Fission of 215Fr studied with gamma spectroscopic methods

K. Miernik, A. Korgul, W. Poklepa, J. N. Wilson, G. Charles, S. Czajkowski, P. Czyż, A. Fijałkowska, L. M. Fraile, P. Garczyński, K. Hauschild, C. Hiver, T. Kurtukian-Nieto, M. Lebois, M. Llanos, A. Lopez-Martens, K. M. Deby Treasa, J. Ljungvall, I. Matea, J. Mielczarek, J. R. Murias, G. Pasqualato, A. Skruch, K. Solak, K. Stoyachev, I. Tsekhanovich
Nuclear Experiment, Nuclear Experiment (nucl-ex)
2023-11-23 00:00:00
Background: Asymmetric fission is known to occur in two regions, the actinides and sub-lead, and is dependent on the fissioning system excitation energy. Experimental evidence in the sub-lead 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 $^{215}$Fr at moderate excitation energy and determine previously unknown independent fission yields and other properties. Method: The compound nucleus was formed in the reaction $^{18}$O + $^{197}$Au. The prompt gamma-rays emitted during the reaction were measured with the high efficiency and high granularity $\nu$-ball2 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 $Z \approx 54-56$, 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 sub-lead region, contrary to the actinides, and so far there is no well-developed explanation of this phenomenon.
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