Measuring what comes before alpha

University of Tennessee, Knoxville physicists and their colleagues have made critical measurements of the lifetime and decay energy of tellurium-104 (Te-104), an important step in answering a century-old question and understanding how hundreds of nuclei decay. The results are published in Nature.

A Particle Determined to Escape

Professor Robert Grzywacz led the experimental team at the Radioactive Isotope Beam Factory (RIBF) at RIKEN in Japan. He explained how the results match decades-old predictions that tellurium-104 is a special case in alpha decay, a process where an alpha particle (a strongly-bound system of two protons and two neutrons) tunnels through the barrier surrounding the nucleus where it resides. Though alpha radioactivity was discovered more than 125 years ago, where the particle comes from is still a mystery, especially in nuclei that have large numbers of protons and neutrons.

“Alpha decay is the oldest decay mode,” Grzywacz said. “The big question is how the alpha particle forms in heavy nuclei, which are known to have uniform matter distribution. There must be a mechanism which causes local ‘clump’ or ‘cluster’ formation.”

Clustering is connected to how a nucleus is structured. Called preformation, it’s a signal an alpha particle is about to make a break for it.

“Once formed,” Grzywacz explained, “the alpha particle will escape from the nucleus.”

He said that this emission is a well-understood quantum mechanical tunneling process that depends on available energy. Since the 1960s scientists have thought that one nucleus—tellurium-104—has a special enhancement that could better explain how it happens.

Following the Decay Chain

While tellurium lives among the metalloids on the periodic table and can be found in nature, the isotope tellurium-104 has to be synthesized. Creating these nuclei is a challenge for multiple reasons. First, they only live for a few nanoseconds. Second, they’re a result of the decay of xenon-108, which in itself is difficult to produce. In this experiment, the team overcame these still-formidable obstacles with technological advances at RIBF. Using four coupled cyclotrons, they accelerated a beam of xenon-124 into a beryllium target. The collision produced fragments of xenon-108, whose decay populates tellurium-104, which is followed in this decay chain by tin-100.

“We have measured the lifetime and energy of this decay and found that the preformation probability is much larger than expected based on predictions, which used available experimental knowledge,” Grzywacz said. “We also found that tellurium-104 is the shortest known alpha particle radioactive nucleus with a 7.2 nanosecond half-life. This very short half-life, corrected for decay energy, gives unusually high alpha particle preformation. It will likely be a single case like that among all nuclei.”

He added the only other case is the well-studied decay of polonium-212 to lead-208, which has preformation probability 10 times smaller than that of tellurium-104.

Grzywacz said that more than half a century ago scientists pictured tellurium-104 having a brief existence as a molecule comprising tin-100 and an alpha particle. Tin-100 is a doubly-magic nucleus, meaning it’s strongly bound, as is an alpha particle. He and the research team attribute tellurium-104’s high preformation to its relation to doubly-magic tin, creating favorable conditions to form an alpha particle.

A Trail Blazed at Oak Ridge

Years of previous studies made these findings possible. Much of that work was rooted at the U.S. Department of Energy’s Oak Ridge National Laboratory (ORNL), where researchers have been at the forefront of exploring the island of alpha-emitting nuclei near tin-100 for decades. In 2006, a team including Grzywacz and ORNL physicists Krzysztof Rykaczewski and Carl Gross used the Recoil Mass Spectrometer at ORNL’s historic Holifield Radioactive Ion Beam Facility (HRIBF) to discover the neighboring xenon-109 to tellurium-105 to tin-101 alpha-decay chain. The measurement suggested that alpha-particle preformation was growing as nuclei approached doubly-magic tin-100. This strengthens the case that tellurium-104 would be the definitive test of the “superallowed” prediction, where the parent nucleus is essentially the doubly-magic plus a preformed alpha particle.

Independently, a 2018 experiment at Argonne National Laboratory (ANL) achieved the first observation of the xenon-108 to tellurium-104 to tin-100 chain, though the two decays could not be fully separated, leaving the individual half-life and energy of tellurium-104 unmeasured. In parallel, the detector technology pioneered at Holifield—fast-response scintillator crystals coupled to position-sensitive photomultiplier tubes—was further developed by Grzywacz’s group and ORNL collaborators at Japan’s Advanced Science Research Center, and proved essential for the present RIKEN experiment.

Rykaczewski, a Distinguished Senior Researcher in ORNL’s Physics Division and co-spokesperson for the RIKEN experiment, played a central role in designing and executing the measurement. Toby King, a UT physics graduate now on ORNL’s staff, was instrumental in building and operating the detection system and data acquisition. Additional ORNL support came from James Allmond and Thomas Ruland, who provided supplemental equipment and on-site experimental assistance.

“The path from the Holifield discovery of the tellurium-105 decay chain to this definitive measurement of tellurium-104 spans nearly two decades of sustained effort between UT and ORNL,” Rykaczewski said. “Each step—new isotopes, new detectors, new accelerator capabilities—brought us closer to this singular nucleus.”

A Strong Foundation for Students

Ian Cox (PhD, 2024) was the paper’s lead author. Now a postdoctoral appointee with ANL, he began working on the project as an undergraduate physics major and handled most of the experimental analysis “in record time,” according to Grzywacz.

“Studying nuclei on the edge of existence presents significant challenges but can also produce profound results,” Cox said.  “It has been a pleasure to start my research career with a result that can greatly impact the field.”  

Following in his footsteps, current Graduate Students Nico Braukman and Donnie Hoskins (physics), as well as Benjamin Kreider (engineering) were all co-authors on the Nature publication.

“Getting exposure to the kind of work that goes into producing high-impact physics results is an important part of being a grad student,” Braukman said. “I’m glad to have had the opportunity to participate in this experiment early in my grad school career.”

Hoskins shared similar sentiments.

“As a graduate student, one of our goals is to learn and participate in research to prepare us for our futures,” he said. “Exposure in prestigious journals, like Nature, increases visibility for me as an independent scientist to set up my own research in the future with a proven strong foundation in nuclear physics.”

The experimental effort included UT Physics Research Assistant Professor Z.Y. Xu, along with partners from ORNL, RIKEN, the University of Tokyo, the University of Warsaw, the National Centre for Nuclear Research (Poland), the Universität zu Köln (Germany), Universidad Complutense de Madrid (Spain), Lawrence Livermore National Laboratory, and the Japan Atomic Energy Agency.

The U.S. Department of Energy Office of Science and the National Science Foundation helped support this work.