Luis M. P. Fernandes

A bstract: The NEXT-100 detector at the LSC aims at the first competitive search for the 0 νββ decay using a high-pressure ¹³⁶ Xe electroluminescent time projection chamber. The first low-background run of NEXT-100 at 3.95 bar has been devoted to the measurement of the radon-induced backgrounds impacting this search. The contributions from both the internal and external airborne radon have been evaluated. The internal ²²² Rn activity is found to be (0.95 ± 0.04(stat) ± 0.09(sys)) Bq/m ³ , while no traces of ²²⁰ Rn have been observed. Most of the ²²² Rn progeny plate-out on the surface of the cathode of the detector, leading to a rate of Rn-induced ²¹⁴ Bi of (0.97 ± 0.05(stat) ± 0.10(sys)) Hz for visible energies above 400 keV. The corresponding background index in the 0 νββ region of interest is evaluated as (7.3 ± 1.5(stat) ± 0.8(sys)) × 10 ^{− 4} counts/(keV · kg · yr) after selection of the fully contained events. This background index is reduced to ∼ 4 × 10 ^{− 5} counts/(keV · kg · yr) by applying a topological selection requiring only one double-electron-like track in the events. This value is one order of magnitude below the total radiogenic background expectation in NEXT-100. By analyzing the correlation of the airborne radon activity and the measured rate of events in NEXT-100, it is concluded that the detector operates in a virtually radon-free environment thanks to the radon abatement system of the LSC.

Gas Proportional Scintillation Counters (GPSCs) are noble gas detectors in which the primary ionization charge generated by radiation interactions is amplified via electroluminescence (EL) in the gas. Under an external electric field, the primary ionization electrons drift into a region where the field exceeds the gas scintillation threshold. Compared to charge amplification via electron avalanches, EL amplification provides higher gain with improved signal-to-noise ratio, no space charge effects, and better immunity to radiofrequency noise and electrical discharges. GPSCs have been used in applications such as x-ray fluorescence analysis and x- and γ-ray astrophysics. Their high resistance to radiation damage, wide operational temperature range, simple yet flexible design, and large detection area capability are notable advantages. However, the solid angle subtended by the photosensor varies with the EL emission position, causing the detected signal to depend on the radiation interaction location. This limits the size of the detector's radiation window relative to the photosensor. To overcome these limitations, we recently proposed a novel and simple GPSC design that ensures a constant solid angle across the scintillation region. In this configuration, a single annular anode electrode, centered on the photosensor axis, defines a localized scintillation region near its surface. As a result, the entire region maintains a fixed solid angle with respect to the photosensor, making the EL signal independent of the interaction position. This design enables a large window-to-photosensor size ratio while maintaining uniform response. Previously, we developed a comprehensive simulation tool incorporating radiation absorption, electron drift, EL emission and light detection, which was validated against an annular-anode GPSC prototype. In this work, we use this simulation tool to explore the optimal geometrical parameters of the annular anode GPSC, aiming to balance energy resolution and the different parameters, mamely the detector window-to-photosensor radius ratio.

IPG K ⁺ chemosensors are effective for Ba ²⁺ detection in aqueous solution and low-background single-ion fluorescence microscopy.

Hydrogen-like light muonic ions, in which one negative muon replaces all of the electrons, are extremely sensitive probes of nuclear structure. Using pulsed laser spectroscopy, we have measured three 2 S -2 P transitions in the muonic helium-3 (μ ³ He ⁺ ) ion, an ion formed by a negative muon and bare helium-3 nucleus. This allowed us to extract the Lamb shift, the 2 P fine structure splitting, and the 2 S -hyperfine splitting in μ ³ He ⁺ . Comparing these measurements with theory, we determined the root-mean-square charge radius of the helion ( ³ He nucleus) to be r _h = 1.97007(94) fm, in good agreement with the value from elastic electron scattering but a factor 15 more accurate. Our results represent benchmarks for few-nucleon theories and open the way for precision quantum electrodynamics tests in He atoms and ions.

A new, simple and robust design for a gas proportional scintillation counter (GPSC) is studied. One sole electrode, the anode, is used to define the electric field in the drift and scintillation regions of the detector volume. The anode has an annular shape aligned with the photosensor axis. Such design allows to keep constant the solid angle subtended by the photosensor relative to the different positions of the scintillation region. Having an oblong anode with a 10 cm inner diameter and a 5 cm photomultiplier tube placed 5 cm below the anode, an energy resolution of 12.0 % FWHM has been achieved for a 10 kV anode bias, the maximum voltage that could be applied to the anode in the present prototype. According to simulations, energy resolutions of ~10 % can be achieved for anode voltages of ~13 kV, a value comparable to the 9–10 % achieved in GPSC using solid angle variation compensation. Independently of having a constant solid angle, the absolute value of the solid angle must be considered, a lower number of EL photons detected at the photosensor due to a reduced solid angle may contribute to the GPSC energy resolution degradation. In addition, non-uniformities present in the photosensor or in the GPSC must be taken into account, since they will contribute to a dependence of the pulse amplitude on the radiation interaction position despite the constant solid angle subtended by the photosensor relative to the scintillation region.

A Correction to this paper has been published: https://doi.org/10.1038/s41586-021-03360-2.