Investigation of Large LGB Detectors for Antineutrino Detection

A detector material or configuration that can provide an unambiguous indication of neutron capture can substantially reduce random coincidence backgrounds in antineutrino detection and capture-gated neutron spectrometry applications. Here we investigate the performance of such a material, a composite of plastic scintillator and $^6$Li$_6^{nat}$Gd$(^{10}$BO$_{3})_{3}$:Ce (LGB) crystal shards of ~1 mm dimension and comprising 1% of the detector by mass. While it is found that the optical propagation properties of this material as currently fabricated are only marginally acceptable for antineutrino detection, its neutron capture identification ability is encouraging.Comment: 6 pages, 7 figures, submitted to Nuclear Instruments and Methods

Observation of Neutrons with a Gadolinium Doped Water Cerenkov Detector

Spontaneous and induced fission in Special Nuclear Material (SNM) such as 235U and 239Pu results in the emission of neutrons and high energy gamma-rays. The multiplicities of and time correlations between these particles are both powerful indicators of the presence of fissile material. Detectors sensitive to these signatures are consequently useful for nuclear material monitoring, search, and characterization. In this article, we demonstrate sensitivity to both high energy gamma-rays and neutrons with a water Cerenkov based detector. Electrons in the detector medium, scattered by gamma-ray interactions, are detected by their Cerenkov light emission. Sensitivity to neutrons is enhanced by the addition of a gadolinium compound to the water in low concentrations. Cerenkov light is similarly produced by an 8 MeV gamma-ray cascade following neutron capture on the gadolinium. The large solid angle coverage and high intrinsic efficiency of this detection approach can provide robust and low cost neutron and gamma-ray detection with a single device.Comment: 7 pages, 4 figures. Submitted to Nuclear Instruments and Methods,

Improved Fast Neutron Spectroscopy via Detector Segmentation

Organic scintillators are widely used for fast neutron detection and spectroscopy. Several effects complicate the interpretation of results from detectors based upon these materials. First, fast neutrons will often leave a detector before depositing all of their energy within it. Second, fast neutrons will typically scatter several times within a detector, and there is a non-proportional relationship between the energy of, and the scintillation light produced by, each individual scatter; therefore, there is not a deterministic relationship between the scintillation light observed and the neutron energy deposited. Here we demonstrate a hardware technique for reducing both of these effects. Use of a segmented detector allows for the event-by-event correction of the light yield non-proportionality and for the preferential selection of events with near-complete energy deposition, since these will typically have high segment multiplicities.Comment: Accepted for publication in Nuclear Instruments and Methods in Physics Research Section

First measurement of θ<inf>13</inf> from delayed neutron capture on hydrogen in the Double Chooz experiment

The Double Chooz experiment has determined the value of the neutrino oscillation parameter θ13 from an analysis of inverse beta decay interactions with neutron capture on hydrogen. This analysis uses a three times larger fiducial volume than the standard Double Chooz assessment, which is restricted to a region doped with gadolinium (Gd), yielding an exposure of 113.1 GW-ton-years. The data sample used in this analysis is distinct from that of the Gd analysis, and the systematic uncertainties are also largely independent, with some exceptions, such as the reactor neutrino flux prediction. A combined rate- and energy-dependent fit finds sin22θ13=0.097±0.034 (stat.)±0.034 (syst.), excluding the no-oscillation hypothesis at 2.0. This result is consistent with previous measurements of sin22θ13

Theory of phase transmission fibre-optic deformation sensing

We present a theory and conceptual examples for fibre-optic deformation sensing based on phase changes of transmitted light. As a first result, we establish an exact relation between observable phase changes and the deformation tensor along the fibre. This relation is nonlinear and includes effects related to both local changes in fibre length and deformation-induced changes of the local refractive index. In cases where the norm of the deformation tensor is much smaller than 1, a useful first-order relation can be derived. It connects phase changes to an integral over in-line strain along the fibre times the local refractive index. When spatial variations of the refractive index are negligible, this permits the calculation of phase change measurements from distributed strain measurements, for instance, from distributed acoustic sensing (DAS). An alternative form of the first-order relation reveals that a directional sensitivity determines the ability of a point along the fibre to measure deformation. This directional sensitivity is proportional to fibre curvature and spatial variability of the refractive index. In a series of simple conceptual examples, we illustrate how a seismic wavefield is represented in a phase change time-series and what the role of higher-order effects may be. Specifically, we demonstrate that variable curvature along the fibre may lead to a multiplication of seismic waves, meaning that a single seismic wave appears multiple times in a recording of optical phase changes. Furthermore, we show that higher-order effects may be observable in specific scenarios, including deformation exactly perpendicular to the fibre orientation. Though higher-order effects may be realized in controlled laboratory settings, they are unlikely to occur in seismic experiments where fibre geometries are irregular and waves asymptotically propagate in all directions with all possible polarizations as a consequence of 3-D heterogeneity. Our results provide the mathematical foundation for the analysis of emerging transmission-based fibre-optic sensing data, and their later use in seismic event characterization and studies of Earth structure. © 2022 The Author(s)

Sensitivity kernels for transmission fibre optics

Fibre-optic sensing based on transmission offer an alternative to scattering-based distributed acoustic sensing (DAS). The ability to interrogate fibres that are thousands of kilometres long opens opportunities for studies of remote regions, including ocean basins. However, by averaging deformation along the fibre, transmission systems produce integrated instead of distributed measurements. They defy traditional interpretations in terms of simple seismic phases, thereby inherently requiring a full-waveform approach. For this, we develop a formalism to calculate sensitivity kernels of transmitted optical phase changes with respect to (Earth) structure using optical phase delay measurements. We demonstrate that transmission-based sensing can effectively provide distributed measurements when optical phase delays are analysed in different time windows. The extent to which a potentially useful sensitivity coverage can be achieved depends on the fibre geometry, and specifically on its local curvature. This work establishes a theoretical foundation for tomographic inversions and experimental design using transmission-based optical sensing. © 2022 The Author(s)

Linking Distributed and Integrated Fiber-Optic Sensing

Distributed Acoustic Sensing (DAS) has become a popular method of observing seismic wavefields: backscattered pulses of light reveal strains or strain rates at any location along a fiber-optic cable. In contrast, a few newer systems transmit light through a cable and collect integrated phase delays over the entire cable, such as the Microwave Frequency Fiber Interferometer (MFFI). These integrated systems can be deployed over significantly longer distances, may be used in conjunction with live telecommunications, and can be significantly cheaper. However, they provide only a single time series representing strain over the entire length of the fiber. This work discusses theoretically how a distributed and integrated system can be quantitatively compared, and we note that the sensitivity depends strongly on points of curvature. Importantly, this work presents the first results of a quantitative, head-to-head comparison of a DAS and the integrated MFFI system using pre-existing telecommunications fibers in Athens, Greece. © 2022 The Authors