C.D.R. Azevedo

First proof of topological signature in high pressure xenon gas with electroluminescence amplification

A bstractThe NEXT experiment aims to observe the neutrinoless double beta decay of 136Xe in a high-pressure xenon gas TPC using electroluminescence (EL) to amplify the signal from ionization. One of the main advantages of this technology is the possibility to reconstruct the topology of events with energies close to Qββ. This paper presents the first demonstration that the topology provides extra handles to reject background events using data obtained with the NEXT-DEMO prototype.Single electrons resulting from the interactions of 22Na 1275 keV gammas and electronpositron pairs produced by conversions of gammas from the 228Th decay chain were used to represent the background and the signal in a double beta decay. These data were used to develop algorithms for the reconstruction of tracks and the identification of the energy deposited at the end-points, providing an extra background rejection factor of 24.3 ± 1.4 (stat.)%, while maintaining an efficiency of 66.7 ± 1.% for signal events.

THGEM-based detectors for sampling elements in DHCAL: laboratory and beam evaluation

We report on the results of an extensive R&}D program aimed at the evaluation of Thick-Gas Electron Multipliers (THGEM) as potential active elements for Digital Hadron Calorimetry (DHCAL). Results are presented on efficiency, pad multiplicity and discharge probability of a 10x10 cm2 prototype detector with 1 cm2 readout pads. The detector is comprised of single- or double-THGEM multipliers coupled to the pad electrode either directly or via a resistive anode. Investigations employing standard discrete electronics and the KPiX readout system have been carried out both under laboratory conditions and with muons and pions at the CERN RD51 test beam. For detectors having a charge-induction gap, it has been shown that even a ~ 6 mm thick single-THGEM detector reached detection efficiencies above 95%, with pad-hit multiplicity of 1.1?1.2 per event; discharge probabilities were of the order of 10?6-10?5 sparks/trigger, depending on the detector structure and gain. Preliminary beam tests with a WELL hole-structure, closed by a resistive anode, yielded discharge probabilities of < 2x10?6 for an efficiency of ~ 95%. Methods are presented to reduce charge-spread and pad multiplicity with resistive anodes. The new method showed good prospects for further evaluation of very thin THGEM-based detectors as potential active elements for DHCAL, with competitive performances, simplicity and robustness. Further developments are in course.

Single photon counting x-ray imaging system using a micro hole and strip plate

A single photon counting x-ray imaging system based on a MicroHole and Strip Plate (MHSP) using a resistive charge division method was implemented. The MHSP is a hybrid microstructure that combines within the same device the features of the Gas Electron Multiplier (GEM) and the Micro-strip Gas Chamber (MSGC). The MHSP presents two multiplication stages thus allowing reaching high gains. Two thin orthogonal resistive lines of about 100 Omega between each strip (top side) and between each anode strip (bottom side) allows us to obtain the actual position in both x and y directions. The readout electronics use only two charge preamplifiers in each dimension and a TNT module with four independent ADCs controlled through FPGAs that allow converting and registering the four signals through the USB port of a computer. Position resolution in both dimensions as a function of the detector parameters are presented for a 30 keV copper-target x-ray tube. Detector position resolution of sigmax=130 mum and sigmay=250 mum were achieved for 8keV, making it suitable for many applications in single photon X-ray imaging. The first x-ray images produced by the system are also presented together with a discussion of the image quality and the future prospects.

Sensitivity of NEXT-100 to neutrinoless double beta decay

A bstractNEXT-100 is an electroluminescent high-pressure xenon gas time projection chamber that will search for the neutrinoless double beta (0νββ) decay of 136Xe. The detector possesses two features of great value for 0νββ searches: energy resolution better than 1% FWHM at the Q value of 136Xe and track reconstruction for the discrimination of signal and background events. This combination results in excellent sensitivity, as discussed in this paper. Material-screening measurements and a detailed Monte Carlo detector simulation predict a background rate for NEXT-100 of at most 4 × 10−4 counts keV−1 kg−1 yr−1. Accordingly, the detector will reach a sensitivity to the 0νββ-decay half-life of 2.8 × 1025 years (90% CL) for an exposure of 100 kg·year, or 6.0 × 1025 years after a run of 3 effective years.

Background rejection in NEXT using deep neural networks

We investigate the potential of using deep learning techniques to reject background events in searches for neutrinoless double beta decay with high pressure xenon time projection chambers capable of detailed track reconstruction. The differences in the topological signatures of background and signal events can be learned by deep neural networks via training over many thousands of events. These networks can then be used to classify further events as signal or background, providing an additional background rejection factor at an acceptable loss of efficiency. The networks trained in this study performed better than previous methods developed based on the use of the same topological signatures by a factor of 1.2 to 1.6, and there is potential for further improvement.

Beam studies of the segmented resistive WELL: a potential thin sampling element for digital hadron calorimetry

Thick Gas Electron Multipliers (THGEMs) have the potential of constituting thin, robust sampling elements in Digital Hadron Calorimetry (DHCAL) in future colliders. We report on recent beam studies of new single- and double-THGEM-like structures; the multiplier is a Segmented Resistive WELL (SRWELL) - a single-faced THGEM in contact with a segmented resistive layer inductively coupled to readout pads. Several 10 10 cm 2 configurations with a total thickness of 5-6 mm (excluding electronics) with 1 cm 2 pads coupled to APV-SRS readout were investigated with muons and pions. Detection e ciencies in the 98% range were recorded with average pad-multiplicity of 1.1. The resistive anode resulted in e cient discharge damping, with potential drops of a few volts; discharge probabilities were 10 7 for muons and 10 6 for pions in the double-stage configuration, at rates of a few kHz/cm 2 . Further optimization work and research on larger detectors are underway.

First proof of topological signature in the high pressure xenon gas TPC with electroluminescence amplification for the NEXT experiment

A bstractThe NEXT experiment aims to observe the neutrinoless double beta decay of 136Xe in a high-pressure xenon gas TPC using electroluminescence (EL) to amplify the signal from ionization. One of the main advantages of this technology is the possibility to reconstruct the topology of events with energies close to Qββ. This paper presents the first demonstration that the topology provides extra handles to reject background events using data obtained with the NEXT-DEMO prototype.Single electrons resulting from the interactions of 22Na 1275 keV gammas and electronpositron pairs produced by conversions of gammas from the 228Th decay chain were used to represent the background and the signal in a double beta decay. These data were used to develop algorithms for the reconstruction of tracks and the identification of the energy deposited at the end-points, providing an extra background rejection factor of 24.3 ± 1.4 (stat.)%, while maintaining an efficiency of 66.7 ± 1.% for signal events.

Application and performance of an ML-EM algorithm in NEXT

The goal of the NEXT experiment is the observation of neutrinoless double beta decay in 136Xe using a gaseous xenon TPC with electroluminescent amplification and specialized photodetector arrays for calorimetry and tracking. The NEXT Collaboration is exploring a number of reconstruction algorithms to exploit the full potential of the detector. This paper describes one of them: the Maximum Likelihood Expectation Maximization (ML-EM) method, a generic iterative algorithm to find maximum-likelihood estimates of parameters that has been applied to solve many different types of complex inverse problems. In particular, we discuss a bi-dimensional version of the method in which the photosensor signals integrated over time are used to reconstruct a transverse projection of the event. First results show that, when applied to detector simulation data, the algorithm achieves nearly optimal energy resolution (better than 0.5% FWHM at the Q value of 136Xe) for events distributed over the full active volume of the TPC.

First in-beam studies of a Resistive-Plate WELL gaseous multiplier

We present the results of the first in-beam studies of a medium size (10 × 10 cm 2 ) Resistive-Plate WELL (RPWELL): a single-sided THGEM coupled to a pad anode through a resistive layer of high bulk resistivity (~109 Ωcm). The 6.2 mm thick (excluding readout electronics) single-stage detector was studied with 150 GeV muons and pions. Signals were recorded from 1×1 cm 2 square copper pads with APV25-SRS readout electronics. The single-element detector was operated in Ne/(5%CH4) at a gas gain of a few times 104 , reaching 99% detection efficiency at average pad multiplicity of ~1.2. Operation at particle fluxes up to ~104 Hz/cm 2 resulted in ~23% gain drop leading to ~5% efficiency loss. The striking feature was the discharge-free operation, also in intense pion beams. These results pave the way towards robust, efficient large-scale detectors for applications requiring economic solutions at moderate spatial and energy resolutions.

THGEM gain calculations using Garfield++: solving discrepancies between simulation and experimental data

Discrepancies between the measured and simulated gain in Thick Micropatterned gaseous detectors (MPGD), namely THGEM, have been observed by several groups. In order to simulate the electron avalanches and the gain the community relies on the calculations performed in Garfield++, known to produce differences of 2 orders of magnitude in comparison to the experimental data for thick MPGDs. In this work, simulations performed for Ne/5%CH4, Ar/5%CH4 and Ar/30%CO2 mixtures shows that Garfield++ is able to perfectly describe the experimental data if Penning effect is included in the simulation. The comparison between the number of excitations which may lead to a Penning transfer, is shown for THGEM and GEM, explaining the less pronounced gain discrepancies observed in GEM.