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HADES produces unexpected spin-off from open-access measurement into realm of Dark Matter

JRC scientists in the 225 m deep underground facility HADES, where the flux of cosmic rays is 10,000 time lower compared to above ground. The gamma-ray detectors re operating inside thick lead/copper shields under the blue dust-covers
JRC scientists in the 225 m deep underground facility HADES, where the flux of cosmic rays is 10,000 time lower compared to above ground. The gamma-ray detectors re operating inside thick lead/copper shields under the blue dust-covers
Apr 30 2020

An excellent example on science at its best, going in unpredicted ways into unchartered territories, was recently published.

A new article in Physical Review Letters  “Search for dark matter induced de-excitation of 180Tam puts forward a new technique to study Dark Matter (DM) interactions. Researchers used existing data from measurements performed in the 225 m deep underground laboratory HADES conducted as a JRC open-access project as seen in Physical Review Letters "Search for the decay of nature's rarest isotope 180mTa" and ScienceNews website.

Tantalum

Tantalum, an intriguing element, was discovered in 1802 by the Swedish chemist Anders Ekberg in the famous rock sample from Ytterby Mine, Sweden. Ekberg named the new element after the Greek half-god Tantalos who was sentenced to an eternity of suffering from hunger and thirst – “partly in allusion to its incapacity, when immersed in acid, to absorb any and be saturated”. The isotope 180mTa counts as being the rarest naturally existing isotope on Earth. Furthermore, 180mTa is the heaviest amongst all stable odd-odd nuclei (referring to the number of neutrons and protons) of which there are only nine.

The second excited state in 180Ta is a very long-lived metastable state (where the “m” in tantalum-180m refers to this 2nd excited state). It is in fact the most long-lived metastable state known to man, >4.5×1017 years. This metastable state has had implications for studies in building a gamma-ray laser as well as for models on its production in stars (stellar nucleosynthesis). The decay of this metastable state has never been detected. However, amongst the ten attempts, the best was made in HADES within the (EUFRAT) open access project number 21-14 by Technical University Dresden.

HADES

HADES is a research infrastructure located 225 m underground. Due to the reduction of cosmic rays by a factor 10,000, it is the perfect place for measurements of weak radioactivity and other rare processes.

Learn more for HADES, Underground laboratory on EU Science Hub and on the European Commision YouTube channel.

Dark Matter

Dark Matter (DM) is surrounding us and makes up 85% of all matter in the Universe. We observe its influence in the movement of stars in galaxies and in the evolution of the Universe as a whole - yet it has never been directly observed on earth. The nature of DM is unknown, and many existing theories predict particle Dark Matter with a wide range of possible masses or even a whole zoo of dark particles. Leading experiments search for DM collisions with detectors in underground laboratories. Such collisions are extremely rare (requiring large detectors and a low radioactivity environment) and the resulting signal is very weak (requiring sensitivity to small energies).

The new spin-off published in Physical Review Letters

A novel DM detection method is presented in the new article in Physical Review Letters  "Search for dark matter induced de-excitation of 180Tam" by a team lead by Björn Lehnert from Lawrence Berkeley Laboratory (previously at TU Dresden). The team propose that DM could interact more strongly than previously thought. In this case, DM with initial galactic velocities collides with the atmosphere and rock above the underground facility before reaching the detector. Most of its kinetic energy is lost and cannot be detected in conventional experiments. But such slow and thermalized DM can still interact with the isomeric state of 180Tam, causing it de-excite and gain a small kick in the process. By searching for gamma-rays that accompany such de-excitations information is obtained about the DM candidate.

The present study looks again at the existing data from HADES, which was initially used to set the best half-life limit for 180mTa - now with the goal of searching for DM. No gamma-rays associated with DM were detected but the researchers could derive the first direct constraint on the life-time of 180mTa against DM-induced transitions of >1.3×1014 years.

This analysis demonstrates that the energy stored in nuclear isomer samples can be used to probe “strongly interacting Dark Matter”. Such particles are too slow to be detected by conventional experiments since they require an “exothermic" process to make them experimentally observable. A different class of DM models, so-called “inelastic Dark Matter”, can be also probed by such “exothermic" isomers. DM becomes a composite particle that can be excited itself. The energy stored in isomers excites the DM significantly increasing its interaction cross-section and detection rate.

In addition to the first application of this new method, the team also excluded untested parameter space and strengthened existing constraints with an entirely different method. Proposing new experiments and other isomers, the researchers aim at further probing DM interactions using this method. One of these ideas is already being discussed at the 2 km -deep underground laboratory SNOLAB in Sudbury, Canada, for initiating a future experiment.

Figure 1:Six hyperpure tantalum discs used for the 180mTa measurements. The discs were cleaned by INFN Padua in Italy to remove surface impurities, then they were stored a year underground for 182Ta to decay before starting measurements. M. Hult JRC