Directionality and head-tail recognition in the keV-range with the MIMAC detector by deconvolution of the ionic signal

Abstract

Directional detection is the only strategy for the unambiguous identification of galactic Dark Matter (DM) even in the presence of an irreducible background such as beyond the neutrino floor. This approach requires measuring the direction of a DM-induced nuclear recoil in the keV-range. To probe such low energies, directional detectors must operate at high gain where 3D track reconstruction can be distorted by the influence of the numerous ions produced in the avalanches. The article describes the interplay between electrons and ions during signal formation in a Micromegas. It introduces SimuMimac, a simulation tool dedicated to high gain detection that agrees with MIMAC measurements. This work proposes an analytical formula to deconvolve the ionic signal induced on the grid from any measurements, with no need for prior nor ad hoc parameter. This deconvolution is experimentally tested and validated, revealing the fine structure of the primary electrons cloud and consequently leading to head-tail recognition in the keV-range. Finally, the article presents how this deconvolution can be used for directionality by reconstructing the spectra of mono-energetic 27 keV and 8 keV neutrons with an angular resolution better than 15°. This novel approach for directionality appears as complementary to the standard one from 3D tracks reconstruction and offers redundancy for improving directional performances at high gain in the keV region.

Directional detection of keV proton and carbon recoils with MIMAC

“…MIMAC relies on the principle of a Time Projection Chamber with a drift region of 25 cm, based on a Micromegas with a large gap of 512 μm for the amplification region. The detector is filled with a mixture of 50% i-C 4 H 10 + 50% CHF 3 at 30 mbar dedicated to keV recoils detection [17] and mainly composed of odd nuclei (H and F) opening the spin-dependent channel for WIMP detection [19]. The primary electrons, resulting from the ionization process by a nuclear recoil, drift towards the anode under an electric field of 84 V/cm leading to a drift velocity 𝑣 drift = 11.5 μm/ns according to the Magboltz simulation code [20].…”

“…A voltage is applied to extract electrons or ions of known kinetic energy from a few hundred of eV up to 50 keV. By interfacing Comimac with a MIMAC chamber, we have studied in detail the signal formation in the detector [17,22,23]. In this work, we use Comimac to calibrate the detector and to measure the Ionization Quenching Factor.…”

“…We analyze measurements performed in 2020 with a 27.24 ± 0.05 keV field and measurements of 2021 with an 8.12 ± 0.01 keV field. We published a first analysis of these measurements in [17].…”

“…The Flash-ADC measures the signal induced on the grid which corresponds to the charge integration as a function of time. In [17], we developed an analytical deconvolution formula to separate the electronic signal from the ionic signal measured by the Flash-ADC without introducing any ad hoc parameter. Since each avalanche induces an electronic signal for a period of about 1 ns, which is lower than the MIMAC time resolution, the extracted electronic signal provides the time distribution of the primary electrons cloud at the grid level, i.e.…”

“…To the best of the authors' knowledge, such directional performance was not achieved in a general configuration, we refer to [13,15,16] for some reviews. Recently, we published MIMAC measurements [17] demonstrating a better than 15 • angular resolution on keV proton tracks produced by mono-energetic neutrons, under the experimental restriction that the incident neutrons (and by extension the WIMPs) must be aligned with the drift direction of the MIMAC detector. The current paper extends this work to a general basis, i.e.…”