Vol.36,

X-ray detectors show potential applications in medical imaging, materials science, and nuclear energy. To achieve high detection efficiency and spatial resolution, many conventional semiconductor materials, such as amorphous selenium, cadmium telluride zinc, and perovskites have been utilized in direct conversion X-ray detectors. However, these semiconductor materials are susceptible to temperature-induced performance degradation, crystallization, delamination, uneven lattice growth, radiation damage, and high dark current. This study explores a new approach by coupling an FC40 electronic fluorinated liquid with a specialized high-resolution and high-readout-speed complementary metal-oxide-semiconductor (CMOS) pixel array, specifically the Topmetal II^- chip, to fabricate a direct conversion X-ray detector. The fluorinated liquid FC40 (molecular formula: C₂₁F₄₈N₂) is an electronic medium that is minimally affected by temperature and displays no issues with uniform conductivity. It exhibits a low dark current and minimal radiation damage and enables customizable thickness in X-ray absorption. This addresses the limitations inherent in conventional semiconductor-based detectors. In this study, simple X-ray detector imaging tests were conducted, demonstrating the excellent coupling capability between FC40 electronic fluorinated liquid and CMOS chips by the X-ray detector. A spatial resolution of 4.0 lp/mm was measured using a striped line par card, and a relatively clear image of a cockroach was displayed in the digital radiography imaging results. Preliminary test results indicated the feasibility of fabricating an X-ray detector by combining FC40 electronic fluorinated liquid and CMOS chips. Owing to the absence of issues related to chip-material coupling, a high spatial resolution could be achieved by reducing the chip pixel size. This method presents a new avenue for studies on novel liquid-based direct conversion X-ray detectors.

321

To monitor nuclear and radiation emergencies, it is crucial to obtain accurate in-situ measurements of the environmental γ radiation dose rate from key radionuclides, particularly for large radioactive surface sources. The methods currently used for measuring dose rates are inadequate for obtaining the dose rates of key radionuclides and have large angular response errors when monitoring surface sources. To address this practical problem, this study proposes three methods for measuring the dose rate: the weighted peak total ratio, mean value regression, and numerical integration methods. These methods are based on energy-spectrum measurement data, and they were theoretically derived and numerically evaluated. Finally, a 1-m-long hexagonal radioactive surface source was integrated into a larger surface source. In-situ measurement experiments were conducted on a large radioactive surface source using a dose-rate meter and a portable HPGe γ spectrometer to analyze the errors of the three aforementioned methods and verify their validity.

276

In current neural network algorithms for nuclide identification in high-background, poor-resolution detectors, traditional network paradigms including back-propagation networks, convolutional neural networks, recurrent neural networks, etc. have been limited in research on γ spectrum analysis because of their inherent mathematical mechanisms. It is difficult to make progress in terms of training data requirements and prediction accuracy. In contrast to traditional network paradigms, network models based on the transformer structure have the characteristics of parallel computing, position encoding, and deep stacking, which have enabled good performance in natural language processing tasks in recent years. Therefore, in this paper, a transformer-based neural network (TBNN) model is proposed to achieve nuclide identification for the first time. First, the Geant4 program was used to generate the basic single-nuclide energy spectrum through Monte Carlo simulations. A multi-nuclide energy spectrum database was established for neural network training using random matrices of γ-ray energy, activity, and noise. Based on the encoder-decoder structure, a network topology based on the transformer was built, transforming the 1024-channel energy spectrum data into a 32×32 energy spectrum sequence as the model input. Through experiments and adjustments of model parameters, including the learning rate of the TBNN model, number of attention heads, and number of network stacking layers, the overall recognition rate reached 98.7%. Additionally, this database was used for training AI models such as back-propagation networks, convolutional neural networks, residual networks, and long short-term memory neural networks, with overall recognition rates of 92.8%, 95.3%, 96.3%, and 96.6%, respectively. This indicates that the TBNN model exhibited better nuclide identification among these AI models, providing an important reference and theoretical basis for the practical application of transformers in the qualitative and quantitative analysis of the γ spectrum.

350

Polymethyl methacrylate (PMMA) is an optically transparent thermoplastic with favorable processing conditions. In this study, a series of plastic scintillators are prepared via thermal polymerization, and the impact of PMMA content on their transparency and pulse shape discrimination (PSD) ability is investigated. The fabricated samples, comprising a polystyrene (PS)-PMMA matrix, 30.0 wt% 2,5-diphenyloxazole (PPO), and 0.2 wt% 9,10-diphenylanthracene (DPA), exhibit high transparency with transmissivity ranging from 70.0% to 90.0% (above 415.0 nm) and demonstrate excellent n/γ discrimination capability. Transparency increased with increasing PMMA content across the entire visible light spectrum. However, the PSD performance gradually deteriorated when the aromatic matrix was replaced with PMMA. The scintillator containing 20.0 wt% PMMA demonstrated the best stability concerning PSD properties and relative light yields.

248

β-ray-induced X-ray spectroscopy (BIXS) is a promising method for tritium detection in solid materials because of its unique advantages, such as large detection depth, nondestructive testing capabilities, and low requirements for sample preparation. However, high-accuracy reconstruction of the tritium depth profile remains a significant challenge for this technique. In this study, a novel reconstruction method based on a backpropagation (BP) neural network algorithm that demonstrates high accuracy, broad applicability, and robust noise resistance is proposed. The average reconstruction error calculated using the BP network (8.0%) was much lower than that obtained using traditional numerical methods (26.5%). In addition, the BP method can accurately reconstruct BIX spectra of samples with an unknown range of tritium and exhibits wide applicability to spectra with various tritium distributions. Furthermore, the BP network demonstrates superior accuracy and stability compared to numerical methods when reconstructing the spectra, with a relative uncertainty ranging from 0 to 10%. This study highlights the advantages of BP networks in accurately reconstructing the tritium depth profile from BIXS and promotes their further application in tritium detection.

201

Shielding materials are critical for downhole pulsed neutron tool design because they directly influence the accuracy of formation measurements. A well-designed shield configuration ensures that the response of the tool is maximally representative of the formation without being affected by the tool and borehole environment. This study investigated the effects of boron-containing materials on neutron and gamma detectors based on a newly designed logging-while-drilling tool that is currently undergoing manufacturing. As the boron content increased, the ability to absorb thermal neutrons increased significantly. Through simulation, it was proven that boron carbide (B₄C)can be used as an effective boron shielding material for thermal neutrons, and is therefore employed in this work. To shield against thermal neutrons migrating from the mud pipes, the optimal shielding thicknesses for the near- and far-neutron detectors were determined to be 5mm and 4mm. At a porosity of 25 p.u., near-neutron sensitivity exhibited a 5.6% increase. Furthermore, to shield the capture gamma generated by thermal neutrons once they enter the tool from the mud pipe and formation, internal and external shields for the gamma detector were evaluated. The results show that the internal shield requires a boron content of 75 %, whereas the external shield has a thickness of 14.2mm thickness and a boron content of 25 % to minimize the tool effect.

241

Neutron-induced gamma-ray imaging is a spectroscopic technique that uses characteristic gamma rays to infer the elemental distribution of an object. Currently, this technique requires the use of large facilities to supply a high neutron flux and a time-consuming detection procedure involving direct collimating measurements. In this study, a new method based on low neutron flux was proposed. A single-pixel gamma ray detector combined with random pattern gamma ray masks was used to measure the characteristic gamma rays emitted from the sample. Images of the elemental distribution in the sample, comprising 30×30 pixels, were reconstructed using the maximum-likelihood expectation-maximization algorithm. The results demonstrate that the elemental imaging of the sample can be accurately determined using this method. The proposed approach, which eliminates the need for high neutron flux and scanning measurements, can be used for in-field imaging applications.

194

The stability of matrix graphite under neutron irradiation and in corrosive environments is crucial for the safe operation of molten salt reactors (MSRs). Raman spectroscopy and a slow positron beam were employed to investigate the effects of He ion irradiation fluences and subsequent annealing on the microstructure and defects of the matrix graphite. He ions with 500 keV energy and fluences ranging from 1.1×10¹⁵ ions/cm² to 3.5×10¹⁷ ions/cm² were used to simulate neutron irradiation at 300 K. The samples with an irradiation fluence of 3.5×10¹⁶ ions/cm² were subjected to isochronal annealing at different temperatures (573 K, 873 and 1173 K) for 3 h. The Raman results revealed that the D peak gradually increased, whereas the intrinsic G peak decreased with increasing irradiation fluence. At the same irradiation fluence, the D peak gradually decreased, whereas the intrinsic G peak increased with increasing annealing temperature. Slow positron beam analysis demonstrated that the density or size of irradiation defects (vacancy type) increased with higher irradiation fluence, but decreased rapidly with increasing annealing temperature. The Raman spectral analysis of sample cross sections subjected to high irradiation fluences revealed the emergence of amorphization precisely at the depth where ion damage was most pronounced, whereas the surface retained its crystalline structure. Raman and positron annihilation analyses indicated that the matrix graphite exhibited good irradiation resistance to He ions at 300 K. However, vacancy-type defects induced by He ion irradiation exhibit poor thermal stability and can be easily removed during annealing.

164

Growth of high-quality Nb₃Sn thin films for superconducting radiofrequency (SRF) applications using the vapor diffusion method requires a uniform distribution of tin nuclei on the niobium (Nb) surface. This study examines the mechanism underlying the observed non-uniform distribution of tin nuclei with tin chloride SnCl₂. Electron backscatter diffraction (EBSD) analysis was used to examine the correlation between the nucleation behavior and orientation of niobium grains in the substrate. The findings of the density functional theory (DFT) simulation are in good agreement with the experimental results, showing that the non-uniform distribution of tin nuclei is the result of the adsorption energy of SnCl₂ molecules by varied niobium grain orientations. Further analysis indicated that the surface roughness and grain size of niobium also played significant roles in the nucleation behavior. This study provides valuable insights into enhancing the surface pretreatment of niobium substrates during the growth of Nb₃Sn thin films using the vapor diffusion method.

221

This study focuses on the electrical properties and microstructure of polypropylene (PP)-based blends used for cable insulation in nuclear power plants (NPPs). The PP-based blend, comprising isotactic PP and propylene-based elastomer (PBE) at concentrations ranging from 0 to 50 wt%, underwent a melt blending process and subsequent cobalt-60 gamma-ray irradiation with doses ranging from 0 kGy to 250 kGy. Electrical conductivity, trap distribution, and alternating (AC) breakdown strength were chosen to assess the insulation performance. These results indicate that the addition of PBE significantly improves the electrical properties of PP under irradiation. For PP, the electrical conductivity increased with irradiation, whereas the trap depth and breakdown strength decreased sharply. Conversely, for the blend, these changes initially exhibit opposite trends. When the irradiation was increased to 250 kGy, the AC breakdown strength of the blend improved by more than 21% compared to that of PP. The physical and chemical structures of the samples were investigated to explore the improvement mechanisms. The results offer insights into the design of new cable-insulation materials suitable for NPPs.

227

Gallium nitride (GaN)-based devices have significant potential for space applications. However, the mechanisms of radiation damage to the device, particularly from strong ionizing radiation, remains unknown. This study investigates the effects of radiation on p-gate AlGaN/GaN high-electron-mobility transistors (HEMTs). Under a high voltage, the HEMT leakage current increased sharply and was accompanied by a rapid increase in power density that caused "thermal burnout" of the devices. In addition, a burnout signature appeared on the surface of the burned devices, proving that a single-event burnout effect occurred. Additionally, degradation, including an increase in the on-resistance and a decrease in the breakdown voltage, was observed in devices irradiated with high-energy heavy ions and without bias. The latent tracks induced by heavy ions penetrated the heterojunction interface and extended into the GaN layer. Moreover, a new type of N₂ bubble defect was discovered inside the tracks using Fresnel analysis. The accumulation of N₂ bubbles in the heterojunction and buffer layers is more likely to cause leakage and failure. This study indicates that electrical stress accelerates the failure rate and that improving heat dissipation is an effective reinforcement method for GaN-based devices.

220

In the scenario of a steam generator tube rupture (SGTR) accident in a lead-cooled fast reactor (LFR), secondary circuit subcooled water under high pressure is injected into an ordinary-pressure primary vessel, where a molten lead-based alloy (typically pure lead or lead bismuth eutectic (LBE)) is used as the coolant. To clarify the pressure build-up characteristics under water jet injection, this study conducted several experiments by injecting pressurized water into a molten LBE pool at Sun Yat-sen University. To obtain a further understanding, several new experimental parameters were adopted, including the melt temperature, water subcooling, injection pressure, injection duration, and nozzle diameter. Through detailed analyses, it was found that the pressure and temperature during the water-melt interaction exhibited a consistent variation trend with our previous water droplet injection mode LBE experiment. Similarly, the existence of a steam explosion was confirmed, which typically results in a much stronger pressure buildup. For the non-explosion cases, increasing the injection pressure, melt-pool temperature, nozzle diameter, and water subcooling promoted pressure build-up in the melt pool. However, a limited enhancement effect was observed when increasing the injection duration, which may be owing to the continually rising pressure in the interaction vessel or the isolation effect of the generated steam cavity. Regardless of whether a steam explosion occurred, the calculated mechanical and kinetic energy conversion efficiencies of the melt were relatively small (not exceeding 4.1% and 0.7%, respectively). Moreover, the range of the conversion efficiency was similar to that of previous water droplet experiments, although the upper limit of the jet mode was slightly lower.

163

The off-situ accurate reconstruction of the core neutron field is an important step in realizing real-time reactor monitoring. The existing off-situ reconstruction method of the neutron field is only applicable to cases wherein a single region changes at a specified location of the core. However, when the neutron field changes are complex, the accurate identification of the individual changed regions becomes challenging, which seriously affects the accuracy and stability of the neutron field reconstruction. Therefore, this study proposed a dual-task hybrid network architecture (DTHNet) for off-situ reconstruction of the core neutron field, which trained the outermost assembly reconstruction task and the core reconstruction task jointly such that the former could assist the latter in the reconstruction of the core neutron field under core complex changes. Furthermore, to exploit the characteristics of the ex-core detection signals, this study designed a global-local feature upsampling module that efficiently distributed the ex-core detection signals to each reconstruction unit to improve the accuracy and stability of reconstruction. Reconstruction experiments were performed on the simulation datasets of the CLEAR-I reactor to verify the accuracy and stability of the proposed method. The results showed that when the location uncertainty of a single region did not exceed nine and the number of multiple changed regions did not exceed five. Further, the reconstructed ARD was within 2%, RD_max was maintained within 17.5%, and the number of RD_≥10% was maintained within 10. Furthermore, when the noise interference of the ex-core detection signals were within ± 2%, although the average number of RD_≥10% increased to 16, the average ARD was still within in 2%, and the average RD_max was within 22%. Collectively, these results show that, theoretically, the DTHNet can accurately and stably reconstruct most of the neutron field under certain complex core changes.

198

Small modular reactors have received widespread attention owing to their inherent safety, low investment, and flexibility. Small pressurized water reactors (SPWRs) have become important candidates for SMRs owing to their high technological maturity. Since the Fukushima accident, research on accident-tolerant fuels (ATFs), which are more resistant to serious accidents than conventional fuels, has gradually increased. This study analyzes the neutronics and thermal hydraulics of an SPWR (ACPR50S) for different ATFs, BeO+UO₂-SiC, BeO+UO₂-FeCrAl, U₃Si₂-SiC, and U₃Si₂-FeCrAl, based on a PWR fuel-management code, the Bamboo-C deterministic code. In the steady state, the burnup calculations, reactivity coefficients, power and temperature distributions, and control-rod reactivity worth were studied. The transients of the control-rod ejection accident for the two control rods with the maximum and minimum reactivity worth were analyzed. The results showed that 5% B-10 enrichment in the wet annular burnable absorbers assembly can effectively reduce the initial reactivity and end-of-life reactivity penalty. The BeO+UO₂-SiC core exhibited superior neutronic characteristics in terms of burnup and negative temperature reactivity compared with the other three cases owing to the strong moderation ability of BeO+UO₂ and low neutron absorption of SiC. However, the U₃Si₂ core had a marginally better power-flattening effect than BeO+UO₂, and the differential worth of each control-rod group was similar between different ATFs. During the transient of a control-rod ejection, the changes in the fuel temperature, coolant temperature, and coolant density were similar. The maximum difference was less than 10 ° for the fuel temperature and 2 ° for the coolant temperature.

241

Higher-order modes of the neutron diffusion/transport equation can be used to study the temporal behavior of nuclear reactors and can be applied in modal analysis, transient analysis, and online monitoring of the reactor core. Both the deterministic method and the Monte Carlo(MC) method can be used to solve the higher-order modes. However, MC method, compared to the deterministic method, faces challenges in terms of computational efficiency and α mode calculation stability, whereas the deterministic method encounters issues arising from homogenization-related geometric and energy spectra adaptation. Based on the higher-order mode diffusion calculation code HARMONY, we developed a new higher-order mode calculation code, HARMONY2.0, which retains the functionality of computing λ and α higher-order modes from HARMONY1.0, but enhances the ability to treat complex geometries and arbitrary energy spectra using the MC-deterministic hybrid two-step strategy. In HARMONY2.0, the mesh homogenized multigroup constants were obtained using OpenMC in the first step, and higher-order modes were then calculated with the mesh homogenized core diffusion model using the Implicitly Restarted Arnoldi method (IRAM), which was also adopted in the HARMONY1.0 code. In addition, to improve the calculation efficiency, particularly in large higher-order modes, event-driven parallelization λ/α domain decomposition methods are embedded in the HARMONY2.0 code to accelerate the inner iteration of λ/α mode using OpenMP. Furthermore, the higher-order modes of complex geometric models, such as Hoogenboom and ATR reactors for λ mode and the MUSE-4 experiment facility for the prompt α mode, were computed using diffusion theory.

166

The accumulation of ²²²Rn and ²²⁰Rn progeny in poorly ventilated environments poses the risk of natural radiation exposure to the public. A previous study indicated that satisfactory results in determining the ²²²Rn and ²²⁰Rn progeny concentrations by measuring the total alpha counts at five time intervals within 560 min should be expected only in the case of high progeny concentrations in air. To complete the measurement within a relatively short period and adapt it for simultaneous measurements at comparatively lower ²²²Rn and ²²⁰Rn progeny concentrations, a novel mathematical model was proposed based on the radioactive decay law. This model employs a nonlinear fitting method to distinguish nuclides with overlapping spectra by utilizing the alpha particle counts of nonoverlapping spectra within consecutive measurement cycles to obtain the concentrations of ²²²Rn and ²²⁰Rn progeny in air. Several verification experiments were conducted using an alpha spectrometer. The experimental results demonstrate that the concentrations of ²²²Rn and ²²⁰Rn progeny calculated by the new method align more closely with the actual circumstances than those calculated by the total count method, and their relative uncertainties are all within ± 16%. Furthermore, the measurement time was reduced to 90 min, representing an acceleration of 84%. The improved capability of the new method in distinguishing alpha particles with similar energies emitted from ²¹⁸Po and ²¹²Bi, both approximately 6 MeV, contributed to realizing more accurate results. The proposed method has the potential advantage of measuring relatively low concentrations of ²²²Rn and ²²⁰Rn progeny in air more quickly via air filtration.

174

By combining experimental α-decay energies and half-lives, the α-particle preformation factor for nuclei around neutron magic numbers N of 126, 152, and 162 were extracted using the two-potential approach. The nuclei around the shell closure were more tightly bound than adjacent nuclei. Additionally, based on the WS4 mass model [Phys. Lett. B 734, 215 (2014)], we extended the two-potential approach to predict the α-decay half-lives of nuclei around N values of 178 and 184 with Z of 119 and 120. We believe that our findings will serve as guidelines for future experimental studies.

184

Schottky mass spectrometry utilizing heavy-ion storage rings is a powerful technique for the precise mass and decay half-life measurements of highly charged ions. Owing to the non-destructive ion-detection features of Schottky noise detectors, the number of stored ions in the ring is determined by the peak area in the measured revolution-frequency spectrum. Because of their intrinsic amplitude-frequency characteristic (AFC), Schottky detector systems exhibit varying sensitivities at different frequencies. Using low-energy electron-cooled stored ions, a new method is developed to calibrate the AFC curve of the Schottky detector system of the Cooler Storage Ring (CSRe) storage ring located in Lanzhou, China. Using the amplitude-calibrated frequency spectrum, a notable refinement was observed in the precision of both the peak position and peak area. As a result, the storage lifetimes of the electron-cooled fully-ionized ⁵⁶Fe²⁶⁺ ions were determined with high precision at beam energies of 13.7 and 116.4 MeV/u, despite of frequency drifts during the experiment. When electron cooling was turned off, the effective vacuum condition experienced by the 116.4 MeV/u ⁵⁶Fe²⁶⁺ ions was determined using amplitude-calibrated spectra, revealing a value of 2×10^-10 mbar, which is consistent with vacuum-gauge readings along the CSRe ring. The method reported herein will be adapted for the next-generation storage ring of the HIAF facility under construction in Huizhou, China. It can also be adapted to other storage-ring facilities worldwide to improve precision and enhance lifetime measurements using many ions in the ring.

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