Cluster Structure of Light Nuclei: Use of polarized beams to probe the alpha particle transfer reaction (7Li,t)
K. W. Kemper, O. Momotyuk, D. Robson (FSU), N. Keeley (Saclay)
J. Millener (BNL) and K. Rusek (Warsaw
Knowledge of the (α,γ) capture strengths for reactions such as 15O(α,γ) are important for the understanding of the origin of light elements. Often the states into which capture takes place have small α widths making their determination extremely difficult. Numerous new studies are proposing to make use of the (6Li,d) reaction in inverse kinematics with radioactive beams to measure α widths, bringing back all the previous uncertainties inherent in obtaining absolute α strengths with this reaction. We carried out an extensive study of the 12C(6Li,d) reaction with a polarized 6Li beam in which all analyzing powers were measured at the two 6Li energies of 34 and 50 MeV. Our analyzing powers show for the first time (1) that the final state interaction between the deuteron and the residual nucleus 16O is extremely strong and greatly affects the extracted α strength. The measured analyzing powers are very sensitive to the presence of multi-step processes. Transitions to the 6.92 MeV 2+ and 10.35 MeV 4+ states are well described by simple direct α transfer whereas those to the 0+ ground and the 3- 6.13 MeV states have significant multistep contributions. This work also showed that multistep transfers become more important as the bombarding energy is increased. Our conclusion based on this work is that the use of (6Li,d) to obtain weak α capture strengths is extremely uncertain.
It has been known for sometime (2) that the 12C(7Li,t) reaction has much greater selectivity in the final states populated in 16O than does (6Li,d) and presumably is a much cleaner “α” transfer reaction. It should be the reaction of choice for use in radioactive beam studies such as 7Li(15O, 19Ne) t. The original reason for its lack of use was the need to carry out finite-range DWBA calculations to extract α strengths, however with modern computing capabilities this should no longer be an impediment to its use.We are proposing to measure all analyzing powers for the reactions 12C and 16O(7Li,t) during the coming grant period. Our observation (3) of a direct correlation between the analyzing powers TT10 and TT30 for elastic and inelastic scattering and also for (7Li, α) proved that these reactions are peripheral and well localized in the reaction plane, exactly the conditions needed for one-step processes. The case for (7Li,t) being a direct α-particle transfer would be considerably strengthened were we to observe this same correlation. A further test of this reaction has been derived assuming no spin-dependent distortions for a 0+→0+ transfer. The prediction is that TT20 = 0.5 for all angles and energies. Also, the maximum TT10 analyzing power is 1.12, again allowing a further proof that this reaction can be used for determining α spectroscopic strengths if this limit is observed.
It is also proposed to measure cross section and analyzing powers for the 6Li(7Li,t) reaction to search for the (sd)4 configurations in 10B. This work will be carried out in conjunction with John Millener of BNL and is part of an effort to fully understand the nature of all states in the mass 10 system. A preliminary run shows distinctive analyzing power shapes and with Millener’s spectroscopic amplitudes it should be possible to characterize all observed states.
(1) N. Keeley et al ,Phys. Rev. C67, 044604 (2003)
(2) P. T. Debevec et al Phys. Rev. C9, 2451 (1974); M. E. Cobern et al Phys. Rev. C14, 491 (1976); F. D. Becchetti et al Nucl. Physi. A305,313 (1978).
(3) E. E. Bartosz et al Phys. Lett. B488, 138 (2000).