Abstract
Fission and gamma decay probabilities of 237 U and 239 Np have been mea- sured, for the first time simultaneously in dedicated experiments, via the surrogate re- actions 238 U( 3 He, 4 He) and 238 U( 3 He,d), respectively. While a good agreement between our data and neutron-induced data is found for fission probabilities, gamma decay prob- abilities 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 popu- lated 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 ob- tained. 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.
Highlights
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
As expected the Pγ is equal to 1 below the neutron emission threshold of 5.1 MeV and it significantly drops above this energy due to the competition with the neutron emission
Our data are compared to neutron-induced data (JENDL 4.0 - full line) and discrepancies a factor 3 at 6.4MeV are observed. We plot both the fission and gamma emission probabilities shown in Figs. 1a and 1b in the region where both decay channels are open simultaneously, and we compare the experimental data to the evaluated neutron induced data. In this excitation energy region we observe a good agreement with the neutron-induced data for the fission probability and a discrepancy of up to a factor 10 for the gamma emission probability
Summary
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. The surrogate reaction method was proposed for the first time in the 70’s by Cramer and Britt [1] It is an indirect method which aims at determining compound nucleus reaction cross sections involving short lived and/or difficult-to-produce targets. The method has the main advantages of allowing one to access short-lived nuclei, not otherwise accessible via direct measurements, and to simultaneously investigate several transfer channels on a broad excitation energy range. The formation of a compound nucleus must take place both in the neutron-induced and in the transfer reactions This means that the formed nucleus loses memory of the entrance channel (except for the conserved quantities, i.e. energy and Jπ) and its decay is independent of its formation. Further details on the method and on the underlying assumptions can be found in Ref.[3]
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