Vol.32,

As an advanced treatment method in the past five years, ultra-high dose rate (FLASH) radiotherapy as a breakthrough and milestone in radiotherapy development has been verified to be much less harmful to healthy tissues in different experiments. FLASH treatments require an instantaneous dose rate as high as hundreds of grays per second to complete the treatment in less than 100 ms. Current proton therapy facilities with the spread-out of the Bragg peak formed by different energy layers, to our knowledge, cannot easily achieve an adequate dose rate for FLASH treatments because the energy layer switch or gantry rotation of current facilities requires a few seconds, which is relatively long. A new design for a therapy facility based on a proton linear accelerator (linac) for FLASH treatment is proposed herein. It is designed under two criteria: no mechanical motion and no magnetic field variation. The new therapy facility can achieve an ultra-high dose rate of up to 300 Gy/s; however, it delivers an instantaneous dose of 30 Gy within 100 ms to complete a typical FLASH treatment. The design includes a compact proton linac with permanent magnets, a fast beam kicker in both azimuth and elevation angles, a fixed gantry with a static superconducting coil to steer proton bunches with all energy, a fast beam scanner using radio-frequency (RF) deflectors, and a fast low-level RF system. All relevant principles and conceptual proposals are presented herein.

356

A 2856-MHz, π-mode, 7-cell standing-wave-deflecting cavity was designed and fabricated for bunch-length measurement in Tsinghua Thomson scattering X-ray source (TTX) facility. This cavity was installed in the TTX and provided a deflecting voltage of 4.2 MV with an input power of 2.5 MW. Bunch-length diagnoses of electron beams with energies up to 39 MeV have been performed. In this article, the RF design of the cavity using HFSS, fabrication, and RF test processes are reviewed. High-power operation with accelerated beams and calibration of the deflecting voltage are also presented.

541

An S-band high-gradient accelerating structure is designed for a proton therapy linear accelerator (linac) to accommodate the new development of compact, single-room facilities and ultra-high dose rate (FLASH) radiotherapy. To optimize the design, an efficient optimization scheme is applied to improve the simulation efficiency. An S-band accelerating structure with 2856 MHz is designed with a low beta of 0.38, which is a difficult structure to achieve for a linac accelerating proton particles from 70 to 250 MeV, as a high gradient up to 50 MV/m is required. A special design involving a dual-feed coupler eliminates the dipole field effect. This paper presents all the details pertaining to the design, fabrication, and cold test results of the S-band high-gradient accelerating structure.

378

We report the secondary X(γ) radiation from the accelerator in a normal operating state and activated X(γ) radiation from the accelerator devices when the accelerator stops operating in the cancer treatment facility of the Shanghai Proton and Heavy Ion Center. These radiation measurements show us the structural radiation distribution along the beam lines and devices inside the accelerator room when the beam is on and off, and can support the radiation protection design of the accelerator facility used for cancer treatment and help evaluate the accumulated radiation dose in the case of an emergency, such as a personal safety system failure or a radiation accident. The radiation dose rate measured in this facility shows that the facility is safe from the radiation protection point. After shooting the quality assurance (QA) beam, the radiation dose rate in the treatment room was also measured to investigate the radiation dose space distribution and decay time dependence. In addition, the time period before safely entering the treatment room after determining the shooting of the QA beam is recommended to be approximately 5 min.

388

A method based on the cross-sectional relationship between ¹⁰B(n, α)⁷Li and ¹H(n, γ)²H was proposed to detect and reconstruct the three-dimensional boron concentration/dose distribution in vivo during boron neutron capture therapy (BNCT). Factors such as the neutron energy, fluence rate, and degree of non-uniform distribution of the boron concentration in a voxel may affect the results of this method. A theoretical analysis of the accuracy of the method using a Monte Carlo simulation shows that the determining error is generally less than 1% under different tumor locations and neutron source configurations. When the voxel size is larger than 0.4 cm, the determining error might be higher for a non-uniformly distributed boron concentration in the voxel because of the changes in the neutron energy and fluence rate. In conclusion, the proposed method enables an accurate three-dimensional boron determination in vivo during BNCT.

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Proton therapy is the most advanced radiotherapy approach in the world, and causes less damage to normal human tissue than traditional radiotherapy. Because the treatment process produces a high-energy proton beam, the personnel safety interlock system mainly considers measures to protect personnel from radiation hazards during beam preparation and the beam release process. Unlike other safety interlock systems, the personnel safety interlock system designed in this study focuses on the safety and stability of the system itself. The hardware and software of important interlock control loops are designed and developed according to the requirements of Safety Integrity Level 3 specified by IEC61508. A set of redundant ring networks was developed to ensure that damage to a certain network line does not affect the normal operation of the system. A set of friendly operation interfaces and data storage systems were developed to ensure that the operator can monitor the data in real time and trace the data. The personnel safety interlock system mainly includes a beam enabling function, clearance function, and emergency stop function. The system was put into actual use and successfully ensured personnel safety.

338

The widespread use of computed tomography (CT) in clinical practice has made the public focus on the cumulative radiation dose delivered to patients. Low-dose CT (LDCT) reduces the X-ray radiation dose, yet compromises quality and decreases diagnostic performance. Researchers have made great efforts to develop various algorithms for LDCT and introduced deep-learning techniques, which have achieved impressive results. However, most of these methods are directly performed on reconstructed LDCT images, in which some subtle structures and details are readily lost during the reconstruction procedure, and convolutional neural network (CNN)-based methods for raw LDCT projection data are rarely reported. To address this problem, we adopted an attention residual dense CNN, referred to as AttRDN, for LDCT sinogram denoising. First, it was aided by the attention mechanism, in which the advantages of both feature fusion and global residual learning were used to extract noise from the contaminated LDCT sinograms. Then, the denoised sinogram was restored by subtracting the noise obtained from the input noisy sinogram. Finally, the CT image was reconstructed using filtered back-projection. The experimental results qualitatively and quantitatively demonstrate that the proposed AttRDN can achieve a better performance than state-of-the-art methods. Importantly, it can prevent the loss of detailed information and has the potential for clinical application.

435

Isochronous mass spectrometry (IMS) of heavy-ion storage rings is a powerful tool for the mass measurements of short-lived nuclei. In IMS experiments, masses are determined through precision measurements of the revolution times of the ions stored in the ring. However, the revolution times cannot be resolved for particles with nearly the same mass-to-charge (m/q) ratios. To overcome this limitation and to extract the accurate revolution times for such pairs of ion species with very close m/q ratios, in our early work on particle identification, we analyzed the amplitudes of the timing signals from the detector based on the emission of secondary electrons. Here, the previous data analysis method is further improved by considering the signal amplitudes, detection efficiencies, and number of stored ions in the ring. A sensitive Z-dependent parameter is introduced in the data analysis, leading to a better resolution of ³⁴Ar¹⁸⁺ and ⁵¹Co²⁷⁺ with A/Z=17/9. The mean revolution times of ³⁴Ar¹⁸⁺ and ⁵¹Co²⁷⁺ are deduced, although their time difference is merely 1.8 ps. The uncorrected, overlapped peak of these ions has a full width at half maximum of 7.7 ps. The mass excess of ⁵¹Co was determined to be -27,332(41) keV, which is in agreement with the previous value of -27,342(48) keV.

404

Neutrons have played a vital role in many nuclear physics fields. In some cases, the inverse kinematics of neutrons colliding with other nuclei are also worth studying. In this study, the inverse kinematics of thermal neutrons colliding with high-energy protons is simulated by using the Monte Carlo method. Thermal neutrons are taken as target particles, whereas protons are incident particles. The simulation implies that, after collision, the energy of the output neutron at 0∘ equals the energy of the incident proton. A possible application of the result is proposed that might yield single-energy neutrons. Some key parameters of the conceptual design were evaluated, demonstrating that the design may reach high-neutron-energy resolution.

387

To mitigate consequences of core melting, an ex-vessel core catcher is investigated in this study. Instructions should be obeyed to cool down the corium caused by core melting. The corium destroys the reactor containment and causes radioactive materials to be released into the environment if it does not cool down well. It is important to build a core catcher system for the reception, localization, and cool down of the molten corium during a severe accident resulting from core melting. In this study, the role of a core catcher in the VVER-1000/v528 reactor containment during a station black out (SBO) accident is evaluated using the MELCOR1.8.6 code. In addition, parametric analyses of the SBO for (i) SBO accidents with emergency core cooling system (ECCS) operation, and (ii) without ECCS operation are performed. Furthermore, thermal-hydraulic analyses in dry and wet cavities with/without water are conducted. The investigations include the reduction of gases resulting from molten-corium–concrete interactions (H₂, CO, CO₂). Core melting, gas production, and the pressure/temperature in the reactor containment are assessed. Additionally, a full investigation pertaining to gas release (H₂, CO, CO₂) and the pressure/temperature of the core catcher is performed. Based on MELCOR simulations, a core cavity and a perimeter water channel are the best options for corium cooling and a lower radionuclide release. This simulation is also theoretically investigated and discussed herein. The simulation results show that the core catcher system in addition to an internal sacrificial material reduce the containment pressure from 689 to 580 kPa and the corresponding temperature from 394 to 380 K. Furthermore, it is observed that the amount of gases produced, particularly hydrogen, decreased from 1698 to 1235 kg. Moreover, the presence of supporting systems, including an ECCS with a core catcher, prolonged the core melting time from 16430 to 28630 s (in an SBO accident) and significantly decreased the gases produced.

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