Attempt to confirm superheavy element production in the ${}^{48}\mathrm{Ca}+{}^{238}\mathrm{U}$ reaction

An attempt to confirm production of superheavy elements in the reaction of ${}^{48}\mathrm{Ca}$ beams with actinide targets has been performed using the ${}^{238}\mathrm{U}$(${}^{48}\mathrm{Ca}$,3$n){}^{283}$112 reaction. Two ${}^{48}\mathrm{Ca}$ projectile energies were used that spanned the energy range where the largest cross sections have been reported for this reaction. No spontaneous fission events were observed. No α decay chains consistent with either reported or theoretically predicted element 112 decay properties were observed. The cross-section limits reached are significantly smaller than the recently reported cross sections.

Figure 1

Average charges of heavy ions passing through dilute He gas. The ordinate shows the experimental average charges, with the sinusoidal correction for the electronic shell structure of the stripped ions removed. The best fit is indicated by the solid line. The dashed lines indicate the ±2$\sigma$ deviations about the fit. The value for expected ${}^{283}$112 EVRs is indicated with the arrow.

Figure 2

Cross-section limits for the combined 230.3- and 235.6-MeV irradiations shown as a function of assumed ${}^{283}$112 magnetic rigidity and assumed centroid for the ${}^{238}\mathrm{U}$(${}^{48}\mathrm{Ca}$,3$n){}^{283}$112 excitation function. Contours are labeled in picobarns and represent an 84% confidence level including a Poisson treatment of counting statistics as well as a 12% systematic error. The crosses indicate ${}^{48}\mathrm{Ca}$ beam-energy ranges in the targets and focal plane detector magnetic rigidity coverage for the individual 230.3- and 235.6-MeV runs.

Figure 3

(a) Probability of observing zero events in the present work, assuming the reported DGFRS production rate (of six events at 2.5 pb) is correct, shown as a function of assumed ${}^{283}$112 magnetic rigidity and assumed centroid for the ${}^{238}\mathrm{U}$(${}^{48}\mathrm{Ca}$,3$n){}^{283}$112 excitation function. This calculation considers counting statistics only and ignores systematic errors. The vertical line indicates the reported DGFRS beam energy. Estimated 1$\sigma$, 2$\sigma$, and 3$\sigma$ uncertainties in the predicted ${}^{283}$112 magnetic rigidity are indicated by the solid, dashed, and dotted horizontal lines, respectively. (b) Same as (a) with the DGFRS production rate adjusted to four events at 1.67 pb.

Figure 4

(Color online) ${}^{208}\mathrm{Pb}$(${}^{48}\mathrm{Ca}$,2$n){}^{254}$No excitation functions measured at BGS, VASSILISSA, and the DGFRS. The BGS and DGFRS centroids agree to within 1 MeV, and the amplitudes agree to within a factor of 1.2, suggesting that systematic errors in beam energy and/or separator sensitivities do not explain the discrepancies in ${}^{238}\mathrm{U}$(${}^{48}\mathrm{Ca}$,3$n){}^{283}$112 production.