GEM-based TPC with CCD imaging for directional dark matter detection

Abstract

The most mature directional dark matter experiments at present all utilize low-pressure gas Time Projection Chamber (TPC) technologies. We discuss some of the challenges for this technology, for which balancing the goal of achieving the best sensitivity with that of cost effective scale-up requires optimization over a large parameter space. Critical for this are the precision measurements of the fundamental properties of both electron and nuclear recoil tracks down to the lowest detectable energies. Such measurements are necessary to provide a benchmark for background discrimination and directional sensitivity that could be used for future optimization studies for directional dark matter experiments. In this paper we describe a small, high resolution, high signal-to-noise GEM-based TPC with a 2D CCD readout designed for this goal. The performance of the detector was characterized using alpha particles, X-rays, gamma-rays, and neutrons, enabling detailed measurements of electron and nuclear recoil tracks. Stable effective gas gains of greater than 1 × 10 5 were obtained in 100 Torr of pure CF 4 by a cascade of three standard CERN GEMs each with a 140 µm pitch. The high signal-to-noise and sub-millimeter spatial resolution of the GEM amplification and CCD readout, together with low diffusion, allow for excellent background discrimination between electron and nuclear recoils down below ∼10 keVee (∼23 keVr fluorine recoil). Even lower thresholds, necessary for the detection of low mass WIMPs for example, might be achieved by lowering the pressure and utilizing full 3D track reconstruction. These and other paths for improvements are discussed, as are possible fundamental limitations imposed by the physics of energy loss.

Gaseous and dual-phase time projection chambers for imaging rare processes

“…3. Some representative images obtained with state of the art TPCs employed outside collider physics: a) β-delayed proton emission from 46 Fe [76]; b) a low energy C or F nucleus (ε = 214 keV) recoiling against a neutron [11]; c) a triple α event produced after the reaction 12 C(γ,α) 8 Be in a 150 mbar CO 2 /N 2 active target [15]; d) a 1. 275 MeV photoelectron from a 22 Na source in a 10 bar xenon/TMA admixture [36]; e) a low energy electron spiraling in a magnetic field, reconstructed in a ∼ 1 cm 3 mini-TPC with an InGrid device [10]; f) a cosmic ray shower obtained with the dual-phase argon TPC of the WA105 collaboration [9]; g) pair production of a 74.3 GeV photon in the HARPO polarimeter, based on pressurized argon [77]; h) electrons with energies above 30 keV, reconstructed in 50 mbar of CF 4 ; i) elastic scattering between two α-particles at around 1 bar, reconstructed with the AT-TPC [78].…”

“…First, we are referring here to situations where reaction products can arrange themselves in a wealth of topologically complex multi-track patterns (and even particle showers) as it often happens with neutrino interactions [8,9]. At the same time, however, TPCs in this category may have to deal with nearly point-like [10,11] or tortuous [12] tracks, too. These broad topological characteristics, together with the need of huge (and, in some cases, flexible) active volumes, can be expected to reduce the feasibility and benefits of a magnet, and indeed the effect of the magnetic field is considerably subtle in some instances (e.g.…”

“…Therefore, and contrary to TPCs at colliders, operation under magnetic field will be seldom found throughout this text. But even in rcases where the event topology is dull and a handful of fairly straight tracks must be detected, the ionization profiles may need to be reconstructed in modern TPCs with a very fine pixelization (mm and sub-mm scale), in order to identify the reaction products [15][16][17], the direction and sense of their momentum vector [11,12] or the polarization of an incoming particle [18,19]. Despite recent progress (e.g.…”

“…The position resolution on the other hand (that is, the precision with which the barycenter of a portion of the track can be reconstructed), can reach similar levels in both types of TPCs, meaning that they will recover each track's geometrical information with a comparable performance. Importantly, in TPCs used for the study of rare interactions, global information like the particle's range (e.g., [11]) and/or the total energy transferred to ionization and scintillation ( [22][23][24][25]) can become crucial particle discriminants. Such assets require, unlike in collider TPCs, full event containment.…”

“…2), making a technological classification increasingly impractical: (classical) charge-readout [16,[34][35][36], optical [12,15,37], negative-ion [38,39], liquid [40][41][42], dual-phase [9,[22][23][24]43], solid [44], Penning-fluorescent [45], Cherenkov [46], radial [47,48], spherical [49,50]... In the case of gaseous and dual-phase chambers (the topic of this review), their operational range of pressures goes from tens of mbar [51] to above 10 bar [12], their temperatures from 87 K [9] to 300 K, their readouts presently include wires [39], GEMs [11], thick GEMs/LEMs [9], Micromegas [36], µ-dot/µ-PIC [52], In-Grid [10], photomultipliers (PMs) [22], silicon PMs [12], and CCD or CMOS cameras [53,54]. Besides imaging the primary ionization with high accuracy they can, in some configurations, retrieve essential information from the gas scintillation, too.…”