Fission of 215Fr studied with spectroscopic methods

Background: Asymmetric fission is known to occur in two regions, the actinides and sublead, and is dependent on the fissioning system excitation energy. Experimental evidence in the sublead region show that this mode is surprisingly persistent with increasing energy and its origin is not fully understood.Purpose: To experimentally study the fusion-fission reaction of 215Fr at moderate excitation energy and deter-mine previously unknown independent fission yields and other properties.Method: The compound nucleus was formed in the reaction 18O + 197Au. The prompt gamma rays emitted during the reaction were measured with the high efficiency and high granularity nu-ball-2 spectrometer. Independent fission yields of even-even nuclei were determined by detecting triple-gamma cascades in the fission fragments.Results: The observed yields, although dominated by a symmetric peak, show maxima for heavy fragment of Z approximate to 54-56, which is consistent with the known results in the actinide region but unexpected for the nuclide of interest, and at the studied excitation energy.Conclusions: The mode of asymmetric fission is present even at relatively high excitation energies in the system studied. This observation matches experimental findings in the sublead region, contrary to the actinides, and so far there is no well-developed explanation of this phenomenon.