Fusion reactions

A fusion reaction occurs when two nuclei come together with sufficient kinetic energy to overcome their mutual electrostatic repulsion (Coulomb barrier) and form a compound nucleus. Depending upon the available kinetic energy, the collision partners will fuse either by passing over or by quantum mechanical tunneling through the Coulomb barrier. Fusion is one of the ways of producing elements and energy during nucleosynthesis in the stars, whereas on earth it is the only way to produce new very short lived elements to extend the periodic table. Further, the fusion between two heavy nuclei at energies around the Coulomb barrier serves as an illustration of various quantum mechanical aspects. Measurements both here and at other laboratories showed that the fusion of two nuclei is a multidimensional tunneling problem which not only depends on the radial separation but is also very strongly governed by their internal structure. Fusion below the barrier energy provides an excellent illustration of the connection between various internal degrees of freedom (direct reactions channels) and the translational degrees of freedom of the two colliding nuclei (elastic scattering). The probability of the nuclei to fuse (compound nucleus formation) can be obtained from measurement of various decay products of the compound nucleus. These could be measurements of characteristic gamma rays originating from the decay of evaporation residues, number of fission fragments or direct measurements of the evaporation residues using a recoil mass separator.

Apart from showing the effect of the internal properties of the interacting nuclei on the fusion process, a new method to obtain the angular momentum of the fused nuclei and its connection with the fusion excitation function have been suggested from experiments done here. Recent availability of unstable beams, also known as Radioactive ion beams (beams of short lived nuclei having low binding energy and a larger extended matter distribution as compared to stable nuclei), at various facilities around the world have prompted the investigation of influence of their properties on the reaction process. A precursor to such studies is the study of stable but weakly bound nuclei. Experiments have been performed to understand if fusion with weakly bound nuclei behaves differently from normal nuclei

Direct reactions

Reactions in which the interaction between projectile and target happens through a specific degree of freedom (either a single nucleon or a collective coordinate) causing a change in that degree of freedom are called direct reactions. Here the two colliding nuclei essentially maintain their identity after the interaction. By studying the energy and angular distributions of the nuclei scattered after the reactions information about the interaction potential (elastic scattering), the internal states, which correspond to a particular state of motion of the constituent nucleons (inelastic scattering) and correlations of nucleons like paring are obtained. Further, these studies provide an insight into how the states of nuclei can be visualized in terms of various internal configurations (transfer reactions). Studies in our lab have focused on understanding the energy dependence of the interaction potential, breakup of weakly bound nuclei, mechanism of transfer for a pair of nucleons to see whether they are transferred sequentially or as a pair, clustering in nuclei and the deviation of one and two nucleon transfer at large relative distance between the interacting nuclei from theoretical models.

Heavy – ion resonances

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In the case of some nuclei, the variation of the cross section for a given process, like elastic or inelastic scattering, as a function of the projectile energy shows fluctuations in the form of high and narrow peaks. A characteristic feature of these resonances, unlike the fluctuations, is that it shows up simultaneously in various outgoing channels and at all angles. These fluctuations can be understood in terms of the formation of long lived, positive energy states of the projectile and target, known as ‘molecular configurations’. It is very difficult to identify these states (because of their larger widths) unless they have a very special structure. Studies here have focused on investigation of such states at high energies in light ystems consisting of “4N” nuclei like ¹⁶O+¹²C, ¹²C+¹²C. The detailed study of excitation functions of various outgoing reaction channels for a given incoming channel has helped in determining their structure. Using the outgoing particle correlations some of these structures have been interpreted as a linear chain of 6 a particles and others have been shown to be highly deformed.

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