Construction and operation of the Next-100 detector

PhD Program

Speaker

Myriam Martinez Vara

When

2025/12/19
10:30

Place

IFIC, Valencia

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PhD Thesis defense by Miryam Martinez Vara

Supervisor: Michel Sorel, Justo Martín-Albo & Juan José Gómez Cadenas
(DIPC, Ikerbasque)

Neutrino Physics

Neutrino physics has significantly advanced since the 20th century, beginning with the theoretical challenges of beta decay. The discovery of the neutrino by Pauli, Fermi's beta decay theory, and the 1956 experimental confirmation by Cowan and Reines established neutrinos as fundamental particles. Subsequent discoveries, like neutrino oscillations, proved they have mass.

A key remaining open question is whether neutrinos are Dirac or Majorana particles. Neutrinoless double beta decay (0nubb), a hypothetical lepton-number-violating process, could reveal the Majorana nature and provide access to the absolute neutrino mass scale. Its detection would have profound implications, but requires detectors with excellent energy resolution, topological reconstruction, and extremely low background.

The NEXT experiment searches for neutrinoless double beta decay in Xe-136 using high-pressure xenon gas TPCs, following the SOFT concept, which separates calorimetric and topological functions. PMTs collect energy data, while SiPMs on DICE boards reconstruct tracks. The current detector, NEXT-100, installed at the Laboratorio Subterráneo de Canfranc, represents a major step toward future large-scale detectors.

NEXT-100 includes a 1.7 cubic meter radiopure vessel with copper and lead shielding, and two planes for energy and tracking. The energy plane consists of PMTs and the tracking plane of an array of SiPMs. These SiPMs have been characterized to ensure optimal performance at approximately 1 V above breakdown voltage, balancing gain, noise, and stability. Optical components like teflon masks and TPB coatings have been tested to improve light collection and spatial resolution, achieving acceptable uniformity despite coating variations. The radiopurity of the detector components was ensured through material screening using gamma spectroscopy, GDMS, and ICP-MS, keeping background contributions below 0.1 counts/year per component.

Energy calibration and resolution studies have been carried out using both Monte Carlo simulations and experimental calibration data. Simulated data with Tl-208 sources yielded energy resolutions of (0.55+-0.01)% FWHM at 1592 keV (DEP) and (0.54+-0.07)% at 2615 keV. Real data from Th-228 calibrations achieved (1.296+-0.024)% and (1.105+-0.07)%, respectively, after applying non-linearity corrections and fiducial cuts. These results confirm that the detector is close to the energy resolution requirement of 1% FWHM at Qbb and validates the reconstruction algorithms.

Finally, a novel method was studied to reconstruct electron kinematics in neutrinoless double beta decay events using only the decay vertex. Simulations show that, combined with barium tagging and sufficient spatial resolution, NEXT can probe different lepton number violating mechanisms beyond the standard mass model.