Multiplicities For Evaporative 4He Emission In Heavy Ion Reactions: Relationships To Spin And Lifetime Expectancy Of The Composite Nucleus

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"We are grateful to the staff of the SuperHILAC for providing us with beams of40Ar and56Fe and also for other assistance during the experiment. This work has been supported by the Division of Nuclear Physics, Office of High Energy and Nuclear Physics, U.S. Department of Energy. One of us (D.Ø.E.) appreciates a travel grant from the Norwegian Council for Science and the Humanities."


Multiplicities are compared for4He evaporation in reactions of40Ar and56Fe projectiles (8.5 MeV/u) with withnatAg and238U targets. Coincidence requirements involving fusion-fission and projectile-like fragments have been used to select separate entrance-channel spin zones. Evaporation-like4He emission from the composite nuclei is shown to be large for mean spins ≦100ℏ, and to decrease dramatically with increasing spin. Coincidence measurements between two4He particles have also been studied for40Ar+natAg. This requirement is shown to select reactions that lead to evaporation residues and therefore to a zone of relatively low spin. The shapes of the4He spectra provide an interesting probe of the mechanism and also of the thermodynamic properties of hot nuclei.

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