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We correct an overestimation of the production rate of 137 Xe in the DARWIN detector operated at LNGS. This formerly dominant intrinsic background source is now at a level similar to the irreducible background from solar 8 B neutrinos, thus unproblematic at the LNGS depth. The projected half-life sensitivity for the neutrinoless double beta decay ( 0 ν β β ) of 136 Xe improves by 22 % compared to the previously reported number and is now T 1 / 2 0 ν = 3.0 × 10 27 yr (90% C.L.) after 10 years of DARWIN operation.

Two-neutrino double electron capture (2 ν ECEC) is a second-order weak-interaction process with a predicted half-life that surpasses the age of the Universe by many orders of magnitude 1 . Until now, indications of 2 ν ECEC decays have only been seen for two isotopes 2 – 5 , 78 Kr and 130 Ba, and instruments with very low background levels are needed to detect them directly with high statistical significance 6 , 7 . The 2 ν ECEC half-life is an important observable for nuclear structure models 8 – 14 and its measurement represents a meaningful step in the search for neutrinoless double electron capture—the detection of which would establish the Majorana nature of the neutrino and would give access to the absolute neutrino mass 15 – 17 . Here we report the direct observation of 2 ν ECEC in 124 Xe with the XENON1T dark-matter detector. The significance of the signal is 4.4 standard deviations and the corresponding half-life of 1.8 × 10 22  years (statistical uncertainty, 0.5 × 10 22  years; systematic uncertainty, 0.1 × 10 22  years) is the longest measured directly so far. This study demonstrates that the low background and large target mass of xenon-based dark-matter detectors make them well suited for measuring rare processes and highlights the broad physics reach of larger next-generation experiments 18 – 20 . Two-neutrino double electron capture is observed experimentally in 124 Xe with the XENON1T detector, yielding a half-life of 1.8 × 10 22 years.

A low-energy electronic recoil calibration of XENON1T, a dual-phase xenon time projection chamber, with an internal$^{37}$$37 Ar source was performed. This calibration source features a 35-day half-life and provides two mono-energetic lines at 2.82 keV and 0.27 keV. The photon yield and electron yield at 2.82 keV are measured to be ($$32.3\\,\\pm \\,0.3$$32.3 ± 0.3 ) photons/keV and ($$40.6\\,\\pm \\,0.5$$40.6 ± 0.5 ) electrons/keV, respectively, in agreement with other measurements and with NEST predictions. The electron yield at 0.27 keV is also measured and it is ($$68.0^{+6.3}_{-3.7}$$68 . 0 - 3.7 + 6.3 ) electrons/keV. The$^{37}$$37 Ar calibration confirms that the detector is well-understood in the energy region close to the detection threshold, with the 2.82 keV line reconstructed at ($$2.83\\,\\pm \\,0.02$$2.83 ± 0.02 ) keV, which further validates the model used to interpret the low-energy electronic recoil excess previously reported by XENON1T. The ability to efficiently remove argon with cryogenic distillation after the calibration proves that$^{37}$$37 Ar can be considered as a regular calibration source for multi-tonne xenon detectors.

We describe the purification of xenon from traces of the radioactive noble gas radon using a cryogenic distillation column. The distillation column was integrated into the gas purification loop of the XENON100 detector for online radon removal. This enabled us to significantly reduce the constant 222 Rn background originating from radon emanation. After inserting an auxiliary 222 Rn emanation source in the gas loop, we determined a radon reduction factor of R > 27 (95% C.L.) for the distillation column by monitoring the 222 Rn activity concentration inside the XENON100 detector.

We describe the purification of xenon from traces of the radioactive noble gas radon using a cryogenic distillation column. The distillation column was integrated into the gas purification loop of the XENON100 detector for online radon removal. This enabled us to significantly reduce the constant [Formula omitted]Rn background originating from radon emanation. After inserting an auxiliary [Formula omitted]Rn emanation source in the gas loop, we determined a radon reduction factor of [Formula omitted] (95% C.L.) for the distillation column by monitoring the [Formula omitted]Rn activity concentration inside the XENON100 detector.

A low-energy electronic recoil calibration of XENON1T, a dual-phase xenon time projection chamber, with an internal 37Ar source was performed. This calibration source features a 35-day half-life and provides two mono-energetic lines at 2.82 keV and 0.27 keV. The photon yield and electron yield at 2.82 keV are measured to be (32.3±0.3) photons/keV and (40.6±0.5) electrons/keV, respectively, in agreement with other measurements and with NEST predictions. The electron yield at 0.27 keV is also measured and it is (68.0+6.3−3.7) electrons/keV. The 37Ar calibration confirms that the detector is well-understood in the energy region close to the detection threshold, with the 2.82 keV line reconstructed at (2.83±0.02) keV, which further validates the model used to interpret the low-energy electronic recoil excess previously reported by XENON1T. The ability to efficiently remove argon with cryogenic distillation after the calibration proves that 37Ar can be considered as a regular calibration source for multi-tonne xenon detectors.

Abstract The DARWIN observatory is a proposed next-generation experiment to search for particle dark matter and for the neutrinoless double beta decay of¹³⁶136 Xe. Out of its 50 t total natural xenon inventory, 40 t will be the active target of a time projection chamber which thus contains about 3.6 t of¹³⁶136 Xe. Here, we show that its projected half-life sensitivity is2.4× 10²⁷ \\hbox year2.4×1027year , using a fiducial volume of 5 t of natural xenon and 10 year of operation with a background rate of less than 0.2 events/(t  ⋅ ·  year) in the energy region of interest. This sensitivity is based on a detailed Monte Carlo simulation study of the background and event topologies in the large, homogeneous target. DARWIN will be comparable in its science reach to dedicated double beta decay experiments using xenon enriched in¹³⁶136 Xe.

Two-neutrino double electron capture (2νECEC) is a second-order weak-interaction process with a predicted half-life that surpasses the age of the Universe by many orders of magnitude . Until now, indications of 2νECEC decays have only been seen for two isotopes , Kr and Ba, and instruments with very low background levels are needed to detect them directly with high statistical significance . The 2νECEC half-life is an important observable for nuclear structure models and its measurement represents a meaningful step in the search for neutrinoless double electron capture-the detection of which would establish the Majorana nature of the neutrino and would give access to the absolute neutrino mass . Here we report the direct observation of 2νECEC in Xe with the XENON1T dark-matter detector. The significance of the signal is 4.4 standard deviations and the corresponding half-life of 1.8 × 10  years (statistical uncertainty, 0.5 × 10  years; systematic uncertainty, 0.1 × 10  years) is the longest measured directly so far. This study demonstrates that the low background and large target mass of xenon-based dark-matter detectors make them well suited for measuring rare processes and highlights the broad physics reach of larger next-generation experiments .

Two-neutrino double electron capture (2νECEC) is a second-order weak-interaction process with a predicted half-life that surpasses the age of the Universe by many orders of magnitude1. Until now, indications of 2νECEC decays have only been seen for two isotopes2,3,4,5, 78Kr and 130Ba, and instruments with very low background levels are needed to detect them directly with high statistical significance6,7. The 2νECEC half-life is an important observable for nuclear structure models8,9,10,11,12,13,14 and its measurement represents a meaningful step in the search for neutrinoless double electron capture—the detection of which would establish the Majorana nature of the neutrino and would give access to the absolute neutrino mass15,16,17. Here we report the direct observation of 2νECEC in 124Xe with the XENON1T dark-matter detector. The significance of the signal is 4.4 standard deviations and the corresponding half-life of 1.8 × 1022 years (statistical uncertainty, 0.5 × 1022 years; systematic uncertainty, 0.1 × 1022 years) is the longest measured directly so far. This study demonstrates that the low background and large target mass of xenon-based dark-matter detectors make them well suited for measuring rare processes and highlights the broad physics reach of larger next-generation experiments18,19,20.

Abstract We correct an overestimation of the production rate of¹³⁷137 Xe in the DARWIN detector operated at LNGS. This formerly dominant intrinsic background source is now at a level similar to the irreducible background from solar⁸8 B neutrinos, thus unproblematic at the LNGS depth. The projected half-life sensitivity for the neutrinoless double beta decay (0ν β β 0 ν β β ) of¹³⁶136 Xe improves by22%22 % compared to the previously reported number and is nowT^(0ν)_(1/2)= 3.0× 10²⁷ \\hbox yrT 1 / 2 0 ν = 3.0 × 10 27 yr (90% C.L.) after 10 years of DARWIN operation.