Vol.33,

A simpler and improved utility approximate point scattered function for thin-film converters currently used in neutron photographic devices is proposed as a correction method to produce clearer, more realistic images. The validity of the model was demonstrated through a simulation experiment. Based on the results, an error analysis was carried out, certain corrections were made to the original model, and the final model achieved a very low relative error in the simulation experiment. The model can also be optimized for quantitative neutron photographic analysis using iterative algorithms to obtain realistic neutron photographic images more quickly. At the end of the article, the model is extended to consider the case of energy spectrum hardening by introducing a temperature correction parameter.

463

Very-high-frequency (VHF) gun photoinjectors, capable of producing high-brightness and high-repetition-rate electron bunches, are some of the best electron sources for driving MHz-class repetition-rate free-electron lasers. In this study, the beam dynamics optimization of a VHF gun photoinjector for Shanghai HIgh Repetition Rate X-ray Free Electron Laser and Extreme Light Facility (SHINE) is systematically demonstrated using a genetic algorithm. Through the inclusion of the solenoid geometry as an optimization variable into the genetic algorithm, the optimum projected normalized emittance for 100 pC bunches with bunch length of 1 mm rms is reduced to 0.1 mm mrad for 100% of the particles and 0.075 mm mrad for 95% of the particles, proving that sub-100 nm emittance can be achieved in the SHINE injector using a single-cell Tsinghua University (THU) VHF gun. This emittance fulfills the requirements not only of SHINE and Linac Coherent Light Source (LCLS)-II but also of LCLS-II-High Energy (LCLS-II-HE). We demonstrate that the optimal emittance in the VHF gun injector is reduced via the optimization of the solenoid geometry, thereby reducing solenoid spherical aberration. Through the inclusion of high-order (H.O.) energy spread among the optimization objectives, the H.O. energy spread can be reduced by a factor of nearly six using a high-harmonic cavity despite a 38% emittance growth. Finally, the beam dynamics in the SHINE main accelerator show that reducing the H.O. energy spread in the injector is of great significance to improving compression efficiency and reducing bunch current spike.

765

The use of carbon-ion radiotherapy (CIRT) is gradually increasing. Owing to the generation of high-energy secondary neutrons during CIRT, its use presents new challenges in radiation protection. Thus, secondary neutron dose distributions must be explored and evaluated under clinical scenarios based on different treatment configurations. However, neutron dose and energy spectrum measurements are often difficult. This can be primarily attributed to the inherent limitations of most neutron detectors, such as their unsuitability for spectral measurements and inaccurate responses to neutrons with energies above 20 MeV. Numerical calculation methods based on probabilistic statistical theory are fast and convenient for neutron dose evaluation. In this study, external secondary neutron doses at the heavy ion medical machine in Wuwei, which is equipped with a passive beam delivery system, were calculated using the Monte Carlo method. The dependence of neutron doses on various treatment parameters (incident carbon-ion beam energy, spatial location, field size, and spread-out Bragg peak (SOBP) width) was investigated. Furthermore, the feasibility of applying an analytical model to predict the ambient dose equivalent was verified. For the combination involving an energy of 400 MeV/u and SOBP width of 6 cm, the ambient dose equivalent per therapeutic dose (H/D) at the isocenter was 79.87 mSv/Gy.. The H/D value decreased rapidly with increasing spatial distance and slightly with increasing aperture size and SOBP width. The H/D values derived from the Monte Carlo simulations were in good agreement with the results reported in the literature. The analytical model could be used to quickly predict the H/D value along the incidence direction of the beam with an error of less than 20 %. Thus, our study contributes to the understanding of the relationship between neutron radiation and treatment configuration parameters, which establishes a basis for predicting non-therapeutic radiation doses in CIRT.

646

Particle accelerators are indispensable tools in both science and industry. However, the size and cost of conventional RF accelerators limits the utility and scope of this technology. Recent research has shown that a dielectric laser accelerator (DLA) made of dielectric structures and driven at optical frequencies can generate particle beams with energies ranging from MeV to GeV at the tabletop level. To design DLA structures with a high acceleration gradient, we demonstrate topology optimization, which is a method used to optimize the material distribution in a specific area based on given load conditions, constraints, and performance indicators. To demonstrate the effectiveness of this approach, we propose two schemes and design several acceleration structures based on them. The optimization results demonstrate that the proposed method can be applied to structure optimization for on-chip integrated laser accelerators, producing manufacturable structures with significantly improved performance compared with previous size or shape optimization methods. These results provide new physical approaches to explore ultrafast dynamics in matter, with important implications for future laser particle accelerators based on photonic chips.

402

Superconducting linear accelerators (SCL) have a high acceleration gradient and are capable of operating in a high-duty factor mode. For high-power and high-intensity SCL, the design of beam dynamics generally follows the principle that the zero-current periodic phase advance (σ₀) of each degree of freedom is less than 90° to avoid envelope instability caused by space charge. However, this principle is obtained under the condition of a completely periodic focusing channel, and it is ambiguous for pseudo-periodic structures, such as linear accelerators. Although transverse beam dynamics without acceleration have been studied by other researchers, it appears that there are some connections between pure 2D and 3D beam dynamics. Based on these two points, five focusing schemes for the solenoid and quadrupole doublet channels were designed to simulate the beam behavior with non-constant σ₀. Among them, the four schemes follow the characteristics of variation in the zero-current longitudinal phase advance (σ_0l) under a constant acceleration gradient and synchronous phase. The zero-current transverse phase advance (σ_0t) is consistent with σ_0l, based on the equipartition requirement. The initial σ_0t was set to 120°, 110°, 100°, and 90°, and was then gradually decreased to approximately 40° at the end of the channel. The last scheme maintains the maximum σ_0t of 88° by reducing the acceleration gradient of the corresponding cavities, until the point at which σ_0t equals 88° with a normal gradient. Using the stopbands obtained from the linearized envelope equations and multiparticle particle-in-cell (PIC) simulations, the transport properties of both continuous and 3D-bunched beams with the acceleration of the five focusing schemes were studied. It was found that for a CW beam, when tune depression > 0.7, σ_0t can break through 90° when the beams were transported in both solenoid and quadrupole doublet periodic focusing channels. When tune depression < 0.7, the conclusions were different. For the solenoid focusing system, σ_0t can partially break through 90°, and the beam quality is not significantly affected. For the quadrupole doublet focusing system, a partial breakthrough of 90° has a greater impact on the beam quality. The same conclusions were obtained for a bunched beam with acceleration.

330

A temperature-measurement device can produce data deviations and can even be damaged in a high-dose radiation environment. To reduce the radiation damage to such a device and improve the temperature-measurement accuracy in a radiation environment, a temperature sensor based on optical-fiber sensing technology is proposed. This sensor has a cascade structure composed of a single-mode fiber (SMF), a dispersion-compensation fiber (DCF), a no-core fiber (NCF), and another SMF (SDNS). The DCF and NCF are coated with a polydimethylsiloxane (PDMS) film, which is a heat-sensitive material with high thermal optical and thermal expansion coefficients. In experiments, PDMS was found to produce an irradiation crosslinking effect after irradiation, which improved the temperature sensitivity of the SDNS sensor. The experimental results showed that within a range of 30-100 ℃, the maximum temperature sensitivity after irradiation was 62.86 pm/℃, and the maximum transmission sensitivity after irradiation was 3.353 × 10^-2 dB/℃, which were 1.22 times and 2.267 times the values before irradiation, respectively. In addition, repeated temperature experiments verified that the SDNS sensor coated with the PDMS film had excellent temperature repeatability. Furthermore, it was found that with an increase in the irradiation intensity, the irradiation crosslinking degree of PDMS increased, and the temperature sensitivity of the sensor was improved. The proposed sensor could potentially be applied to temperature measurement in a nuclear-radiation environment.

372

Multiple-bit upsets (MBUs) have become a threat to modern advanced field-programmable gate arrays (FPGAs) applications in radiation environments. Hence, many investigations have been conducted using medium-energy heavy ions to study the effects of MBU radiation. However, high-energy heavy ions (HEHIs) greatly affect the size and percentage of MBUs because their ionization track structures differ from those of medium-energy heavy ions. In this study, the different impacts of high-energy and medium-energy heavy ions on MBUs in 28 nm FPGAs as well as their mechanisms are thoroughly investigated. With the Geant4 calculation, more serious energy effects of HEHIs on MBU scales were successfully demonstrated. In addition, we identified worse MBU responses resulting from lowered voltages. The MBU orientation effect was observed in the radiation of different dimensions. The broadened ionization tracks for tilted tests in different dimensions could result in different MBU sizes. The results also revealed that the ionization tracks of tilted HEHIs have more severe impacts on the MBU scales than medium-energy heavy ions with much higher linear energy transfer. Therefore, comprehensive radiation with HEHIs is indispensable for effective hardened designs to apply high-density 28 nm FPGAs in deep space exploration.

616

Configurational information entropy (CIE) analysis has been shown to be applicable for determining the neutron skin thickness (δ_np) of neutron-rich nuclei from fragment production in projectile fragmentation reactions. The BNN + FRACS machine learning model was adopted to predict the fragment mass cross-sections (σ_A) of the projectile fragmentation reactions induced by calcium isotopes from ³⁶Ca to ⁵⁶Ca on a ⁹Be target at 140 MeV/u. The fast Fourier transform was adopted to decompose the possible information compositions in σ_A distributions and determine the quantity of CIE (S_A [f]). It was found that the range of fragments significantly influences the quantity of S_A [f], which results in different trends of S_A [f] ~ δ_np correlation. The linear S_A [f] ~ δ_np correlation in a previous study [Nucl. Sci. Tech. 33, 6 (2022)] could be reproduced using fragments with relatively large mass fragments, which verifies that S_A [f] determined from fragment σ_A is sensitive to the neutron skin thickness of neutron-rich isotopes.

348

The self-consistent quadruple potential is deduced within the relativistic mean-field (RMF) framework and substituted into the Hamiltonian, which is calculated using the complex momentum representation (CMR). Considering even-even titanium isotopes as an example, this study investigated various properties, including the resonant states of neutron-rich nuclei in the RMF-CMR model, and used them to describe the binding energy. The abrupt decrease in the two-neutron separation energy (S_2n) corresponds to the traditional magic number. The resonant and bound states are simultaneously exposed in the complex moment plane, where the continuum is along the integration contour. The four oblate neutron-rich nuclei ^72-78Ti are weakly bound or resonant because their Fermi energies are approximately 0 MeV. The root-mean-square (RMS) radii of these nuclei increase suddenly compared with those of others (neutron number N < 48). Moreover, ⁷⁸Ti and ⁷⁶Ti are determined as drip-line nucleons by the value of S_2n and the energy levels, respectively. Finally, the weak-bounded character can be represented by diffuse density probability distributions.

438

SiC fibers were irradiated by 414.4-MeV ¹¹²Sn^27.3+ ions to different fluences (5.0×10¹², 6.0×10¹³, 1.6×10¹⁴, and 1.92×10¹⁵ ions/cm²). ¹¹²Sn^27.3+ deposited its energy mainly via electron energy loss and passed through the SiC fiber. Then, the mechanical properties and surface characteristics of fibers were studied using a specific single filament tensile test and field emission scanning electron microscopy (FE-SEM). Results revealed that the carbon concentration on the fiber surface increased while the silicon concentration decreased. Moreover, the addition of oxygen was found to correlate with an increase in ion fluence. Meanwhile, the surface fiber morphology of the least fluence (5.0×10¹² ions/cm²) irradiated specimen displayed no obvious changes and its diameter was slightly reduced. With successive increases of ion fluence, large grains/bubbles on the fiber surface first appeared and then disappeared, and the diameter of fibers evidently increased. Moreover, at the highest fluence (1.92×10¹⁵ Sn ions/cm²) irradiated specimen, some fibers were brittle fractured. As a result, the mean tensile strength and the average elastic modulus of the fibers generally decreased with respect to the ion fluence. The degradation mechanisms of mechanical properties of SiC fibers under irradiation are discussed in detail.

384

The Shanghai soft X-ray Free-Electron Laser (SXFEL) user facility project started in 2016 and is expected to be open to users by 2022. It aims to deliver ultra-intense coherent femtosecond X-ray pulses to five endstations covering a range of 100 to 620 eV for ultrafast X-ray science. Two undulator lines are designed and constructed, based on different lasing modes: self-amplified spontaneous emission and echo-enabled harmonic generation. The coherent scattering and imaging (CSI) endstation is the first of five endstations to be commissioned online. It focuses on high-resolution single-shot imaging and the study of ultrafast dynamic processes using coherent forward scattering techniques. Both the single-shot holograms and coherent diffraction patterns were recorded and reconstructed for nanoscale imaging, indicating the excellent coherence and high peak power of the SXFEL and the possibility of "diffraction before destruction" experiments at the CSI endstation. In this study, we report the first commissioning results of the CSI endstation.

644

The multilayer Laue lens (MLL) is a diffractive focusing optical element which can focus hard X-rays down to the nanometer scale. In this study, a WSi₂/Si multilayer structure consisting of 1736 layers, with a 7.2-nm-thick outermost layer and a total thickness of 17μm, is prepared by DC magnetron sputtering. Regarding the thin film growth rate calibration, we correct the long-term growth rate drift from 2% to 0.6%, as measured by the grazing incidence X-ray reflectivity (GIXRR). A one-dimensional line focusing resolution of 64 nm was achieved, while the diffraction efficiency was 38% of the -1 order of the MLL Shanghai Synchrotron Radiation Facility (SSRF) with the BL15U beamline..

381

To study the interaction between molten non-eutectic alloys and subcooled water during severe nuclear accidents, an experimental investigation was carried out by injecting molten Lead-Bismuth Non-Eutectic alloy (LBNE, 70% Pb-30% Bi) into the water in a free-fall style using the Visualized Thermo-hydraulic characteristics in Melt Coolant Interaction (VTMCI) facility. The effects of various experimental parameters, including water temperature, melt temperature, melt penetration velocity, and water depth, on the molten LBNE jet fragmentation characteristics were studied. The research shows that compared with Lead-Bismuth Eutectic, larger fragments, less spherical fragments, and more porous debris beds are generated for LBNE. Higher water or melt temperatures facilitate molten LBNE fragmentation, resulting in higher debris bed sphericity and lower debris bed porosity. A higher melt temperature leads to smaller fragment sizes, except that a cake-like debris bed is formed for very high alloy superheat and very low water subcooling. More approximate spherical particles were generated in the film boiling zone than in the thermal interaction zone. Fragment size decreases with an increase in the melt penetration velocity, but the debris bed porosity and sphericity variation were not obvious. The effect of water depth on molten LBNE fragmentation behavior was not obvious under the current experimental conditions. Compared with other fragmentation theories, the Weber number theory better predicted the fragment volume mean diameter. In addition, a melt-jet behavior mode diagram that describes the competition between the hydrodynamic and thermal interactions and diagrams of debris bed porosity and sphericity, that show the influence of thermal factors were analyzed in this study.

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