A “dark” environment was established at the Radioactive Ion Beam Line in Lanzhou (RIBLL), China, to look for a soft flash of light as evidence of isomer depletion. Such depletion is needed to harness nuclear energy conserved in long-lived isomeric states through nuclear excitation by electron capture (NEEC).
Yet, in this independent experiment, an indication of isomer depletion was not observed. Outcomes were published in Physical Review Letters on June 17th, with the NEEC proportions measured at less than 2×10-5, thus creating doubt on the first reported experimental statement of NEEC in 2018.
The first experimental statement of NEEC was documented in 2018 with a measured excitation probability of 1.0 (3)%, nearly nine orders of magnitude larger than the theoretical objective. Later, researchers noted a possible overestimation owing to contamination from the enormous gamma background.
For reinvestigating isomer depletion, researchers at the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS) and their partners utilized an isomer beam to prevent contamination from the heavy gamma background.
“Our experiment was designed to escape from the gamma background. In the previous work, researchers searched for the scarce special photons in a harsh light, like seeking a firefly in the sunshine. For a refined survey, we drove it into the dark,” said Guo Song, an experimenter at IMP.
“The experiment they report is demonstrably superior to the earlier work in that the 93mMo nucleus formed in a fusion evaporation reaction was separated from the incident beam via traversal through the RIBLL separator. This allows for a low-background signal to be obtained that conclusively limited the NEEC rate,” announced one editor for Physical Review Letters.