Critical Te-104 decay measurements may help answer century-old alpha particle formation question

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.

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