Constraints on the Decay of ${}^{180m}\mathrm{Ta}$

${}^{180m}\mathrm{Ta}$ is a rare nuclear isomer whose decay has never been observed. Its remarkably long lifetime surpasses the half-lives of all other known $\beta$ and electron capture decays due to the large K-spin differences and small energy differences between the isomeric and lower-energy states. Detecting its decay presents a significant experimental challenge but could shed light on neutrino-induced nucleosynthesis mechanisms, the nature of dark matter, and K-spin violation. For this study, we repurposed the Majorana Demonstrator, an experimental search for the neutrinoless double-beta decay of ${}^{76}\mathrm{Ge}$ using an array of high-purity germanium detectors, to search for the decay of ${}^{180m}\mathrm{Ta}$. More than 17 kg, the largest amount of tantalum metal ever used for such a search, was installed within the ultralow-background detector array. In this Letter, we present results from the first year of Ta data taking and provide an updated limit for the ${}^{180m}\mathrm{Ta}$ half-life on the different decay channels. With new limits up to $1.5\times {10}^{19}\mathrm{yr}$, we improved existing limits by 1–2 orders of magnitude which are the most sensitive searches for a single $\beta$ and electron capture decay ever achieved. Over all channels, the decay can be excluded for $T_{1/2}<0.29\times {10}^{18}\mathrm{yr}$.

Figure 1

Level diagram of the decay modes of ${}^{180m}\mathrm{Ta}$ (red arrows) based on data from Ref. [16]. Certain decay modes can also be observed indirectly when the ground state of ${}^{180}\mathrm{Ta}$ is populated that then decays further (pink dashed arrows). The observation of $\gamma$ rays at characteristic energies can be used to identify the different signatures (blue arrows, blue dashed for the not yet observed 37.7-keV transition). The nomenclature follows Eq. (1), and all energies are given in keV.

Figure 2

Left: the detector module during assembly. Right: technical drawing of two of the seven installed strings detector arrangement with three and four HPGe detectors (teal) and the tantalum sample disks (gray).

Figure 3

Count rate in real time for the region between 100 and 500 keV, where the ${}^{180m}\mathrm{Ta}$ signatures are expected. The count rate, not lifetime corrected, is due primarily to radioactivity in the Ta samples: ${}^{182}\mathrm{Ta}$ decay, the decay of other short-lived cosmogenic isotopes, and a constant rate from the $\mathrm{U}/\mathrm{Th}$ decay chains.

Figure 4

Simulated detection efficiency averaged per detector (blue dashed line) compared to the intensities of $\gamma$ rays from ${}^{182}\mathrm{Ta}$ decays with branching ratio greater than 1% (orange points). Since the absolute ${}^{182}\mathrm{Ta}$ activity is not known, the points are multiplied with an arbitrary constant $C$ to compare the distribution to the curve shape. The green points show efficiencies determined by the ${}^{208}\mathrm{Tl}$ coincidence method. The simulation is normalized to these points [scaling factor 0.95(6)], whereas the band represents the uncertainty due to scaling.

Figure 5

Summed spectra for all detectors for two selected ROI for the 37.7-keV (top) and the 332.3-keV (bottom) $\gamma$ rays, respectively. The yellow line shows the best fit of the background peaks and flat background. The red curve shows the best fit of the signal peak.

Figure 6

Top: 90% C.L. exclusion limits on the per-nucleon cross section for strongly interacting DM assuming a fractional relic density of $f_{\mathrm{DM}}={10}^{-4}$. The limit based on the nonobservation of the 93.3-keV line (blue dashed) is sensitive to DM-induced deexcitations to the ${}^{180}\mathrm{Ta}$ first excited ($2^{+}$) and ground ($1^{+}$) states. This can be compared to limits from Ref. [47] (gray dashed line), which is based on the 103.5-keV signature. The limit derived from the nonobservation of the 39.5-keV line (dashed orange) is sensitive only to deexcitations to the first excited state. Both cover phase space not covered by other experimental approaches [53]. Bottom: 90% C.L. exclusion limits on the per-nucleon cross section for inelastic DM with mass splitting $\delta M$. Color coding is identical to the top plot (the gray shaded region is based on Ref. [54]).