In nuclear physics, the decay of excited states of nuclei provides insights into the deformation of nuclei when they are in those states. Scientists have extensively explored the variations in shape and the consequent formation of the short-lived isotopes, also known as isomers. However, experts still have not thoroughly understood one of the most extreme cases: the decay of Ta-180m.
Mystery of Ta-180m
Tantalum is considered one of the rarest elements with multiple stable isotopes. Its least abundant tantalum isotope, Ta-180, is naturally found in a long-lived excited state. Such a feature is very unique to this isotope. In excited states. The protons or neutrons of a nucleus can have higher than normal energy levels.
The radioactive decay of the excited state in Ta-180m is energetically possible, but it has never been observed. Nuclear physicists can use nuclear theory in predicting the decay of Ta-180m based on their knowledge of shorter-lived isomers. However, they still find it hard to measure the least abundant tantalum isotope.
The remarkably long lifetime of Ta-180m surpasses the half-lives of all other known beta and electron capture decays because of the large K-spin differences. It is also attributed to the small energy differences between the isomeric and lower-energy states. The exceptional stability of Ta-180m challenges the existing theories and models of nuclear structure and decay.
This means that if experts successfully measure decay in Ta-180m, then they can have an unprecedented opportunity to contribute to nuclear theory. Detecting the elusive decay also sheds light on K-spin violation, the nature of dark matter, and nucleosynthesis mechanisms induced by neutrinos.
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Search for the Elusive Decay
Researchers have recently conducted experiments which aim to measure the radioactive decay of Ta-180m. They even expect it to have a lifetime approximately one million times longer than the age of the universe.
In 2023, researchers have finally devised an experiment which has the required sensitivity to reach the predicted half-lives of Ta-180m. This study, entitled "Constraints on the Decay of
Ta-180m", has produced initial data and established the longest limits ever reached in the study of nuclear isomers.
In this study, scientists restructured the MAJORANA ultra-low background facility located at the Sanford Underground Research Facility in South Dakota. They introduced a substantially larger tantalum sample compared to any previously used in similar studies.
Over the course of one year, the physicists gathered data using germanium detectors which boast exceptional energy resolution. Analysis methods were also developed in such a way that they are specifically tailored to find multiple anticipated decay signatures.
The combined efforts have allowed the researchers to establish unprecedented limits, which fall within the range of 10^18 to 10^19 years. This level of sensitivity marks the first instance where the predicted half-life values from nuclear theory have become reachable.
While the process of radioactive decay for Ta-180m has not yet been observed, these advancements have significantly enhanced existing limits by one to two orders of magnitude. Additionally, this progress has enabled the scientists to dismiss particular parameter ranges related to different potential dark matter particles.
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