Can weakly absorbing surface transparent nucleus-nucleus interaction ensure orbiting?

Abstract

We find that in contrast to the large orbiting yield seen in Si+' C, Mg+ 0, Mg+ C, and Ne+' C reactions, the back-angle, strongly damped binary yields from He+ Al and He+ Ca reactions are in agreement with the predictions of statistical compound nuclear reaction. These results disagree with the observation from heavy-ion reaction systematics that the orbiting yield will dominate whenever the number of open channels is small, and suggest that the weak absorption and surface transparency of nucleus-nucleus interaction are not sufficient conditions for the dominance of orbiting in back-angle, strongly damped reaction. PACS number(s): 25.70. -z, 24. 10. -i, 24.30. -v, 24.60. -kWe know from studies [1 -4] of fully energy-damped binary yields produced in light heavy-ion reactions at energies around 4 -8 MeV/nucleon that in certain systems a deepinelastic orbiting mechanism dominates binary yields and in others fusion-fission process dominates binary yields. In the case of fusion-fission process, such as Cl+ ' C, Na+ Mg, and P+ 0, an equilibrated compound nucleus [1 -3] is formed and it has a certain probability of decaying into entrance channel depending on available phase-space and barrier penetration probabilities. In the case of orbiting reactions such as Si+ C, Mg+' 0, etc. , the two nuclei form a dinuclear orbiting complex [4,5] where they maintain their separate identities and have a large probability (4 -5 times larger than the predictions of the statistical compound nucleus model) of breaking into the entrance channel. Both orbiting and fusion-fission process have long lifetime and as a result characteristic 1/sin0, angular distribution in the center-of-mass frame is observed. However, orbiting is characterized by large binary damped yield [4] (4 -5 times larger than expected from compound nuclear reaction), entrance channel dependence [5] and its characteristic y-ray angular correlation [6].In order to obtain a better understanding of the orbiting reaction mechanism, it is important to understand the basic criteria which determine whether orbiting or fusion-fission mechanism will dominate the binary yield of breaking apart into the entrance channel. Recently, Beck et al. [7] made an observation from the heavy-ion reaction systematics that the orbiting mechanism dominates back-angle binary yield when the number of open channels for the system is small and the fusion-fission mechanism dominates when the number of open channels for the system is large. Beck et aI. 's observation is in agreement with the dominance of orbiting in Si+ C Mg+ 60 4Mg+ & C and Ne+ C reacti and that of fusion-fission in P+' 0, Cl+' C, and Na+ Mg reactions. The number of open channels is a qualitative measure of absorption of partial waves by the nuclear potential due to the direct reaction processes. So Beck et al. 's observation implies that the orbiting yield is dominant in the case of the surface transparent weakly absorbing nucleus-nucleus interaction. On the other hand, strongly absorbing nucleus-nucleus interaction suppresses or...

Nuclear orbiting and anomalies in nuclear reactions

“…The number of open channels argument also cannot explain our earlier observations [6] regarding the lack of any orbiting effect for He · ¾ Al and He · ¼ Ca reactions.…”