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

Periodic Reporting for period 2 - INITIUM (an Innovative Negative Ion TIme projection chamber for Underground dark Matter searches)

Reporting period: 2020-09-01 to 2022-02-28

INITIUM goal is to boost the development of gaseous Time Projection Chamber (TPC),in particular in direct Dark Matter (DM) searches.

The presence of DM is nowadays an established, yet still mysterious paradigm: deciphering its essence is one of the most compelling tasks for fundamental physics. Weakly Interacting Massive Particles (WIMPs) are well motivated DM candidates, independently predicted by Standard Model extensions and Big Bang cosmology. Direct DM searches look for very low energy (1-100 keV) nuclear recoils due to the elastic scattering of Weakly Interactive Massive Particles (WIMPs) in the active volume of the detector. The rarity of the expected interaction requires any backgrounds indistinguishable from the DM signal to be strictly controlled and minimised. For this reason, experiments are typically located underground to suppress cosmogenic backgrounds and are manufactured from materials with excellent radio-purity.

The expected WIMP scattering in the detector is due to the Earth’s relative motion with respect to the galactic halo, that is believed to contain high concentration of DM from measurement of the rotational curves of our Galaxy. This implies that an apparent WIMP wind coming from the Cygnus constellation is expected to be observable on our planet, with a change in direction of about 90 degrees for every 12 sidereal hours due to Earth’s axis orientation with respect to DM wind. The determination of the incoming direction of the WIMP particle can provide a correlation with an astrophysical source that no background whatsoever can mimic and therefore offers an unique key for a positive, unambiguous identification of a DM signal.

INITIUM focuses on the development of a 1 m3 Negative Ion TPC (NITPC) with Gas Electron Multipliers (GEMs) and optical readout with sCMOS cameras and PMTs at Laboratori Nazionali del Gran Sasso (LNGS) for directional DM searches. The combination of 3D optical sCMOS + PMT readout with reduced diffusion and fiducialization offered by the Negative Ion Drift (NID) can offer superior means for discrimination of electron recoils (ERs) from nuclear recoils (NRs) and directionality down to keV energies. Negative Ion Drift is a peculiar modification of the TPC principle in order to overcome the restrictions due to electron diffusion. When an highly electronegative dopant is added to the gas mixture, electrons liberated by the track ionisation process are quickly captured at very short distances O(10) um by the dopant molecules, that become negative ions, making this a NITPC. Due to the applied electric field, the anions drift to the anode acting as the effective track image carriers instead of electrons. Thanks to their higher mass anions thermalise during the drift, largely reducing longitudinal and transverse diffusion. Once at the anode amplification plane, the strong electric field strips the electron from the anion and a standard electron avalanche is generated. This characteristic allows for the use of longer drift distances, combined with improved tracking performances.

INITIUM sets into the context of the international CYGNUS proto-collaboration effort, to establish a multi-site multi-target Galactic Directional Recoil Observatory at the ton-scale that can test the DM hypothesis beyond the Neutrino Floor and measure the coherent scattering of neutrinos from the Sun and Supernovae.
We manufactured two prototypes to focus each on different sets of studies: MANGO, 10x10 cm2 readout, 5 cm drift, 1 sCMOS+1 PMT (Fig1), and LIME, 33x33 cm2 readout, 50 cm drift, 1 sCMOS+4 PMTs (Fig2). MANGO will study innovative gas mixtures and GEM structures for improved light yield and reduced diffusion, and LIME (identical to one of the modules foreseen to comprise INITIUM, Fig3) will verify on realistic dimensions underground INITIUM expected performances, testing materials and construction techniques. During their assembling, an old prototype LEMOn (20 x 24 cm2 readout, 20 cm drift) was used to evaluate the approach performances. Given the challenge of NID, we started with He:CF4 to first characterise the prototypes and better understand means for improvement before adding SF6.

With LEMOn we tested the response to low energy nuclear recoils (NR) induced by AmBe, comparing it to low energy electron recoils (ER) from Fe55 (Measur.Sci.Tech. 32 (2021) 2, 025902). A multivariate approach to improve on this results is under development and already showing very encouraging results.

With MANGO we are testing different GEM thicknesses and stacking, with different He:CF4:(SF6) ratios, to optimise light yield versus tracking (on going). Within these tests, we demonstrated for the first time electroluminescence by non-ionising electrons in He:CF4 (JINST 15 (2020) 08, P08018), by creating an electric field after the last GEM to accelerate the avalanche electrons to produce additional light after the charge amplification, and recently extended these studies to higher fields with improved results (paper in preparation).

LIME was commissioned overground with radioactive sources and cosmic rays before being installed in a hut in LNGS Tir Tunnel (Fig. 4), together with gas and DAQ auxiliary systems, in March 2022. This will allow to study INITIUM underground performances and, with dedicated shielding, characterise internal and external backgrounds and perform a preliminary measurement of environmental neutron flux.
Progress beyond the state of the art achieved up to now

-We obtained the first experimental evidence of electroluminescence by non-ionising electrons in He:CF4. While this phenomenon is well known and already exploited in Argon and Xenon based detector, this is the first time it has been observed with Helium and CF4. This opens new windows of opportunities for any gas detector based on this mixture and optically read out. The possibility of increasing the light production without the need for additional electric gain can in fact significantly help in reducing the experimental energy threshold, as well as improve the energy resolution.

-We started for the first time a systematic evaluation of sCMOS radioactivity and of its internal components and development of low-radioactivity objectives.This is very important for any sCMOS application when low radioactivity budget is required.

-We managed to stretch LIME 33 x 33 cm2 GEMs on low radioactivity PMMA rather than on CERN PCBs, known to be significantly radioactive, a very interesting development for GEM based gas detectors requiring low radioactivity budget.

Expected results until the end of the project

By the end of the project we expect to demonstrate the possibility of NID operation with an optical readout at atmospheric pressure and to test this technique in an underground environment with LIME and the upcoming 1 m3 INITIUM detector. We expect to establish the feasibility and proof-of-principle of the NID optical readout approach with sCMOS + PMT readout towards the realisation of a large scale detector for directional DM searches and solar neutrinos measurements. We expect to perform a precise spectral and directional measurement of underground LNGS neutron flux and establish the first directional limit for WIMP searches with Spin Independent coupling with sensitivity to O(GeV) WIMP masses.
Underground LIME location and hut layout
INITIUM 1 m3 detector preliminary design
MANGO prototype
LIME prototype