nuclear forces and radioactivity
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Nuclear forces and Radioactivity. The nucleus is a competition between opposing forces Radioactivity is a result of imbalance between the forces. Learning objectives. Describe the basic forces and particles involved in nuclear structure - PowerPoint PPT PresentationTRANSCRIPT
Nuclear forces and Nuclear forces and RadioactivityRadioactivity
The nucleus is a competition between The nucleus is a competition between opposing forcesopposing forces
Radioactivity is a result of imbalance Radioactivity is a result of imbalance between the forcesbetween the forces
Learning objectivesLearning objectives Describe the basic forces and particles involved in Describe the basic forces and particles involved in
nuclear structurenuclear structure Describe principles behind nuclear decay and Describe principles behind nuclear decay and
radioactivityradioactivity Describe the particles emitted in nuclear decayDescribe the particles emitted in nuclear decay Define half-life and apply the concept to simple Define half-life and apply the concept to simple
problemsproblems Describe the relationship between energy and matterDescribe the relationship between energy and matter Identify the differences between nuclear fission and Identify the differences between nuclear fission and
fusion and their importance in generation of nuclear fusion and their importance in generation of nuclear powerpower
Forces act in opposing directionsForces act in opposing directions 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
Stability is not achieved in one step: Stability is not achieved in one step: products also decayproducts also decay
Atomic number Atomic number increasesincreases Neutron:proton ratio Neutron:proton ratio decreasesdecreases
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
Beta particle emitted
Summary of types of radiationSummary of types of radiation
Radioactive series are complexRadioactive series are complex
The decay series from uranium-238 to lead-206Each nuclide is radioactive and undergoes nuclear decayLeft-pointing longer arrows (red) are alpha emissions
M and Z decreaseRight-pointing shorter arrows (blue) are beta emissions
M constant, Z increases
Half-life measures rate of decayHalf-life measures rate of decay Concentration of nuclide is halved after the Concentration of nuclide is halved after the
same time interval regardless of the initial same time interval regardless of the initial amount – Half-lifeamount – Half-life
Can range from fractions of a second to Can range from fractions of a second to millions of yearsmillions of years
Fission and fusion: Fission and fusion: Radical nuclear engineeringRadical nuclear engineering
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 nucleon Nuclear fission produces nuclei with lower nucleon
massmass
One neutron produces three: the basis for a chain One neutron produces three: the basis for a chain reaction – explosive potentialreaction – explosive potential
Neutrons must be obtained from other nuclear Neutrons must be obtained from other nuclear processes such as bombardment of aluminum with processes such as bombardment of aluminum with alpha particlesalpha particles
nBaKrUn 10
14256
9136
23592
10 3
Chain reactions require rapid multiplication Chain reactions require rapid multiplication of speciesof species
Chain reactions have the potential Chain reactions have the potential for nuclear explosionsfor nuclear explosions
Bomb requires creation of high rate of Bomb requires creation of high rate of collisions in small volumecollisions in small volume
How to achieve this at the desired time in a How to achieve this at the desired time in a controlled manner?controlled manner?
The importance of U-235The importance of U-235 U-235 is less than 1 % of naturally occurring U-235 is less than 1 % of naturally occurring
uranium, but undergoes fission with much uranium, but undergoes fission with much greater efficiency than U-238greater efficiency than U-238
Fission can follow many paths: over 200 Fission can follow many paths: over 200 different isotopes have been observeddifferent isotopes have been observed
Each process produces more neutrons than it Each process produces more neutrons than it consumesconsumes
nBaKrUn 10
14256
9136
23592
10 3
nTeZrUn 10
13752
9040
23592
10 2
Enrichment of uraniumEnrichment of uranium Weapons grade uranium requires a high Weapons grade uranium requires a high
concentration of U-235concentration of U-235 This is achieved by isotope separationThis is achieved by isotope separation The lighter U-235 diffuses more rapidly than The lighter U-235 diffuses more rapidly than
the heavier U-238 in the gas phase as UFthe heavier U-238 in the gas phase as UF66
Total of mass and energy is Total of mass and energy is conserved but are inter-changeableconserved but are inter-changeable
Fission: combined mass of smaller nuclei is less than Fission: combined mass of smaller nuclei is less than the original nucleusthe original nucleus
A A B + C B + C MMAA > M > MBB + M + MCC
Loss in mass equals energy released:Loss in mass equals energy released:E = mcE = mc2 2 (Einstein’s relation)(Einstein’s relation)
Smaller nuclei are more stableSmaller nuclei are more stable Fission of U-235: 0.08 % of mass is converted into Fission of U-235: 0.08 % of mass is converted into
energyenergy
Comparison of nuclear and chemical Comparison of nuclear and chemical energy sourcesenergy sources
Conversion of tiny amount of matter into Conversion of tiny amount of matter into energy produces masses:energy produces masses:
1 gram 1 gram 10 101414 J J Chemical process:Chemical process:
1 gram fuel produces 101 gram fuel produces 1033 J J Nuclear process:Nuclear process:
1 gram uranium at 0.08 % produces 101 gram uranium at 0.08 % produces 101111 J J
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 deuterium – tritium Example is the deuterium – 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 The sun’s energy derives from the fusion of
hydrogen atoms to give heliumhydrogen atoms 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