Schottky barrier diodes were fabricated on (001) monoclinic β-Ga2O3 wafers with low doped epitaxial layers of 7.0 × 1015 cm−3. Circular Ni Schottky contacts with area 2 mm2 were deposited by electron beam evaporation. Devices were characterized electrically by performing forward and reverse current-voltage sweeps with a range of −100 V–2 V, as well as capacitance-voltage sweeps to −30 V. The breakdown voltage was also determined to be −180 V for the devices. Experiments measuring the electrical response from incident X-ray radiation was performed. A response time to X-ray radiation of less than 1 s was recorded and a decay time of approximately 2 s after removing X-ray source, which primarily attribute to X-ray switching on and off time. Energy spectra of alpha particles from a 0.9 μCi 241Am button source was collected at various voltage biases using devices with the lowest measured leakage current while reverse biased. The total count rate was observed to increase linearly with increasing device bias. The peak channel number was observed to increase with increasing bias with the best resolution of 9.5% at −100 V reverse bias.

Schottky barrier diodes were fabricated on (001) monoclinic (3- Ga 2 O 3 wafers with low doped epitaxial layers of 7.0 x 1015 15 cm-- 3 . Circular Ni Schottky contacts with area 2 mm2 2 were deposited by electron beam evaporation. Devices were characterized electrically by performing forward and reverse current-voltage sweeps with a range of-100 V-2 V, as well as capacitance-voltage sweeps to-30 V. The breakdown voltage was also determined to be-180 V for the devices. Experiments measuring the electrical response from incident X-ray radiation was performed. A response time to X-ray radiation of less than 1 s was recorded and a decay time of approximately 2 s after removing X-ray source, which primarily attribute to X-ray switching on and off time. Energy spectra of alpha particles from a 0.9 mu Ci 241 Am button source was collected at various voltage biases using devices with the lowest measured leakage current while reverse biased. The total count rate was observed to increase linearly with increasing device bias. The peak channel number was observed to increase with increasing bias with the best resolution of 9.5% at-100 V reverse bias.

An optical fiber-based gamma thermometer (OFBGT) is being developed in a collaborative effort by The Ohio State University and Texas A&M University. With an array of OFBGTs within a reactor core, one can infer the power distribution among the fuel assemblies using a data analytic methodology. With this data analytic methodology, both the code Monte Carlo N-Particle Transport (MCNP) and experimentally determined parameters are used as input, and energy balance is employed to determine estimates of power for each fuel assembly segment, from each OFBGT segment. Then, these estimates are averaged together using a weighting scheme, and an iterative solution is used so that the inferred power distribution is self-consistent. The data analytic methodology is intended to be used to determine the power distribution in power reactors at steady state, utilizing MCNP data. However, the OFBGTs that we have constructed are being tested using the Ohio State University Research Reactor (OSURR), and the OSURR is not operated for intervals that are long enough to establish equilibrium concentrations of gamma emitting fission and activation products. The impact of the transient nature of the reactor power on OFBGT measurements and the data analytics that is associated with the OFBGT is investigated in this paper using MCNP. The results of our analysis show that if an OFBGT's output is processed using response functions that are appropriate for a steady-state distribution of fission products, then the OFBGT can significantly underpredict the power in the reactor. The magnitude of the underprediction is largest for a clean core, which operationally means for measurements that are made early in the day for a research reactor that is operated intermittently. For a clean core, the underprediction may be as large as a factor of 0.8 for fuel assembly segments that are r=5cm from the OFBGT thermal mass segment.

The goal of this work was to investigate the in-core performance of sapphire optical fiber temperature sensors and to develop clad sapphire optical fibers for in-core instrumentation.We fabricated clad sapphire optical fibers and evaluated the distributed sensing performance of these sensors via optical backscatter reflectometry under high fluence and combined radiation and temperature effects.A series of irradiations was completed to evaluate the effect of irradiation on sapphire optical fiber temperature sensors and to determine the operational limits of these sensors.• Objective 1: Fabricate sapphire optical fiber sensors.• Objective 2: Evaluate the clad sapphire fiber to verify single-mode behavior and determine and characterize the light modes supported by optical fibers.• Objective 3: Characterize the in-core temperature sensing of sapphire optical fiber, as well as the combined temperature and irradiation effects.• Objective 4: Evaluate the lifetime and performance of the sensor under irradiation to high neutron fluence.Objectives 1, 2, and 3 were completed during the first 2 years of the project.Due to the Covid pandemic, Objective 4, a high-fluence irradiation performed at the Massachusetts Institute of Technology Research Reactor (MITR), was delayed, as partner facilities were subject to mandatory shutdowns and required a 1 year, no-cost extension.This irradiation was eventually completed on December 12, 2022.This work indicates that sapphire optical fiber sensors may be a solution for ultra-high-temperature applications in which traditional silica optical fibers are prone to fail.Sapphire sensors are potentially suitable for experiments featuring temperatures above 700℃ for long periods of time, or for any length of time above 1000℃.Experiments featuring a low total fluence, such as irradiations conducted in the Transient Reactor Test (TREAT) facility, also represent good applications for sapphire optical sensors.Additional work is required to characterize the sapphire fiber cladding performance, which falls outside the scope of this project, as well as the effects of high temperatures on the response of the fiber.A comprehensive material study is recommended as future work to evaluate the attenuation in sapphire under irradiation, and how that attenuation changes with irradiation temperature.The drift and attenuation in the fiber at temperatures of up to 1600℃ and a total fluence of up to 2.9 x 10 17 n/cm 2 was minimal, and the fibers returned to baseline after being heated to 1600℃ under irradiation.This is promising for the future use of sapphire optical fibers in advanced reactors.

The use of single-crystal sapphire optical fibers has been considered to extend fiber-optic sensing to the extreme temperature (> 1000 degrees C) environments encountered in nuclear applications. However, before these sapphire fiber-based sensors can be deployed, their optical transmission and dimensional stability (which impacts drift of some sensors) must be characterized under representative testing conditions. Data regarding the optical transmission of sapphire following high-dose neutron irradiation at temperatures > 100 degrees C is extremely limited. This work provides measurements of optical density (i.e., attenuation) and directional dimensional changes in bulk single-crystal sapphire materials irradiated to a fast neutron fluence of 2.4 x 10(21) n/cm(2) (3.5 displacements per atom) at temperatures ranging from 95 to 688 degrees C. Optical density measured after irradiation at 95 and 298 degrees C showed ultraviolet and visible absorption bands corresponding to known defect centers and temperature trends that were generally consistent with previous ex situ and in situ measurements made at much lower neutron fluence. However, optical density measured after irradiation at 688 degrees C was as much as two orders of magnitude higher, indicating that the fundamental mechanism for radiation-induced attenuation changes at this irradiation temperature. Additional analysis and comparison with previous works suggest that the attenuation may result from void formation, leading to increased Rayleigh scattering losses in the material and increased swelling that would also result in drift of Bragg grating-based sensors in sapphire fibers. These results pose serious questions regarding the feasibility of sapphire fiber-based sensors for high-temperature nuclear applications. (C) 2021 Elsevier B.V. All rights reserved.

This paper is an examination of the effect of an axially varying fluid temperature on the measurement of $\Delta {T}({z})$ , the difference in the temperature between the optical fiber within the thermal mass and the optical fiber within the capillary tube that is attached to the outer sheath of an Optical Fiber Based Gamma Calorimeter (OFBGC). Our analysis indicates that for an OFBGC with the general design of the OFBGC that was built and tested in the Ohio State University Research Reactor (OSURR), a sinusoidal variation with spatial frequency ${k}_{B}$ in the bulk fluid temperature ${T}_{B}$ along the length of the OFBGC will create a sinusoidal variation in $\Delta {T}({z})$ , which could be misinterpreted as a sinusoidal variation in the linear energy deposition rate within the OFBGC’s thermal mass. However, for the specific design of the OFBGC that was built and tested in the OSURR, for the slowly varying spatial variation of ${T}_{B}({z})$ along the length of the OSURR’s Water Irradiation Facility, within which the OFBGC was tested, the perturbation in $\Delta {T}({z})$ is negligibly small.

Optical frequency domain reflectometry measurements in internally clad single crystal sapphire fiber have received attention in recent years due to their high temperature distributed sensing potential. As work with these fibers has proceeded, there have been some inconsistencies in the results. Deeper investigation and testing has identified two critical considerations for the proper functioning of these fibers. First, users must address the propagation of multimode light along the outer surface of the fiber. By observing the far-field image of an internally clad sapphire fiber when adding index matching fluid to the outer fiber surface, we demonstrate the effects of removing the higher order modes from these fibers. The addition of index matching fluid resulted in nearly single mode performance where multimode performance was previously observed. Second, users must address the effect that coupling the fiber to the interrogator via silica based fiber has on the internally clad sapphire fiber's performance. Direct fusion splicing of silica to sapphire, as has been used in the recent work with these fibers, has a mode filtering effect which can be beneficial towards the modal behavior of the fibers. However, in this paper we demonstrate that the splicing can cause a sensing failure due to little or no low order mode light, that is useful for sensing, returning to the detector. The positive results from recent years have demonstrated that optical frequency domain reflectometry sensing performance will be successful in clad sapphire fiber; but only when the considerations described herein are addressed properly.

Optical frequency domain reflectometry (OFDR) is a family of optical techniques which can be used to produce distributed temperature measurements from the spectral shift of an interference pattern based on the Rayleigh backscatter signature of an optical fiber. Adaptive signal processing techniques have recently been used with OFDR to record meaningful spectral shift data from commercially available SMF-28 optical fibers heated beyond 950 °C. However, a correlation between the measured spectral shift and temperature has not yet been developed at these high temperatures. To extend the measurable temperature range of OFDR in SMF-28, this work describes the development of such a correlation from room temperature (22 °C) to 1000 °C. The relationship between spectral shift and temperature change over this range was found to be best characterized by the fourth-order polynomial $\Delta {T}=(-4.241\ast 10^{-11})\text {S}^{4} +(-2.017\ast 10^{-7})\text {S}^{3} +(-3.677\ast 10^{-4})\text {S}^{2}+(-0.8057)\text{S}$ , where $\Delta {T}$ represents the temperature difference compared to the reference temperature, and S represents the spectral shift measured by the fibers. The calibration developed in this work assumes that the fiber has been fully annealed by heating the fiber to 1000 °C for a few hours. This paper is the first to demonstrate the calibration and use of SMF-28 distributed optical fiber sensors up to 1000 °C, enabled using adaptive OFDR-based signal processing.

Recent advancements in fiber optic manufacturing, sensor design, and fiber optic interrogators have provided significant opportunities towards the development of cross-cutting fiber optic sensing solutions across the nuclear industry. The addressable harsh nuclear environment markets include Gen II, II+ and IV nuclear reactors, fusion reactors, and accelerator systems. In this work the authors present a series of developments towards the implementation of singlefiber, multipoint, temperature and pressure sensors, test results in high-temperature and high-radiation environments, cryogenic environments, material compatibility studies for sensor packaging, and future development needs to address technical challenges towards sensor commercialization.

This paper describes the parametric analysis of an optical fiber-based gamma thermometer (OFBGT) that is intended to be used to infer the power distribution in the Ohio State University Research Reactor (OSURR). The OFBGT measures the radial temperature difference Delta T between an optical fiber that is within the thermal mass and an optical fiber that is within the capillary tube that is attached to the exterior of the outer sheath of the OFBGT. This gas gap acts as a thermal resistance to volumetric gamma heating q''' of the OFBGT thermal mass. Of the six that are analyzed, one is deemed most appropriate for operation in the OSURR Central Irradiation Facility. This design produces a maximum Delta T of similar to 50 degrees C at full reactor power (450 kW). A comparison of the six OFBGT designs generally shows how modifications of the design that increase Delta T suffer from decreased spatial resolution.

A 4-step data analytic methodology has been devised for the purpose of inferring the distribution of power in a reactor core, based on the response of an array of optical fiber based gamma thermometers (OFBGTs). This data analytic methodology is crucial for the development of a system of OFBGTs for the purpose of calibrating local power range monitors in boiling water reactors. Such a system would be an improvement to the present calibration system in boiling water reactors, in terms of safety, efficiency, and permanence. The first step of this methodology is to establish an energy balance method. In this method, one uses MCNP to determine response functions, which allow one to convert from gamma thermometer response to power. The gamma thermometers and the reactor core are segmented, such that each gamma thermometer segment provides an estimate of power for each reactor core segment. The estimates of power for the reactor core segments (hereafter referred to as fuel assembly segments) are calculated based on the measured response of the gamma thermometers. The second step of the methodology is to employ a weighting scheme to combine the estimates of the power of the various fuel assembly segments, based on the response of the various OFGBT segments and the incremental dose rates of the various fuel assembly segments to the OFBGT segments. The third step is to iteratively calculate the estimates of the power of the fuel assembly segments, until a convergence criterion is met, which indicates that the calculation has converged. The fourth and final step of the methodology is to estimate the uncertainty of the power for each fuel assembly segment. The mathematical basis for this step is not the focus of this paper. We have used a 3D homogeneous reactor model to demonstrate the data analytic methodology; and have found that the data analytic methodology operates as intended.