Nuclear fission and fusion Types of decay process Types of decay process Rates of decay Rates of...
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Transcript of Nuclear fission and fusion Types of decay process Types of decay process Rates of decay Rates of...
Nuclear fission and fusionNuclear fission and fusion
Types of decay processTypes of decay processRates of decayRates of decayNuclear stabilityNuclear stabilityEnergy changesEnergy changesFission and fusionFission and fusion
Forces at work in the nucleusForces at work in the nucleus
Electrostatic repulsion: pushes protons apartElectrostatic repulsion: pushes protons apart
Strong nuclear force: pulls protons togetherStrong nuclear force: pulls protons together
Nuclear force is much shorter range: protons must be close togetherNuclear force is much shorter range: protons must be close together
Neutrons only experience the strong Neutrons only experience the strong nuclear forcenuclear force
Proton pair experiences both forcesProton pair experiences both forces
Neutrons experience only the strong nuclear forceNeutrons experience only the strong nuclear force
But: neutrons But: neutrons alonealone are unstable are unstable
Neutrons act like nuclear glueNeutrons act like nuclear glue
Helium nucleus contains 2 protons and 2 Helium nucleus contains 2 protons and 2 neutrons – increase attractive forcesneutrons – increase attractive forces Overall nucleus is stableOverall nucleus is stable
As nuclear size increases, As nuclear size increases, electrostatic repulsion builds upelectrostatic repulsion builds up
There are electrostatic repulsions between There are electrostatic repulsions between protons that don’t have attractive forcesprotons that don’t have attractive forces
More neutrons requiredMore neutrons required
Long range repulsive force
with no compensation from attraction
Neutron to proton ratio increases Neutron to proton ratio increases with atomic numberwith atomic number
Upper limit of stability
Upper limit to nuclear stabilityUpper limit to nuclear stability
Beyond atomic number 83, all nuclei are Beyond atomic number 83, all nuclei are unstable and decay via radioactivityunstable and decay via radioactivity
Radioactive decay (Radioactive decay (TransmutationTransmutation) – ) – formation of new elementformation of new element
HeThU 42
23490
23892
HeThU 42
23490
23892
Atomic number
decreases
Alpha particle emitted
Mass number
Atomic number
Odds and sodsOdds and sods All elements have a All elements have a
radioactive isotoperadioactive isotope Only H has fewer neutrons Only H has fewer neutrons
than protonsthan protons The neutron:proton ratio The neutron:proton ratio
increases with Zincreases with Z All isotopes heavier than All isotopes heavier than
bismuth-209 are radioactivebismuth-209 are radioactive Most nonradioactive isotopes Most nonradioactive isotopes
contain an even number of contain an even number of neutrons (207 out of 264). 156 neutrons (207 out of 264). 156 have even protons and neutrons; have even protons and neutrons; 51 have even protons and odd 51 have even protons and odd neutrons; 4 have odd protons and neutrons; 4 have odd protons and neutronsneutrons
Nuclear processes relieve instabilityNuclear processes relieve instability
Chemical reactions involve electrons; nuclear Chemical reactions involve electrons; nuclear reactions involve the nucleusreactions involve the nucleus
Isotopes behave the same in chemical Isotopes behave the same in chemical reactions but differently in nuclear onesreactions but differently in nuclear ones
Rate of nuclear process independent of T,P, Rate of nuclear process independent of T,P, catalystcatalyst
Nuclear process independent of state of the Nuclear process independent of state of the atom – element, compoundatom – element, compound
Energy changes are massiveEnergy changes are massive
Types of radiationTypes of radiation
Alpha particle emissionAlpha particle emission
238 4 23492 2 90U He Th
92 protons146 neutrons238 nucleons
2 protons2 neutrons4 nucleons
90 protons144 neutrons234 nucleons
Beta particle emissionBeta particle emission
131 0 13153 1 54I e Xe
53 protons78 neutrons
131 nucleons
54 protons77 neutrons
131 nucleons
0 nucleons-1 charge
Other decay processesOther decay processes
Positron emission: the Positron emission: the conversion of a proton into conversion of a proton into a neutron plus positive a neutron plus positive electronelectron Decrease in z with no Decrease in z with no
decrease in mdecrease in m
Electron capture: the Electron capture: the capture of an electron by a capture of an electron by a proton to create a neutronproton to create a neutron Decrease in z with no Decrease in z with no
decrease in mdecrease in m
40 40 019 18 1K Ar e
19 protons21 neutrons40 nucleons
18 protons22 neutrons40 nucleons
0 nucleons+1 charge
197 0 19780 1 79Hg e Au
80 protons117 neutrons197 nucleons
79 protons118 neutrons197 nucleons
0 nucleons-1 charge
ProcessProcess SymbolSymbol Change Change in atomic in atomic numbernumber
Change Change in mass in mass numbernumber
Change Change in in neutron neutron numbernumber
Alpha Alpha αα -2-2 -4-4 -2-2
BetaBeta ββ-- +1+1 00 -1-1
GammaGamma γγ 00 00 00
PositronPositron ββ++ -1-1 00 +1+1
Electron Electron capturecapture
E.C.E.C. -1-1 00 +1+1
42 He
01e
01e
00
Summary of processes and notationSummary of processes and notation
Measuring decayMeasuring decay
Rates of radioactive decay vary enormously – Rates of radioactive decay vary enormously – from fractions of a second to billions of yearsfrom fractions of a second to billions of years
The rate equation is the same first order The rate equation is the same first order process process
Rate = k x NRate = k x N
ktN
N
o
ln
Half-life measures rate of decayHalf-life measures rate of decay
Concentration of Concentration of nuclide is halved after nuclide is halved after the same time interval the same time interval regardless of the initial regardless of the initial amount – Half-lifeamount – Half-life
Can range from Can range from fractions of a second to fractions of a second to millions of yearsmillions of years
0
0.2
0.4
0.6
0.8
1
1.2
0 5 10 15 20 25 30 35 40
Mathematical jiggery pokeryMathematical jiggery pokery
Calculating half life from decay rateCalculating half life from decay rate
t = 0, N = Nt = 0, N = Noo; t = t; t = t1/21/2, N = N, N = Noo/2/2
Calculating residual amounts from half lifeCalculating residual amounts from half life
kt
2ln2/1
2/1
2lnlnt
t
N
N
o
Magic numbersMagic numbers
Certain numbers of protons and/or neutrons Certain numbers of protons and/or neutrons convey unusual stability on the nucleusconvey unusual stability on the nucleus
2, 8, 20, 28, 50, 82, 1262, 8, 20, 28, 50, 82, 126 There are ten isotopes of Sn (Z=50); but only There are ten isotopes of Sn (Z=50); but only
two of In (Z=49) and Sb (Z=51)two of In (Z=49) and Sb (Z=51) Magic numbers are associated with the nuclear Magic numbers are associated with the nuclear
structure, which is analogous to the electronic structure, which is analogous to the electronic structure of atomsstructure of atoms
Correlation of neutron:proton ratio Correlation of neutron:proton ratio and decay processand decay process
Stability is not achieved in one step: Stability is not achieved in one step: products also decayproducts also decay
Here atomic number actually increases, but Here atomic number actually increases, but serves to reduce the neutron:proton ratioserves to reduce the neutron:proton ratio
Beta Beta particle emission occurs with particle emission occurs with neutronneutron-excess nuclei-excess nuclei Alpha Alpha particle emission occurs with particle emission occurs with protonproton-heavy nuclei-heavy nuclei
ePaTh 01
23491
23490
Radioactive series are complexRadioactive series are complex
The decay series from uranium-238 to lead-206. Each nuclide except for the last is radioactive and undergoes nuclear decay. The left-pointing, longer arrows (red) represent alpha emissions, and the right-pointing, shorter arrows (blue) represent beta emissions.
Energy changes and nuclear decayEnergy changes and nuclear decay
In principle there will be an energy associated In principle there will be an energy associated with the binding of nuclear particles to form a with the binding of nuclear particles to form a nucleusnucleus
Experimentally demanding!Experimentally demanding!
HenH 42
10
11 22
Use Einstein’s relationshipUse Einstein’s relationship
E = mcE = mc22
Consider the He nucleus:Consider the He nucleus:Mass of individual particles = 4.03188 amuMass of individual particles = 4.03188 amuMass of He nucleus = 4.00150 amuMass of He nucleus = 4.00150 amuMass loss = 0.03038 amuMass loss = 0.03038 amu The “lost” mass is converted into energy – the The “lost” mass is converted into energy – the
bindingbinding energy, which is released during the nuclear energy, which is released during the nuclear processprocess
For the example above, the energy is 2.73 x 10For the example above, the energy is 2.73 x 1099 kJ/mol kJ/mol
Inter-changeability of mass and Inter-changeability of mass and energyenergy
Loss in mass equals energy given outLoss in mass equals energy given out
E = mcE = mc22
Tiny amount of matter produces masses of energy:Tiny amount of matter produces masses of energy:
1 gram 1 gram 10 101414 J J Energy Energy and and mass are conserved, but can be inter-mass are conserved, but can be inter-
changedchanged Binding energy per nucleon presents the total binding Binding energy per nucleon presents the total binding
energy as calculated previously per nuclear particleenergy as calculated previously per nuclear particle Usually cited in eV, where 1 eV = 1.6x10Usually cited in eV, where 1 eV = 1.6x10-19-19JJ
Average mass per nucleon varies with atomic number Average mass per nucleon varies with atomic number Average Nuclear
Binding En/Nucleon
0
1
2
3
4
5
6
7
8
9
10
0 50 100 150 200 250
Mass Number (A)
MeV
H
HeFe
UNucleon mass
The binding energy per nucleon for the most stable isotope of each naturally occurring element. Binding energy reaches a maximum of 8.79 MeV/nucleon at 56Fe. As a result, there is an increase in
stability when much lighter elements fuse together to yield heavier elements up to 56Fe and when much heavier elements split apart to yield lighter elements down to 56Fe, as indicated by the arrows.
Mass changes in chemical reactions?Mass changes in chemical reactions?
Conservation of mass and energy means that Conservation of mass and energy means that energy changes in chemical processes involve energy changes in chemical processes involve concomitant changes in massconcomitant changes in mass
Magnitude is so small as to be undetectableMagnitude is so small as to be undetectable A A ΔΔH of -436 kJ/mol corresponds to a weight H of -436 kJ/mol corresponds to a weight
loss of 4.84 ng/molloss of 4.84 ng/mol
Fission and fusion: ways to harness Fission and fusion: ways to harness nuclear energynuclear energy
Attempts to grow larger Attempts to grow larger nuclei by bombardment nuclei by bombardment with neutrons yielded with neutrons yielded smaller atoms instead.smaller atoms instead. Distorting the nucleus causes the Distorting the nucleus causes the
repulsive forces to overwhelm repulsive forces to overwhelm the attractivethe attractive
The foundation of The foundation of nuclear energy and the nuclear energy and the atomic bombatomic bomb
Nuclear fissionNuclear fission
Nuclear fission produces nuclei with lower Nuclear fission produces nuclei with lower nucleon massnucleon mass
One neutron produces three: the basis for a One neutron produces three: the basis for a chain reaction – explosive potentialchain reaction – explosive potential
Many fission pathways – 800 fission products Many fission pathways – 800 fission products from U-235from U-235
nBaKrUn 10
14256
9136
23592
10 3
Chain reactions require rapid multiplication Chain reactions require rapid multiplication of speciesof species
Nuclear fusion: opposite of fissionNuclear fusion: opposite of fission
Small nuclei fuse to yield larger onesSmall nuclei fuse to yield larger ones Nuclear mass is lostNuclear mass is lost
Example is the Example is the deuterium – tritiumdeuterium – tritium reactionreaction
About 0.7 % of the mass is converted into About 0.7 % of the mass is converted into energyenergy
+ E
The sun is a helium factoryThe sun is a helium factory
The sun’s energy derives from the fusion of hydrogen The sun’s energy derives from the fusion of hydrogen atoms to give heliumatoms to give helium
eeHeH 01
01
42
11 224
201
01 ee
Fusion would be the holy grail if...Fusion would be the holy grail if...
The benefitsThe benefits:: High energy output (10 x more output than fission)High energy output (10 x more output than fission) Clean products – no long-lived radioactive waste or toxic heavy metalsClean products – no long-lived radioactive waste or toxic heavy metals
The challenge:The challenge: Providing enough energy to start the process – positive Providing enough energy to start the process – positive
charges repelcharges repel Reproduce the center of the sun in the labReproduce the center of the sun in the lab
Fusion is demonstrated but currently consumes Fusion is demonstrated but currently consumes rather than produces energyrather than produces energy
Useful radioisotopes and half-livesUseful radioisotopes and half-livesRadioisotopeRadioisotope SymbolSymbol
RadiationRadiationHalf-Half-lifelife
UseUse
TritiumTritium ββ-- 12.33 y12.33 y Biochemical tracerBiochemical tracer
Carbon-14Carbon-14 ββ-- 5730 y5730 y Archeologocial datingArcheologocial dating
Phosphorus-32Phosphorus-32 ββ-- 14.25 d14.25 d Leukemia therapyLeukemia therapy
Potassium-40Potassium-40 ββ-- 1.28 x 101.28 x 1099 y y Geological datingGeological dating
Cobalt-60Cobalt-60 ββ--,,γγ 5.27 y5.27 y Cancer therapyCancer therapy
Technecium-Technecium-9999mm
γγ 6.01 h6.01 h Brain scansBrain scans
Iodine-123Iodine-123 γγ 13.27 h13.27 h Thyroid therapyThyroid therapy
Uranium-238Uranium-238 αα 7.04 x 107.04 x 1088 y y Power generationPower generation
31 H146C3215 P4019 K6027Co9943Tc
12353 I
23892U
Radioisotopes have wide range of usesRadioisotopes have wide range of uses
H-3 Triggering nuclear weapons, luminous paints and H-3 Triggering nuclear weapons, luminous paints and gauges, biochemical tracergauges, biochemical tracer
I-131 Thyroid treatment and medical imagingI-131 Thyroid treatment and medical imaging Co-60Co-60 Food irradiation, industrial applications, Food irradiation, industrial applications,
radiotherapyradiotherapy Sr-90 Tracer in medical and agricultural studiesSr-90 Tracer in medical and agricultural studies U-235/238 Nuclear power generation, depleted U U-235/238 Nuclear power generation, depleted U
used in weapons and shieldingused in weapons and shielding Am-241 Thickness and distance gauges, smoke Am-241 Thickness and distance gauges, smoke
detectorsdetectors
Nuclear power prevalent in EuropeNuclear power prevalent in Europe
Different units for measuring Different units for measuring radiationradiation
UnitUnit Quantity measuredQuantity measured DescriptionDescription
Becquerel (Bq)Becquerel (Bq) Decay eventsDecay events Amount of sample that Amount of sample that undergoes 1 undergoes 1 disintegration/sdisintegration/s
Curie (Ci)Curie (Ci) Decay eventsDecay events Amount of sample that Amount of sample that undergoes 3.7 x 10undergoes 3.7 x 101010 disintegrations/sdisintegrations/s
Gray (Gy)Gray (Gy) Energy absorbed per Energy absorbed per kg tissuekg tissue
1 Gy = 1J/kg tissue1 Gy = 1J/kg tissue
RadRad Energy absorbed per Energy absorbed per kg tissuekg tissue
1 rad = 0.01 Gy1 rad = 0.01 Gy
Sievert (Sv)Sievert (Sv) Tissue damageTissue damage 1 Sv = 1 J/kg1 Sv = 1 J/kg
RemRem Tissue damageTissue damage 1 rem = 0.01 Sv1 rem = 0.01 Sv
Radiation is nastyRadiation is nasty
Dose (rem)Dose (rem) Biological effectsBiological effects
0 – 250 – 25 No detectable effectsNo detectable effects
25 – 10025 – 100 Temporary decrease in white Temporary decrease in white blood cell countblood cell count
100 – 200100 – 200 Nausea, vomiting, longer-term Nausea, vomiting, longer-term decrease in white blood cell countdecrease in white blood cell count
200 – 300200 – 300 Vomiting, diarrhea, loss of Vomiting, diarrhea, loss of appetiteappetite
300 – 600300 – 600 Vomiting, diarrhea, hemorrhaging, Vomiting, diarrhea, hemorrhaging, eventual death in some caseseventual death in some cases
> 600> 600 Death in nearly all casesDeath in nearly all cases