Vol.30,

A real-time Bonner sphere spectrometer (BSS) has been developed for spectral neutron measurements with the HL-2A Tokamak. To correct and verify the accuracy of the neutron spectrum from the BSS, the BSS system was calibrated using monoenergetic neutron beams in the energy range of 100 keV–5 MeV. The response function of the BSS was corrected based on the calibration results, and the corrected BSS system was verified by unfolding monoenergetic neutron spectra. Fusion neutron spectra on the HL-2A have been obtained from the calibrated BSS system for the first time.

511

A Long Short-Term Memory (LSTM) neural network has excellent learning ability applicable to time series’ of nuclear pulse signals. It can accurately estimate parameters associated with amplitude, time, and so on, in digitally shaped nuclear pulse signals—especially signals from overlapping pulses. By learning the mapping relationship between Gaussian overlapping pulses after digital shaping and exponential pulses before shaping, the shaping parameters of the overlapping exponential nuclear pulses can be estimated using the LSTM model. Firstly, the Gaussian overlapping nuclear pulse (ONP) parameters which need to be estimated received Gaussian digital shaping treatment, after superposition by multiple exponential nuclear pulses. Secondly, a dataset containing multiple samples was produced, each containing a sequence of sample values from Gaussian ONP, after digital shaping, and a set of shaping parameters from exponential pulses before digital shaping. Thirdly, the Training Set in the dataset was used to train the LSTM model. From these data sets, the values sampled from the Gaussian ONP were used as the input data for the LSTM model, and the pulse parameters estimated by the current LSTM model were calculated by forward propagation. Next, the loss function was used to calculate the loss value between the network-estimated pulse parameters and the actual pulse parameters. Then, a gradient-based optimization algorithm was applied, to feedback the loss value and the gradient of the loss function to the neural network, to update the weight of the LSTM model, thereby achieving the purpose of training the network. Finally, the sampled value of the Gaussian ONP for which the shaping parameters needed to be estimated was used as the input data for the LSTM model. After this, the LSTM model produced the required nuclear pulse parameter set. In summary, experimental results showed that the proposed method overcame the defect of local convergence encountered in traditional methods, and could accurately extract parameters from multiple, severely overlapping Gaussian pulses, to achieve optimal estimation of nuclear pulse parameters in the global sense. These results support the conclusion that this is a good method for estimating nuclear pulse parameters.

624

Radiochromic film is a useful tool for beam quality assurance, but accurate response assessment of the film is still a problem. In this study, the response uncertainties of HDV2 film were investigated using a flatbed scanner from both the scanning settings and interscan variability. Scanning settings are fixed conditions for scanning, including scanning resolution and focus setting. In this study, multipeak distributions of pixel values were found under some dots-per-inch values, which should be avoided, and the optimal setting of 2000 dpi without this problem was selected. By changing the focus setting, the relative standard deviation of pixel values was reduced by 36% to 50%. The influence of the interscan variability induced by three factors was investigated, including the outside illumination intensity, film homogeneity, and operating temperature. Scanning the film before and after irradiation at the same position was recommended. Moreover, the suitable operating-temperature range for the scanner was found to be 15°C to 24°C, which results in stable film responses. Regarding the studied factors, correction methods and strategies were proposed, and the accurate response assessment of HDV2 film was realized. Finally, a standard operating procedure for response assessment of films was introduced. It can help other researchers study more scanners, films, and particle types.

619

The main purpose of this work was to perform a rigorous computational study on scintimammography with a Mura-mask based on Monte Carlo simulation of voxelized breast phantoms. Three main objectives were addressed: (i) verification of Geant4 version 10.4, (ii) optimization of the imaging setup, and (iii) small tumor detection and localization. We successfully verified the Geant4-based imaging of a commonly used phantom in the field. We used a Mura-mask with a 41× 41 array pattern with adjustable thickness, material, and hole shape (box and cylinder); a low energy high resolution collimator with different hole shapes (cylinder and hexagon); and a voxelized breast phantom with different sizes (small, medium, and large) and glandularity percentages (low, medium, and high). We also compared the detector crystal outputs of CdZnTe and NaI(Tl). The simulation was followed by a deconvolution procedure, and the data (images) were statistically emphasized. Statistical metrics indicate that the Mura-mask (W material with 1.5 mm thickness and box holes) combined with a CdZnTe detector leads to the optimum point spread function. Finally, a preliminary study on small sized tumor detection and localization was conducted with different tumor-to-background ratios (from 2 to 12). Tumors with diameters of 5 and 8 mm could be detected, while those of 2 mm were undetectable. Nevertheless, this study enhances our understanding of the early detection of tumors in the field of scintimammography.

450

An experimental muon source (EMuS) is planned for the China Spallation Neutron Source (CSNS). A simplified beamline with a limited number of magnets is achieved using a FODO lattice for implementation in a future preliminary stage. The yield of the muon delivered to the experimental sample is slightly larger than 10⁵ μ⁺/s within the FWHM beam spot (~ Ø30 mm) from a thick muon target. In addition, the beam polarization is 92% and the contamination that is mainly formed by positrons is approximately a fraction of 1%.

471

The protein complex crystallographic beamline BL19U1 at the Shanghai Synchrotron Radiation Facility is one of the five beamlines dedicated to protein sciences operated by National Facility for Protein Science (Shanghai, China). The beamline, which features a small-gap in-vacuum undulator, has been officially open to users since March 2015. This beamline delivers X-ray in the energy range 7-15 keV. With its high flux, low divergence beam and a large active area detector, BL19U1 is designed for proteins with large molecular weight and large crystallographic unit cell dimensions. Good performance and stable operation of the beamline have allowed the number of Protein Data Bank (PDB) depositions and the number of articles published based on data collected at this beamline to increase steadily. To date, over 300 research groups have collected data at the beamline. More than 600 PDB entries have been deposited at the PDB (pdb.org). More than 300 papers have been published that include data collected at the beamline, including 21 research articles published in the top-level journals Cell, Nature, and Science.

685

In neutron and photon transport problems, anisotropic scattering is of great importance for the particle flux, especially when the angular flux has a strong forward peak in shielding analyses. The conventional Legendre expansion is widely-used in discrete ordinates transport codes because of algebraic simplifications with spherical harmonics for the scattering source. However, negative cross-sections caused by the finitely truncated expansion may give rise to a negative source and flux. A simple method is adopted, based on integrating functions of scattering moments, to evaluate anisotropy and convergence of expanded functions. A series of problems were designed with angular fluxes of different anisotropy, and numerical simulations were performed using the ARES transport code to study different treatments and algorithms for scattering. Results show that the diagonal transport approximation is more stable, and obtains a similar accuracy with the extended approximation. A conservative fix-up for the negative source could ensure particle balance and improve computational accuracy significantly for photon transport. The effect of anisotropic scattering is problem-dependent, and no distinct differences among various methods are observed for volume source problems with a continuous energy source. For beam source problems, flux results are sensitive to negative scattering functions, and strictly non-negative cross-sections need to be implemented.

550

Semi-implicit direct kinetics (SIDK) is an innovative method for the temporal discretization of neutronic equations proposed by J. Banfield. The key approximation of the SIDK method is to substitute a time-averaged quantity for the fission source term in the delayed neutron differential equations. Hence, these equations are decoupled from prompt neutron equations and an explicit analytical representation of precursor groups is obtained, which leads to significant reduction in computational cost. As the fission source is not known in a time-step, the original study suggested using a constant quantity pertaining to the previous time-step for this purpose, and a reduction in the size of the time-step was proposed to lessen the imposed errors. However, this remedy notably diminishes the main advantage of the SIDK method. We discerned that if the original method is properly introduced into the algorithm of the point-implicit solver along with some modifications, the mentioned drawbacks will be mitigated adequately. To test this idea, a novel multi-group, multi-dimensional diffusion code using the finite-volume method and a point-implicit solver is developed which works in both transient and steady states. In addition to the SIDK, two other kinetic methods, i.e. direct kinetics and higher-order backward discretization (HOBD), are programmed into the diffusion code for comparison with the proposed model. The final code is tested at different conditions of two well-known transient benchmark problems. Results indicate that while the accuracy of the improved SIDK is comparable with the best available kinetic methods, it reduces the total time required for computation by up to 24%.

469

The HTR-10 is a small modular high temperature reactor (HTR) located in Tsinghua University, in China. After the reactor ran continuously for 72 h at full power in the commissioning stage, a full load rejection test was conducted by manually disconnecting the generator from the grid. In this study, reactor transients were analyzed using the THERMIX model. Some of the important thermal-fluid phenomena that occurred after the test initiation are discussed here, including the natural convection of helium in the core and the temperature redistribution in the reactor. Temperatures reproduced for the measuring points arranged in the internals were in agreement with the test data. This demonstrated that the code and calculation model are suitable for post-test analysis applications. Regarding the safety features of the reactor, there was a large margin between the predicted maximum fuel temperature of 997 ^oC and the safety limit of 1620 ^oC.

580

NG-CT-10 and NG-CT-20 are newly developed grades of nuclear-grade graphite from China. In this study, their oxidation behaviors were experimentally investigated using thermal gravimetric analysis. Microstructural evolution before and after oxidation was investigated using scanning electron microscope, mercury intrusion and Raman spectroscopy. The apparent activation energy of NG-CT-10 nuclear graphite is 161.4 kJ/mol in a reaction temperature range of 550–700 °C and that of NG-CT-20 is 153.5 kJ/mol in a temperature range of 550–650 °C. The activation energy in the inner diffusion control regime is approximately half that in the kinetics control regime. At high temperatures, the binder phase is preferentially oxidized over the filler particles and small pores are generated in the binder. No new large or deep pores are generated on the graphite surfaces. Oxygen can diffuse along the boundaries of filler particles and through the binder phase, but cannot diffuse into the spaces between the nanocrystallites in the filler particles. Filler particles are oxidized starting at their outer surfaces and the sizes of nanocrystallites do not decrease following oxidation.

715

To optimize the temperature coefficient of reactivity (TCR) for a graphite-moderated and liquid-fueled molten salt reactor, the effects of fuel salt composition on the fuel salt temperature coefficient of reactivity (FSTC) were investigated in our earlier work. In this study, we aim to provide a more comprehensive analysis of the TCR by considering the effects of the graphite moderator temperature coefficient of reactivity (MTC). The effects of ²³⁵U enrichment and heavy metal (HM) proportion in the salt mixture on the MTC are investigated from the perspective of the six-factor formula based on a full-core model. For the MTC (labeled "α_TM"), the temperature coefficient of the fast fission factors (α_TM(ε)) is positive, while those of the resonance escape probability (α_TM(p)), the thermal reproduction factor (α_TM(η)), the thermal utilization factor (α_TM(f)), and the total non-leakage probability (α_TM(Λ)) are negative. The results reveal that the magnitudes of α_TM(ε) and α_TM(p) for the MTC are similar. Thus, variations in the MTC with ²³⁵U enrichment for different HM proportions are mainly dependent on α_TM(η), α_TM(Λ), and α_TM(f), but especially on the former two. To obtain a more negative MTC, a lower HM proportion and/or a lower ²³⁵U enrichment is recommended. Together with our previous studies on the FSTC, a relatively soft neutron spectrum could strengthen the TCR with a sufficiently negative MTC.

544

The current study describes the application of a new extraction method for efficient uranium adsorption via cost-effective hydrazine-impregnated activated carbon. Various experimental parameters such as time, adsorbent weight, temperature (^оC), and uranium concentration were thoroughly investigated. The synthesized adsorbent was characterized via X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FT-IR), Scanning electron microscopy (SEM) and Thermogravimetric analysis (TGA). The results showed 86% uranium extraction under optimized conditions (20% P₂O₅ at 25 °C, 120 min). The obtained findings fit well with thermodynamic and isothermal (Langmuir and Freundlich isotherms) models and pseudo second-order kinetics. In thermodynamic studies, the negative sign of (�G°) specified the spontaneity of process, the negative sign of (�H°) revealed endothermicity and the positive sign of (�S°) showed high randomness after adsorption.

492

Intensity-modulated proton therapy (IMPT) is becoming essential for proton therapy and is under rapid development. However, for IMPT, the lateral penumbra of the spot-scanning proton beam is still an urgent issue to be solved. Patient-specific block collimators (PSBCs), which can block unnecessary doses, play a crucial role in passive scattering delivery technology but are rarely used in spot scanning. One objective of this study is to investigate the lateral penumbra variations of intensity-modulated spot scanning with and without a PSBC. For fields with varying degrees of sharpness and at varying depths in a water phantom, the lateral penumbral widths were calculated using a Monte Carlo-based dose engine from RayStation 6. The results suggest that the lateral penumbral widths can be reduced by more than 30% for uniform target volumes, regardless of whether a range-shifter is used, and that the maximum dose beyond the field edges can be reduced significantly. The results of patient cases show that the doses in organs-at-risk near the edge of the target volume decrease if a PSBC is implemented. This study demonstrates that intensity-modulated spot scanning with a PSBC can effectively reduce the lateral penumbra and block unnecessary doses and is therefore promising for clinical applications in spot-scanning proton therapy.

498

Two-dimensional (2D) transition metal carbides known as MXenes belong to a new branch of 2D material family and their fundamental properties vary with their compositions and surface functionalizations. In this study, the structural and ideal mechanical properties of M₂C-type MXenes and their functionalized M₂CT₂ MXenes (M=Ti, Zr, Hf; T=O, F, OH) were systematically examined via first-principles methods. The stress–strain (SS) curves of the MXenes under homogenous biaxial and uniaxial tension are identified, and the fundamental quantities (e.g., Young’s modulus, in-plane stiffness, and Poisson’s ratio) are addressed. With significantly higher strength and extended critical strains, the M₂CO₂ MXenes exhibit optimal flexibility when compared with that of M₂C, M₂CF₂, and M₂C(OH)₂. Additionally, Hf₂CT₂ exhibits optimal tensile performance under uniaxial or biaxial tension when compared to that of Ti₂CT₂ and Zr₂CT₂. The Young’s modulus, in-plane stiffness, and Poisson’s ratio of MXenes with different surface functionalization increase in a sequence corresponding to OH < F < O. Furthermore, the effects of vacancy on the mechanical properties of MXenes are further explored and indicate that vacancy can significantly weaken the tensile properties of MXenes that are considered. Moreover, vacancy also results in a certain anisotropy of stress along armchair and zigzag directions even under the biaxial tension condition.

473