Insights into the mechanisms and time-scales of breakup of 6,7Li

D.H Luong,M Dasgupta,D.J Hinde,R Du Rietz,R Rafiei,C.J Lin,M Evers,A Diaz-Torres

doi:10.1016/j.physletb.2010.11.007

D.H Luong, M Dasgupta + Show 6 more

Open Access

https://doi.org/10.1016/j.physletb.2010.11.007

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Abstract

Using a back-angle detector array covering 117° to 167°, coincidence measurements of breakup fragments at sub-barrier energies have enabled the complete characterisation of the breakup processes in the reactions of 6,7Li with 208Pb. Those breakup processes fast enough (∼10−22 s) to affect fusion are identified through the measured relative energy of the two breakup fragments. The majority of these prompt breakup events are triggered by transfer of a neutron from 6Li, and of a proton to 7Li. These mechanisms, rather than breakup following direct projectile excitation, should thus be responsible for the majority of the ∼30% suppression of complete fusion observed at above-barrier energies. Breakup characteristics thus depend both on the properties of the initial nucleus and its neighbours. Quantitative modelling of this two-step process will require development of a complete reactions model, relevant for reactions involving both α-cluster nuclei, and exotic nuclei near the neutron and proton drip-lines.

Highlights

The stability of both the helium atom and its nucleus results from the filling of the lowest energy quantum state by a pair of electrons, or pairs of protons and neutrons respectively.This stability can cause nuclei to behave as though they contain αparticles
This work aims to provide a complete picture of breakup in reactions of weakly-bound 6,7Li nuclei, and to identify experimentally those breakup processes fast enough to affect fusion
Unlike excited states of the heavy reaction partner, which typically decay by emission of γ -rays in >10−12 s, breakup of the light partner must occur before γ -ray emission, the energy of the excited states appears in the fragment kinetic energies, and so cannot be determined from the Q -spectra

Summary

Introduction

The stability of both the helium atom and its nucleus (the αparticle) results from the filling of the lowest energy quantum state by a pair of electrons, or pairs of protons and neutrons respectively. This stability can cause nuclei to behave as though they contain αparticles. Current quantum models of nuclear reactions are of limited use as they cannot separate complete and incomplete fusion, cannot model the effect of breakup on complete fusion [20,21] For these experimental and theoretical reasons, a quantitative understanding of breakup and the suppression of complete fusion has not yet been attained. This work aims to provide a complete picture of breakup in reactions of weakly-bound 6,7Li nuclei, and to identify experimentally those breakup processes fast enough to affect fusion

Experimental details and results

Breakup time-scale

Interpretation of measured Erel

Conclusion