an Innovative Negative Ion TIme projection chamber for Underground dark Matter searches

Project Information

INITIUM

Grant agreement ID: 818744

DOI

10.3030/818744

Project closed

EC signature date 18 February 2019

Start date 1 March 2019

End date 31 August 2025

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 995 719,00

EU contribution

€ 1 995 719,00

Coordinated by

GRAN SASSO SCIENCE INSTITUTE
Italy

Periodic Reporting for period 4 - INITIUM (an Innovative Negative Ion TIme projection chamber for Underground dark Matter searches)

Reporting period: 2023-09-01 to 2025-08-31

Summary of the context and overall objectives of the project

The presence of Dark Matter (DM) is an established, yet still mysterious paradigm: deciphering its essence is one of the most compelling tasks for fundamental physics. INITIUM goal is to boost the development of gaseous Time Projection Chambers (TPCs) in this field, by enabling the measurement of both energy and direction of low energy nuclear recoils, which offers the key for a positive identification of a DM signal by correlating with its galactic origin.

INITIUM innovative concept relies on the combination of Photomultipliers (PMTs) and sCMOS camera to optically readout a gaseous Time Projection Chamber (TPC) and achieve 3D tracking at keV energies over large volumes. Within this approach, INITIUM pioneers the atmospheric pressure operation of Negative Ion Drift (NID) mode, where electronegative dopants capture ionisation electrons and drift as negative ions, drastically reducing diffusion and enabling longer drift distances.

INITIUM was carried out in synergy with the CYGNO effort, which employs the same TPC architecture with electron drift (ED) in He:CF4 (a mixture of known reduced diffusion), and shared infrastructures, data analysis, R&D activities and long-term goals towards a large scale directional DM experiment. Running both branches in parallel was a deliberate risk-mitigation strategy to guarantee scientific output while pushing a more innovative technology.

INITIUM reached its main objectives, namely the first demonstration of NID operation with optical TPC readout at atmospheric pressure and the long term operation of prototypes in both ED and NID. These achievements were enabled by the R&Ds with the small MANGO detector, and the long-term underground data taking campaign performed with larger 50 L LIME prototype. Thanks to these, INITIUM demonstrated low energy tracks reconstruction, directionality and particle identification, together with a detailed understanding of internal and external backgrounds over large volumes.

All these results are now converging into a 0.4 m3 detector representing the final INITIUM objectives currently under construction, with foreseen underground installation in Jan 2026. Its goal is to demonstrate the scalability of INITIUM approach and the potentialities of a O(30) m3 detector to provide competitive and directional physics sensitivity. By demonstrating the maturity and scalability of large optical gaseous TPCs at atmospheric pressure (both in electron drift and NID regimes), INITIUM not only fulfils its goals, but marks a decisive breakthrough opening the way to large volumes directional rare events experiments towards a ton-scale CYGNUS Galactic Directional Recoil Observatory.

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

INITIUM achieved its goals by employing 3 prototypes:

--MANGO, 10 × 10 cm2 area, 1 to 15 cm drift, 1 sCMOS+1 PMT, R&D studies;

--LEMOn, 20 × 24 cm2 area, 20 cm drift, 1 sCMOS+1 PMT, medium size realisation;

--LIME, 33 × 33 cm2 area, 50 cm drift, 1 sCMOS+4 PMTs, full-scale module of INITIUM concept operated for > 2 years underground with full auxiliary systems.

With these:

--we performed systematic studies of ED and NID mixtures and amplification strategies (arXiv:2507.02474 arXiv:2505.06362 Eur.Phys.J.C 84 (2024) 10, 1122, Phys.Lett.B 855 (2024) 138759, JINST 19 (2024) 06, P06021, J.Phys.Conf.Ser. 1498 (2020) 012017, JINST 15 (2020) 08, P08018, JINST 14 (2019) P07011 + papers in preparation on NID atmospheric measurements);

--we developed original sCMOS images and PMT waveforms analysis techniques (JINST 15 (2020) 12, T12003, Measur.Sci.Tech. 34 (2023) 12, 125024, Measur.Sci.Tech. 34 (2023) 12, 125145, CERN-THESIS-2023-323, CERN-THESIS-2023-315, arXiv: 2509.10890) and simulations (arXiv:2505.06362);

--we characterised optical TPC approach response to low energy electron recoils, nuclear recoils and alpha particles, including directionality and particle identification capabilities (Eur.Phys.J.C 83 (2023) 10, 946, arXiv:2505.06362 arXiv: 2509.10890 arXiv:2408.03760 CERN-THESIS-2024-211, Astrophys.Space Sci.Proc. 60 (2023) 43-47, Measur.Sci.Tech. 32 (2021) 2, 025902);

--we validated LIME as INTIUM/CYGNO-04 basic module underground, including automated operation and monitoring;

--we characterised LIME internal and external backgrounds and validated their MonteCarlo simulation on data;

--we performed the first atmospheric pressure optical NID underground operation with a 50 L detector;

These results were employed in the realisation of the 0.4 m3 demonstrator (thanks also to the crucial support of the material radioactivity measurements of Special Techniques Division of LNGS), the final detector foreseen in Annex 1 currently under construction, adapted in dimensions to the assigned space (doi:10.15161/oar.it/76967). The volume reduction do no impact ERC objectives, since the physics reach is only minimally affected, while the scalability and cost-effectiveness are improved by the reduced number of sCMOS per unit volume.

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)

--INITIUM achieved the first optical NID operation at atmospheric pressure, with a novel PMT analysis (arXiv: 2509.10890) demonstrated the presence of minority carriers in He:CF4:SF6 (NID mobility paper in preparation), developed a phenomenological model of NID GEM amplification (NID diffusion papers in preparation) and operated a 50 L detector underground in atmospheric pressure NID mode;

--The studies in ED mode led to the first comprehensive characterisation and optimisation of optical TPCs response in He:CF4 at atmospheric pressure (arXiv:2507.02474 Eur. Phys. J. C 84 (2024) 1122, JINST 15 (2020) 08 P08018, JINST 14 (2019) P07011, Eur. Phys. J. C 83 (2023) 946, Phys.Lett.B 855 (2024) 138759).

--LIME operation provided the first underground characterisation of optical TPC internal and external backgrounds (arXiv: 2509.10890) and the first underground LNGS environmental neutron flux directional measurement (analysis on going);

--The synergy between the NID and ED lines enabled a shared development of:
(a) original sCMOS images and PMT waveform reconstruction and simulation (JINST 15 (2020) 12, T12003, Measur.Sci.Tech. 34 (2023) 12, 125024, Measur.Sci.Tech. 34 (2023) 12, 125145, CERN-THESIS-2023-323, CERN-THESIS-2023-315, arXiv: 2509.10890 arXiv:2505.06362 arXiv:2408.03760);
(b) the first demonstration of low energy electron and nuclear recoils and alphas directionality (arXiv:2408.03760 CERN-THESIS-2024-211, arXiv: 2509.10890 arXiv:2506.04973) with an optical TPC;
(c) the first demonstration of background rejection with sCMOS images (Measur.Sci.Tech. 32 (2021) 2, 025902, Astrophys.Space Sci.Proc. 60 (2023));
(d) the first demonstration of WIMP sensitivity below 1 GeV (Instruments 6 (2022) 1, 6, Phys.Lett.B 855 (2024) 138759) and feasibility of solar neutrino spectroscopy (arXiv:2408.03760 Eur.Phys.J.C 84 (2024) 10, 1021) with a directional detector.

--The upcoming underground operation of the 0.4 m3 detector realises the final pioneering INITIUM objective of a scalable high-precision optical TPC, marking a breakthrough in the advancement of directional DM experiment and establishing the proposed experimental technique as pathfinder and benchmark of the ton-scale CYGNUS Galactic Directional Recoil Observatory (Eur.Phys.J.C 84 (2024) 10, 1021).

--INITIUM development generates a compelling long-term impact on high precision tracking detector across domains, already contributing to significant advancements in hard x-ray polarimetry space missions (arXiv:2510.26239).

GSSI PhD students and me working on MANGO

0.4 m3 detector underground infrastructure in Hall F at LNGS

0.4 m3 detector exploded technical drawing

LIME underground installation and shieldings

MANGO prototype

LIME prototype

0.4 m3 detector PMMA vessel and field cage foil under construction

0.4 m3 detector schematics