Post on 04-Apr-2018
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Radioactivity
Whataresomeofthethingsthatcometomindwhenyouhearthewordradioac5vity?
Whatisitusedfortoday? Energyproduc5on Cancertreatment Radioac5veda5nginArchaeology Steriliza5onandDisinfec5on
Radioac'vityinanutshell:Unstableatomstrytobecomestableagain.OnewaytheycandothisisbygivingoffmaHerandenergy.Thisiswhatwecallradia5on.
Amaterialwithunstableatomsissaidtoberadioac've. Threetypesofradia5on:,, 1
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Discoveryofradioactivity
Radioac5vitydiscoveredbyBecquerelin1896 MarieCurie(1867-1934)
discoveredseveralotherradioac5veelements coinedthetermradio-ac5vity discoveredthatradioac5vitycankillhumancells inventedradiotherapyforcancertreatment enriBecquerel
MarieCurie
Nothinginlifeistobefeared,itisonlytobeunderstood.Nowis
the8metounderstandmore,so
thatwemayfearless.2
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Howdoesradioactivedecay
work? Whatmakesanelementradioac5ve?
Atomicnucleimadeofprotonsandneutrons Nuclearforcekeepstheprotonspackedtogether Electric(coulomb)forcemakesprotonsrepeleach
other
Forbigandheavyatomicnucleitheelectricforcebecomestoolargeforthenuclearforceto
counteractandthenucleusbecomesunstable
MaHerandenergyarereleasedasthenucleusdecaysradioac5velyintoamorestablestate
Radioac5vedecayisaspontaneousandrandomprocess
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Typesofradioactivedecay
Whenanatomicnucleusisunstableitwilldecayinoneofthreeways:
emit-par'cle(alpharadia5on) emit-par'cle(betaradia5on) emit-rays(gammaradia5on)
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Typesofradioactivedecay
Whenanatomicnucleusisunstableitwilldecayinoneofthreeways:
emit-par'cle(alpharadia5on) emit-par'cle(betaradia5on) emit-rays(gammaradia5on)
-par'cles:
areeliumnuclei(e):2protons+2neutrons areposi5velycharged travelslowly lowpenetra5ngpower onlytravelforafewcminair(shortrange)
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Typesofradioactivedecay
Whenanatomicnucleusisunstableitwilldecayinoneofthreeways:
emit-par'cle(alpharadia5on) emit-par'cle(betaradia5on) emit-rays(gammaradia5on)
-par'cles:
arehighenergyelectrons arenega5velycharged travelfast mediumpenetra5ngpower travelforabout3metersinair(mediumrange) 6
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Typesofradioactivedecay
Whenanatomicnucleusisunstableitwilldecayinoneofthreeways:
emit-par'cle(alpharadia5on) emit-par'cle(betaradia5on) emit-rays(gammaradia5on)
-rays:
areelectromagne5cwaves havenomassorcharge travelatthespeedoflight highpenetra5ngpower nopuresources(&par5clesalsoemiHed) 7
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Propertiesofradioactive
radiations Alltypesofradioac5vitycanpoten5allydamagelivingcells Theydosobyionizingatoms Ioniza5ontakesplacewhen
electronsareknockedoffatoms(atombecomesaposi'veion) electronsareabsorbedbyatoms(atombecomesanega'veion)
Asingleradioac5vepar5clehasenoughenergytoproduce10ionsinair
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Whichtypeismostdangerous?
Iftheradioac5vesourceisinsidethebody: -radia5onisthemostdangerous(itiseasilyabsorbedbycells) and-radia5onarelessdangerous(lesslikelytobeabsorbedby
acell)
Iftheradioac5vesourceisoutsidethebody: -radia5onislessdangerousbecauseitisunlikelytoreachliving
cellsinsidethebody(itcanttravelfarandiseasilystopped)
and-radia5onaremoredangerous(theycanpenetratetheskinanddamagethecellsinside)
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Propertiesofradioactive
radiations owusradia5onaffectedinthepresenceofelectromagne5c
fields?
+ + + + + +
- - - - - - 10
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Detectingradiation
GeigerMllercounter Par5cledetectorthatmeasures
ionizingradia5on
Countsthenumberofpar5clesdetectedperminute
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I 1
r2
RadiationIntensity
Radia'onintensityistheenergypersecondpassingthroughagivenareathatisperpendiculartothedirec5onofradia5on
travel
Radia5onfromaradioac5vesourceisdistributedequallyinalldirec5onsandobeysaninversesquarelaw
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Isotopes
Isotopesarevariantsofpar5cularchemicalelements Isotopesofanelementhavethesamechemicalproper'es
Theyallhavethesamenumberofprotonsandelectronsbutdifferinthenumberofneutrons intheirnucleus
ForexampleandaretwoisotopesofUranium Theabundanceofanisotopeisameasure(in%)ofhow
commonthatpar5cularisotopeisinnature
Someisotopesarestablebutotherscanberadioac8ve
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Radioactivedecayequations
-par'cleemission
-par'cleemission
-rayemission Nochargeormasssodoesnotproducenewelements
Z
AX2
4He+Z2
A4Y
88
226Ra
2
4He+
86
222Rn
Z
AX
1
0e+
Z+1
AY
6
14C
1
0e+
7
14N
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Example
Athoriumnucleusdecaysbyemission.Whatelementdoesitdecayinto?
ACobaltnucleusdecaysbyemission.Whatelementdoesitdecayinto?
90
228Th ?
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60Co ?
a) RadiumRa(Z=88)b) Ac5niumAc(Z=89)c) Proac5niumPa(Z=91)d) UraniumU(Z=92)?
a) ManganeseMn(Z=2)b) IronFe(Z=26)c) NickelNi(Z=28)d) CopperCu(Z=29)? 1
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RadioactivedecayandHalf-life
Radioac5vedecayisaspontaneousandrandomprocess owlongdowehavetowaitun5lhalfofouroriginal
radioac5vematerialhasdecayed?
Thenumberofnucleidecayingpersecondisdirectlypropor5onaltothenumberofnucleipresentinoursampleatthatgiven5me:
where=theradioac5vedecayconstantoftheatomconcerned
-dN/dtiscalledtheac'vityofthesource(measuredinBecquerelBq)
dN
dt= N
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RadioactivedecayandHalf-life
NumberofundecayedatomsNaer5met
Half-lifeT1/2:the5metakenfortheatomstodisintegratetohalftheirini5alnumber
dN
NN0
N
=
dt
0
t
lnN
[ ]N0
N
=
t
lnN lnN0= t ln
N
N0
$
%&
'
()= t
N=
N0e
t
Radioac5vedecayequa5on
T1/2=
0.693
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Example
Afreshlypreparedsampleofaradioac5veisotopeXcontains
1020atoms.Thehalf-lifeofthatisotopeis12hours.Calculate
a) Theini5alac5vityb) Thenumberofradioac5veatomsofXremainingaer
1. 1hour2. 24hours
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Radiocarbondating
Carbonhasaradioac5veisotope:C-14 FormedwhenneutronsreactwithNitrogenintheair
Usedtodeterminetheagesofcarbon-bearingorganismsanddateorganicremainsfoundinarcheologicalsites
AgecanbedeterminedifweknowtheC-14ac5vityinlivingandancient(dead)samplesofthesameorganism
7
14N+
0
1n
6
14C+
1
1H
N
N0
= et ln
N
N0
#
$%
&
'(= t=
0.693
T1/2
t
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Example
LivingwoodhasanC-14ac5vityof1.3countsperminuteper
gramofcarbon.Asampleofgramsofdeadwoodisfoundto
haveC-14ac5vityof17.0countsperminute.owoldisthe
sampleofthewood?(Thehalf-lifeofCarbon-14is68years)
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Example
Apointsourceof-radia5onhasahalf-lifeof30minutes.The
ini5alcountrate,recordedbyaGeigercounterplaced2mfrom
thesourceis360s-1.Thedistancebetweenthecounterandthe
sourceischanged.Aer1.hoursthecountraterecordedis
s-1
.Whatisthenewdistancebetweenthecounterandthesource?
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ToDo
Readchapter28[p.7-98Radioac5vity] Assignmentwk:Prac'calDC2
Where?Physicslabs(G.O.Jones2ndfloor) When?Friday1thFebbetween1-6pm(makesureyougoatthe
right5me)
anditinnolaterthan4:00pmontheWednesdayaerthereadingweek-LATEWORKWILLNOTBEACCEPTED
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