Search for spontaneous-fission isomerism in nuclei of medium mass

Spontaneous-fission isomers have been searched for among the recoil products from the following reactions: ^209Bi + ^1H, ^209Bi + ^2H, ^209Bi + ^4He, Pb + ^2H, ^206Pb + ^4He, Ir + ^14N, ^169Tm + ^14N, ^154Gd + ^4He, and ^141Pr + ^14N. The major products were ^198–208Po, ^199–208Bi, ^149–151Dy, At, and Os. In no case have isomers been observed unambiguously. Limiting cross sections for fission isomers are ≅10⁻³¹ to 10⁻³³ cm² compared to product cross sections of 10⁻²⁴ to 10⁻²⁵ cm². The limiting isomer ratios are ≅10⁻⁶ to 10⁻⁸ for decay periods of a fraction of a nanosecond to many hours. These ratios are generally less than ≅1 to 0.01% of the corresponding ratios for fission isomers in heavier nuclei.