Neutron-induced capture and fission reactions together (Vol. 43 No. 3)

The accurate knowledge of (n,gamma) neutron-capture cross-sections for fissile isotopes is highly relevant for next-generation applications of nuclear technology. However, accurate measurements are difficult due to the gamma-ray background generated in competing (n,f) fission reactions. Scientists at the CERN n_TOF facility have developed a new experimental setup that is capable of simultaneously measuring and identifying the capture and fission reactions.

The setup combines the existing 4pi BaF₂ Total Absorption Calorimeter (TAC) with a set of three MicroMegas detectors (MGAS). A successful test experiment was performed using moderated spallation neutrons in an energy region of 6-22 eV with a ²³⁵U sample.

Neutron captures were measured in the TAC while fission reactions were simultaneously detected in the TAC and MGAS. Coincident events recorded in the TAC and MGAS were selected and the specific TAC response to capture and fission events was exploited to select the two components by imposing conditions on sum energy and event multiplicity to disentangle them.

It was important to precisely determine the different detector efficiencies for both reaction types, which in turn allowed to derive capture and fission cross-sections, as well as their ratios. The comparison of the experimentally determined values with evaluated cross-sections showed good agreement between measurement and evaluation for both types of reaction.

Thus the experimental method was validated for an unambiguous extraction of cross-sections from a simultaneous measurement of the capture and fission reaction.

With the successful commissioning of the combined experimental setup, the n_TOF collaboration has now developed a tool that will enable them to target the accurate measurement of neutron-induced capture cross-sections of fissile isotopes.

This new method brings interesting perspective for the necessary improvement of nuclear data for next-generation nuclear technology applications. The figure shows the neutron capture measurements of actinides performed at n_TOF (^233,234U, ²³⁷Np, ²⁴⁰Pu and ²⁴³Am) and those intended for future experiments (236,238U and 241Am)