7/29/2019 Radioactivity - P3, week 6

1/22

Radioactivity

Whataresomeofthethingsthatcometomindwhenyouhearthewordradioac5vity?

Whatisitusedfortoday? Energyproduc5on Cancertreatment Radioac5veda5nginArchaeology Steriliza5onandDisinfec5on

Radioac'vityinanutshell:Unstableatomstrytobecomestableagain.OnewaytheycandothisisbygivingoffmaHerandenergy.Thisiswhatwecallradia5on.

Amaterialwithunstableatomsissaidtoberadioac've. Threetypesofradia5on:,, 1

7/29/2019 Radioactivity - P3, week 6

2/22

Discoveryofradioactivity

Radioac5vitydiscoveredbyBecquerelin1896 MarieCurie(1867-1934)

discoveredseveralotherradioac5veelements coinedthetermradio-ac5vity discoveredthatradioac5vitycankillhumancells inventedradiotherapyforcancertreatment enriBecquerel

MarieCurie

Nothinginlifeistobefeared,itisonlytobeunderstood.Nowis

the8metounderstandmore,so

thatwemayfearless.2

7/29/2019 Radioactivity - P3, week 6

3/22

Howdoesradioactivedecay

work? Whatmakesanelementradioac5ve?

Atomicnucleimadeofprotonsandneutrons Nuclearforcekeepstheprotonspackedtogether Electric(coulomb)forcemakesprotonsrepeleach

other

Forbigandheavyatomicnucleitheelectricforcebecomestoolargeforthenuclearforceto

counteractandthenucleusbecomesunstable

MaHerandenergyarereleasedasthenucleusdecaysradioac5velyintoamorestablestate

Radioac5vedecayisaspontaneousandrandomprocess

3

7/29/2019 Radioactivity - P3, week 6

4/22

Typesofradioactivedecay

Whenanatomicnucleusisunstableitwilldecayinoneofthreeways:

emit-par'cle(alpharadia5on) emit-par'cle(betaradia5on) emit-rays(gammaradia5on)

4

7/29/2019 Radioactivity - P3, week 6

5/22

Typesofradioactivedecay

Whenanatomicnucleusisunstableitwilldecayinoneofthreeways:

emit-par'cle(alpharadia5on) emit-par'cle(betaradia5on) emit-rays(gammaradia5on)

-par'cles:

areeliumnuclei(e):2protons+2neutrons areposi5velycharged travelslowly lowpenetra5ngpower onlytravelforafewcminair(shortrange)

7/29/2019 Radioactivity - P3, week 6

6/22

Typesofradioactivedecay

Whenanatomicnucleusisunstableitwilldecayinoneofthreeways:

emit-par'cle(alpharadia5on) emit-par'cle(betaradia5on) emit-rays(gammaradia5on)

-par'cles:

arehighenergyelectrons arenega5velycharged travelfast mediumpenetra5ngpower travelforabout3metersinair(mediumrange) 6

7/29/2019 Radioactivity - P3, week 6

7/22

Typesofradioactivedecay

Whenanatomicnucleusisunstableitwilldecayinoneofthreeways:

emit-par'cle(alpharadia5on) emit-par'cle(betaradia5on) emit-rays(gammaradia5on)

-rays:

areelectromagne5cwaves havenomassorcharge travelatthespeedoflight highpenetra5ngpower nopuresources(&par5clesalsoemiHed) 7

7/29/2019 Radioactivity - P3, week 6

8/22

Propertiesofradioactive

radiations Alltypesofradioac5vitycanpoten5allydamagelivingcells Theydosobyionizingatoms Ioniza5ontakesplacewhen

electronsareknockedoffatoms(atombecomesaposi'veion) electronsareabsorbedbyatoms(atombecomesanega'veion)

Asingleradioac5vepar5clehasenoughenergytoproduce10ionsinair

8

7/29/2019 Radioactivity - P3, week 6

9/22

Whichtypeismostdangerous?

Iftheradioac5vesourceisinsidethebody: -radia5onisthemostdangerous(itiseasilyabsorbedbycells) and-radia5onarelessdangerous(lesslikelytobeabsorbedby

acell)

Iftheradioac5vesourceisoutsidethebody: -radia5onislessdangerousbecauseitisunlikelytoreachliving

cellsinsidethebody(itcanttravelfarandiseasilystopped)

and-radia5onaremoredangerous(theycanpenetratetheskinanddamagethecellsinside)

9

7/29/2019 Radioactivity - P3, week 6

10/22

Propertiesofradioactive

radiations owusradia5onaffectedinthepresenceofelectromagne5c

fields?

+ + + + + +

- - - - - - 10

7/29/2019 Radioactivity - P3, week 6

11/22

Detectingradiation

GeigerMllercounter Par5cledetectorthatmeasures

ionizingradia5on

Countsthenumberofpar5clesdetectedperminute

11

7/29/2019 Radioactivity - P3, week 6

12/22

I 1

r2

RadiationIntensity

Radia'onintensityistheenergypersecondpassingthroughagivenareathatisperpendiculartothedirec5onofradia5on

travel

Radia5onfromaradioac5vesourceisdistributedequallyinalldirec5onsandobeysaninversesquarelaw

12

7/29/2019 Radioactivity - P3, week 6

13/22

Isotopes

Isotopesarevariantsofpar5cularchemicalelements Isotopesofanelementhavethesamechemicalproper'es

Theyallhavethesamenumberofprotonsandelectronsbutdifferinthenumberofneutrons intheirnucleus

ForexampleandaretwoisotopesofUranium Theabundanceofanisotopeisameasure(in%)ofhow

commonthatpar5cularisotopeisinnature

Someisotopesarestablebutotherscanberadioac8ve

13

7/29/2019 Radioactivity - P3, week 6

14/22

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

14

7/29/2019 Radioactivity - P3, week 6

15/22

Example

Athoriumnucleusdecaysbyemission.Whatelementdoesitdecayinto?

ACobaltnucleusdecaysbyemission.Whatelementdoesitdecayinto?

90

228Th ?

27

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

7/29/2019 Radioactivity - P3, week 6

16/22

RadioactivedecayandHalf-life

Radioac5vedecayisaspontaneousandrandomprocess owlongdowehavetowaitun5lhalfofouroriginal

radioac5vematerialhasdecayed?

Thenumberofnucleidecayingpersecondisdirectlypropor5onaltothenumberofnucleipresentinoursampleatthatgiven5me:

where=theradioac5vedecayconstantoftheatomconcerned

-dN/dtiscalledtheac'vityofthesource(measuredinBecquerelBq)

dN

dt= N

16

7/29/2019 Radioactivity - P3, week 6

17/22

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

17

7/29/2019 Radioactivity - P3, week 6

18/22

Example

Afreshlypreparedsampleofaradioac5veisotopeXcontains

1020atoms.Thehalf-lifeofthatisotopeis12hours.Calculate

a) Theini5alac5vityb) Thenumberofradioac5veatomsofXremainingaer

1. 1hour2. 24hours

18

7/29/2019 Radioactivity - P3, week 6

19/22

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

19

7/29/2019 Radioactivity - P3, week 6

20/22

Example

LivingwoodhasanC-14ac5vityof1.3countsperminuteper

gramofcarbon.Asampleofgramsofdeadwoodisfoundto

haveC-14ac5vityof17.0countsperminute.owoldisthe

sampleofthewood?(Thehalf-lifeofCarbon-14is68years)

20

7/29/2019 Radioactivity - P3, week 6

21/22

Example

Apointsourceof-radia5onhasahalf-lifeof30minutes.The

ini5alcountrate,recordedbyaGeigercounterplaced2mfrom

thesourceis360s-1.Thedistancebetweenthecounterandthe

sourceischanged.Aer1.hoursthecountraterecordedis

s-1

.Whatisthenewdistancebetweenthecounterandthesource?

21

7/29/2019 Radioactivity - P3, week 6

22/22

ToDo

Readchapter28[p.7-98Radioac5vity] Assignmentwk:Prac'calDC2

Where?Physicslabs(G.O.Jones2ndfloor) When?Friday1thFebbetween1-6pm(makesureyougoatthe

right5me)

anditinnolaterthan4:00pmontheWednesdayaerthereadingweek-LATEWORKWILLNOTBEACCEPTED

22