A thick ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ target has been bombarded with ${}_{}{}^{63,65}{}_{}{}^{}\mathrm{Cu}$ ions at an incident beam energy of 9.6 MeV/nucleon. After radiochemical separations into CdS, HgS, and La-oxalate fractions, a search for evidence of the production of superheavy elements was undertaken using Si surface-barrier detectors and mica foils simultaneously for detection of spontaneous-fission events and high-energy $\alpha$ emission. No evidence for the presence of superheavy elements in the samples was found. Upper limits for production cross sections for superheavy elements in each of the three chemical samples, assuming an arbitrary half-life of 100 days and a target thickness equal to the entire reaction range of the incident ions from initial energy to the reaction barrier, are as follows: (a) CdS ($Z=108-115\mathrm{}$): 1 × ${10}^{-34\mathrm{}}$ ${\mathrm{cm}}^2$; (b) HgS ($Z=112\mathrm{}$): 6 × ${10}^{-35\mathrm{}}$ ${\mathrm{cm}}^2$; ($c$) Laoxalate (${}_{}{}^{254}{}_{}{}^{}\mathrm{Cf}$): 8 × ${10}^{-35\mathrm{}}$ ${\mathrm{cm}}^2$.

NUCLEAR REACTIONS ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$(${}_{}{}^{63,65}{}_{}{}^{}\mathrm{Cu}$, $X$) superheavy elements and ${}_{}{}^{254}{}_{}{}^{}\mathrm{Cf}$, $E=9.6\mathrm{}$ MeV/nucleon; measured activities; deduced upper limits for $\sigma$ of superheavy element products and ${}_{}{}^{254}{}_{}{}^{}\mathrm{Cf}$.

• Received 17 May 1976

DOI:https://doi.org/10.1103/PhysRevC.15.319