Background: There is a long-term interest in running the fission reaction backward, i.e., studying the “inverse fission” of uranium. The recent availability of beams of $n$-rich fission fragments has stimulated interest in this endeavor.

Purpose: To search for inverse fission in the reactions ${}^{124,132}$Sn + ${}^{100}$Mo.

Method: In the ${}^{124}$Sn + ${}^{100}$Mo reaction, evaporation residues were searched for using in-beam detection of evaporation residues, in-beam $\alpha$ spectroscopy, and post-irradiation $\alpha$ spectroscopy, while in the ${}^{132}$Sn + ${}^{100}$Mo reaction, the evaporation residue ${}^{230}$U was searched for using post-irradiation $\alpha$ spectroscopy.

Results: No evidence for the occurrence of the inverse fission reactions was found. The upper-limit cross section for the latter reaction is $\sim$550 $\mu$b, while the experimental upper-limit cross section for the former reaction is about 21${}_{-21}^{+38}$ nb.

Conclusions: The intensity of suitable radioactive beams is not high enough at present to detect inverse fission. For the ${}^{124}$Sn + ${}^{100}$Mo reaction, the observed upper limits are below the estimates of current models for these reactions, probably due to fusion hindrance.

• Received 18 February 2012

DOI:https://doi.org/10.1103/PhysRevC.85.044620