Large-scale optical neutrino and dark-matter detectors rely on large-area photomultiplier tubes (PMTs) for cost-effective light detection. The new R14688-100 8-inch PMT developed by Hamamatsu provides state-of-the-art timing resolution of around 1 ns (FWHM), which can help improve vertex reconstruction and enable Cherenkov and scintillation light separation in scintillation-based detectors. This PMT also provides excellent charge resolution, allowing for precision photoelectron counting and improved energy reconstruction. The Eos experiment is the first large-scale optical detector to utilize these PMTs. In this manuscript we present a characterization of the R14688-100 single photoelectron response, such as the transit-time spreads, the dark-rates, and the afterpulsing. The single photoelectron response measurements are performed for the 206 PMTs that will be used in Eos.

Conventional silicon junction detectors encounter significant carrier recombination within the heavily doped p(+) and n(+) layers, as well as beneath the metal contact regions, creating the so-called "dead layers", especially on the detector side. In this study, we present the tunnel oxide passivating contact with doped polysilicon on oxide, which demonstrates exceptional surface passivation and carrier selectivity. The key innovation lies in an ultra-thin (similar to 1.5 nm) interfacial oxide layer that facilitates efficient majority carrier transportation via tunneling while effectively block minority carriers. Remarkably low saturation current densities, ranging from 5 to 10 fA/cm(2) even with the metal contact, underscore the superiority of both n-type and p-type tunnel oxide passivating contacts. In contrast, conventional p-n junction or high-low junction exhibit saturation current densities ranging from 10 to 90 fA/cm(2) in the studied p(+) and n(+) layers with surface passivation schemes due to Auger recombination and surface recombination, and 1000-6000 fA/cm(2) with metal contacts due to intense metal-induced recombination at the interface. These findings indicate the potential and superiority of implementing n-type tunnel oxide passivating contact on the detector side and p-type contact on the back side for advanced silicon radiation detectors. This approach would enable thorough collection of generated charge carriers along the track of incident ionizing radiation particles, leading to improved energy resolution and reduced noise levels.

This article presents a parallel log-domain particle filtering algorithm combined with gamma spectrum unfolding to perform localization, identification, and evaluation of multiple point sources of various isotopes in an environment with attenuating obstacles. The method uses sets of precomputed attenuation kernels that map the attenuation characteristics of the environment. These kernels are specific to the energy level of a photopeak of interest. The spectral measurements are deconvolved into count measurements of each photopeak. These count measurements are fed into a set of parallel particle filters using attenuation kernels computed for that photopeak's energy level. The individual regularized particle filters perform all likelihood calculations in the logarithmic domain to mitigate the effects of particle degeneracy. The output of each particle filter is combined to estimate which isotopes are present as well as their positions and strengths. The performance of the algorithm is characterized in a lab-scale environment using a mobile robot equipped with a gamma ray spectrometer in the presence of up to three different radioactive isotopes simultaneously. The sources were localized to within 10 cm, and their strengths were estimated within 10% of their true values. The isotopes were all correctly identified, and no spurious sources were reported.

Three boron-10 enriched aromatic molecules have been synthesized and incorporated into two different commercial polysiloxane resins, Shin Etsu KER-6000 and Wacker SilRes H62-C. Scintillating fluorophores, 9,9-dimethyl-2-phenylfluorene (PhF) and 2,5-diphenyloxazole (PPO), were tested in combination with each resin and boron-10 molecule for the simultaneous detection of gamma rays, fast neutrons, and thermal neutrons. The H62-C resin was able to solubilize a large amount of PhF, in excess of 20 wt%. Cure kinetics were controlled through the addition of divinylbenzene (DVB) and phenyl tris(dimethylsiloxy)silane (PTDMSS) crosslinker solution to the H62-C resin, with rheology experiments demonstrating a large reduction in time to gelation when 20 wt% crosslinker solution was added, from more than 4 h to less than 1 h. These polysiloxane resins can be cured in 3 h under air, while common poly(vinyltoluene) scintillators require at least four days of heating and oxygen-free conditions. PhF-doped KER-6000 with tolyl boronate pinacol ester exhibited the best overall performance with a light yield (LY) of 62% relative to EJ-200 and thermal neutron capture at energies up to 103 keVee (84 keVee for EJ254B-5). In addition, four samples exhibited LYs surpassing an industry-standard boron-doped plastic scintillator, Eljen's EJ254B-5. Over the course of ten months, the KER-6000 samples showed precipitation of dopant molecules, which reduced their LY by 15% on average, while H62-C proved to be more stable with only a 6% reduction.

This work investigates how increased temperature affects neutron/γ discrimination and light yield (LY) in several different polysiloxane-based scintillators doped with either 9,9-dimethyl-2-phenylflourene (PhF) or 2,5-diphenyloxazole (PPO) as a primary fluorophore and 9,9-dimethyl-2,7-di((E)-styryl) fluorene (SFS) as the secondary fluorophore. The polysiloxane matrices were prepared from the commercial resins Wacker Lumisil 579 or Shin-Etsu KER-6000. Control scintillators were prepared from poly(vinyltoluene) (PVT), the industry standard matrix, as a reference point for LY and pulse shape discrimination (PSD) measurements. Samples with PhF and PPO dopant concentrations of 1 wt% and 5 wt% in the polysiloxanes and 3 wt% and 5 wt% in the PVT samples were tested at 20 °C, 35 °C, and 50 °C. In the polysiloxane samples, the KER-6000 resin outperformed the Wacker 579 and PhF proved to be a better dopant than PPO in both LY and PSD capabilities. Polysiloxane scintillators showed slight decreases in LY and increases in neutron/γ discrimination at increased temperatures, while PVT scintillators showed a similar LY decrease with little to no improvement in neutron/γ discrimination at increased temperatures. Overall, polysiloxane scintillators may not require recalibration in applications where temperatures increase up to 50 °C.

The ability to reconstruct fine-resolution images in a high-count-rate environment is an ongoing challenge to the fields of nuclear security, medicine, and high energy physics. This study presents the characterization and performance of an image reconstruction algorithm and detector array in such an environment. The detector array is composed of quartz Cherenkov radiators and lutetium-yttrium oxyorthosilicate inorganic scintillators detector elements with light collection via silicon photomultipliers (SiPM). The reconstruction algorithm was evaluated using ANSI testing standard N42.46-2008 for imaging performance of active interrogation systems for national security applications; this included spatial resolution, wire detection, and penetration studies. The array was tested using a 6-MVp pulsed photon beam where test objects were translated through the detector field of view demonstrating a capability to resolve a 2.05-mm wire at a source standoff of 2.2 m, a horizontal spatial resolution of 3 mm, and a contrast sensitivity of 1.5%.