Search for new islands of fission isomerism

Abstract

Abstract The spontaneously fissioning isomers of Am and neighboring elements have been explained as nuclei trapped in a secondary minimum in the fission barrier profile. Some recently published calculations suggest the possible existence of another group of such shape isomers near Z = 85, N = 118. An attempt was made to produce isomers of Po, At, Rn and Fr by bombardment of 197 Au with beams of 11 B, 12 C, 14 N, 16 O, from the Berkeley HILAC and to observe their fission mode with Si semiconductor detectors. The cross-section limits were set at 70 nb or less for the half-life range 2 × 10 −9 to 2 × 10 3 s. An attempt was also made to observe fission fragments from nuclei decaying in flight after recoiling out of targets of Ce, Pr, Tm and Au bombarded with heavy ions. No fission fragments were observed from products in the half-life range 1–50 ns above a cross-section limit of ≈ 50 nb. The possible existence of isomers decaying primarily by γ-transitions to the first minimum was not excluded in either set of measurements. The apparatus was tested with known isomers produced by the reactions 238 U(d, pn) 238m U (200 ns) and 238 U( 12 C, α3n) 243m Cm (38 ns).