Transcript Fusion
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. neutrino: related to electrons by the hydrogen nuclei with one proton neutron weak nuclear force each 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 MeV. • Q = (0.0282 u)(931.5 MeV/u) • Q = 26.27 MeV 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