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4.4.1RadioactivedecaySomeatomicnucleiareunstable.Unstablenucleidecaybyemittingalphaandbetaparticles,andgammarays.Theydothistobecomemorestable.AlphaemissionAnalphaparticleconsistsof2protonsand2neutrons.(1)!Whatcharge(incoulombs)doesanalphaparticlehave?Whatisits‘relativecharge’comparedtoaproton?(Thechargeononeproton=+1.6 × 10&'(𝐶.)Wecanuse‘𝛼’asthesymbolforanalphaparticle.(2)!Sometimesthesymbol 𝐻𝑒-
. isusedinstead.Whydoyouthinkthatis?Becausethethorium-229losestwoprotons(and2neutrons),itmovestwoplacesdownintheperiodictableandbecomesanisotopeofradium( 𝑅𝑎11
--2 ,orradium-225).(3)!Radium-226decaysbyalphaparticleemission.Whatisotopeisitconvertedinto?(Hint:Itloses2protons,andsomovesdowntwoplacesintheperiodictable.Itloses4particlesfromitsnucleus,soitsnucleonnumber(ormassnumber)decreasesby4.)
𝑇ℎ(5--(
Thorium-229isunstable.Tobecomemorestable,itemitsanalpha(𝛼)particle.
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BetaemissionAbetaparticleconsistsofanelectronmovingahighspeed.(4)!Whatcharge(incoulombs)doesbetaparticlehave?Whatisits‘relativecharge’comparedtoaproton?(5)!Carbon-14decaysbybetaemission.Whatisotopedoesitchangeinto?(Hint:Itgainsoneproton,somovesuponeplaceintheperiodictable.Becauseaneutronchangestoaproton,thenucleonnumber(ormassnumber)remainsthesame.)Gammarayemission Theparticlecompositionofthenucleusisunchangedbytheemissionofagammaray.
Caesium-137isunstable.Tobecomemorestable,aneutroninthenucleuschangestoaproton+anelectron.Theelectronisemitted.Thisiscalleda‘beta(𝛽)particle’.
𝛽
𝐶𝑠22'89
Thishappensinthenucleus.
Gammarayemissionoftenaccompaniesalphaorbetadecay.Alphaandbetadecayoftenleavethenucleusinanexcitedstate.Thenucleustransitionstoalowerenergystatebyemittingagamma(𝛾)rayphoton.Gammaraysarehighenergy,electromagneticradiation.
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DetectingemissionsWecanuseadevicecalledaGeigerCountertodetectalpha,betaandgammaemissions. TheGeigerCounterconsistsofadetectortubeconnectedtoacounter.Whenionisingradiationentersthroughthemicawindowontheendofthedetectingtube,apulseofcurrentisproduced.Thisisrecordeda‘count’onthecounter.‘Countrate’isthenumberofcountspersecond.Propertiesofalpha,betaandgammaThefollowingtablesummarisesthepropertiesofthe3radioactiveemissions:Emission Rangeinair Stoppedby Ionisingpoweralpha <fewcm sheetofpaper highbeta <1m thinsheetof
aluminiummedium
gamma unlimited fewcmoflead lowThe‘rangeinair’ishowfareachoftheemissionswilltravelinair.(6)!Whatdoyouthinkcausesalphaandbetaparticlestobestoppedbyair?Theemissionscanbe‘stoppedby’materialsputintheirpath.(7)!Usingtheequipment(above),describeamethodtoshowthataradioactivematerialisnotemittingalphaparticles.
detectortube
counter
thinmicawindow
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‘Ionisingpower’istheabilityoftheradiationtoproduceionsinthematerialsthattheypenetrate.(8)!Whatare‘ions’?(9)!Whatdoyouthinkcouldhappenifthematerialabsorbingtheionisingradiationisthepartofthehumanbody?
