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20- nuclear reactions (compound reactions (Branching ratios and widths…
20- nuclear reactions
compound reactions
general
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nucleons will try to rearrange themselves to find a stable state, passing through several configurations
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2-stage process comprising the formation of a relatively long-lived intermediate nucleus followed by decay
relatively slow reaction
time taken at least longer than it would take for the projectile to cross the diameter of the target nucleus
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intermediate state nucleus lifetime is at least as long as the reaction time, can be much longer
activation reaction- activated a nucleus, unstable isotope later decays
characteristics
Once intermediate state is formed, decay is independent of the mode of formation
decay probability to a particular final state does not depend on the creating reaction, just the amount of excitation energy it has
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evidence
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when uncertainty relation applied, a lifetime of ~10^-16 is implied, long
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(α,n) reactions
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use to make neutron source, AmBe
resonance
low e scattering
when most likely to get compound reactions, showiing up as peaks in xsection vs energy coresponding to compound states.
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high e scattering
smoother continuum, varies slowly with energy
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at A~20 10MeV, A~200 100keV transition
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elastic deflects projectile, inelastic other changes occur, e.g. projectile loses energy (gamma emission)
chances of a particular type of reaction occurring will likely depend on the kinetic energy of projectile