Unexpected abundance of deuteron clusters inside atomic nuclei discovered by RIKEN and partners

According to the liquid-drop model proposed by Niels Bohr, neutrons and protons exist separately inside an atomic nucleus, behaving much like a uniform liquid. However, reality appears to be far more diverse, with clusters of multiple neutrons and protons coexisting within the nucleus. An international joint research group has discovered that "deuteron clusters," which are tightly bound pairs consisting of one proton and one neutron, exist inside ¹²C and ¹⁶O nuclei at a much higher probability than previously assumed. The team includes Group Director Tomohiro Uesaka and Research Scientist Yuki Kubota of the Nuclear Dynamics Research Group at the RIKEN Nishina Center for Accelerator-Based Science, Associate Professor Juzo Zenihiro and Graduate Student Ryotaro Tsuji (at the time of the research) at the Graduate School of Science, Kyoto University, Assistant Professor Junki Tanaka at the Research Center for Nuclear Physics, the University of Osaka, and Professor Kazuyuki Ogata at the Graduate School of Science, Kyushu University. The findings were published in the online edition of Progress of Theoretical and Experimental Physics.

Located at the center of an atom, which measures roughly one ten-billionth of a meter, the atomic nucleus itself is incredibly tiny, spanning only about one quadrillionth of a meter. Yet it accounts for 99.97% of the atom's total mass. Bohr proposed the concept that the state inside this nucleus is uniform, like a liquid, and this has remained the traditional view in nuclear physics. However, alpha decay, discovered at the end of the 19th century, is difficult to explain using the liquid-drop model alone.

George Gamow later explained this phenomenon by assuming that alpha particles are formed inside the nucleus and are then ejected outward via the quantum tunneling effect. Alpha decay serves as the origin of helium gas on Earth and is applied in practical technologies such as cancer treatments and smoke detectors. However, exactly how alpha particles are formed inside an atomic nucleus remains unknown.

To explore the interior of atomic nuclei, the international joint research group launched the "Onokoro Project." This method involves firing high-energy protons at a nucleus to knock out clusters of particles existing inside, much like the traditional Japanese game of Daruma Otoshi (a game where stacked blocks are swiftly knocked out from the bottom without disturbing the rest of the tower), thereby removing them without affecting the remaining portion of the nucleus.

In this study, the team utilized a cyclotron at the University of Osaka's Research Center for Nuclear Physics to strike ¹²C and ¹⁶O with a proton beam accelerated to 226 MeV (approximately 60% the speed of light) and subsequently analyzed the knocked-out deuterons and protons. Firing a proton at an atomic nucleus is comparable to hitting a single marble placed inside the Tokyo Dome with a single adzuki bean.

In this experiment, approximately one quadrillion protons were fired, around 100,000 of them successfully struck a nucleus, and the Daruma Otoshi effect was achieved approximately 2,000 times. The results revealed that deuteron clusters account for 40% of the outermost orbit in ¹²C and 32% in ¹⁶O.

Traditional nuclear physics had never assumed that such a large number of deuteron clusters existed. This discovery clearly demonstrates that the non-uniformity within atomic nuclei is far more diverse and manifests at a much higher probability than previously believed.

Uesaka noted, "Thanks to advancements in reaction theory and the excellence of our experimental setup, we succeeded in extracting this information beautifully."

Ogata, who constructed the reaction theory, explained the challenges they faced, "Because deuteron clusters have low stability, conventional wisdom dictated that striking them with a proton would break them apart, making them unobservable. In fact, if a student presented this topic at an academic conference, he or she would likely face harsh criticism from the senior experts. It was a phenomenon that simply wasn't believed. To address this, we integrated the probability of the cluster breaking apart into our theory, allowing us to calculate the specific percentage of clusters that would survive unbroken. I believe this helped guarantee the physical reliability of our experimental data."

The Onokoro Project has also successfully knocked out alpha clusters from calcium-40 nuclei, as well as tritium and 3He clusters from calcium isotopes.

Looking ahead, Uesaka stated, "Based on these results, we want to expand our research to heavier nuclei that are closer to undergoing alpha decay, as well as unstable nuclei that only exist on Earth for a split second. Ultimately, we hope to observe the alpha particles inside nuclei that serve as the 'seeds' of alpha decay, and obtain new information regarding stability and reactivity that connects to the very fundamentals of atomic nuclei."

Zenihiro reflected on the broader implications: "It is only natural to think that atomic nuclei are non-uniform. Living things, including ourselves, naturally possess diversity. Similar phenomena are occurring across various hierarchical layers of nature. It might even be a shared, universal law. We have finally reached a standpoint where we can begin to ask where such universal patterns come from."

Journal Information
Publication: Progress of Theoretical and Experimental Physics
Title: Proton-Induced Deuteron Knockout (p,pd) on ¹²C and ¹⁶O at 226 MeV
DOI: 10.1093/ptep/ptag046