Low background techniques on particle detectors for Rare Event Searches

Project Information

LOBRES

Grant agreement ID: 101026819

DOI

10.3030/101026819

Project closed

EC signature date 16 April 2021

Start date 1 September 2022

End date 18 January 2025

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 160 932,48

EU contribution

€ 160 932,48

Coordinated by

UNIVERSIDAD DE ZARAGOZA
Spain

Periodic Reporting for period 1 - LOBRES (Low background techniques on particle detectors for Rare Event Searches)

Reporting period: 2022-09-01 to 2024-08-31

Summary of the context and overall objectives of the project

The nature of Dark Matter (DM), which composes 27% of the Universe is one of the biggest open questions in modern physics. The most attractive DM candidates arise from extensions of the Standard Model (SM): axions and WIPMs. Weakly Interacting Massive Particles (WIMPs) appear in well-motivated extensions of the SM, like supersymmetry, and have been extensively searched over the last two decades. On the other hand, the axion arises from extensions of the SM as a solution to the strong-CP problem. Axions and WIMPs could have been produced in an early stage of the Universe and could separately explain the total amount of DM in the Universe.
The use of Micromegas readouts has been recently introduced for axion and WIMPs searches and proposed for experiments like TREX-DM and IAXO. TREX-DM is located at the Canfranc Underground Laboratory (LSC) and looking for low-mass WIMPs (∼10 GeV), in a mass range that is currently attracting the interest of all mainstream experiments. IAXO is a new generation solar axion helioscope that aims to improve current sensitivities by more than one order of magnitude; the collaboration is currently building an intermediate version of the experiment, called BabyIAXO, with discovery potential. The low counting rates expected in these experiments require the experimental background to be reduced to the lowest possible level. Another important aspect to enhance the physics potential of IAXO and TREX-DM is the reduction of the low energy threshold, which would improve TREX-DM sensitivities to lower WIMP masses. It would also make IAXO sensitive to very low energy solar axions produced in axion-electron conversion processes.
The objectives of the project are to reduce the background levels and to lower the energy threshold in IAXO and TREX-DM, with the aim of achieving sensitivity scenarios with discovery potential for axions and low-mass WIMPs. Different research lines have been developed to achieve these objectives: simulations to understand the different background contributions; the study of new Micromegas architectures, such as a Gas Electron Multiplier (GEM) pre-amplification stage and the development of different test-benches for the study of the background at surface level and in the LSC. The measurements carried out at the LSC showed record background levels of 10-7counts keV-1cm-2s-1. This level proves that the intrinsic background achievable with the Micromegas detectors meets the requirements for BabyIAXO. On the other hand, although a record surface level background of about 8.6 x10-7counts keV-1cm-2s-1 was also achieved, experimental data and simulations results indicate that new active shielding strategies are required to enable the tagging of cosmic-induced neutrons, which are considered to be the main contribution to the background at surface level. Finally, the development of a GEM preamplifier stage on top of the Micromegas readout planes showed excellent results with record thresholds down to 50 eV.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Simulations have been extensively used in this project, in which a background model has been developed to understand the effect of cosmic induced events. Simulation results show that the Micromegas background at surface level is limited by cosmic induced neutrons, while current active shielding strategies, which are based on plastic scintillators do not have capabilities to tag these neutrons. Therefore, these results were the starting point of a new research line for the development of new active shielding strategies with neutron tagging capabilities.
Different studies on novel Micromegas architectures were performed with the aim to improve the low energy threshold, including studies on segmented mesh Micromegas and the combination of a Micromegas with a GEM preamplifier stage. The GEM preamplifier stage showed promising results, with gain factors up to 100, while low energy thresholds of about 50 eV has been already achieved in the TREX-DM experiment, thus increasing TREX-DM sensitivity to low mass WIMPs.
Various test benches were developed with the aim of understanding the different contributions at surface level and at the LSC. The Micromegas operating at the LSC has been taking data using Xe and Ar-based mixtures and obtaining record background levels of 10-7counts keV-1cm-2s-1. The test-bench at surface level consist of a Micromegas detector fully covered by plastic scintillators as active muon veto, obtaining background levels of 8.6 x counts keV-1cm-2s-1. This background level is still far from BabyIAXO requirements, yet this value is the lowest background reached at surface level; simulations point out at cosmic-induced neutrons as the limiting factor of this background.
The project results have been disseminated in 4 publications (two more are in preparation); the results of the project and the methodology have been presented in the TAUP2023 international conference. The know-how on low background techniques has been disseminated during several IAXO Collaboration Meetings. For the general public, an article in a scientific inlet of a large circulation newspaper was published, while dedicated outreach talks and activities during the European Researcher’s Night for a broader audience have also been given. On the other hand, the project results are being exploited in the new research lines which have arisen from this project, such as the development of new active shielding strategies. In addition, the excellent results obtained in the reduction of the low energy threshold using a GEM preamplification stage can open new avenues and research lines. Besides, low WIMP mass searches, low energy threshold detectors are essential in other fields, such as reactor neutrino-nucleus scattering experiments.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

This project has set the baseline for a new state-of-the-art background in Micromegas detectors for rare event searches. The background level achieved at the LSC may be limited by 39Ar present in the conversion gas of the Micromegas. Regarding the Xe-based mixtures, the background level is currently limited by 222Rn contamination from the filters in the gas recirculation system, and different strategies are being implemented in order to mitigate the effect of the 222Rn contamination. Background levels below 10-7counts keV-1cm-2s-1 are deemed feasible after adopting these strategies. On the other hand, the development of an efficient active veto with neutron-tagging capabilities could lead to a background reduction of the Micromegas detectors operating at surface to their intrinsic levels. More importantly, the development of a GEM preamplification stage for the Micromegas detectors has set a new state-of-the-art threshold for the Micromegas detectors, thus improving TREX-DM sensitivity to low mass WIMPs. These results will be extrapolated to axion searches, and will make IAXO sensitive to solar axions produced in axion-electron conversion processes, enhancing the discovery potential of IAXO to solar axions.
The project had a foremost impact in the career of the researcher and secondarily at the hosting group. The researcher has gained notoriety and visibility on the field and he has become an independent researcher. On the part of the hosting group, the new research lines which have arisen from this project, such as the development of new active shielding strategies with neutron-tagging capabilities, have reinforced and diversified the research activities at the hosting group.

Demonstrator set-up at the Underground Laboratory of Canfranc