Fragment emission studies in low energy light heavy-ion reactions

Fragment emission mechanisms have been studied in 12C on 12C and 13C on 12C reactions at same excitation energy. The inclusive energy distributions of the complex fragments (3≤ Z ≤ 5) emitted from the composite system have been measured in the angular range 140 to 360. The present experiments have been performed with the motivation to study the isotopic dependence of fragment yields in these two reactions. From the preliminary analysis, it has been observed that fragments are emitted from a completely equilibrated and long lived composite system for both 12C + 12C and 13C + 12C reactions. It has also been observed that the emission of neutron-rich fragments are more in 13C + 12C compared to 12C + 12C reaction.


Introduction
Study of fragment emission mechanisms for light heavyions (A pro j. + A target ≤ 60) collisions, at low energies (≤ 10 MeV/u), is a subject of great interest for many years [1].The origin of these fragments extends from quasielastic, deep-inelastic transfer and orbiting to fusion -fission (FF) processes.The occurrence of FF and orbiting like processes have been observed experimentally in some light as well as medium heavy ion collision at beam energy well above the barrier [1-7].The FF process appears to be less competitive in systems for which the number of open channels or available phase space in exit channel is less, however, a di-nuclear configuration survives for a longer period through an orbiting trajectory [2].For reactions involving α-clustered nuclei, e.g, 20 Ne + 12 C, 24 Mg + 12 C, 28 Si + 12 C, etc., enhancement in the yield and/or resonance-like excitation function in a few outgoing channels (around the entrance channel) have indicated the role played by deep inelastic orbiting process in fragments emission [3-6].In the FF mechanism, a completely equilibrated compound nucleus (CN) is formed, which decays into various exit channels.The decay probability is governed by the available phase space and barrier penetration probabilities for the respective decay channels.However, deep inelastic orbiting may be described in terms of the formation of a long-lived, di-nuclear molecular complex [7], which acts as a "door way to fusion" with a strong memory of the entrance channel.In this picture, the interacting ions are trapped in a more deformed configuration than that of the compound nucleus (trapped in the pocket of the ion-ion interaction potential due to coma e-mail: tapan@vecc.gov.inbined effects of Coulomb and centrifugal barriers).Both orbiting and fusion-fission processes occur on similar time scales.In addition to that, for the light heavy-ion systems, the shapes of the orbiting di-nuclear complexes are quite similar to the saddle and scission shapes obtained in course of evolution of the FF process.Therefore, it is difficult to differentiate the signatures of the two processes.Study of fragments emission mechanism is considered to be an important tool to study such entrance channel effects; however, such data are scarce at low energies.Here we report our measurements of fragments emission in 12 C (80 MeV) on 12 C and 13 C (78.5MeV) on 12   [2] and most likely to form long lived di-nuclear type composite system.In a recent study by Morelli et al. [8], residual deviations from the statistical behaviour have been seen in two specific exit channels (carbon with 3α's and oxygen with 2 α's channels) for the reaction 12 C + 12 C, which have tentatively been assigned due to the presence of direct reactions and/or α-clustering effects.Anomalously high branching ratios with respect to the statistical model calculation for these channels also have been observed [9].The present experiments have been performed with the motivation to see the isotopic dependence of fragments yield in these two reactions.

Experimental details
The experiments have been performed at Pelletron-Linac facility, Mumbai, using 80 MeV 12 C and 78.5 13 C ion beams on 12 C target (thickness ∼70µg/cm 2 ).The emitted fragments have been identified using two telescopes, each consisting of ∼ 50µm ∆E single-sided silicon strip detector (SSSD), ∼1000µm E double-sided silicon strip detector (DSSD) and backed by four CSI(Tl) detectors, each of thickness 6 cm.Typical experimental setup has been shown in figure 1.The angular resolutions were 0.8 0 and 1 0 for the right and left telescope in figure 1, respectively.Typical two dimensional plots of ∆E versus E particle identification spectra for strip detectors are shown in figure 2 and in figure 3 for 12 C + 12 C and for 13 C + 12 C reactions, respectively.Very good isotopic separation for different fragments have been observed.Inclusive energy distributions for various fragments (3 ≤ Z ≤ 5) have been measured in the angular range of 14 0 to 36 0 .Energy calibration of the telescopes have been performed using elastically scattered 12,13 C ions from 209 Bi target at different energies ( 12 C beam of 70 and 80 MeV , 13 C beam of 78.5 and 82 MeV) and α -particles from the 229 Th α-source.

Results and discussions
Typical inclusive energy spectra (θ = 14 0 ) of different isotopes of the fragments Li, and Be obtained in the reactions 12 C + 12 C and 13 C + 12 C have been shown in figure 4 and figure 5 by solid and dotted lines respectively.The energy distributions are nearly Gaussian in shape (excluding the transfer channel), having their centroid at the expected kinetic energies for the fission fragments obtained from the Viola systematics corrected by the corresponding asymmetry factors [10] (shown by arrows in figure 4 and figure 5).This suggests that, in all cases, the fragments are emitted from a fully energy relaxed composite as expected for both FF and orbiting processes.the assumption of a two body kinematics averaged over total kinetic energy distributions.The cm angular distributions (dσ/dΩ), of different isotopes of Li and Be obtained in the above two reactions have been shown in figure 6 and figure 7, respectively.The angular distributions of all the fragments emitted in both the reactions are found to follow ∼ 1/sinθ cm dependence in cm (shown by solid lines (red) in figure 6 and figure 7), which further conjectured the emission of fragments are from a completely equilibrated, long-lived, composite system.It is seen from figure 6 and figure 7 that, at each angle, the differential cross-sections of neutron-rich fragments, viz. 7Li and 9 Be, obtained in 13 C + 12 C reaction are more than those obtained in 12 C + 12 C reaction.However, the cross-sections of 6 Li and 7 Be are approximately same for both the reactions.The enhanced yield of 9 Be in the case of 13 C + 12 C compared to 12 C + 12 C reaction may be due to the most probable binary splitting channel, i. e. 25 Mg splitting into 9 Be + 16 O.
For the present study, all detected 2α events (for a particular angle of the telescope for both reactions) have been extracted from the inclusive event-by-event data to reconstruct the excitation energy spectra of the 8 Be decay [11, 12].The 2α events originate mainly from (i) decay of particle unbound 8 Be states and (ii) decay of particle unbound excited states of 12 C ( 13 C) → 3α (+n) emission, either directly or through the sequential process 12 C ( 13 C) → 8 Be ( 9 Be) + α → 3 α (+n).The decay of 8 Be was identified by reconstructing the 8 Be excitation energy spectrum (shown in Figure 8) for all events in which two alpha particles hit two separate strips within the detector.The peaks at excitation energy (E x ) = 0 MeV correspond to decays of the particle-unstable ground state of 8 Be (J p = 0 + ) formed in both the reactions; however the yield is more in the case of 12 C + 12 C reaction.The pronounced bump at excitation energy ∼ 0.5 MeV observed in the reaction 13 C + 12 C is due to the decay of the 2.43 MeV state in 9 Be, which is not at all visible for 12 C + 12 C reaction.The broad peak observed at E x ∼ 1.5 MeV, is due to the sequential breakup channel of 12 C and is more pronounced for 12 C + 12 C reaction.It is seen from figure 8 that there is significant difference in the cluster state formation in these two reactions and the yield of neutron rich Be isotope i.e. 9 Be is much more in 13 C + 12 C reaction.The large cross-section 8 Be/ 9 Be in 12 C + 12 C/ 13 C + 12 C also support the formation of clusters as observed in references [8, 9].The electromagnetic transition is another probe which can be used to study the clustering effect in light nuclei [13].

Conclusions
Fragment emission mechanisms have been studied for 12  Gaussian in shapes with their centroid at the expected kinetic energies for the binary breakup obtained from the Viola systematic corrected by corresponding asymmetric factors.The cm angular distributions (dσ/dΩ), of these fragments are found to follow ∼ 1/sinθ cm , which signifies the emission of these fragments are from a long lived equilibrated composite.The differential cross-sections of neutron-rich fragments, viz. 7Li and 9 Be, obtained in 13 C + 12 C reaction are found to be more than those obtained in 12 C + 12 C reaction at each angle.On the other hand, the differential cross-sections of 6 Li and 7 Be are approximately same for both the reactions.The enhanced yield of 9 Be observed in the 13 C + 12 C reactions may be due to the most probable binary splitting of the compound nucleus 25 Mg into 9 Be + 16 O channel.A pronounced bump has been observed at excitation energy ∼ 0.5 MeV in the excitation energy spectrum plot generated from 2α detected events for the reaction 13 C + 12 C, which is due to the decay of the 2.43 MeV state in 9 Be and is not at all visible for 12 C + 12 C reaction.It has also been seen from the 2α detected events that the production of neutron-rich isotope viz. 9 Be is more in 13 C + 12 C compared to those produced in 12 C + 12 C reaction.Further details analysis for these two systems are in progress.

Figure 3 .
Figure 3. Same as figure 2 for the reaction 13 C + 12 C.

Figure 4 .Figure 5 .
Figure 4. Typical energy spectra of isotopes of Li in the reactions 12 C + 12 C (solid-line) and 13 C + 12 C (dotted-line).Arrows indicate the energy of the fragments as obtained from Viola systematics corrected by asymmetric factor.
C (80 MeV) + 12 C and 13 C (78.5 MeV) + 12 C reactions.From the preliminary analysis, it has been observed that the energy distributions of different isotopes of Li and Be, obtained in the above two reactions, are found to be nearly