Direct measurement of the breakup process

NUCLEAR REACTIONS

Direct measurement of the breakup process

F. A. Souza; R. Liguori Neto; M. M. de Moura; M. G. Munhoz; A. A. P. Suaide; E. M. Szanto; J. Takahashi; A. Szanto de Toledo; N. Carlin

Instituto de Física - Universidade de São Paulo, Departamento de Física Nuclear, Laboratório Pelletron, Caixa Postal 66318, 05315-970, São Paulo - SP, Brazil

ABSTRACT

In order to better understand the breakup contribution to fusion, we perfomed light particle coincidence measurements for the systems ^6,7Li+¹²C,⁵⁹Co at energies near and above the barrier. A three body kinematics analysis is performed with the objective of separating the contributions of different reaction mechanisms. For small angular differences between the light particles, the sequential and direct breakup seem to dominate. On the other hand, for large detector separations, a sequential decay following transfer is likely to dominate.

The recent availability of radioactive nuclei beams motivated the investigation of fusion reactions involving weakly bound nuclei. The cross section enhancement generally observed at sub-barrier energies is understood in terms of dynamical processes arising from couplings to collective inelastic excitations of the target and/or projectile [1]. However, in the case of reactions where at least one of the colliding nuclei has a sufficient low binding energy so that breakup becomes an important process, conflicting experimental and theoretical results are reported in recent papers [2–10].

The ^6,7Li + ¹²C,⁵⁹Co fusion reactions are used to investigate the effect of breakup on the fusion cross section [11]. These measurements help to establish the influence of the projectile breakup on the fusion process at near-barrier energies and contribute to the determination of how the mass of the target affects the process, as well as the influence of the incomplete fusion yield.

It is important to have clear the reference used when an enhancement and/or a suppression is defined[5–7]. A final tuning for the coupling of the breakup channel, as well as the correct description of the reaction dynamics, requires the explicit measurement of precise elastic scattering data as well as yields leading to breakup itself [12–14].

The identification of the breakup products has been achieved measuring the three body final state correlations. Coincidence data displayed in Fig. 1a show the identification of the process Q-value in order to gate exclusively on the projectile breakup channel. Furthermore, the system excitation energy as well as the projectile fragment relative energy (see Fig. 1b) are used to identify the exit channel with no ambiguity. Based on those filters, angular correlations (see Fig. 2) are obtained to identify the several processes. This is complemented by measurements of relative energy of the fragments using different rest frame references (target like, projectile, target + fragment) in order to disentangle the contribution of breakup, incomplete fusion and/or transfer-reemission process. These experimental results are compared to three body kinematics calculations. As an example Fig. 3 displays the relative energy between a – d for the ⁶Li breakup on a ¹²C target. The region between the dashed vertical lines represent the angular interval in which the three body kinematics predict a constant relative energy. The fact that this experimental trajectory is not constant out of this region indicates the presence of other processes. The fact that the yield for coincidences is not negligible out of this region (see Figs. 2) indicates that the other processes producing, as in this case a – d coincidences, are relevant and must be identified. Fig. 4 shows the a energy as a function of the d emission angle. We notice that the a energy is constant on the angular range which does not correspond to the breakup contribution. This constant energy suggests the presence of a binary intermediate process ^6,7Li+⁵⁹Co ® a+⁶¹Ni^*® a+d+⁵⁹Co. From the average a energy, we determined the excitation energy of the intermediate nucleus ⁶¹Ni, as being 25MeV. Similar data were taken for ^6,7Li+¹¹⁵In. This procedure in unfolding the several light particle emission processes has not been exploited so far in the literature.

Acknowledgments

This work was supported by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)-Brasil.

Received on 15 November, 2004

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Publication Dates

• Publication in this collection
  07 Nov 2005
• Date of issue
  Sept 2005

History

• Received
  15 Nov 2004