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© 2016 flippedaroundphysics.com 4.4.1 Radioactive decay Some atomic nuclei are unstable. Unstable nuclei decay by emitting alpha and beta particles, and gamma rays. They do this to become more stable. Alpha emission An alpha particle consists of 2 protons and 2 neutrons. (1)! What charge (in coulombs) does an alpha particle have? What is its ‘relative charge’ compared to a proton? (The charge on one proton = +1.6 × 10 &’( .) We can use ‘’ as the symbol for an alpha particle. (2)! Sometimes the symbol - . is used instead. Why do you think that is? Because the thorium-229 loses two protons (and 2 neutrons), it moves two places down in the periodic table and becomes an isotope of radium ( 11 --2 , or radium-225). (3)! Radium-226 decays by alpha particle emission. What isotope is it converted into? (Hint: It loses 2 protons, and so moves down two places in the periodic table. It loses 4 particles from its nucleus, so its nucleon number (or mass number) decreases by 4.) ℎ (5 --( Thorium-229 is unstable. To become more stable, it emits an alpha() particle.

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Page 1: 4.4.1 Radioactive decay - Flipped Around Physics...4.4.1 Radioactive decay Some atomic nuclei are unstable. Unstable nuclei decay by emitting alpha and beta particles, and gamma rays

© 2016 flippedaroundphysics.com

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.

Page 2: 4.4.1 Radioactive decay - Flipped Around Physics...4.4.1 Radioactive decay Some atomic nuclei are unstable. Unstable nuclei decay by emitting alpha and beta particles, and gamma rays

© 2016 flippedaroundphysics.com

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.

Page 3: 4.4.1 Radioactive decay - Flipped Around Physics...4.4.1 Radioactive decay Some atomic nuclei are unstable. Unstable nuclei decay by emitting alpha and beta particles, and gamma rays

© 2016 flippedaroundphysics.com

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

Page 4: 4.4.1 Radioactive decay - Flipped Around Physics...4.4.1 Radioactive decay Some atomic nuclei are unstable. Unstable nuclei decay by emitting alpha and beta particles, and gamma rays

© 2016 flippedaroundphysics.com

‘Ionisingpower’istheabilityoftheradiationtoproduceionsinthematerialsthattheypenetrate.(8)!Whatare‘ions’?(9)!Whatdoyouthinkcouldhappenifthematerialabsorbingtheionisingradiationisthepartofthehumanbody?


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