Lecture 14

Fission and Fusion. Elementary Particles.

• Nuclear Fission

• Nuclear Fusion

• Fundamental Interaction (Forces)

• Elementary Particles

Nuclear Fission

In 1939 was discovered that a 23592U nucleus undergoes

fission when struck by a neutron.235

92U + n 23692U 94

38Sr + 14054 Xe + -rays + n

Products of fission reactions are radioactive with long half-lives.

23 neutrons are released in a fission act and can cause other atoms of uranium to split a chain reaction occurs.A chain reaction was first demonstrated in 1942 (Fermi).

Chain Fission ReactionIf not each neutron produces a U split, the reaction dies.If each neutron stimulates 1 split, the reaction is sustained.If more than 1 neutron from each fission act causes other fissions, the reaction is out of control - explosion.

Natural uranium contains 0.7% of the fissionable isotope U-235.The rest is U-238 which does not undergo fission.It captures fast neutrons from the reaction, but not slow ones.If the fast neutrons are slowed down, they will produce more fissions.

Plutonium

Fissionable nuclides can be created from nonfissionable ones by absorbing neutrons.

238U + n 239U 239Np + e 239Pl

Neptumium (93Np) and Plutonium (94Pl) are transuranium elements.

Plutonium is produced in a uranium-fueled reactor and can serve a reactor fuel itself.

Nuclear FusionFusion of small nuclei to form large ones can give out even more energy per unit mass of starting material.

Three condition for a fusion reactor:High temperature (100 million K) - fast collisions High concentration of the nuclei - frequent collisionsLong time of operation - positive energy balance

21H + 3

1H 42He + n + 17.6 MeV

2 practical approaches to fusion:Strong magnetic fields hold the particles together.Using energetic beams to both heat and compress small DT-pellets.

Elementary particles

Protons, neutrons, and electrons are elementary particles.

Protons and neutrons consist of quarks, smaller particles.

Almost each elementary particle has an antiparticle, that has the same mass, but the electric charge of an opposite sign (e and e+, electron and positron).

When a particle and its antiparticle come together, they destroy each other (annihilation).The lost mass reappears as energy in the form of rays.

Fundamental InteractionsElementary particles interact with each other in 4 ways.

The strong interaction holds protons and neutrons together.The strong force acts over sizes of ~1015 cm.Electrons are not affected by the strong interaction.

The electromagnetic interaction, which gives rise to electric and magnetic forces between charged particles.It is responsible for the structure of matter.It is 100 times weaker than the strong interaction at short distances, but is unlimited in range and acts on electrons.

Fundamental InteractionsThe weak interaction, which affects all particles and helps determine the compositions of atomic nuclei (beta-decay).It acts over a shorter range than the strong interaction and 10 trillion times weaker.

The gravitational interaction, responsible for the attractive force one mass exerts on another.It dominates on a large scale and is the weakest on the smallest scales.

Leptons and Hadrons

All elementary particles fall into 2 broad categories with respect to their response to the strong interaction.

Leptons (light) are not affected and seem to be point particles with no internal structure (electron).

Hadrons (heavy) are affected by the strong interaction, have definite sizes (~1015 cm), and have structure (proton and neutron).

Quarks

Quarks are particles which make up hadrons.

Only 6 kinds of quarks are needed to account for all hadrons.

The proton, neutron, and heavier hadrons consist of 3 quarks.Quarks have fractional electric charges 1/3e and 2/3e.They do not seem to exist outside hadrons.All the evidence for quarks is indirect, but the theory correctly predicts new hadrons.

SummaryFission reactions are controllable, produce a lot of energy, and radioactive waste.

Fusion reactions produce even more energy, but much harder to control.

There are only 4 fundamental interactions between elementary particles: strong, weak, electromagnetic, and gravitational.

Quarks are building blocks for hadrons, but have not been detected yet.