheunitdis/seciscalledthe

becquerel

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

curie(Ci)

namedaftertheFrenchscientistsPierreandMarieCuriewhostudiedradium(镭).Thecurieis3.7

1010dis/sec,

whichisanearlymeasuredvalueoftheactivitypergramofradium.9第9页3.3MEASUREMENTOFHALF-LIFE

(半衰期测量)Findingthehalf-lifeofanisotopeprovidespartofitsidentification(判定),neededforbeneficialuseorforprotectionagainstradiationhazard.Letuslookatamethodformeasuringthehalf-lifeofaradioactivesubstance.AsinFig.3.2,adetector

探测器

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

(计数率)iscomputed.Itisproportionalto

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

(半对数坐标纸)

asinFig.3.3,andastraightlinedrawnthroughtheobservedpoints.Fromanypairsofpointsontheline

andtH=0.693/

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

ofbackgroundradiation(本底辐射),whichisduetocosmicrays

(宇宙射线)fromoutsidetheearthorfromthedecayofmineralsinmaterialsofconstruction(建筑物)

orintheearth.Itisalwaysnecessarytomeasurethebackgroundcounts

(本底计数)andsubtract(减去)

themfromthoseobservedintheexperiment.

14第14页3.4SUMMARY(总结)Manyelementsthatarefoundinnatureorareman-madeareradioactive,emittingalphaparticles,betaparticles,andgammarays.Theprocessisgovernedbyanexponential(指数）relation,suchthathalfofsampledecaysinatimecalledthehalf-life

tH.ValuesoftHrangefromfractions(小部分,片断)

ofasecondtobillionsofyearsamongthehundredsofradioisotopes(放射性同位素)

known.Measurementoftheactivity(放射性测量),asthedisintegrationrateofasample,yieldshalf-lifevalues,ofimportanceinradiationuseandprotection.15第15页补充与拓展radioact