Fusion Light Nuclei Light nuclei have relatively high

Fusion

Light Nuclei ] Light nuclei have relatively high rest masses. • • • H-1: 1. 008665 u H-2: 2. 014102 u He-3: 3. 016029 u He-4: 4. 002603 u C-12: 12. 000000 u ] Energy is available compared to heavy mass nuclei. • Si-28: 27. 976926 • Fe-56: 55. 934942

The Sun ] The energy output of the sun is inconsistent with chemical processes. • Chemical bonds too weak ] Gravitational contraction would have to be too fast. • Sun would last 100 My ] Nuclear fusion of light nuclei explains the energy output.

Nuclear Fusion ] Electric charge causes positive nuclei to repel. ] At high temperatures nuclei get close enough for the nuclear force to pull them together. heavier nucleus starting nuclei fusion other particles

Proton-Proton Cycle ] Fusion takes place two particles at a time. ] Step 1: two protons form a deuterium nucleus with positron and a neutrino. hydrogen nuclei with one proton each neutron neutrino: related to electrons by the weak nuclear force positron: positive charged electron; annihilates to form photons electron photons

Proton Fusion 2 ] Step 2: A deuterium nucleus absorbs a proton and becomes helium-3. ] The helium-3 is in an excited state and emits a photon when it goes to a ground state. neutron proton photon

Proton Fusion 3 ] Step 3: Two helium-3 nuclei collide. ] They rearrange particles so that very stable helium-4 is formed with two extra protons spit out. proton helium-3 helium-4 next

Excess Energy ] The fusion reaction in a star is exothermic. • H-1: 1. 0078 u • He-4: 4. 0030 u ] This is less than 4 hydrogen masses so there is energy released. ] Find the mass energy difference. • Q = 4(1. 0078)-(4. 0030) • Q = 0. 0282 u ] Convert to Me. V. • Q = (0. 0282 u)(931. 5 Me. V/u) • Q = 26. 27 Me. V ] This is per single fusion reaction.

Thermonuclear Blast ] Fusion of deuterium into helium is exothermic. ] The high temperature of an uncontrolled fission reaction can overcome the nuclear charge. • A fission bomb to ignite fusion device next