[Paper Review] The Hunt for neutrinoless double beta decay with the NEXT experiment

D. Lorca|arXiv (Cornell University)|Nov 3, 2014

Neutrino Physics Research4 references4 citations

TL;DR

The NEXT experiment proposes using a high-pressure xenon gas time projection chamber with electroluminescent amplification to search for neutrinoless double beta decay in 136Xe, aiming to probe the Majorana nature of neutrinos and measure the effective electron neutrino mass. A prototype, NEXT-DEMO, achieved 0.62% FWHM energy resolution at the 136Xe Q-value, validating the technology for the upcoming NEXT-100 detector, which targets a sensitivity of 5.9×10²⁵ years and a sensitivity to mββ ≈ 100 meV.

ABSTRACT

The NEXT-100 detector will search for the neutrinoless double beta decay of $^{136}$Xe using an electroluminescent high-pressure xenon gas TPC filled with 100 kg of enriched Xe. An observation of this hypothetical process would establish a Majorana nature for the neutrino and prove the violation of lepton number. A scaled-down prototype, NEXT-DEMO, has been built to demonstrate the feasibility of the technology. NEXT-DEMO includes an energy plane made of PMTs and a tracking plane made of SiPMs. X-ray energy depositions, produced by the de-excitation of xenon atoms after their interaction with gamma rays, have been used to characterize the detector response. With this method, the released energy by gammas coming from $^{22}$Na source has been corrected, achieving an energy resolution of 5.691% FWHM and 1.62% FWHM at the 29.7 keV and 511 keV peaks respectively, which extrapolate to 0.62% FWHM and 0.73% FWHM at Q$_{ββ}$ value of Xenon.

Motivation & Objective

• To develop a scalable, low-background detector technology for observing neutrinoless double beta decay (ββ0ν), a process that would prove neutrinos are Majorana particles.
• To demonstrate the feasibility of high-pressure xenon gas electroluminescent time projection chambers (HPXe-EL-TPC) for large-scale neutrino mass measurements.
• To achieve sub-1% energy resolution at the 136Xe Q-value (2451 keV) to distinguish rare ββ0ν events from background.
• To reduce background rates through radiopure materials and topological event reconstruction, enabling sensitivity to effective Majorana neutrino masses below 100 meV.
• To validate the NEXT-100 design through a prototype, NEXT-DEMO, and prepare for operation at the Canfranc Underground Laboratory.

Proposed method

• The NEXT-100 detector uses a 100 kg, 90%-enriched 136Xe high-pressure xenon gas TPC operating at 15 bar, with a 32.5% PMT coverage for energy measurement.
• Ionization electrons drift toward the anode, where electroluminescence produces VUV photons detected by a plane of SiPMs (tracking plane) and PMTs (energy plane) for 3D event reconstruction.
• Xenon K-shell X-ray depositions from 22Na sources are used to calibrate spatial and energy response, correcting for non-uniformities and electron losses.
• Energy resolution is measured via the 29.7 keV and 511 keV peaks from 22Na, with corrections applied for electron attachment and pressure/temperature drifts.
• The detector employs a 12 cm thick radio-pure copper shield and radiopure materials to suppress backgrounds, achieving an expected rate of 5×10⁻⁴ kg⁻¹ keV⁻¹ year⁻¹.
• Theoretical sensitivity is projected using the relation T₁/₂ ∝ a·ε·√(M·t / (ΔE·B)), with ΔE extrapolated from prototype data to the 136Xe Q-value.

Experimental results

Research questions

• RQ1 Can a high-pressure xenon gas electroluminescent TPC achieve sub-1% energy resolution at the 136Xe Q-value, enabling discrimination of rare ββ0ν events?
• RQ2 Can X-ray depositions from 22Na be used effectively to calibrate spatial and energy response in a gaseous xenon TPC, especially for correcting non-uniformities and electron losses?
• RQ3 Can the NEXT-100 detector achieve a background rate low enough to reach a sensitivity of 5.9×10²⁵ years for ββ0ν decay?
• RQ4 What is the achievable energy resolution in NEXT-100, extrapolated from prototype measurements, and does it meet the requirement for resolving the 2451 keV Q-value peak?
• RQ5 Can the combination of calorimetry and tracking in NEXT-100 enable topological discrimination of ββ0ν events from dominant background sources?

Key findings

• NEXT-DEMO achieved an energy resolution of 5.691% FWHM at 29.7 keV and 1.62% FWHM at 511 keV, with corrections for detector non-uniformities and electron losses.
• After extrapolation to the 136Xe Q-value (2451 keV), the expected energy resolution is 0.62% FWHM for the 29.7 keV peak and 0.73% FWHM for the 511 keV peak, assuming Poisson-limited photon shot noise.
• The NEXT-100 detector is projected to achieve a sensitivity of 5.9×10²⁵ years after 5 years of data taking, corresponding to a sensitivity to the effective Majorana neutrino mass of approximately 100 meV.
• Xenon K-shell X-ray depositions were successfully used as a calibration tool across the detector volume, enabling spatial calibration and response equalization.
• The use of radiopure materials and a copper shield reduced the expected background rate to 5×10⁻⁴ kg⁻¹ keV⁻¹ year⁻¹, supporting the feasibility of the low-background design.
• The NEXT-100 detector is approved for installation at the Laboratorio Subterráneo de Canfranc, with underground operations using non-enriched xenon planned for 2015 and enriched xenon physics runs scheduled for early 2016.