Tantalum-180m (180mTa), a rare isotope of tantalum whose decay has never been observed, is expected to have a lifetime approximately 1 million times longer than the age of the Universe.
The modified Majorana module in the assembly glovebox with germanium detector crystals and tantalum samples installed. Image credit: Majorana Collaboration.
Tantalum, a chemical element with the symbol Ta and atomic number 73, is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion-resistant.
It has multiple stable isotopes: two stable and 35 artificial radioisotopes.
The least abundant isotope, tantalum-180, is found naturally in a long-lived excited state.
In excited states, a nuclei’s protons or neutrons have higher than normal energy levels.
Although energetically possible, the radioactive decay of this excited state in tantalum-180m has never been observed.
Nuclear physicists from the Majorana Collaboration are now conducting experiments that aim to measure this decay, which is expected to have a lifetime approximately 1 million times longer than the age of the Universe.
For their experiments, they repurposed the Majorana Demonstrator at the Sanford Underground Research Facility.
Additionally, they introduced a substantially larger tantalum sample compared to any previously used in similar studies.
Over the course of a year, they collected data using an array of high-purity germanium detectors boasting exceptional energy resolution.
They also developed analysis methods specifically tailored to detect multiple anticipated decay signatures.
These combined efforts have enabled them to establish unprecedented limits, falling within the range of 1018 to 1019 years.
This level of sensitivity marks the first instance where predicted half-life values from nuclear theory have become reachable.
Although the decay process has not yet been observed, these advancements have significantly enhanced existing limits by one to two orders of magnitude.
Furthermore, this progress has allowed the Majorana team to dismiss certain parameter ranges associated with various potential dark matter particles.
“With new limits up to 1.5*1019 years, we improved existing limits by 1-2 orders of magnitude which are the most sensitive searches for a single β and electron capture decay ever achieved,” the authors said.
“Over all channels, the decay can be excluded for T1/2<0.29*1018 years.”
The results appear in the journal Physical Review Letters.
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I.J. Arnquist et al. (Majorana Collaboration). Constraints on the Decay of 180mTa. Phys. Rev. Lett 131 (15): 152501; doi: 10.1103/PhysRevLett.131.152501
