A. L. Ferreira

Solid–fluid coexistence of the Lennard-Jones system from absolute free energy calculations

The thermodynamic properties of coexisting solid and fluid phases are computed for the Lennard-Jones system. We make absolute Helmholtz free energy calculations for fluid and solid reference thermodynamic states and we compute differences in free energy relatively to these states in order to obtain thermodynamic properties in a range of temperatures and densities. For the free energy difference calculations we use a previously developed method [A. L. Ferreira and M. A. Barroso, Phys. Rev. E 61, 1195 (2000)]. Our results are compared with others available in the literature. Furthermore, we present absolute values of the Helmholtz free energy along solid–fluid coexistence lines.

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.

Present Status and Future Perspectives of the NEXT Experiment

NEXT is an experiment dedicated to neutrinoless double beta decay searches in xenon. The detector is a TPC, holding 100 kg of high-pressure xenon enriched in the 136Xe isotope. It is under construction in the Laboratorio Subterraneo de Canfranc in Spain, and it will begin operations in 2015. The NEXT detector concept provides an energy resolutionbetter than 1% FWHM and a topological signal that can be used to reduce the background. Furthermore, the NEXT technology can be extrapolated to a 1 ton-scale experiment.

Initial results of NEXT-DEMO, a large-scale prototype of the NEXT-100 experiment

This work was supported by the following agencies and institutions: the Ministerio de Economia y Competitividad of Spain under grants CONSOLIDER-Ingenio 2010 CSD2008-0037 (CUP) and FPA2009-13697-C04-04; the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under contract no. DE-AC02-05CH11231; and the Portuguese FCT and FEDER through the program COMPETE, project PTDC/FIS/103860/2008. J. Renner (LBNL) acknowledges the support of a US DOE NNSA Stewardship Science Graduate Fellowship under contract no. DE-FC52-08NA28752.

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.

Radiopurity control in the NEXT-100 double beta decay experiment: procedures and initial measurements

We deeply acknowledge LSC directorate and staff for their strong support for performing the measurements at the LSC Radiopurity Service. The NEXT Collaboration acknowledges funding support from the following agencies and institutions: the Spanish Ministerio de Economia y Competitividad under grants CONSOLIDER-Ingenio 2010 CSD2008-0037 (CUP), Consolider-Ingenio 2010 CSD2007-00042 (CPAN), and under contracts ref. FPA2008-03456, FPA2009-13697-C04-04; FCT(Lisbon) and FEDER under grant PTDC/FIS/103860/2008; the European Commission under the European Research Council T-REX Starting Grant ref. ERC-2009-StG-240054 of the IDEAS program of the 7th EU Framework Program; Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under contract no. DE-AC02-05CH11231. Part of these grants are funded by the European Regional Development Fund (ERDF/FEDER). J. Renner (LBNL) acknowledges the support of a US DOE NNSA Stewardship Science Graduate Fellowship under contract no. DE-FC52-08NA28752. F.I. acknowledges the support from the Eurotalents program.

A simulation toolkit for electroluminescence assessment in rare event experiments

Abstract A good understanding of electroluminescence is a prerequisite when optimising double-phase noble gas detectors for Dark Matter searches and high-pressure xenon TPCs for neutrinoless double beta decay detection. A simulation toolkit for calculating the emission of light through electron impact on neon, argon, krypton and xenon has been developed using the Magboltz and Garfield programs. Calculated excitation and electroluminescence efficiencies, electroluminescence yield and associated statistical fluctuations are presented as a function of electric field. Good agreement with experiment and with Monte Carlo simulations has been obtained.

Ionization and scintillation response of high-pressure xenon gas to alpha particles

High-pressure xenon gas is an attractive detection medium for a variety of applications in fundamental and applied physics. In this paper we study the ionization and scintillation detection properties of xenon gas at 10 bar pressure. For this purpose, we use a source of alpha particles in the NEXT-DEMO time projection chamber, the large scale prototype of the NEXT-100 neutrinoless double beta decay experiment, in three different drift electric field configurations. We measure the ionization electron drift velocity and longitudinal diffusion, and compare our results to expectations based on available electron scattering cross sections on pure xenon. In addition, two types of measurements addressing the connection between the ionization and scintillation yields are performed. On the one hand we observe, for the first time in xenon gas, large event-by-event correlated fluctuations between the ionization and scintillation signals, similar to that already observed in liquid xenon. On the other hand, we study the field dependence of the average scintillation and ionization yields. Both types of measurements may shed light on the mechanism of electron-ion recombination in xenon gas for highly-ionizing particles. Finally, by comparing the response of alpha particles and electrons in NEXT-DEMO, we find no evidence for quenching of the primary scintillation light produced by alpha particles in the xenon gas.

Phase diagram of C60 from ab initio intermolecular potential

Recently a new intermolecular potential for C60 was derived from ab initio calculations. Using this new interaction potential we numerically study the phase diagram of fullerite. Several numerical techniques are used in order to ascertain the correctness of the results. We predict that C60 can be found in the liquid state for densities between 0.468 and 0.845 nm−3 and temperatures between 1881 and 2012 K.

Characterization of a medium size Xe/TMA TPC instrumented with microbulk Micromegas, using low-energy gamma-rays

NEXT-MM is a general-purpose high pressure (10 bar, ~ 25 l active volume) Xenon-based TPC, read out in charge mode with an 0.8 cm × 0.8 cm-segmented 700 cm2 plane (1152 ch) of the latest microbulk-Micromegas technology. It has been recently commissioned at University of Zaragoza as part of the R&D of the NEXT 0νββ experiment, although the experiments first stage is currently being built based on a SiPM/PMT-readout concept relying on electroluminescence. Around 2 million events were collected during the last months, stemming from the low energy γ-rays emitted by a 241Am source when interacting with the Xenon gas (Eγ = 26, 30, 59.5 keV). The localized nature of such events around atmospheric pressure, the long drift times, as well as the possibility to determine their production time from the associated α particle in coincidence, allow the extraction of primordial properties of the TPC filling gas, namely the drift velocity, diffusion and attachment coefficients. In this work we focus on the little explored combination of Xe and trimethylamine (TMA) for which, in particular, such properties are largely unknown. This gas mixture offers potential advantages over pure Xenon when aimed at Rare Event Searches, mainly due to its Penning characteristics, wave-length shifting properties and reduced diffusion, and it is being actively investigated by our collaboration. The chamber is currently operated at 2.7 bar, as an intermediate step towards the envisaged 10 bar. We report here its performance as well as a first implementation of the calibration procedures that have allowed the extension of the previously reported energy resolution to the whole readout plane (10.6% FWHM@30 keV).