Abstract：The 232Th(n,f) cross-section is very important in basic nuclear physics and applications based on the Th/U fuel cycle. Using the time-of-flight method and a multi-cell fast fission ionization chamber, a novel measurement of the 232Th(n,f) cross-section relative to 235U in the 1–200 MeV range was performed at the China Spallation Neutron Source Back-n white neutron source (Back-n). The fission event-neutron energy spectra of 232Th and 235U fission cells were measured in the single-bunch mode. Corrected 232Th/235U fission cross-section ratios were obtained, and the measurement uncertainties were 2.5–3.7% for energies in the 2–20 MeV range and 3.6–6.2% for energies in the 20–200 MeV range. The 232Th(n,f) cross-section was obtained by introducing the standard cross-section of 235U(n,f). The results were compared with those of previous theoretical calculations, measurements, and evaluations. The measured 232Th fission cross-section agreed with the main evaluation results in terms of the experimental uncertainty, and 232Th fission resonances were observed in the 1–3 MeV range. The present results provide 232Th(n,f) cross-section data for the evaluation and design of Th/U cycle nuclear systems.
Keywords：232Th(n,f) cross section;Fast-fission ionization chamber;Back-n white neutron source
Abstract：The nuclear charge radius plays a vital role in determining the equation of state of isospin asymmetric nuclear matter. Based on the correlation between the differences in charge radii of mirror-partner nuclei and the slope parameter (L) of symmetry energy at the nuclear saturation density, an analysis of the calibrated slope parameter L was performed in finite nuclei. In this study, relativistic and non-relativistic energy density functionals were employed to constrain the nuclear symmetry energy through the available databases of the mirror-pair nuclei 36Ca-36S, 38Ca-38Ar, and 54Ni-54Fe. The deduced nuclear symmetry energy was located in the range 29.89–31.85 MeV, and L of the symmetry energy essentially covered the range 22.50–51.55 MeV at the saturation density. Moreover, the extracted Ls at the sensitivity density was located in the interval range 30.52–39.76 MeV.
Abstract：Based on the covariant density functional theory, by employing the core–quasiparticle coupling (CQC) model, the nuclear level density of odd-A nuclei at the saddle point is achieved. The total level density is calculated via convolution of the intrinsic level density and the collective level density. The intrinsic level densities are obtained in the finite-temperature covariant density functional theory, which takes into account the nuclear deformation and pairing self-consistently. For saddle points on the free energy surface in the plane, the entropy and the associated intrinsic level density are compared with those of the global minima. By introducing a quasiparticle to the two neighboring even–even core nuclei, whose properties are determined by the five-dimensional collective Hamiltonian model, the collective levels of the odd-A nuclei are obtained via the CQC model. The total level densities of the 234-240U agree well with the available experimental data and Hilaire’s result. Furthermore, the ratio of the total level densities at the saddle points to those at the global minima and the ratio of the total level densities to the intrinsic level densities are discussed separately.
Keywords：Level density;Covariant density functional theory;Core-quasiparticle coupling model;Saddle point
Abstract：Maximum likelihood estimation (MLE) is an effective method for localizing radioactive sources in a given area. However, it requires an exhaustive search for parameter estimation, which is time consuming. In this study, heuristic techniques were employed to search for radiation source parameters that provide the maximum likelihood by using a network of sensors. Hence, the time consumption of MLE would be effectively reduced. First, the radiation source was detected using the k-sigma method. Subsequently, the MLE was applied for parameter estimation using the readings and positions of the detectors that have detected the radiation source. A comparative study was performed in which the estimation accuracy and time consumption of the MLE were evaluated for traditional methods and heuristic techniques. The traditional MLE was performed via a grid search method using fixed and multiple resolutions. Additionally, four commonly used heuristic algorithms were applied: the firefly algorithm (FFA), particle swarm optimization (PSO), ant colony optimization (ACO), and artificial bee colony (ABC). The experiment was conducted using real data collected by the Low Scatter Irradiator facility at the Savannah River National Laboratory as part of the Intelligent Radiation Sensing System program. The comparative study showed that the estimation time was 3.27 s using fixed resolution MLE and 0.59 s using multi-resolution MLE. The time consumption for the heuristic-based MLE was 0.75, 0.03, 0.02, and 0.059 s for FFA, PSO, ACO, and ABC, respectively. The location estimation error was approximately 0.4 m using either the grid search-based MLE or the heuristic-based MLE. Hence, heuristic-based MLE can provide comparable estimation accuracy through a less time consuming process than traditional MLE
Abstract：Radio-frequency (RF) breakdown analysis and location are critical for successful development of high-gradient traveling-wave (TW) accelerators, especially those expected to generate high-intensity, high-power beams. Compared with commonly used schemes involving dedicated devices or complicated techniques, a convenient approach for breakdown locating based on transmission line (TL) theory offers advantages in the typical constant-gradient TW-accelerating structure. To deliver such an approach, an equivalent TL model has been constructed to equate the TW-accelerating structure based on the fundamental theory of the TL transient response in the time domain. An equivalence relationship between the TW-accelerating structure and the TL model has been established via analytical derivations associated with grid charts and verified by TL circuit simulations. Furthermore, to validate the proposed fault-locating method in practical applications, an elaborate analysis via such a method has been conducted for the recoverable RF-breakdown phenomena observed at an existing prototype of a TW-accelerating-structure-based beam injector constructed at the Huazhong University of Science and Technology. In addition, further considerations and discussion for extending the applications of the proposed method have been given. This breakdown-locating approach involving the transient response in the framework of TL theory can be a conceivable supplement to existing methods, facilitating solution to construction problems at an affordable cost.
Keywords：Traveling-wave structure;RF breakdown;Fault locating;Transmission line
Abstract：Neutron computed tomography (NCT) is widely used as a noninvasive measurement technique in nuclear engineering, thermal hydraulics, and cultural heritage. The neutron source intensity of NCT is usually low and the scan time is long, resulting in a projection image containing severe noise. To reduce the scanning time and increase the image reconstruction quality, an effective reconstruction algorithm must be selected. In CT image reconstruction, the reconstruction algorithms used can be divided into three categories: analytical algorithms, iterative algorithms, and deep learning. Because the analytical algorithm requires complete projection data, it is not suitable for reconstruction in harsh environments, such as strong radiation, high temperature, and high pressure. Deep learning requires large amounts of data and complex models, which cannot be easily deployed, as well as has a high computational complexity and poor interpretability. Therefore, this paper proposes the OS-SART-PDTV iterative algorithm, which uses the ordered subset simultaneous algebraic reconstruction technique (OS-SART) algorithm to reconstruct the image and the first-order primal-dual algorithm to solve the total variation (PDTV), for sparse-view NCT three-dimensional reconstruction. The novel algorithm was compared with other algorithms (FBP, OS-SART-TV, OS-SART-AwTV, and OS-SART-FGPTV) by simulating the experimental data and actual neutron projection experiments. The reconstruction results demonstrate that the proposed algorithm outperforms the FBP, OS-SART-TV, OS-SART-AwTV, and OS-SART-FGPTV algorithms in terms of preserving edge structure, denoising, and suppressing artifacts.
Abstract：With the advancement in X-ray astronomical detection technology, various celestial polarization detection projects have been initiated. To meet the calibration requirements of polarimeters on the ground, a polarized X-ray radiation facility was designed for this study. The design was based on the principle that X-rays incident at 45° on a crystal produce polarized X-rays, and a second crystal was used to measure the polarization of the X-rays produced by the facility after rotation. The effects of different diaphragm sizes on the degree of polarization were compared, and the facility produced X-rays with polarization degrees of up to 99.55±0.96% using LiF200 and LiF220 crystals. This result revealed that the polarization of incident X-rays is one of the factors affecting the diffraction efficiency of crystals. The replacement of different crystals can satisfy the calibration requirements of polarized X-ray detectors with more energy points in the energy range (4–10) keV. In the future, the facility should be placed in a vacuum environment to meet the calibration requirements at lower energies.
Abstract：Proton computed tomography (CT) has a distinct practical significance in clinical applications. It eliminates 3–5% errors caused by the transformation of Hounsfield unit (HU) to relative stopping power (RSP) values when using X-ray CT for positioning and treatment planning systems (TPSs). Following the development of FLASH proton therapy, there are increased requirements for accurate and rapid positioning in TPSs. Thus, a new rapid proton CT imaging mode is proposed based on sparsely sampled projections. The proton beam was boosted to 350 MeV by a compact proton linear accelerator (linac). In this study, the comparisons of the proton scattering with the energy of 350 MeV and 230 MeV are conducted based on GEANT4 simulations. As the sparsely sampled information associated with beam acquisitions at 12 angles is not enough for reconstruction, X-ray CT is used as a prior image. The RSP map generated by converting the X-ray CT was constructed based on Monte Carlo simulations. Considering the estimation of the most likely path (MLP), the prior image-constrained compressed sensing (PICCS) algorithm is used to reconstruct images from two different phantoms using sparse proton projections of 350 MeV parallel proton beam. The results show that it is feasible to realize the proton image reconstruction with the rapid proton CT imaging proposed in this paper. It can produce RSP maps with much higher accuracy for TPSs and fast positioning to achieve ultra-fast imaging for real-time image-guided radiotherapy (IGRT) in clinical proton therapy applications.
Abstract：To guarantee the exact proton dose applied to patients and ensure treatment safety while disrupting and destroying tumor cells, it is essential to accurately monitor the proton beam current in real-time during patient treatment. Because clinical treatment requires a proton beam current in the ñA range, nondestructive beam current monitors (BCMs) are preferred to minimize the degradation of beam quality. However, this poses significant challenges in accurately monitoring such extremely low beam intensities. This study proposes a cavity-type BCM equipped with a dielectric plate to reduce its dimensions and achieve sufficient measurement sensitivity for practical requirements. A prototype cavity BCM was fabricated, and offline testing was performed using a metal wire to simulate the beam to study its performance. Both the simulation and experimental results showed that the cavity BCM could measure ultralow proton beam currents with a resolution up to 0.03 nA.
Abstract：The in-core self-powered neutron detector (SPND) acts as a key measuring device for the monitoring of parameters and evaluation of the operating conditions of nuclear reactors. Prompt detection and tolerance of faulty SPNDs are indispensable for reliable reactor management. To completely extract the correlated state information of SPNDs, we constructed a twin model based on a generalized regression neural network (GRNN) that represents the common relationships among overall signals. Faulty SPNDs were determined because of the functional concordance of the twin model and real monitoring systems, which calculated the error probability distribution between the model outputs and real values. Fault detection follows a tolerance phase to reinforce the stability of the twin model in the case of massive failures. A weighted K-nearest neighbor model was employed to reasonably reconstruct the values of the faulty signals and guarantee data purity. The experimental evaluation of the proposed method showed promising results, with excellent output consistency and high detection accuracy for both single- and multiple-point faulty SPNDs. For unexpected excessive failures, the proposed tolerance approach can efficiently repair fault behaviors and enhance the prediction performance of the twin model.
Abstract：Lanthanum bromide (LaBr3) crystal has a high energy resolution and time resolution and has been used in Compton cameras (CCs) over the past few decades. However, LaBr3 crystal arrays are difficult to process because LaBr3 is easy to crack and break; thus, few LaBr3-based CC prototypes have been built. In this study, we designed and fabricated a large-pixel LaBr3 CC prototype and evaluated its performance with regard to position, energy, and angular resolution. We used two 10 × 10 LaBr3 crystal arrays with a pixel size of 5 mm × 5 mm, silicon photomultipliers (SiPMs), and corresponding decoding circuits to construct our prototype. Additionally, a framework based on a Voronoi diagram and a lookup table was developed for list-mode projection data acquisition. Monte Carlo (MC) simulations based on Geant4 and experiments were conducted to evaluate the performance of our CC prototype. The lateral position resolution was 5 mm, and the maximum deviation in the depth direction was 2.5 and 5 mm for the scatterer and absorber, respectively. The corresponding measured energy resolutions were 7.65% and 8.44%, respectively, at 511 keV. The experimental results of 137Cs point-like sources were consistent with the MC simulation results with regard to the spatial positions and full widths at half maximum (FWHMs). The angular resolution of the fabricated prototype was approximately 6° when a point-like 137Cs source was centrally placed at a distance of 5 cm from the scatterer. We proposed and investigated a large-pixel LaBr3 CC for the first time and verified its feasibility for use in accurate spatial positioning of radiative sources with a high angular resolution. The proposed CC can satisfy the requirements of radiative source imaging and positioning in the nuclear industry and medical applications.
Abstract：The use of radioactive isotopes, such as Cs-137, to measure formation density is a common practice; however, it poses high risks such as environmental contamination from lost sources. To address these challenges, the use of pulsed neutron sources for density measurements, also known as "source-less density", has emerged as a promising alternative. By collecting gamma counts at different time gates according to the duty cycle of the pulsed sequence, the inelastic gamma component can be isolated to obtain more accurate density measurements. However, the collection of gamma rays during the neutron burst-on period often contains a proportion of capture gamma rays, which can reduce the accuracy of density measurements. This proportion can vary depending on the formation environment and neutron duty cycle. To address these challenges, an adaptive capture gamma correction method was developed for density measurements. This method distinguishes between "burst-on" and "burst-off" periods based on the gamma time spectra, and derives the capture ratio in the burst-on period by iteratively fitting the capture gamma time spectra, resulting in a more accurate net inelastic gamma. This method identifies the end of the pulse by automatically calculating the differential, and fits the capture gamma time spectra using Gaussian process regression, which considers the differences in formation attenuation caused by different environments. The method was verified through simulations with errors of below 0.025 g/cm3, demonstrating its adaptability and feasibility for use in formation density measurements. Overall, the proposed method has the potential to minimize the risks associated with radioactive isotopes and improve the accuracy of density measurements in various duty cycles and formation environments.
Keywords：Neutron-induced gamma;Adaptive correction;Source-less density
Abstract：Environmental radon emanates from the exhalation and release of soil, rocks, and building materials. Environmental radon contamination tracing and radon pollution prevention and control require the measurement of the radon exhalation rate on media surfaces. Reliable measurements of the radon exhalation rate cannot be achieved without regular calibration of the measuring instrument with a high-performance reference device. In this study, a reference device for the calibration of radon exhalation rate measuring instruments was developed using a diffusion solid radon source with a high and stable radon emanation coefficient, an integrated diffusion component composed of a plasterboard and a high-density wooden board, an air pressure balance device, a radon accumulation chamber, and a support structure. The uniformity and stability of the reference device were evaluated using the activated carbon-γ spectrum and open-loop method, respectively, to measure the radon exhalation rate. The reference device achieved different radon exhalation rates by using different activities of diffusion solid radon sources. Nineteen measurement points were regularly selected on the radon exhalation surface of the reference device, and the uniformity of the radon exhalation rate exceeded 5%. The short-term stability of the reference device was better than 5% under different environmental conditions and was almost unaffected by the ambient air pressure, environmental temperature, and relative humidity.
Abstract：A deep learning-based automated Kirkpatrick–Baez mirror alignment method is proposed for synchrotron radiation. We trained a convolutional neural network (CNN) on simulated and experimental imaging data of a focusing system. Instead of learning directly from bypass images, we use a scatterer for X-ray modulation and speckle generation for image feature enhancement. The smallest normalized root mean square error on the validation set was 4%. Compared with conventional alignment methods based on motor scanning and analyzer setups, the present method simplified the optical layout and estimated alignment errors using a single-exposure experiment. Single-shot misalignment error estimation only took 0.13 s, significantly outperforming conventional methods. We also demonstrated the effects of the beam quality and pretraining using experimental data. The proposed method exhibited strong robustness, can handle high-precision focusing systems with complex or dynamic wavefront errors, and provides an important basis for intelligent control of future synchrotron radiation beamlines.
Abstract：The accurate measurement of parameters such as the cavity-loaded quality factor (QL) and half bandwidth (f0.5) is essential for monitoring the performance of superconducting radio-frequency (SRF) cavities. However, the conventional "field decay method" employed to calibrate these values requires the cavity to satisfy a "zero-input" condition. This can be challenging when the source impedance is mismatched and produce nonzero forward signals (Vf) that significantly affect the measurement accuracy. To address this limitation, we developed a modified version of the "field decay method" based on the cavity differential equation. The proposed approach enables the precise calibration of f0.5 even under mismatch conditions. We tested the proposed approach on the SRF cavities of the Chinese Accelerator Driven System Front-End Demo Superconducting Linac and compared the results with those obtained from a network analyzer. The two sets of results were consistent, indicating the usefulness of the proposed approach.
Keywords：Loaded quality factor;Field decay method;Superconducting cavity;Mismatch;Calibration;Cavity differential equation;Measurement;Accelerator driven system