Measurement of the ^(232)Th(n,f)cross section in the 1-200 MeV range at the CSNS Back-n

The ^(232)Th(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^(232)Th(n,f)cross sec-tion relative to^(235)U 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^(232)Th and^(235)U fission cells were measured in the single-bunch mode.Corrected 232Th/235U fission cross-sectional 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^(232)Th(n,f)cross section was obtained by introducing the standard cross section of^(235)U(n,f).The results were compared with those of previous theoreti-cal 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^(232)Th(n,f)cross-sectional data for the evaluation and design of Th/U cycle nuclear systems.

Silicon photomultiplier based scintillator thermal neutron detector for China Spallation Neutron Source(CSNS)

The energy-resolved neutron imaging spectrometer(ERNI)will be installed in 2022 according to the spectrometer construction plan of the China Spallation Neutron Source(CSNS).The instrument requires neutron detectors with the coverage area of approximately 4 m^(2)in 5°-170°neutron diffraction angle.The neutron detection efficiency needs to be better than 40%at 1 A neutron wavelength.The spatial resolution should be better than 3 mm×50 mm in the horizontal and vertical directions respectively.We develop a one-dimensional scintillator neutron detector which is composed of the^(6)Li F/Zn S(Ag)scintillation screens,the wavelength-shifting fiber(WLSF)array,the silicon photomultipliers(Si PMs),and the self-designed application-specific integrated circuit(ASIC)readout electronics.The pixel size of the detector is designed as 3 mm×50 mm,and the neutron-sensitive area is 50 mm×200 mm.The performance of the detector prototype is measured using neutron beam 20#of the CSNS.The maximum counting rate of 247 k Hz,and the detection efficiency of63%at 1.59 A are obtained.The test results show that the performance of the detector fulfills the physical requirements of the ERNI under construction at the CSNS.

Back-n white neutron source at CSNS and its applications

Back-streaming neutrons from the spallation target of the China Spallation Neutron Source(CSNS)that emit through the incoming proton channel were exploited to build a white neutron beam facility(the so-called Back-n white neutron source),which was completed in March 2018.The Back-n neutron beam is very intense,at approximately 29107 n/cm2/s at 55 m from the target,and has a nominal proton beam with a power of 100 kW in the CSNS-I phase and a kinetic energy of 1.6 GeV and a thick tungsten target in multiple slices with modest moderation from the cooling water through the slices.In addition,the excellent energy spectrum spanning from 0.5 eV to 200 MeV,and a good time resolution related tothe time-of-flight measurements make it a typical white neutron source for nuclear data measurements;its overall performance is among that of the best white neutron sources in the world.Equipped with advanced spectrometers,detectors,and application utilities,the Back-n facility can serve wide applications,with a focus on neutron-induced cross-sectional measurements.This article presents an overview of the neutron beam characteristics,the experimental setups,and the ongoing applications at Backn.

Measurement of the neutron total cross section of carbon at the Back-n white neutron beam of CSNS

To verify the performance of the neutron total cross-sectional spectrometer, the neutron total cross section of carbon is initially measured in the energy range of 1 eV to 20 MeV using the time-of-flight method. The measurement is performed at the Back-n white neutron source with a 76-m time-of-flight path using the China Spallation Neutron Source. A multilayer fast fission chamber with 235U and 238U is employed as the neutron detector. The diameter and thickness of the natural graphite sample are 70 mm and 40 mm, respectively. Signal waveforms are collected using a data acquisition system. Off-line data processing was used to obtain the neutron time-of-flight spectra and transmissions. The uncertainty of the counting statistics is generally approximately 3% for each bin in the energy range of 1–20 MeV. It is determined that the results for the neutron total cross section of carbon obtained using ^235U cells are in good agreement with the results obtained using 238U cells within limits of statistical uncertainty. Moreover, the measured total cross sections show good agreement with the broadening evaluated data.

A large area ^(3)He tube array detector with vacuum operation capacity for the SANS instrument at the CSNS

The small-angle neutron scattering(SANS)instrument,one of the first three instruments of the China Spallation Neutron Source(CSNS),is designed to probe the microscopic and mesoscopic structures of materials in the scale range 1–100 nm.A large-area ^(3)He tube array detector has been constructed and operates at the CSNS SANS instrument since August 2018.It consists of 120 linear position-sensitive detector tubes,each 1 m in length and 8 mm in diameter,and filled with ^(3)He gas at 20 bar to obtain a high detection efficiency.The ^(3)He tubes were divided into ten modules,providing an overall area of 1000 mm×1020 mm with a high count rate capability.Because each tube is installed independently,the detector can be quickly repaired in situ by replacing damaged tubes.To reduce air scattering,the SANS detector must operate in a vacuum environment(0.1 mbar).An all-metal sealing technique was adopted to avoid high-voltage breakdown by ensuring a high-voltage connection and an electronic system working in an atmospheric environment.A position resolution of 7.8±0.1 mm(full width at maximum)is measured along the length of the tubes,with a high detection efficiency of 81±2% at 2A.Operating over the past four years,the detector appears to perform well and with a high stability,which supports the SANS instrument to finish approximately 200 user scientific programs.

Graded composition and doping p-i-n Al_(x)Ga_(1−x)As/GaAs detector for unbiased voltage operation

p-i-n Al_(x)Ga_(1−x)As/GaAs detectors with graded compositions and graded doping were grown and prepared.From the current-voltage and capacitance-voltage measurement results,the devices had good p-n junction diode characteristics,and the electric field strength under an unbiased voltage was 1.7×10^(5) Vcm^(-1).The full width at half maximum and charge collection efficiency of the detectors obtained from energy spectrum measurements of 5.48-MeV alpha particles were 3.04 and approximately 93%,respectively.In this study,we created the most advanced and promising state-of-the-art unbiased detector reported to date.

Compact lithium-glass neutron beam monitor for SANS at CSNS

A small-angle scattering neutron spectrometer for material research is under construction at the China Spallation Neutron Source. An intervening neutron beam monitor behind the sample is needed to measure the beam intensity in order to reduce the measurement uncertainty caused by beam fluctuation. Considering the mobility requirement and limited space, we proposed a compact monitor using a type of lithium-glass scintillator provided by China Building Materials Academy. Its performance was studied experimentally using ^(252)Cf and ^(60)Co sources.The neutron light yield of the selected scintillator was measured to be 5:3 × 10~3 photons/neutron. The feasibility of n-gamma discrimination using the charge comparison method was verified. By using the Geant4 toolkit, themonitor was modeled with precise physical processes including neutron tracking, scintillation, and optical photon transmission. The gamma sensitivity and detection efficiency were investigated in the simulation. It was concluded that a 0.5-mm-thick lithium-glass scintillator with natural lithium is an appropriate choice to satisfy both the neutron detection efficiency and gamma elimination requirements.

Synthesis of hexagonal boron nitride films by dual temperature zone low-pressure chemical vapor deposition

Hexagonal boron nitride(h-BN)films are synthesized by dual temperature zone low-pressure chemical vapor deposition(LPCVD)through using a single ammonia borane precursor on non-catalytic c-plane Al_(2)O_(3)substrates.The grown films are confirmed to be h-BN films by various characterization methods.Meanwhile,the growth rates and crystal quality of h-BN films at different positions in the dual temperature zone are studied.It is found that the growth rates and crystal quality of the h-BN films at different positions on the substrate are significantly different.The growth rates of the h-BN thin films show their decreasing trends with the rearward position,while the crystal quality is improved.This work provides an experimental basis for the preparation of large area wafer thick h-BN films by LPCVD.

Advances of immune checkpoint blockade in the treatment of advanced gastric cancer

The treatment of advanced gastric cancer has been a worldwide problem, and the curative effects of chemotherapy and targeted therapy are limited for advanced gastric cancer. With the widespread use of immune checkpoint blockade in clinical studies, immunotherapy provides new opportunities for patients with advanced gastric cancer. Cytotoxic T lymphocyte associated protein 4 (CTLA-4), programmed death protein 1 (PD-1) and its ligand (PD-L1) are the most thoroughly studied in the immune checkpoints, and the latest research data show good treatment prospects. Whether it is used as single therapy or combined with other treatment, immunological checkpoint inhibitors have achieved good results. How to choose the appropriate immunotherapy as well as the appropriate dose, combination, interval and length of treatment is the problem we will have to pay attention to in the next step.

Development of 3He-filled linear position-sensitive detector for neutron scattering instruments at CSNS

Background Helium-3-filled linear position-sensitive detectors(LPSDs)have been widely applied to neutron scattering instruments in recent decades owing to the high detection efficiency,the excellent neutron/gamma discrimination,and the ability to construct the detector with large area coverage.More than 65%of neutron instruments at the China Spallation Neutron Source(CSNS)require ^(3)He LPSDs for the building of the detector system.The detector of a neutron scattering instrument is normally an array detector composed of a large number of ^(3)He LPSDs.However,no appropriate substitute detector is available for the ^(3)He LPSD for operation over a short period,and its specifications need to be customized to satisfy the requirements of different instruments.This necessitates the development of ^(3)He LPSDs with different specifications for building and upgrading the detectors of neutron scattering instruments at CSNS.Purpose To meet the general requirements specified by the neutron instruments at CSNS,a ^(3)He LPSD was developed and its performances were investigated by using the neutron beam.Methods The neutron beam experiments for this ^(3)He LPSD,including measurements of counting rate plateau curve,position calibration,and position resolution along the tube path,were carried out at the beamline-20 of CSNS.To evaluate its performance in the instrument operation,a detector module made of this ^(3)He LPSD has been installed in the multi-physics instrument(MPI),and the comparison test with the commercial ^(3)He LPSD has been conducted at neutron scattering experiments.Results and conclusion The experimental measurements for this ^(3)He LPSD based on the neutron beam showed that it satisfies the requirements for use in neutron scattering instruments.It reaches a plateau slope of 2.6%/100 V with a plateau range from 1750 to 1950 V and attained a minimal position resolution of 5.3 mm(FWHM),superior to the one achieved by the commercial ^(3)He LPSD.In addition,this ^(3)He LPSD detector module applied to the MPI has been working steadily for half of year and is available for use in experiments.These results provide a sound basis for the subsequent construction and upgradation of the detector of neutron scattering instruments at CSNS.

Test of a ^(3)He neutron detector readout electronics prototype for CSNS multipurpose physics neutron diffractometer

Background Large areas of 3He neutron detector array and corresponding readout electronics are used in the MPI built at China Spallation Neutron Source(CSNS)to measure the position and flight time of scattered neutrons,and MPI has a high requirement for the position resolution of 3He neutron detector.A 3He neutron detector readout electronics prototype composed of two front-end boards and one digital readout board is designed to meet the above requirements.Purpose Testing the whole readout electronics system before its mass production to ensure it can run correctly and fulfill the functional and performance requirements.Methods The test of the readout electronics prototype is implemented in the laboratory,and a signal generator is used to generate electronic signals instead of actual neutron signals.The position resolution test of the readout electronics prototype with 3He tubes is carried out in the No.20 beamline measurement room of CSNS.Results and Conclusion The functional and performance tests in the laboratory state that the readout electronics prototype can fulfill the readout requirements.The typical value of charge resolution is 0.85 fC;the integral nonlinearity of charge is 0.094%in the input charge range from 100 to 800 fC.The position resolution test result of the prototype with 3He tubes is better than the design requirement of 10 mm and also quite good compared with that of some other commercial products.These tests provide a reliable basis for the mass production of the readout electronics prototype.

Simulation and optimization of the boron-lined MWPC neutron detectors

Purpose Boron-lined gas detectors are prospective alternatives to helium-3-based neutron detectors.For boron-lined multiwire proportional chamber(MWPC)with neutron grazing incident angle geometry,high neutron detection efficiency and 3D position sensitive can be realized.In this paper,a multi-layer boron-lined MWPC was studied.Methods Influences of the boron coating thickness,coating layer numbers and neutron incident angle on the neutron detection efficiency were studied and optimized with Geant4 simulation.The output signal properties including induced signal width on the readout plane,the time resolution and gas gain uniformity of the detector were studied with Garfield program.Results and conclusion The results show that the neutron detection efficiency can be significantly increased by using neutron grazing incident geometry.For 4 layers of 10B4Cwith thickness of 1μm,the neutron detection efficiency can reach to 54.56%,49.17%and 44.36%when neutron incident angle is 6°,8°and 10°,respectively.For detector with curved geometry,the gas gain is nonuniform among the anode wires,and using separate high voltage for each anode wire or wires group can effectively reduce the nonuniform of the gas gain.The results of this work can be used to optimize the detector design.

The research of laser-based rapid measurement system for scintillator neutron detectors arrays

Purpose Scintillator Neutron Detectors Arrays(SNDA)were successfully installed at General Purpose Powder Diffractome-ter(GPPD)at the China Spallation Neutron Source(CSNS).The inhomogeneity of the detection efficiency in each detector module,which caused by the gain nonuniformity of the multi-anode photo-multiplier tubes(MA-PMTs)and the inconsistency of the wave-length shifting fibers in collecting scintillation photons,need to be mitigated before the installation.Methods An automated rapid measurement system based on the blue laser and the two-dimensional mobile platform was developed to calibrate the light response of each channel in detector modules.According to the test results of this system,the electronics threshold of each channel of the SNDA is adjusted.Before the installation of the all 40 SNDA modules in GPPD,the electronics thresholds of each channel are adjusted according to the measurement results of this rapid measurement system.Results and Conclusion Compared with the unadjusted detector module,the adjusted one obtained a better uniformity of the neutron detection efficiency.The inhomogeneity of the detection efficiency is improved from 27.4%to 10.9%.The test result of the diffraction peak of the standard sample Si showed that the adjusted SNDA works well in GPPD.