[Paper Review] Directional detection of Dark Matter with the MIcro-tpc MAtrix of Chambers

C. Couturier, O. Guillaudin+11 more|arXiv (Cornell University)|Jul 29, 2016

Dark Matter and Cosmic Phenomena4 references3 citations

TL;DR

This paper presents the MIMAC directional dark matter detector, which uses a low-pressure gas Time Projection Chamber with pixelated Micromegas to reconstruct 3D tracks of nuclear recoils from weakly interacting massive particles (WIMPs). It demonstrates successful reconstruction of 19F ion tracks down to 5 keV with 25 keV kinetic energy, achieving 6.5 mm track lengths and 9 keV ionization energy, enabling directional discrimination to suppress neutron backgrounds.

ABSTRACT

Particles weakly interacting with ordinary matter, with an associated mass of the order of an atomic nucleus (WIMPs), are plausible candidates for Dark Matter. The direct detection of an elastic collision of a target nuclei induced by one of these WIMPs has to be discriminated from the signal produced by the neutrons, which leaves the same signal in a detector. The MIMAC (MIcro-tpc MAtrix of Chambers) collaboration has developed an original prototype detector which combines a large pixelated Micromegas coupled with a fast, self-triggering, electronics. Aspects of the two-chamber module in operation in the Modane Underground Laboratory are presented: calibration, characterization of the $^{222}$Rn progeny. A new test bench combining a MIMAC chamber with the COMIMAC portable quenching line has been set up to characterize the 3D tracks of low energy ions in the MIMAC gas mixture: the preliminary results thereof are presented. Future steps are briefly discussed.

Motivation & Objective

• Develop a directional dark matter detector capable of distinguishing WIMP-induced nuclear recoils from neutron and background events.
• Overcome limitations of non-directional detectors by exploiting the anisotropic angular distribution of WIMP-induced recoils due to Earth's motion.
• Achieve precise energy and direction reconstruction of low-energy nuclear recoils using a low-pressure gas TPC with pixelated Micromegas.
• Characterize ionization quenching factors (IQF) for accurate energy measurement in the MIMAC gas mixture at low energies (<50 keV).
• Demonstrate the feasibility of directional detection with a prototype two-chamber module in the Modane Underground Laboratory.

Proposed method

• Use a 50 mbar gas mixture of 70% CF4, 28% CHF3, and 2% C4H10 in a low-pressure Time Projection Chamber (TPC) to detect ionization from nuclear recoils.
• Employ a 256×256 pixelated Micromegas with 256×256 channels at 180 V/cm drift field to amplify primary electrons and record 3D track coordinates.
• Implement a self-triggered electronics system sampling at 50 MHz to resolve time-slices of 20 ns, enabling depth (Z) reconstruction of tracks.
• Use a dual-chamber design with a shared 12 µm aluminized mylar cathode to improve signal-to-noise and directional sensitivity.
• Calibrate the detector weekly using X-rays from 5.9 keV (55Fe) and 6.4 keV (222Rn progeny) and 8.1 keV (Cu) foils to maintain stability.
• Utilize the COMIMAC portable ion beam facility to measure ionization quenching factors (IQF) for 19F ions at energies from 5 to 30 keV.

Experimental results

Research questions

• RQ1 Can a directional TPC with pixelated Micromegas achieve sub-keV sensitivity for nuclear recoils in a low-pressure gas mixture?
• RQ2 To what extent does the ionization quenching factor (IQF) in the MIMAC gas mixture deviate from theoretical models at low energies (<50 keV)?
• RQ3 What is the minimum kinetic energy of 19F ions that can be reliably reconstructed in terms of track length and ionization energy?
• RQ4 How well is the initial recoil direction preserved and measurable in the 3D track reconstruction using the current MIMAC setup?
• RQ5 Can the COMIMAC facility provide accurate IQF measurements for low-energy ions in the MIMAC gas mixture to enable precise energy reconstruction?

Key findings

• The MIMAC detector successfully reconstructed 19F ion tracks with a kinetic energy of 25 keV, measuring a corresponding ionization energy of 9 keV.
• The longest reconstructed 19F tracks reached 6.5 mm (13 timeslices of 20 ns), corresponding to a maximum occurrence at 6.5 mm in the track length distribution.
• The detector achieved stable operation for over two years with weekly calibration, demonstrating long-term gas quality stability using a circulating pump system.
• Ionization quenching factors (IQF) were measured using the COMIMAC facility, revealing discrepancies with existing models at low energies (<50 keV), necessitating direct measurement.
• The system demonstrated the ability to reconstruct 19F ion tracks down to 5 keV kinetic energy, with ongoing efforts to extend sensitivity to 1 keV.
• The dual-chamber design with a shared cathode enabled effective background suppression and improved directional sensitivity in the Modane Underground Laboratory.