Mechanisms and time-scales in breakup of 6,7Li

Abstract

The current and near future of nuclear physics is being directed by the availability of radioactive ion beams and exotic nuclei near the neutron drip line. Reactions with the weakly-bound but stable {u2076}Li and {u2077}Li nuclei were thus studied as a testbed for relating nuclear structure of weakly-bound and unstable nuclei to nuclear reaction outcomes within a coherent framework, an important goal in nuclear reaction theory. Coincidence measurements of charged fragments produced from the reactions of {u2076},{u2077}Li with high-Z targets were carried out, at sub-barrier energies, using a large-area position sensitive detector array at back-angles. The wide and continuous angular coverage of the detector, and the choice of sub-barrier energies, were crucial in obtaining clear conclusions. For the reactions with {u2076}Li, the observed {u03B1} + d and {u03B1} + p coincidences show direct cluster breakup ({u2076}Li {u21E2}{u03B1} + d), but also large yields of breakup triggered by n-transfer ({u2076}Li {u21E2}{u2075}Li {u21E2}{u03B1} + p). Coincidences between {u03B1} + {u03B1} were also observed indicating breakup triggered by d-transfer ({u2076}Li {u21E2}{u2078}Be {u21E2}{u03B1} + {u03B1}). For the {u2077}Li-induced reactions, direct cluster breakup ({u2077}Li {u21E2}{u03B1} + t) was observed, but more probable was p-pickup by the projectile resulting in {u03B1} + {u03B1} coincidences ({u2077}Li {u21E2}{u2078}Be {u21E2}{u03B1} + {u03B1}). The measured relative energy gives an indication of the time-scales for each breakup mode, allowing a comparison with the time-scales for direct nuclear reactions and fusion. These measurements demonstrate that the reaction dynamics and outcomes are determined not only by the properties of the two colliding nuclei, but also by the ground-state and excited state properties of their neighbours. This is a key insight for understanding and predicting reactions of weakly-bound nuclei near the limits of nuclear existence. Furthermore, the results suggest that in sub-barrier collisions of {u2076}Li and {u2077}Li with all but the lightest nuclei, the most likely nuclear reactions will lead to breakup of the projectile-like nucleus, forming elements lighter than Li. This needs to be tested experimentally for collisions with much lighter nuclei, and possible implications for lithium abundances in cosmological processes investigated.