A2Physics物理出國(guó)英語(yǔ)RadioactivityPart5Summaryactivities

1.Q:Whatisthedefinitionofradioactivedecay?

A:Radioactivedecayisaspontaneousprocessinwhichanunstableatomicnucleuslosesenergybyemittingradiation.Thiscaninvolvetheemissionofalphaparticles(heliumnuclei),betaparticles(electronsorpositrons),orgammarays(highenergyphotons).

2.Q:Explaintheconceptofhalflife.

A:Thehalflifeofaradioactivesubstanceisthetimeittakesforhalfoftheradioactivenucleiinasampletodecay.Forexample,ifwestartwith1000radioactivenucleiandthehalflifeis(t_{1/2}),aftertime(t_{1/2}),therewillbe500radioactivenucleiremaining.Afteranotherhalflife((2t_{1/2})),therewillbe250remaining,andsoon.

3.Q:Aradioactivesamplehasaninitialactivityof(A_0).Afterthreehalflives,whatistheremainingactivity(A)?

A:Afteronehalflife,theactivityis(frac{A_0}{2}).Aftertwohalflives,itis(frac{A_0}{2^2}).Afterthreehalflives,theremainingactivity(A=frac{A_0}{2^3}=frac{A_0}{8}).

4.Q:Whatisanalphaparticle?

A:Analphaparticleisaheliumnucleus,consistingoftwoprotonsandtwoneutrons.Itisrepresentedas(_2^4He).Whenanatomemitsanalphaparticle,itsatomicnumberdecreasesby2anditsmassnumberdecreasesby4.

5.Q:Writethenuclearequationforthealphadecayof(_{92}^{238}U).

A:Thealphadecayof(_{92}^{238}U)resultsintheemissionofanalphaparticle((_2^4He))andadaughternucleus.Theequationis(_{92}^{238}Urightarrow_{90}^{234}Th+_2^4He).Here,theatomicnumberofuranium((Z=92))decreasesby2togivethorium((Z=90)),andthemassnumberdecreasesby4.

6.Q:Whatisabetaminusparticle?

A:Abetaminusparticleisanelectron.Inbetaminusdecay,aneutroninthenucleusisconvertedintoaproton,anelectron,andanantineutrino.Theelectronisemittedfromthenucleusasthebetaminusparticle.

7.Q:Writethenuclearequationforthebetaminusdecayof(_{6}^{14}C).

A:Inbetaminusdecayof(_{6}^{14}C),aneutronisconvertedintoaproton.Theequationis(_{6}^{14}Crightarrow_{7}^{14}N+_{1}^0e+bar{nu}),where(bar{nu})istheantineutrino.Theatomicnumberincreasesby1(from6to7),whilethemassnumberremainsthesame.

8.Q:Whatisabetaplusparticle?

A:Abetaplusparticleisapositron.Inbetaplusdecay,aprotoninthenucleusisconvertedintoaneutron,apositron,andaneutrino.Thepositronisemittedfromthenucleus.

9.Q:Writethenuclearequationforthebetaplusdecayof(_{11}^{22}Na).

A:Thebetaplusdecayequationof(_{11}^{22}Na)is(_{11}^{22}Narightarrow_{10}^{22}Ne+_{+1}^0e+nu),where(nu)istheneutrino.Theatomicnumberdecreasesby1(from11to10),andthemassnumberstaysthesame.

10.Q:Whataregammarays?

A:Gammaraysarehighenergyelectromagneticradiation.Theyareemittedfromthenucleuswhenthenucleusisinanexcitedstateandtransitionstoalowerenergystate.Gammarayshavenochargeandnomass.

11.Q:Howdoestheemissionofgammaraysaffecttheatomicnumberandmassnumberofanucleus?

A:Theemissionofgammaraysdoesnotchangetheatomicnumberorthemassnumberofanucleus.Itonlychangestheenergystateofthenucleusfromahigherenergystatetoalowerenergystate.

12.Q:Whatistheactivityofaradioactivesample?

A:Theactivity(A)ofaradioactivesampleisthenumberofradioactivedecaysperunittime.Itismeasuredinbecquerels(Bq),where1Bq=1decaypersecond.

13.Q:Theactivityofasampleisproportionaltowhat?

A:Theactivity(A)ofaradioactivesampleisproportionaltothenumberofradioactivenuclei(N)presentinthesampleatthattime.Mathematically,(A=lambdaN),where(lambda)isthedecayconstant.

14.Q:Whatisthedecayconstant(lambda)?

A:Thedecayconstant(lambda)istheprobabilityofdecayperunittimeforasingleradioactivenucleus.Itisacharacteristicpropertyofaparticularradioactiveisotope.

15.Q:Howisthehalflife(t_{1/2})relatedtothedecayconstant(lambda)?

A:Therelationshipbetweenthehalflife(t_{1/2})andthedecayconstant(lambda)isgivenby(t_{1/2}=frac{ln2}{lambda}approxfrac{0.693}{lambda}).

16.Q:Aradioactivesubstancehasadecayconstant(lambda=0.02s^{1}).Whatisitshalflife?

A:Usingtheformula(t_{1/2}=frac{ln2}{lambda}),with(lambda=0.02s^{1}),weget(t_{1/2}=frac{0.693}{0.02}s=34.65s).

17.Q:Whatisthelawofradioactivedecay?

A:Thelawofradioactivedecaystatesthatthenumberofradioactivenuclei(N)inasampleattime(t)isgivenby(N=N_0e^{lambdat}),where(N_0)istheinitialnumberofradioactivenuclei,(lambda)isthedecayconstant,and(t)isthetimeelapsed.

18.Q:Ifasamplehasaninitialnumberofradioactivenuclei(N_0=1000)andadecayconstant(lambda=0.1s^{1}),howmanynucleiremainafter(t=10s)?

A:Usingtheformula(N=N_0e^{lambdat}),wesubstitute(N_0=1000),(lambda=0.1s^{1}),and(t=10s).So(N=1000timese^{(0.1times10)}=1000timese^{1}approx1000times0.368=368).

19.Q:Whatisbackgroundradiation?

A:Backgroundradiationisthelowlevelradiationthatisconstantlypresentintheenvironment.Itcomesfromnaturalsourcessuchascosmicraysfromspace,radioactiveisotopesintheEarth'scrust(e.g.,radongas),andfrommanmadesourcessuchasmedicalXraysandnuclearpowerplants.

20.Q:Howcanbackgroundradiationbemeasured?

A:BackgroundradiationcanbemeasuredusingaradiationdetectorsuchasaGeigerMullertube.Thedetectorisleftintheenvironmentforacertainperiodoftime,andthenumberofcountsisrecorded.Theaveragecountrategivesthebackgroundcountrate.

21.Q:Whyisitimportanttomeasurebackgroundradiationwhenmeasuringtheactivityofaradioactivesample?

A:Whenmeasuringtheactivityofaradioactivesample,themeasuredcountrateincludesthecontributionfrombackgroundradiation.Toobtainthetrueactivityofthesample,thebackgroundcountratemustbesubtractedfromthetotalmeasuredcountrate.

22.Q:Whatarethehealthrisksassociatedwithexposuretoradioactiveradiation?

A:Exposuretoradioactiveradiationcancausedamagetolivingcells.Lowlevelexposureoveralongperiodcanincreasetheriskofcancer.Highlevelexposurecancauseacuteradiationsickness,whichmayincludesymptomssuchasnausea,vomiting,hairloss,anddamagetotheimmunesystem.

23.Q:Howcanweprotectourselvesfromradioactiveradiation?

A:Wecanprotectourselvesfromradioactiveradiationbyusingshielding.Foralphaparticles,asheetofpaperorafewcentimetersofaircanprovidesufficientshielding.Forbetaparticles,afewmillimetersofaluminumcanbeused.Forgammarays,thickleadorconcreteshieldsarerequired.Also,increasingthedistancefromthesourceandreducingthetimeofexposurecanreducetheradiationdose.

24.Q:Whatistheprinciplebehindcarbon14dating?

A:Carbon14datingisbasedonthefactthatcarbon14((_{6}^{14}C))isaradioactiveisotopewithahalflifeofabout5730years.Livingorganismstakeincarbonfromtheenvironment,andtheratioof(_{6}^{14}C)to(_{6}^{12}C)intheirbodiesisthesameastheratiointheatmosphere.Whenanorganismdies,itstopstakingincarbon,andtheamountof(_{6}^{14}C)initsbodystartstodecay.Bymeasuringtheremainingamountof(_{6}^{14}C)inasampleandcomparingittotheinitialamount,theageofthesamplecanbeestimated.

25.Q:Afossilhas1/4oftheamountof(_{6}^{14}C)comparedtoalivingorganism.Howoldisthefossil?

A:Sincetheamountof(_{6}^{14}C)is1/4oftheinitialamount,twohalfliveshavepassed.Giventhehalflifeof(_{6}^{14}C)is(t_{1/2}=5730)years,theageofthefossilis(2times5730=11460)years.

26.Q:Whatisthedifferencebetweennaturalandartificialradioactivity?

A:Naturalradioactivityoccursspontaneouslyinnaturallyoccurringradioactiveisotopes.TheseisotopeshaveexistedsincetheformationoftheEarth.Artificialradioactivityisproducedbybombardingstablenucleiwithparticlessuchasneutrons,protons,oralphaparticlesinanuclearreactororaparticleaccelerator.

27.Q:Whatisanuclearfissionreaction?

A:Anuclearfissionreactionisaprocessinwhichaheavynucleus(suchas(_{92}^{235}U))splitsintotwoormorelighternuclei,alongwiththereleaseofalargeamountofenergyandseveralneutrons.Forexample,when(_{92}^{235}U)absorbsaneutron,itcanundergofissiontoproducenucleisuchas(_{56}^{141}Ba)and(_{36}^{92}Kr)alongwith3neutrons.

28.Q:Whatisachainreactioninnuclearfission?

A:Inanuclearfissionchainreaction,theneutronsreleasedinonefissioneventcangoontocausefurtherfissioneventsinotheruraniumnuclei.Ifthenumberofneutronscausingfissionissufficient,aselfsustainingchainreactionoccurs.Thisistheprinciplebehindnuclearpowerplantsandnuclearweapons.

29.Q:Howisachainreactioncontrolledinanuclearpowerplant?

A:Inanuclearpowerplant,controlrodsmadeofmaterialssuchascadmiumorboronareusedtoabsorbneutrons.Byadjustingthepositionofthecontrolrods,thenumberofneutronsavailableforcausingfissioncanberegulated,andthechainreactioncanbecontrolledtomaintainasteadypoweroutput.

30.Q:Whatisnuclearfusion?

A:Nuclearfusionisaprocessinwhichtwolightnucleicombinetoformaheaviernucleus,releasingalargeamountofenergy.Forexample,inthesun,hydrogennuclei((_{1}^{1}H))fusetoformheliumnuclei((_{2}^{4}He))throughaseriesofreactions.

31.Q:WhyisnuclearfusiondifficulttoachieveonEarth?

A:Nuclearfusionrequiresextremelyhightemperaturesandpressurestoovercometheelectrostaticrepulsionbetweenthepositivelychargednuclei.OnEarth,itisdifficulttocreateandmaintainthenecessaryconditions.Highenergyparticlebeamsandmagneticconfinementtechniquesareusedinattemptstoachievecontrolledfusion,butitisstillamajortechnologicalchallenge.

32.Q:Whataretheadvantagesofnuclearfusionovernuclearfission?

A:Nuclearfusionhasseveraladvantagesovernuclearfission.Thefuelforfusion(e.g.,hydrogenisotopes)isabundant.Fusionreactionsproducelessradioactivewastecomparedtofissionreactions.Also,thereisnoriskofarunawaychainreactionasinfissionreactors.

33.Q:WhatistheQvalueofanuclearreaction?

A:TheQvalueofanuclearreactionistheenergyreleasedorabsorbedinthereaction.Itiscalculatedasthedifferenceintherestmassenergyofthereactantsandtheproducts.If(Q>0),thereactionisexothermic(energyisreleased),andif(Q<0),thereactionisendothermic(energyisabsorbed).

34.Q:Inanuclearreaction,howcantheQvaluebecalculatedfromthemassesofthereactantsandproducts?

A:TheQvalue(Q=(m_{reactants}m_{products})c^{2}),where(m_{reactants})isthetotalmassofthereactants,(m_{products})isthetotalmassoftheproducts,and(c)isthespeedoflightinavacuum.

35.Q:WhatisthesignificanceoftheQvalueinanuclearfissionreaction?

A:Inanuclearfissionreaction,thepositiveQvaluerepresentsthelargeamountofenergyreleasedintheformofkineticenergyofthefissionfragmentsandtheenergyoftheemittedneutronsandgammarays.Thisenergycanbeharnessedforpowergeneration.

36.Q:Howdoestheenergyreleasedinanuclearreactioncomparetotheenergyreleasedinachemicalreaction?

A:Theenergyreleasedinanuclearreactionismuchgreaterthantheenergyreleasedinachemicalreaction.Inachemicalreaction,theenergychangesareduetotherearrangementofelectronsintheatoms,whileinanuclearreaction,theenergychangesareduetochangesinthenucleus.Forexample,theenergyreleasedperatominanuclearfissionreactionisaboutamilliontimesgreaterthantheenergyreleasedpermoleculeinatypicalchemicalreaction.

37.Q:Whatisthebindingenergyofanucleus?

A:Thebindingenergyofanucleusistheenergyrequiredtocompletelyseparateallthenucleons(protonsandneutrons)inthenucleus.Itisameasureofthestabilityofthenucleus.

38.Q:Howisthebindingenergypernucleonrelatedtothestabilityofanucleus?

A:Nucleiwithhigherbindingenergypernucleonaremorestable.Thebindingenergypernucleonreachesamaximumaroundiron((_{26}^{56}Fe)).Nucleilighterthanironcanundergofusiontoincreasetheirbindingenergypernucleon,andnucleiheavierthanironcanundergofissiontoincreasetheirbindingenergypernucleon.

39.Q:Whatisthemassdefectofanucleus?

A:Themassdefect(Deltam)ofanucleusisthedifferencebetweenthesumofthemassesoftheindividualnucleonsandtheactualmassofthenucleus.Themassdefectisrelatedtothebindingenergy(E_b)ofthenucleusby(E_b=Deltamc^{2}).

40.Q:Calculatethemassdefectofahelium4nucleus((_{2}^{4}He))giventhemassofaproton(m_p=1.007276u),themassofaneutron(m_n=1.008665u),andthemassofthe(_{2}^{4}He)nucleus(m_{He}=4.001506u).

A:A(_{2}^{4}He)nucleushas2protonsand2neutrons.Thesumofthemassesoftheindividualnucleonsis(2m_p+2m_n=2times1.007276u+2times1.008665u=4.031882u).Themassdefect(Deltam=(4.0318824.001506)u=0.030376u).

41.Q:Whatistheuseofradioactivetracersinmedicine?

A:Radioactivetracersareusedinmedicinefordiagnosticpurposes.Asmallamountofaradioactiveisotopeisintroducedintothebody,anditsmovementanddistributioncanbetrackedusingaradiationdetector.Forexample,technetium99miscommonlyusedasatracertoimagevariousorgansinthebody.

42.Q:Howareradioactiveisotopesusedinradiotherapy?

A:Inradiotherapy,highenergyradiationfromradioactiveisotopessuchascobalt60isusedtokillcancercells.Theradiationistargetedatthetumor,andthehighenergyphotonsorparticlesdamagetheDNAofthecancercells,preventingthemfromdividingandgrowing.

43.Q:Whatisthedifferencebetweenastableandanunstablenucleus?

A:Astablenucleusdoesnotundergoradioactivedecayspontaneously.Theforceswithinthenucleus,suchasthestrongnuclearforceandtheelectrostaticforce,arebalanced.Anunstablenucleushasanexcessofenergyoranimbalanceofforces,anditwilldecaytoamorestablestatebyemittingradiation.

44.Q:Howdoestheneutrontoprotonratioaffectthestabilityofanucleus?

A:Forlightnuclei,aneutrontoprotonratio((n/p))ofapproximately1isstable.Astheatomicnumberincreases,thestable(n/p)ratioincreases.Nucleiwitha(n/p)ratiothatistoohighortoolowareunstableandmayundergoradioactivedecaytoachieveamorestableratio.

45.Q:Whatistherelationshipbetweentheenergyofagammarayphotonandtheenergylevelsofthenucleus?

A:Theenergyofagammarayphoton(E=hf)(where(h)isPlanck'sconstantand(f)isthefrequencyofthephoton)isequaltothedifferenceinenergybetweentheinitialandfinalenergylevelsofthenucleus.Whenthenucleustransitionsfromahigherenergystate(E_2)toalowerenergystate(E_1),theenergyoftheemittedgammarayphotonis(E=E_2E_1).

46.Q:Whydosomeradioactiveisotopesdecaybyalphaemissionwhileothersdecaybybetaemission?

A:Thetypeofdecay(alphaorbeta)dependsonthenatureoftheinstabilityofthenucleus.Nucleithatareveryheavyandhavealargenumberofprotonsandneutronsmaydecaybyalphaemissiontoreducetheirmassandatomicnumber.Nucleiwithanimbalanceinthe