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Chapter 5:Chapter 5:
RadioactivityRadioactivity
Form 5Form 5
1
PhysicsPhysics
Next >
The study ofThe study of
matter matter
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yy
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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
2
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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
Radioactive decayRadioactive decay
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
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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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Radioactive decayRadioactive decay
"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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Radioactive decayRadioactive decay
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
H li ti T iti
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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
H li ti T iti
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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