Boosting background suppression in the NEXT experiment through Richardson-Lucy deconvolution

Abstract

Next-generation neutrinoless double beta decay experiments aim for half-life sensitivities of ∼ 1027 yr, requiring suppressing backgrounds to < 1 count/tonne/yr. For this, any extra background rejection handle, beyond excellent energy resolution and the use of extremely radiopure materials, is of utmost importance. The NEXT experiment exploits differences in the spatial ionization patterns of double beta decay and single-electron events to discriminate signal from background. While the former display two Bragg peak dense ionization regions at the opposite ends of the track, the latter typically have only one such feature. Thus, comparing the energies at the track extremes provides an additional rejection tool. The unique combination of the topology-based background discrimination and excellent energy resolution (1% FWHM at the Q-value of the decay) is the distinguishing feature of NEXT. Previous studies demonstrated a topological background rejection factor of ∼ 5 when reconstructing electron-positron pairs in the 208Tl 1.6 MeV double escape peak (with Compton events as background), recorded in the NEXT-White demonstrator at the Laboratorio Subterráneo de Canfranc, with 72% signal efficiency. This was recently improved through the use of a deep convolutional neural network to yield a background rejection factor of ∼ 10 with 65% signal efficiency. Here, we present a new reconstruction method, based on the Richardson-Lucy deconvolution algorithm, which allows reversing the blurring induced by electron diffusion and electroluminescence light production in the NEXT TPC. The new method yields highly refined 3D images of reconstructed events, and, as a result, significantly improves the topological background discrimination. When applied to real-data 1.6 MeV e−e+ pairs, it leads to a background rejection factor of 27 at 57% signal efficiency.

Measurement of the Xe136 two-neutrino double- β -decay half-life via direct background subtraction in NEXT

“…The ratio of the signal efficiency over the square root of the background acceptance ranges from ≈2.3 to ≈3.1 for events between 1 and 3 MeV energy, consistent with Ref. [15]. According to this selection, the top and bottom panels of Fig.…”

“…The deconvolved 3D hits are then grouped into volume elements of (5 mm) 3 , which are used to build tracks following the connectivity criteria established by a breadth-first search algorithm [16]. The energies of the endpoints of each track, hereafter "blobs," are defined by integrating the energy of the hits contained within spheres of 18 mm in radius centered in the identified extremes [15]. Figure 3 shows examples of two observed tracks of 1.7 MeV.…”

Toward the discovery of matter creation with neutrinoless double-beta decay

“…Tracking information is obtained from a SiPM array at the anode, while PMTs at the cathode provide optimal energy resolution. NEXT-White (Monrabal et al, 2018), a proof-of-principle detector with 5 kg of Xe at 10 bar, demonstrated an energy resolution of σ = 10 keV at Q ββ (Renner et al, 2019), and tracking performance capable of discriminating sin-gle and double electron track events, retaining 57% of signal events while rejecting background by a factor of 27 (Simón et al, 2020). A second stage, NEXT-100 (Alvarez et al, 2012;Martín-Albo et al, 2016), with a pressure of 15 bar and containing 87 kg of 136 Xe, is currently under construction.…”

Data Compression in the NEXT-100 Data Acquisition System

“…On the other hand, for high-energy events, the threshold is increased to 10 photoelectrons per 1 µs time slice. This value was identified as the optimal threshold for track reconstruction [ 23 ]. Still, given the non-reversibility nature of lossy compression, a heavily conservative ZS configuration was chosen for standard data-taking.…”