Direct observation of muonic molecules for first time in the world

An international joint research group, centered around Designated Assistant Professor Yuichi Toyama and Professor Shinji Okada of the Center for Muon Science and Technology at Chubu University, and Associate Professor Takuma Yamashita and Professor Yasushi Kino of the Department of Chemistry, Graduate School of Science and Faculty of Science at Tohoku University, has successfully achieved the world's first direct observation of the resonance states of muonic molecules, which govern the reaction rate of muon catalyzed fusion (µCF). For this observation, the group employed high-resolution X-ray spectroscopy with a cryogenic detector. Furthermore, they quantitatively identified the abundance ratio for each quantum state. This clarifies the actual image of the muonic molecule formation process, which had been unclear, and resolves long-standing discrepancies between theory and experiment. It represents a major step forward in the foundation for increasing the efficiency of muon catalyzed fusion. The findings were published in Science Advances.

In fusion power generation, which fuses hydrogen nuclei together, methods such as generating plasma at extremely high temperatures and confining it with magnetic fields or instantaneously compressing fuel with lasers to achieve high-temperature, high-density plasma, are typically used.

In contrast, µCF involves replacing the electrons in hydrogen molecules with muons to create muonic hydrogen molecules about 1/200th the size. Within these muonic molecules, the nuclei are confined at extremely close distances, allowing nuclear fusion to occur at room temperature without the need for plasma.

To induce µCF efficiently, it is crucial to promptly generate muonic atoms and muonic molecules. However, the atomic and molecular reaction processes leading to muonic molecule formation have long suffered from discrepancies between theory and experiment, and the role of the resonance states of muonic molecules remained unexplained.

Precise theoretical research by Kino and Yamashita quantitatively demonstrated the possibility of resolving these discrepancies through reaction pathways involving resonance states and predicted a characteristic X-ray spectrum indicating the generation of these resonance states.

In this study, by using a Transition-Edge Sensor (TES) microcalorimeter, a superconducting detector with an energy resolution more than ten times superior to conventional semiconductor detectors, the researchers successfully detected and separated X-ray components originating from muonic molecules and muonic atoms. Furthermore, by comparing the observed spectrum with high-precision theoretical calculations, they identified the vibrational quantum states of muonic molecules (ddµ) consisting of two deuterium nuclei (d) and a muon (µ) in a resonance state, and successfully evaluated their abundance ratios quantitatively.

As a result of this quantitative identification, they demonstrated that the reaction pathway via the resonance state, which had not been previously considered, plays a role as a primary molecule formation process in µCF. Furthermore, they obtained results suggesting the existence of a "fast track" that bypasses the muonic molecule formation reaction, the rate-limiting step of µCF, and transitions directly to a state where nuclear fusion occurs.

These results were also consistent with theoretical predictions. Reaching the stage where muonic molecules can be directly observed and identified at the quantum state level means that µCF research has advanced significantly from a stage dependent on unclear theoretical models to a new stage where reaction processes based on quantum states can be verified through precise experiments.

Journal Information
Publication: Science Advances
Title: Direct observation of muonic molecules in resonance states critical to muon catalyzed fusion
DOI: 10.1126/sciadv.aed3321