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Variable Energy Cyclotron Centre, Kolkata

Nuclear Orbiting Reaction

Nuclear orbiting means that the two nuclei instead of fusing have formed a quasi-bound dinuclear system which would preferentially break apart into the entrance channel without ever undergoing complete fusion. It is generally assumed that the thermal equilibrium of the dinuclear orbiting complex is achieved quickly, but still the two nuclei maintain their identities and their complete fusion is inhibited.

In the literature, the words “Nuclear orbiting reaction” had been used for the last 30-35 years to describe different observed anomalies of nuclear reactions (in the projectile energy region 3-8 MeV/A) that could not be explained by standard statistical model calculations. Usually whenever the observed yields of the emitted fragments were found to be higher than those expected from the standard statistical model calculations or the observed width of the mass distribution of the fragments was found to be wider than that expected from the statistical model calculation, the qualitative idea of nuclear orbiting was invoked in one way or the other. On the other hand, the believers of the statistical model (Moretto and others) tried to show that most of these observations can be explained by adjusting the parameters of the statistical model, using more elaborate statistical model codes and by simple extension of the statistical model, without requiring this idea of orbiting.  There was also the question regarding the fundamental difference between a highly deformed rotating compound nucleus and an orbiting complex formed in the entrance channel. Of course, in the exit channel, there is no difference between the two, as the final breakup must be from a dinuclear system.

            We differentiate between the formations of an orbiting complex and deformed spinning compound nucleus in the entrance channel by observing entrance channel dependence of the emitted fragments when the same composite is formed by two different reactions at the same excitation energy with very similar spin distributions. At the same time, we also ensure that the reaction mechanism is not a simple direct reaction by observing 1/sinq_c.m angular distribution at the back-angles, which should mean the formation of a long-lived intermediate complex. If we find entrance channel dependence (preferential breakup into the entrance channel), in addition to 1/sinq_c.m angular distribution at the back-angles, then we consider the formation of an orbiting complex in the entrance channel. Otherwise it should be a compound nucleus in the entrance channel. Using these criteria, it has been found that only a few systems such as ²⁴Mg+¹⁶O, ²⁸Si+¹²C, ¹⁶O+⁸⁹Y, ¹⁶O+⁹³Nb (at projectile bombarding energy 4-8 MeV/A) form orbiting complexes at high orbital angular momenta (near the critical angular monetum) and preferentially break apart into the entrance channel. Although a theoretical understanding of the process is still lacking, we are experimentally studying the temperature and the decay process of the orbiting complexes and highly deformed spinning compound nuclei.