Naturally occurring isotopes are often considered stable for most practical purposes. Yet scientists believe that some forms of extremely rare radioactivity may still be waiting to be discovered in nature.

In a recent study published in Physics Letters B, scientists have predicted 12 naturally occurring candidate nuclei for cluster radioactivity that are expected to be observable in future experiments.

The study was carried out by researchers from the Heavy Ion Science and Technology Key Laboratory, the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS), and the KTH Royal Institute of Technology in Sweden.

Cluster radioactivity refers to a process in which an atomic nucleus emits a cluster composed of several nucleons. The helium-4 nucleus, or alpha particle, is the most common emitted cluster, while heavier nuclei such as carbon-14 and oxygen-20 can also be emitted. Over the past two decades, experimental studies of alpha radioactivity in naturally occurring isotopes such as bismuth-209 and tungsten-180 have demonstrated that some seemingly stable nuclei can undergo extremely long-lived radioactive decay.

"Since naturally occurring isotopes are often available in large quantities and are suitable for long-term low-background measurements, they provide a favorable platform for searching for rare cluster radioactivity," said Dr. ZHANG Wenqiang from IMP.

The researchers performed the first systematic investigation of cluster radioactivity in all even-even naturally occurring isotopes using the universal decay law. They predicted 12 alpha-decay candidates, of which hafnium-176 has the shortest predicted half-life of about 2.79×10¹⁸ years, making it one of the most promising targets for future observation of natural alpha decay.

For heavier cluster radioactivity, the study adopted 10³⁰ years as an ideal upper limit. The researchers found that osmium-186, which emits the doubly magic calcium-48 cluster, is the only even-even naturally occurring nucleus below the Z=82 shell closure with a predicted half-life below this limit. This makes osmium-186 one of the most noteworthy natural candidates for heavy-cluster decay searches.

Furthermore, the study revealed that natural alpha emitters below Z=82, characterized by extremely low alpha-decay energies, can provide a stringent test of semiempirical decay laws.

"These findings provide a roadmap for future searches for new long-lived cluster-radioactive nuclei. Once the alpha decays of these nuclei are experimentally confirmed, they will provide critical data for testing semiempirical decay laws under extremely low decay-energy conditions," said Dr. QI Chong from the KTH Royal Institute of Technology.