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

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. [ABSTRACT FROM AUTHOR]