arXiv:nucl-ex/0608039 v1 22 Aug 2006 Study of Dissipative Collisions of 20 Ne (∼7-11 MeV/nucleon) + 27 Al Aparajita Dey, C. Bhattacharya, S. Bhattacharya, T. K. Rana, S. Kundu, K. Banerjee, S. Mukhopadhyay, S. R. Banerjee, D. Gupta, R. Saha Variable Energy Cyclotron Centre, Sector - 1, Block - AF, Bidhan Nagar, Kolkata - 700 064, India. The inclusive energy distributions of complex fragments (3 ≤Z ≤ 9) emitted in the reactions 20 Ne (145, 158, 200, 218 MeV) + 27 Al have been measured in the angular range 10 o - 50 o . The fusion- fission and the deep-inelastic components of the fragment yield have been extracted using multiple Gaussian functions from the experimental fragment energy spectra. The elemental yields of the fusion-fission component have been found to be fairly well exlained in the framework of standard statistical model. It is found that there is strong competition between the fusion-fission and the deep-inelastic processes at these energies. The time scale of the deep-inelastic process was estimated to be typically in the range of ∼ 10 −21 - 10 −22 sec., and it was found to decrease with increasing fragment mass. The angular momentum dissipations in fully energy damped deep-inelastic process have been estimated from the average energies of the deep-inelastic components of the fragment energy spectra. It has been found that, the estimated angular momentum dissipations, for lighter fragments in particular, are more than those predicted by the empirical sticking limit. PACS number(s): 25.70.Jj, 24.60.Dr, 25.70.Lm I. INTRODUCTION Complex fragment emission in heavy-ion induced reactions involving light nuclei (A target + A projectile < ∼ 60) at bombarding energies well above the Coulomb barrier has been studied quite extensively in the recent years [1–16] to understand the origin of fragment emission and the role of underlying dynamics. It is well known that different types of reaction mechanisms contribute to fragment emission at different energy regions. At bombarding energies near the Coulomb barriers, complete fusion (CF) process is the dominant reaction mechanism. At higher energies, this process is limited by the contributions of other competing processes, such as quasi-elastic (QE) and deep-inelastic (DI) collisions. The CF cross-section increases with incident energy at lower energies and reaches a near-saturation value at higher energies. On the other hand, non-fusion processes become increasingly dominant at higher energies. Thus, the fragments emitted in light heavy-ion collisions at energies well above the Coulomb barrier may have different origins, which extend from partially relaxed processes, such as, quasi-elastic (QE) collision / projectile break-up [17,18], deep- inelastic (DI) transfer and orbiting [8,9,19–22], to fully relaxed fusion-fission (FF) [23–28] process. In some cases, the structure of the nuclei has also been found to play an important role. Therefore, the characterization of the origin of fragments is of utmost importance to extract information on the relaxation of various degrees of freedom (energy and angular momentum dissipation, for example) in heavy ion collision in this energy domain. However, for light systems, the distinction between different reaction mechanisms, FF and orbiting or DI processes in particular, is very difficult as there is strong overlap in the elemental distributions of the fragments emitted in these processes. Binary decay of the light composite system 47 V has been investigated quite extensively in the past years. In some cases, (where the composite system 47 V was produced through inverse kinematical reactions, like, 35 Cl + 12 C [4,5,29,30], 31 P+ 16 O [7], 23 Na + 24 Mg [6]) it was found that the 47 V composite system deexcites statistically. In these cases, the emitted fragment yields show 1/sin Θ c.m. - like angular dependence and have angle-independent mean total kinetic energy (TKE) values in agreement with the decay of a fully energy equilibrated composite system. The experimental cross-sections are well explained with the predictions of the extended Hauser-Feshbach method (EHFM) [25] and thus suggest a fusion-fission origin. It was further concluded that orbiting process [9] does not play any significant role in the decay of 47 V composite system. On the other hand, studies on the same system, produced through direct kinematical reactions ( 20 Ne + 27 Al [31–33]), showed that the angular distributions of fully damped fragments are forward peaked and fall off faster than 1/sinΘ c.m. , which are characteristic of DI processes. Subsequently, assuming the fragment yield to be of DI origin (and assuming the sticking limit for the angular momentum dissipation), a highly elongated configuration for the 20 Ne + 27 Al di-nuclear system was conjectured [32]). It is clearly evident from the above that some degree of ambiguity prevails over the interpretation of the fragment yield data in the decay of 47 V composite system. To resolve the ambiguity, it is necessary to understand the roles played by various competing processes in this energy regime. For example, there is strong competition between FF and DI processes at these energies, which should be decifered properly to extract meaningful information about the reaction mechanism. In recent years, we have developed a scheme for the decomposition of FF and DI components of the fragment yield [14,20] in order to study systematically the competition between FF and DI processes in light heavy ion collisions at energies well above the barrier; In this paper, we report an experimental study of fragment 1