China's first pulsed neutron source

Evaluating the broad neutron spectrum of ANIS

The Atmospheric Neutron Irradiation Spectrometer (ANIS) is a new member of the worldwide neutron irradiation facilities. It was developed at the China Spallation Neutron Source (CSNS), producing the atmospheric-like neutrons from meV to GeV with high neutron flux simultaneously. Because of the differences between ANIS spectrum and the atmospheric neutron spectrum in the real environment, the neutron spectrum at ANIS should be evaluated to ensure the validity for the accelerated single event effects (SEEs) testing of electronics. In this study, the broad neutron spectrum at ANIS was obtained from the Monte Carlo simulation. An analytic expression of this neutron spectrum was provided. Calculations of the single error rate (SER) for the SRAM and DRAM devices with specific cross-section were also performed. By calculating the SER ratio compared with JESD89A and IEC TS 62396-1 standards, the deviation of the broad neutron spectrum at ANIS from that in the atmospheric neutron environment can be well predicted. This study evaluates the reliability of ANIS neutron spectrum for the first time.

A neutron beam monitor based on ceramic gas electron multiplier with high spatial resolution for low flux measurements

Neutron scattering instruments play an important role in studying the inner structure of materials. A neutron beam monitor is a detector commonly used in a neutron scattering instrument. The detection efficiency for most neutron beam monitors is quite low (10-4-10-6). However, in some experiments with a low neutron flux, such as small angle neutron scattering (SANS) and inelastic neutron scattering experiments, a neutron beam monitor with a higher detection efficiency (∼1% for thermal neutrons) is required to reduce the duration of the experiment. To meet this requirement, a ceramic gas electron multiplier-based neutron beam monitor equipped with a 1 µm 10B4C neutron converter was developed in this study. Its performance was determined both experimentally and in simulations. The detection efficiency in the wavelength range of 1.8-5.5 Å was measured experimentally and was confirmed by the simulation results. An algorithm based on event selection and position reconstruction was developed to improve the spatial resolution to about 1 mm full-width-half-maximum. The wavelength spectrum was measured in beamline 20 (BL20) and agreed well with the results obtained using a commercial monitor. The maximum counting rate was 1.3 MHz. The non-uniformity over the whole 100 × 100 mm2 active area was determined to be 1.4%. Due to the excellent performance of this monitor, it has been used in several neutron instruments, such as the SANS and the High-Energy Direct-Geometry Inelastic Spectrometer instruments in the China spallation neutron source.

Measurement of the [Formula: see text]Ta([Formula: see text]) cross sections up to stellar s-process temperatures at the CSNS Back-n

The neutron capture cross section of [Formula: see text]Ta is relevant to s-process of nuclear astrophysics, extraterrestrial samples analysis in planetary geology and new generation nuclear energy system design. The [Formula: see text]Ta([Formula: see text]) cross section had been measured between 1 eV and 800 keV at the back-streaming white neutron facility (Back-n) of China spallation neutron source(CSNS) using the time-of-flight (TOF) technique and [Formula: see text] liquid scintillator detectors. The experimental results are compared with the data of several evaluated libraries and previous experiments in the resolved and unresolved resonance region. Resonance parameters are extracted using the R-Matrix code SAMMY in the 1-700 eV region. The astrophysical Maxwell average cross section(MACS) from kT = 5 to 100 keV is calculated over a sufficiently wide range of neutron energies. For the characteristic thermal energy of an astrophysical site, at kT = 30keV the MACS value of [Formula: see text]Ta is 834 ± 75 mb, which shows an obvious discrepancy with the Karlsruhe Astrophysical Database of Nucleosynthesis in Stars (KADoNiS) recommended value 766 ± 15 mb. The new measurements strongly constrain the MACS of [Formula: see text]Ta([Formula: see text]) reaction in the stellar s-process temperatures.

Study of a position-sensitive scintillator neutron detector prototype based on 6LiF/ZnS scintillator and silicon photomultiplier arrays readout

Due to the shortage of the ³He gas and its rapidly increasing price, scintillator detectors, the advantages of which are high spatial resolution and capability of detection in real time, become widely used in many neutron instruments. In this work, a position-sensitive neutron detector consisting of a ⁶LiF/ZnS scintillation screen and a silicon photomultiplier array linked to a capacitive network to detect the positions of incident neutrons, is constructed and tested. To evaluate the detector performance, a series of neutron beam experiments with the detector prototype were performed in the BL20 at the China Spallation Neutron Source. The spatial resolution was measured, and the energy-selective neutron imaging and Bragg edge measurements of a 316L stainless steel sample were performed. A sub-millimeter spatial resolution could be obtained for the detector prototype under study. The detector with such a high spatial resolution is promising for applications in neutron scattering experimental installations, especially for neutron single-crystal diffractometers.

Online monitoring of air activation at the China spallation neutron source

During the operation of high-energy proton accelerators, the air in the tunnel is activated with the production of radionuclides. For CSNS (China Spallation Neutron Source), the first pulsed neutron source in China for multidisciplinary research, an online air activation monitoring system was developed to evaluate the radiation safety of the staff and the public, which consisted of a NaI detector, Pb shielding, an MB container and a control system. With the monitoring system, gamma spectra of the activated air from controlled areas are measured, and the activity concentration and immersion dose rates of radionuclides in air are calculated and displayed in real time. The system has been in stable operation since February 2020, and results have been obtained for the evaluation of the radiation risk from activated air.

Atomic Regulation of PGM Electrocatalysts for the Oxygen Reduction Reaction

With the increasing enthusiasm for the hydrogen economy and zero-emission fuel cell technologies, intensive efforts have been dedicated to the development of high-performance electrocatalytic materials for the cathodic oxygen reduction reaction (ORR). Some major fundamental breakthroughs have been made in the past few years. Therefore, reviewing the most recent development of platinum-group-metal (PGM) ORR electrocatalysts is of great significance to pushing it forward. It is known that the ORR on the fuel cell electrode is a heterogeneous reaction occurring at the solid/liquid interface, wherein the electron reduces the oxygen along with species in the electrolyte. Therefore, the ORR kinetic is in close correlation with the electronic density of states and wave function, which are dominated by the localized atomic structure including the atomic distance and coordination number (CN). In this review, the recent development in the regulation over the localized state on the catalyst surface is narrowed down to the following structural factors whereby the corresponding strategies include: the crystallographic facet engineering, phase engineering, strain engineering, and defect engineering. Although these strategies show distinctive features, they are not entirely independent, because they all correlate with the atomic local structure. This review will be mainly divided into four parts with critical analyses and comparisons of breakthroughs. Meanwhile, each part is described with some more specific techniques as a methodological guideline. It is hoped that the review will enhance an insightful understanding on PGM catalysts of ORR with a visionary outlook.

Enhancing the Electrochemical Performance and Structural Stability of Ni-Rich Layered Cathode Materials via Dual-Site Doping

Ni-rich layered cathode materials are considered as promising electrode materials for lithium ion batteries due to their high energy density and low cost. However, the low rate performance and poor electrochemical stability hinder the large-scale application of Ni-rich layered cathodes. In this work, both the rate performance and the structural stability of the Ni-rich layered cathode LiNi_0.8Co_0.1Mn_0.1O₂ are significantly improved via the dual-site doping of Nb on both lithium and transition-metal sites, as revealed by neutron diffraction results. The dual-site Nb-doped LiNi_0.8Co_0.1Mn_0.1O₂ delivers 202.8 mAh·g^-1 with a capacity retention of 81% after 200 electrochemical cycles, which is much higher than that of pristine LiNi_0.8Co_0.1Mn_0.1O₂. Moreover, a discharge capacity of 176 mAh·g^-1 at 10C rate illustrates its remarkable rate capability. Through in situ X-ray diffraction and electronic transport property measurements, it was demonstrated that the achievement of dual-site doping in the Ni-rich layered cathode can not only suppress the Li/Ni disordering and facilitate the lithium ion transport process but also stabilize the layered structure against local collapse and structural distortion. This work adopts a dual-site-doping approach to enhance the electrochemical performance and structural stability of Ni-rich cathode materials, which could be extended as a universal modification strategy to improve the electrochemical performance of other cathode materials.

Radionuclides in target station coolant in the China Spallation Neutron Source

The China Spallation Neutron Source (CSNS) is the first pulsed neutron source in China for multidisciplinary research. After operation with 80 kW proton beam for 4 months, 3 circuits of target station coolant, light water 1/2/3 were sampled, and radionuclides in coolants were measured. The results showed that, activity concentration of H-3 in coolant can be up to the magnitude of 1.00E+06 Bq/L, and the H-3 amount in light water 1 was the highest and the amount in light water 3 was the lowest, agreeing with the radiation field exposed by coolants. For Be-7, due to the complicated filtering and trapping process, amount of Be-7 in coolant differed from a minimum of 7.15E+01 Bq/L to a maximum of 4.58E+03 Bq/L. Comparison of the results with former measurements and simulated results were conducted. Permitted volumes for coolant discharge were presented. And the work time in the equipment room of cooling system after the beam is shut off is safe. Results in this research could provide reference data and measurement methods for similar accelerator devices.

Introduction of a C6D6detector system on the Back-n of CSNS

Radiative neutron capture cross sections are very important in the field of basic physics research and nuclear device R&D. The Back-n white neutron beam line of China Spallation Neutron Source (CSNS) is the first spallation neutron beam line in China. On the purpose for radiative neutron capture cross section measurement, a C6D6detector system was built in the Back-n experimental station. The pulse height weighting technique (PHWT) was used to make the system’s detective efficiency independent of the cascade path and the energy of cascade gamma rays. The neutron energy spectrum was measured for the energy between 1eV and 80keV with a6Li loaded ZnS scintillation detector. Besides, a testing experiment with197Au and169Tm samples was carried out to examine this system. According to the preliminary results, this C6D6detector system can be used to perform neutron capture cross section measurement.

Neutron sub-micrometre tomography from scattering data

Neutrons are valuable probes for various material samples across many areas of research. Neutron imaging typically has a spatial resolution of larger than 20 µm, whereas neutron scattering is sensitive to smaller features but does not provide a real-space image of the sample. A computed-tomography technique is demonstrated that uses neutron-scattering data to generate an image of a periodic sample with a spatial resolution of ∼300 nm. The achieved resolution is over an order of magnitude smaller than the resolution of other forms of neutron tomography. This method consists of measuring neutron diffraction using a double-crystal diffractometer as a function of sample rotation and then using a phase-retrieval algorithm followed by tomographic reconstruction to generate a map of the sample's scattering-length density. Topological features found in the reconstructions are confirmed with scanning electron micrographs. This technique should be applicable to any sample that generates clear neutron-diffraction patterns, including nanofabricated samples, biological membranes and magnetic materials, such as skyrmion lattices.

RESEARCH ON DOSE MONITORING IN BACKSCATTERING NEUTRON HALL IN CHINA SPALLATION NEUTRON SOURCE

The back-n project in China Spallation Neutron Source (CSNS) was launched primarily for nuclear data measurements. In the backscattering neutron hall, the neutron and gamma monitors were used for dose monitoring. Because of the dead time problem of monitors, performance of the monitors in such pulsed radiation field needs to be analyzed. In this research, experiments with dose monitors and personal dosemeters were conducted, and simulation by Monte Carlo code FLUKA was performed. Results showed that the values by monitors are smaller, and the larger the dose, the larger the difference. The reasons in term of energy response and dead time have been analyzed, and corrections were discussed. After corrections, the measured value can agree with the simulation results in the range of about a factor 3. Totally speaking, the values recorded by neutron and gamma monitors can be a reference for radiation safety management in CSNS.

Measurement of the neutron energy spectrum of Back-n #ES1 at CSNS

The China spallation neutron source (CSNS) was built and started running since 2018. It produces neutrons by impinging 1.6 GeV protons onto a tungsten target with 25 Hz repetition frequency. A beam line exploiting the back-streaming neutrons (Back-n) was built mainly for nuclear data measurement and started commissioning simultaneously with CSNS in 2018. There are two experimental endstations along the Back-n beam line: endstation 1 (#ES1) with a neutron flight path of about 55 m and endstation 2 (#ES2) with about 76 m. The neutron energy spectra of both #ES1 and #ES2 were measured since it is important for feasibility study and analysis. In this paper, the measurement of the neutron energy spectrum of Back-n #ES1 is reported. It is measured by a multi-layer fission chamber using the 235U samples as the neutron converters. The neutron energy spectrum from 0.1 eV to 30 MeV is obtained. The integral neutron flux (from 0.1 eV to 30 MeV) normalized to the proton beam power of 100 kW is 1.55×107 neutrons/cm2/s.

Measurement of Gamma-Ray from Inelastic Neutron Scattering on 56Fe

In nuclear reactors, inelastic neutron scattering is a significant energy-loss mechanism which has deep impacts on designments of nuclear reactor and radiation shielding. Iron is an important material in reactor. However, for the existing nuclear data for iron, there exists an obvious divergence for the inelastic scattering cross sections and the related gamma production sections. Therefore the precise measurements are urgently needed for satisfying the demanding to design new nuclear reactors (fast reactors), Accelerator Driven Subcritical System (ADS), and other nuclear apparatus. In this paper, we report a new system with an array of HPGe detectors, electronics and acquisition system. Experiments had been carried out on three neutron facilities.

Back-n white neutron facility at CSNS and first-year nuclear data measurements

Back-streaming neutrons through the incoming proton channel at the spallation target station of China Spallation Neutron Source (CSNS) were exploited to build a white neutron beam line (the so-called Back-n). With a proton beam of 100 kW in beam power and 1.6 GeV in kinetic energy and a thick tungsten target and modest moderation by the cooling water through the target slices, the neutron beam is very intense which is in the order of 7.0×106 n/cm2/s at 77 m from the target and has an excellent energy spectrum spanning from 0.5 eV to 200 MeV. In addition, the time resolution related to the time-of-flight measurements is very good for neutron energy determination. Altogether, it makes the CSNS Back-n one of the best white neutron sources in the world and very suitable for nuclear data measurements, especially for neutron-induced cross-section measurements. Since the completion of the Back-n beamline and four physics spectrometers in March 2018, the first batches of experiments on nuclear data measurements have been carried out, which are summarized in this article.