First simultaneous measurement of fission and gamma probabilities of 237U and 239Np via surrogate reactions

Abstract

Fission and gamma decay probabilities of ²³⁷U and ²³⁹Np have been measured, for the first time simultaneously in dedicated experiments, via the surrogate reactions ²³⁸U(³He, ⁴He) and ²³⁸U(³He,d), respectively. While a good agreement between our data and neutron-induced data is found for fission probabilities, gamma decay probabilities are several times higher than the corresponding neutron-induced data for each studied nucleus. We study the role of the different spin distributions populated in the surrogate and neutron-induced reactions. The compound nucleus spin distribution populated in the surrogate reaction is extracted from the measured gamma-decay probabilities, and used as input parameter in the statistical model to predict fission probabilities to be compared to our data. A strong disagreement between our data and the prediction is obtained. Preliminary results from an additional dedicated experiment confirm the observed discrepancies, indicating the need of a better understanding of the formation and decay processes of the compound nucleus.