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Conceived to host 5 kg of xenon at a pressure of 15 bar in the fiducial volume, the NEXT- White (NEW) apparatus is currently the largest high pressure xenon gas TPC using electroluminescent amplification in the world. It is also a 1:2 scale model of the NEXT-100 detector scheduled to start searching for $betabeta 0 u$ decays in 136Xe in 2019. Both detectors measure the energy of the event using a plane of photomultipliers located behind a transparent cathode. They can also reconstruct the trajectories of charged tracks in the dense gas of the TPC with the help of a plane of silicon photomultipliers located behind the anode. A sophisticated gas system, common to both detectors, allows the high gas purity needed to guarantee a long electron lifetime. NEXT-White has been operating since October 2017 at the Canfranc Underground Laboratory (LSC), in Spain. This paper describes the detector and associated infrastructures.
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One of the major goals of the NEXT-White (NEW) detector is to demonstrate the energy resolution that an electroluminescent high pressure xenon TPC can achieve for high energy tracks. For this purpose, energy calibrations with 137Cs and 232Th sources have been carried out as a part of the long run taken with the detector during most of 2017. This paper describes the initial results obtained with those calibrations, showing excellent linearity and an energy resolution that extrapolates to approximately 1% FWHM at Q$_{betabeta}$.
In experiments searching for neutrinoless double-beta decay, the possibility of identifying the two emitted electrons is a powerful tool in rejecting background events and therefore improving the overall sensitivity of the experiment. In this paper we present the first measurement of the efficiency of a cut based on the different event signatures of double and single electron tracks, using the data of the NEXT-White detector, the first detector of the NEXT experiment operating underground. Using a TO calibration source to produce signal-like and background-like events with energies near 1.6 MeV, a signal efficiency of $71.6 pm 1.5_{textrm{ stat}} pm 0.3_{textrm{ sys}} %$ for a background acceptance of $20.6 pm 0.4_{textrm{ stat}} pm 0.3_{textrm{ sys}} %$ is found, in good agreement with Monte Carlo simulations. An extrapolation to the energy region of the neutrinoless double beta decay by means of Monte Carlo simulations is also carried out, and the results obtained show an improvement in background rejection over those obtained at lower energies.
The NEXT-White (NEW) detector is currently the largest radio-pure high-pressure xenon gas time projection chamber with electroluminescent readout in the world. NEXT-White has been operating at Laboratorio Subterraneo de Canfranc (LSC) since October 2016. This paper describes the calibrations performed with $^{83m}mathrm{Kr}$ decays during a long run taken from March to November 2017 (Run II). Krypton calibrations are used to correct for the finite drift-electron lifetime as well as for the dependence of the measured energy on the event position which is mainly caused by variations in solid angle coverage. After producing calibration maps to correct for both effects we measure an excellent energy resolution for 41.5 keV point-like deposits of (4.553 $pm$ 0.010 (stat.) $pm$ 0.324 (sys.)) % FWHM in the full chamber and (3.804 $pm$ 0.013 (stat.) $pm$ 0.112 (sys.)) % FWHM in a restricted fiducial volume. Using naive 1/$sqrt{E}$ scaling, these values translate into resolutions of (0.516 $pm$ 0.0014 (stat.) $pm$ 0.0421 (sys.)) % FWHM and (0.4943 $pm$ 0.0017 (stat.) $pm$ 0.0146 (sys.)) % FWHM at the $Q_{betabeta}$ energy of xenon double beta decay (2458 keV), well within range of our target value of 1%.
Excellent energy resolution is one of the primary advantages of electroluminescent high pressure xenon TPCs, and searches for rare physics events such as neutrinoless double-beta decay ($betabeta0 u$) require precise energy measurements. Using the NEXT-White detector, developed by the NEXT (Neutrino Experiment with a Xenon TPC) collaboration, we show for the first time that an energy resolution of 1% FWHM can be achieved at 2.6 MeV, establishing the present technology as the one with the best energy resolution of all xenon detectors for $betabeta0 u$ searches.
A high pressure xenon gas time projection chamber with electroluminescent amplification (EL HPGXe TPC) searching for the neutrinoless double beta ($0 ubetabeta$) decay offers: excellent energy resolution ($0.5-0.7%$ FWHM at the $Q_{betabeta}$), by amplifying the ionization signal with electroluminescent light, and tracking capabilities, as demonstrated by the NEXT collaboration using two kg-scale prototypes. The NEXT collaboration is building an EL HPGXe TPC capable of holding 100 kg (NEXT-100) of xenon isotopically enriched in ${{}^{136}rm Xe}$. The installation and commissioning of the NEXT-100 detector at the Laboratorio Subterraneo de Canfranc (LSC) is planned for 2018. The current estimated background level for the NEXT-100 detector is of $4times10^{-4}$ counts/keV-kg-yr or less in the energy region of interest. Assuming an energy resolution of 0.75$%$ FWHM at the $Q_{betabeta}$ and a $0 ubetabeta$ signal efficiency of about 28$%$, this gives an expected sensitivity (at 90$%$ CL) to the $0 ubetabeta$ decay half life of $T^{0 u}_{1/2}>6.0times10^{25}$ yr for an exposure of 275 kg yr. A first phase of the NEXT experiment, called NEW, is currently being commissioned at the LSC. The NEW detector is a scale 1:2 in size (1:10 in mass) of the NEXT-100 detector using the same materials and photosensors and will be used to perform a characterization of the $0 ubetabeta$ backgrounds and a measurement of the standard double beta decay with neutrinos (${2 ubetabeta}$). An 8 sigma significance for the ${2 ubetabeta}$ signal in the NEW detector has been estimated for a 100-day run.
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