Modelling the behaviour of microbulk Micromegas in Xenon/trimethylamine gas

We model the response of a state of the art micro-hole single-stage charge amplication device (`microbulk' Micromegas) in a gaseous atmosphere consisting of Xenon/trimethylamine at various concentrations and pressures. The amplifying structure, made with photo-lithographic techniques similar to those followed in the fabrication of gas electron multipliers (GEMs), consisted of a 100 um-side equilateral-triangle pattern with 50 um-diameter holes placed at its vertexes. Once the primary electrons are guided into the holes by virtue of an optimized field configuration, avalanches develop along the 50 um-height channels etched out of the original doubly copper-clad polyimide foil. In order to properly account for the strong field gradients at the holes' entrance as well as for the fluctuations of the avalanche process (that ultimately determine the achievable energy resolution), we abandoned the hydrodynamic framework, resorting to a purely microscopic description of the electron trajectories as obtained from elementary cross-sections. We show that achieving a satisfactory description needs additional assumptions about atom-molecule (Penning) transfer reactions and charge recombination to be made

The potential of discrimination methods in a high pressure xenon TPC for the search of the neutrinoless double-beta decay of Xe-136

In the search for the neutrinoless double beta decay of 136Xe, a high pressure xenon time projection chamber (HPXe-TPC) has two advantages over liquid xenon TPCs: a better energy resolution and the access to topological features, which may provide extra discrimination from background events. The PandaX-III experiment has recently proposed a 200 kg HPXe-TPC based on Micromegas readout planes, to be located at the Jinping Underground Laboratory in China. Its detection concept is based on two results obtained within the T-REX project: Micromegas readouts can be built with extremely low levels of radioactivity; and the operation in xenon-trimethylamine at 10 bar in realistic experimental conditions has proven an energy resolution of 3% FWHM at the region of interest. In this work, two discrimination methods are applied to simulated signal and background data in a generic 200 kg HPXe-TPC, based on two well-known algorithms of graph theory: the identification of connections and the search for the longest path. Rejection factors greater than 100 are obtained for small pixel sizes and a signal efficiency of 40%. Moreover, a new observable (the blob charge density) rejects better surface contaminations, which makes the use of a trigger signal (T0) not imperative in this experiment

Lessons from the operation of the "Penning-Fluorescent" TPC and prospects

We have recently reported the development of a new type of high-pressure Xenon time projection chamber operated with an ultra-low diffusion mixture and that simultaneously displays Penning effect and fluorescence in the near-visible region (300 nm). The concept, dubbed `Penning-Fluorescent' TPC, allows the simultaneous reconstruction of primary charge and scintillation with high topological and calorimetric fidelity

Background assessment for the TREX Dark Matter experiment

TREX-DM is conceived to look for low-mass Weakly Interacting Massive Particles (WIMPs) using a gas Time Projection Chamber equipped with micromegas readout planes at the Canfranc Underground Laboratory. The detector can hold in the active volume 20 l of pressurized gas up to 10 bar, corresponding to 0.30 kg of Ar or 0.16 kg of Ne. The micromegas are read with a self-triggered acquisition, allowing for thresholds below 0.4 keV (electron equivalent). A low background level in the lowest energy region is another essential requirement. To assess the expected background, all the relevant sources have been considered, including the measured fluxes of gamma radiation, muons and neutrons at the Canfranc Laboratory, together with the activity of most of the components used in the detector and ancillary systems, obtained in a complete assay program. The background contributions have been simulated by means of a dedicated application based on Geant4 and a custom-made code for the detector response. The background model developed for the detector presently installed in Canfranc points to levels from 1 to 10 counts keV-1 kg-1 d-1 in the region of interest, making TREX-DM competitive in the search for low-mass WIMPs. A roadmap to further decrease it down to 0.1 counts keV-1 kg-1 d-1 is underway.Comment: Final version, as accepted in Eur. Phys. J.

New CAST limit on the axion-photon interaction

Hypothetical low-mass particles, such as axions, provide a compelling explanation for the dark matter in the universe. Such particles are expected to emerge abundantly from the hot interior of stars. To test this prediction, the CERN Axion Solar Telescope (CAST) uses a 9 T refurbished Large Hadron Collider test magnet directed towards the Sun. In the strong magnetic field, solar axions can be converted to X-ray photons which can be recorded by X-ray detectors. In the 2013-2015 run, thanks to low-background detectors and a new X-ray telescope, the signal-to-noise ratio was increased by about a factor of three. Here, we report the best limit on the axion-photon coupling strength (0.66 × 10 -10 GeV -1 at 95% confidence level) set by CAST, which now reaches similar levels to the most restrictive astrophysical bounds

Accurate gamma and MeV-electron track reconstruction with an ultra-low diffusion Xenon/TMA TPC at 10 atm

We report the performance of a 10 atm Xenon/trimethylamine time projection chamber (TPC) for the detection of X-rays (30 keV) and gamma-rays (0.511-1.275 MeV) in conjunction with the accurate tracking of the associated electrons. When operated at such a high pressure and in similar to 1%-admixtures, trimethylamine (TMA) endows Xenon with an extremely low electron diffusion (1.3 +/- 0.13 mm-sigma (longitudinal), 0.95 +/- 0.20 mm-sigma (transverse) along 1 m drift) besides forming a convenient Penning-Fluorescent' mixture. The TPC, that houses 1.1 kg of gas in its fiducial volume, operated continuously for 100 live-days in charge amplification mode. The readout was performed through the recently introduced microbulk Micromegas technology and the AFTER chip, providing a 3D voxelization of 8 mm x 8 mm x 1.2 mm for approximately 10 cm/MeV-long electron tracks. Resolution in energy (epsilon) at full width half maximum (R) inside the fiducial volume ranged from R = 14.6% (30 keV) to R = 4.6% (1.275 MeV). This work was developed as part of the R&D program of the NEXT collaboration for future detector upgrades in the search of the neutrino-less double beta decay (beta beta 0 nu) in Xe-136, specifically those based on novel gas mixtures. Therefore we ultimately focus on the calorimetric and topological properties of the reconstructed MeV-electron tracks. In particular, the obtained energy resolution has been decomposed in its various contributions and improvements towards achieving the R =1.4%root MeV/epsilon levels obtained in small sensors are discussedThe NEXT collaboration acknowledges funding support from the following agencies and institutions: European Research Council under Advanced Grant 339787-NEXT and Starting Grant 240054-TREX, Spanish Ministerio de Economia y Competitividad under grants Consolider-Ingenio 2010 CSD2008-0037 (CUP) and CSD2007-00042 (CPAN), contracts FPA2008-03456 and FPA2009-13697; Portuguese Fundacao para a Ciencia e a Tecnologia; European FEDER under grant PPTDC/FIS/103860/2008; US Department Of Energy under contract DE-AC02-05CH11231.Gonzalez Diaz, D.; Álvarez Puerta, V.; Borges, FIG.; Camargo, M.; Carcel, S.; Cebrian, S.; Cervera, A.... (2015). Accurate gamma and MeV-electron track reconstruction with an ultra-low diffusion Xenon/TMA TPC at 10 atm. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. 804:8-24. https://doi.org/10.1016/j.nima.2015.08.033S82480

The potential of discrimination methods in a high pressure xenon TPC for the search of the neutrinoless double-beta decay of Xe-136

In the search for the neutrinoless double beta decay of 136Xe, a high pressure xenon time projection chamber (HPXe-TPC) has two advantages over liquid xenon TPCs: a better energy resolution and the access to topological features, which may provide extra discrimination from background events. The PandaX-III experiment has recently proposed a 200 kg HPXe-TPC based on Micromegas readout planes, to be located at the Jinping Underground Laboratory in China. Its detection concept is based on two results obtained within the T-REX project: Micromegas readouts can be built with extremely low levels of radioactivity; and the operation in xenon-trimethylamine at 10 bar in realistic experimental conditions has proven an energy resolution of 3% FWHM at the region of interest. In this work, two discrimination methods are applied to simulated signal and background data in a generic 200 kg HPXe-TPC, based on two well-known algorithms of graph theory: the identification of connections and the search for the longest path. Rejection factors greater than 100 are obtained for small pixel sizes and a signal efficiency of 40%. Moreover, a new observable (the blob charge density) rejects better surface contaminations, which makes the use of a trigger signal (T0) not imperative in this experiment