Looking for the decay of nature's rarest isotope: tantalum-180m

Tantalum is one of the rarest elements and has many stable isotopes. The least abundant tantalum isotope, Ta-180, is naturally found in a long-lived excited state, a feature unique to this isotope. In excited states, the protons or neutrons of nuclei have higher than normal energy levels.

Although energetically possible, radiative decay of this excited state has never been observed in Ta-180m. Researchers are now conducting experiments to measure this decay, which is expected to have a lifetime of about 1 million times the age of the universe.

The decay of excited states of nuclei provides insight into how nuclei decay into those states. Nuclear physicists have extensively studied the shape variations and formation of these short-lived isotopes, called isomers. However, they did not fully study one of the more extreme cases, Ta-180m decay.

Physicists can use atomic theory to predict Ta-180m decay based on knowledge of short-lived isomers, but this particular isomer has not been measured. Its exceptional stability challenges existing theories and models of atomic structure and decay. This means that measuring decay in Ta-180m is an unprecedented opportunity to contribute to atomic theory.

Now, for the first time, scientists have developed a test with the sensitivity needed to achieve the predicted half-life. The experiment produced initial data and established the longest ranges ever achieved in nuclear isomer studies. Research is Published In the journal Physical review letters.

In this project, physicists rearranged Majorana A very low background facility at the Sanford Underground Research Facility in South Dakota. Additionally, they introduced a significantly larger tantalum sample than previously used in similar studies.

Over the course of a year, the researchers collected data using germanium detectors that boasted exceptional energy resolution. They also developed analysis methods specifically designed to detect several expected decay signatures. These combined efforts enabled them to set unprecedented limits within the 10 range.¹⁸ up to 10¹⁹ years. This sensitivity level represents the first instance where half-life values ​​predicted from atomic theory can be reached.

Although the degradation process has not yet been observed, these developments have significantly improved the existing limits by one to two orders of magnitude. Furthermore, this advance has allowed researchers to rule out some of the parameter limitations associated with various possible dark matter particles.