Nuclear isomers are long lived excited states of atomic nuclei that can store large amounts of energy. They are considered candidates for applications such as nuclear batteries, gamma ray lasers and highly precise nuclear clocks, but controlled and rapid release of their stored energy remains a major challenge.
The isomer Mo 93m has attracted particular interest as a potential high energy density storage medium. Earlier work proposed that nuclear excitation by electron capture could efficiently trigger its energy release, but later theoretical and experimental studies raised doubts about whether that process really dominates in realistic environments.
To resolve the issue, the researchers produced a purified high energy beam of Mo 93m ions using the radioactive ion beam line at the Heavy Ion Research Facility in Lanzhou. They also developed a low background, high sensitivity technique to track how the population of the isomer changed during interactions with different target materials.
After purification, the Mo 93m ions were implanted into detectors covered with either lead foil or carbon foil. By detecting characteristic gamma rays emitted as the ions slowed down and interacted with the target, the team measured the probability that the isomer would be depleted in each case.
The depletion probability was about 2 in 100000 for ions implanted in lead and about 5 in 1000000 for ions implanted in carbon. These values matched theoretical expectations for inelastic nuclear scattering but were far larger than predicted for nuclear excitation by electron capture under the same conditions.
"This indicates that the observed isomer depletion in Mo 93m is dominated by inelastic nuclear scattering, rather than the previously proposed NEEC mechanism," said first author Dr. DING Bing of the Institute of Modern Physics. The result provides experimental clarity in a debate that has persisted for years.
The work delivers benchmark data for understanding how nuclear isomers behave in environments such as plasmas, astrophysical sites and inertial confinement fusion targets. It also constrains scenarios in which isomer based energy storage or release might be harnessed in future technologies.
According to corresponding author Prof. ZHOU Xiaohong, nuclear excitation by electron capture still remains a promising pathway in principle for triggering the release of energy from isomers. However, future attempts to detect and use NEEC may require specially optimized conditions, including plasma environments or collisions between intense electron beams and ion beams.
Research Report:Isomer Depletion of 93mMo Triggered by Inelastic Nuclear Scattering Rather than Nuclear Excitation by Electron Capture
Related Links
Institute of Modern Physics of the Chinese Academy of Sciences
Understanding Time and Space
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