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

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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

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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

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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

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10 3

nTeZrUn 10

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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

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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