Investigation of γ-ray emission preceding isomeric fission of 236U

Measurements were made to detect γ rays preceding isomeric fission in ²³⁶U induced by eV-range neutrons captured in ²³⁵U. A limit of ≤ 6 × 10⁻⁵ was placed on the ratio of the rate of isomeric fission events with prefission γ rays to the rate for prompt fission events. This experiment provides direct evidence that the penetration of the outer barrier is much greater than that for the inner barrier for 3⁻ and 4⁻ states in ²³⁶U.