f5c5 radioactivity

7/18/2019 F5C5 Radioactivity

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Chapter 5:Chapter 5:

RadioactivityRadioactivity

Form 5Form 5

1

PhysicsPhysics

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The study ofThe study of

matter matter

http://en.wikipedia.org/wiki/Physics





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Objectives:Objectives:((what you will learnwhat you will learn))

1) understanding nucleus of atom

2) radioactive decay

3) uses of radioisotopes

4) nuclear energy

5) management of radioactive

substances

Physics: ChapterPhysics: Chapter

55

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Nucleus of atomNucleus of atom

An atom consists of a central nucleus wheremost of the mass of the atom is concentrated

!rbiting around nucleus are electrons

"he nucleus is composed of protons thatare positively charged# and neutrons thatare neutral

n

n

%%

&

& % proton

n neutro

n& electro

n

A helium

atom4 'e2



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Nucleus of atomNucleus of atomproton number () $ the number of protons in nucleusnucleon number (A) $ the number of nucleons (protons neutrons)

in nucleusnuclide $ a nucleus species with a certain proton number certain

nucleon number

4 'e2

represents nucleus with proton number nucleon A

A *

represents nuclide with 2 protons 4 nucleons "he number of neutrons is 4 & 2 $ 2

Isotopes $ nuclides with same proton number# di+erentnucleon numbers

,sotopes of an element have the same chemical propertiesbut dierent physical properties# such as mass

1' 2' 3'Example isotopes- (tritiu

m)(deuterium)

(hydrogen)



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Nucleus of atomNucleus of atomRutherford.s alpha/particle (0/particle) scatteringeperiment

Rutherford bombarded gold foil with 0/particlesost 0/particles go through gold foil undeected as thenucleus is very tiny (occupies a small fraction of the volumeof atom)

ome 0/particles are slightly deected# others are deectedthrough large angles "he positive 0/particles are repelledby a massive# positively charged nucleus

0/particlesource

0/particledeecte

d

6oldfoil

7luorescentscreen

"elescope

vacuum



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Radioactive decayRadioactive decayRadioactivity $ spontaneous disintegration ofunstable nuclei accompanied by emission of energeticparticles or radiations (photons)

pontaneous disinte!ration $ emissions of theparticles or photons are not planned in advance

Radioactive decay is random because it is not possible topredict which nuclei the number of nuclei that would decay at a particularinstantRadioactive decay is not aected by

physical conditions such as temperature and pressure# chemical composition

"he particles emitted in radioactive decays are 0/particlesand 8/particles# and the radiation emitted is gamma/ray (γ/ray)

Apparatus used to detect radioactive emissions include



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Radioactive decayRadioactive decay

"he trac9s of radioactive emissions can beobserved in a cloud chamber

0/particles. trac9s- thic9 because of their high ioni:ing power

straight because of the comparative large mass all of same length because they are emitted with thesame speed

8/particles. trac9s- thin because of their wea9 ioni:ing power wavy because of the comparative small mass long because of its relative long range in air

γ/rays. trac9s-

identical to 8/particles. trac9s but are short the trac9s are those of electrons produced from ionisation



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doctronics


6eiger/uller tube;hen connected to a counter# it will count thenumber of 8/particles or γ/ray photons that entersit

;hen connected to a ratemeter# it will give thenumber of particles per seconds that enter the6/tube

"he 6/tube is unable to detect 0/particles which

cannot penetrate the window of the tube

http://www.doctronics.co.uk/scope.htm

http://www.doctronics.co.uk/scope.htm



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Radioactive decayRadioactive decay<hanges to structure of nucleus during radioactivedecay

"lpha#decayA *

A/4 =/2

% 4 'e2

(0/particle)

$eta#decay

A *

A =%1

% > e/1

(8/particle)

%ammaray?o changes in the proton number and nucleon number

@roton number increases by 1?ucleon number unchanged

@roton number decreases by 2?ucleon number decreases by 4



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"he half#life# " of a radioisotope is the time ta9enfor half of the number of nuclei in a sample todecay

,t is also the time ta9en for the rate of decay of a

sample to become half

N > N > B N >

T T

A> A>B A>

T T



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Radioactive decayRadioactive decayRadioisotope $ an isotope that isradioactiveCses in medicine(a) γ/rays from cobalt/D>-

/ radiotherapy to destroy cancerous cells/ sterili:ation to destroy bacteria or germs

(b) Radioactive tracers-/ iodine/131 to evaluate function of thyroid gland/ sodium/24 to estimate volume of blood inpatientCses in a!riculture

(a) Radioactive tracers used in plant nutrientresearch

(b) γ/rays used to sterili:e insects# destroypestsEbacteria in foodEfruits

Cses in archaeolo!y(a) Carbon#&' datin!- @roportion of </14 to </12

in living organism is the same as that of the

atmosphere ;hen an organism dies# itsproportion decreases ,ts age is estimated by



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Cses in industry(a) %au!e control

6/tube connected to ratemeter measuresthic9ness of paper by its constant count rate

(b) (ea) tracer

odium/24 used as tracer to locate damagedunderground pipes 6/tube is used to detecthigh count rate from lea9s in the pipe

(c) *uality control γ/rays (<obalt/D>) used to detect aws in Foints

between pipes carrying natural gas(d) mo)e detector

Americium/241 emits 0/particles which ioni:esair particles# allowing current to ow betweencharged plates mo9e particles which reduces

current ow by deecting 0/particles can thenbe detected

li i i i



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Nuclear +ner!yNuclear +ner!y

"he unit of mass used for measuring the mass ofatoms#

"tomic mass unit (amu)#

u $ (mass of an atom of carbon/12)

1 u $ 1DD 1>/2G 9g

1

12

Hinstein.s energy/mass relation

"he energy eIuivalent E of mass m is given by

Hnergy# E $ mc2

where c $ 3> 1>J m s/1

?uclear ,ssion $ splitting of a nucleus into two nuclei

low neutrons are used to split the nucleus

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-ana!ement-ana!ement

2 negative e+ects of radioactive materials/ omatic dama!e- near/term death of cells ofsensitive organs such as eyes

/ %enetic dama!e- long/term e+ectK mutation ofcells in subseIuent generations

0/particles-

Luite harmless outside body due to short range and wea9penetration power ,nside body# they are the mostdamaging due to their strong ioni:ing power

8/particles-

'armful both outside and inside body due to strongerpenetration power# but moderate ioni:ing power

γ/rays-

'armful outside body due to strong penetration power

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ummaryummary

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.hat you have learned:.hat you have learned:

1 understanding nucleus of

atom

/han) 0ou/han) 0ou

2 radioactive decay3 uses of radioisotopes

4 nuclear energy

5 management of radioactivesubstances

f5c5 radioactivity

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