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Alpha capture reactions mainly important in stars

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Unformatted text preview: ide decays with a half-life of 10-16 sec. because • Without this process, entire universe would be only H and He… Step 1: Step 2: 4He + 4He 8Be + 4He Be 8Be Overall: Three helium nuclei (triple %) produce one carbon triple 12C +$ Fusion of Heavier Elements ! Alpha capture reactions • Mainly important in stars more massive than our Sun • Concentric shells inside the star are dominated by different fusion reactions, yielding products that are fused in the adjacent inward shell of gas • Produces elements up to the Fe-group (first row transition elements) • Examples: 12C + 4He 16O + 4He 52Fe + 4He 16O 20Ne 56Ni • Some products undergo fission due to nucleon bombardment, neutron capture… leads to a state of “nuclear statistical equilibrium” • Does not lead to elements heavier than Ni…. electrostatic repulsion between positively-charged nuclei and alpha particles dampens fusion Fusion of Elements Heavier than Ni ! ! Neutron capture, principally s-process: neutron flux in 2nd generation stars or red giants is low s-process: neutron enough that nuclides can undergo # decay between capture events enough • Can yield nuclides up to 209Bi Example step 1: 62Ni + 1n step 2: 63Ni Ni ! 63Ni +$ 63Cu + #- + " r-process: high neutron flux can allow sequential capture of many r-process: high neutrons before decay can occur • Occurs in final minutes of a red giant, during supernova detonation ! p-process: low-probability reactions where multiple protons are captured (by bombardment) simultaneously • Characteristic of supernova detonations • Yields particular isotopes such as 74Se, 92Mo Explaining Cosmic Abundances ! H and He are most abundant • ! Exponential decrease in abundance of first 50 elements • ! Caused by sluggishness of neutron capture reactions in normal stars Greater stability of nuclides with even-numbered numbers of protons and/or neutrons • ! Reflects decreased productivity of He capture processes Lower abundances of heavier elements • ! Because they were formed in the Big Bang Nucleons with paired spins have stronger nuclear binding affinities Low abundances of Li, Be, B • Because production processes tend to bypass these elements, also they are destroyed by nucleon bombardment inside the star...
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