Study on collimation and shielding of the back-streaming neutrons at the CSNS target

The back-streaming neutrons from the spallation target at CSNS are very intense, and can pose serious damage problems for the devices in the accelerator-target interface region. To tackle the problems, a possible scheme for this region was studied, namely a specially designed optics for the proton beam line produces two beam waists, and two collimators are placed at the two waist positions to maximize the collimation effect of the back-streaming neutrons. Detailed Monte Carlo simulations with the beams in the two different CSNS phases show the effectiveness of the collimation system, and the radiation dose rate decreases largely in the interface section. This can ensure the use of epoxy coils for the last magnets and other devices in the beam transport line with reasonable lifetimes, e.g., thirty years. The design philosophy for such an accelerator-target interface region can also be applicable to other high-power proton beam applications.     

Study of shielding design for SANS at CSNS

The small angle neutron scattering (SANS) instrument is presently being constructed at Chinese Spallation Neutron Source (CSNS) in China, and the biological shielding design is needed to prevent the instrument from causing excessive dose rates in accessible locations. In this paper, the study of shielding design for SANS that relies on Monte Carlo simulation is introduced. Beam line shielding calculations are performed considering both scenarios of closed versus open T0 chopper. The basic design scheme of the beam stop is discussed. The size of the T0 chopper rotor is also estimated.     

Preliminary shielding analysis for the CSNS target station monolith

The construction of the China Spallation Neutron Source (CSNS) has been initiated at Dongguan,Guangdong,China.In spallation neutron sources the target station monolith is contaminated by a large number of fast neutrons whose energies can be as large as those of the protons of the proton beam directed towards the tungsten target.A detailed radiation transport analysis of the target station monolith is important for the construction of the CSNS.The analysis is performed using the coupled Monte Carlo and multi-dimensional discrete ordinates method.Successful elimination of the primary ray effects via the two-dimensional uncollided flux and first collision source methodology is also illustrated.The dose at the edge of the monolith is calculated.The results demonstrate that the doses received by the hall staff members are below the required standard limit.     

Preliminary shielding analysis for the CSNS target station monolith

The construction of the China Spallation Neutron Source （CSNS） has been initiated at Dongguan,Guangdong,China.In spallation neutron sources the target station monolith is contaminated by a large number of fast neutrons whose energies can be as large as those of the protons of the proton beam directed towards the tungsten target.A detailed radiation transport analysis of the target station monolith is important for the construction of the CSNS.The analysis is performed using the coupled Monte Carlo and multi-dimensional discrete ordinates method.Successful elimination of the primary ray effects via the two-dimensional uncollided flux and first collision source methodology is also illustrated.The dose at the edge of the monolith is calculated.The results demonstrate that the doses received by the hall staff members are below the required standard limit.     

Application of the first collision source method to CSNS target station shielding calculation

Ray effects are an inherent problem of the discrete ordinates method. RAY3 D, a functional module of ARES, which is a discrete ordinates code system, employs a semi-analytic first collision source method to mitigate ray effects. This method decomposes the flux into uncollided and collided components, and then calculates them with an analytical method and discrete ordinates method respectively. In this article, RAY3 D is validated by the Kobayashi benchmarks and applied to the neutron beamline shielding problem of China Spallation Neutron Source(CSNS)target station. The numerical results of the Kobayashi benchmarks indicate that the solutions of DONTRAN3 D with RAY3 D agree well with the Monte Carlo solutions. The dose rate at the end of the neutron beamline is less than10.83 μSv/h in the CSNS target station neutron beamline shutter model. RAY3 D can effectively mitigate the ray effects and obtain relatively reasonable results.     

Preliminary shielding analysis in support of the CSNS target station shutter neutron beam stop design

The construction of China Spallation Neutron Source (CSNS) has been initiated in Dongguan, Guangdong, China. Thus a detailed radiation transport analysis of the shutter neutron beam stop is of vital importance. The analyses are performed using the coupled Monte Carlo and multi-dimensional discrete ordinates method. The target of calculations is to optimize the neutron beamline shielding design to guarantee personal safety and minimize cost. Successful elimination of the primary ray effects via the two-dimensional uncollided flux and the first collision source methodology is also illustrated. Two-dimensional dose distribution is calculated. The dose at the end of the neutron beam line is less than 2.5 μSv/h. The models have ensured that the doses received by the hall staff members are below the standard limit required.     

Optimization of the collimation system for CSNS/RCS with the robust conjugate direction search algorithm

The Robust Conjugate Direction Search(RCDS) method is used to optimize the collimation system for the Rapid Cycling Synchrotron(RCS) of the China Spallation Neutron Source(CSNS). The parameters of secondary collimators are optimized for a better performance of the collimation system. To improve the efficiency of the optimization, the Objective Ring Beam Injection and Tracking(ORBIT) parallel module combined with MATLAB parallel computing is used, which can run multiple ORBIT instances simultaneously. This study presents a way to find an optimal parameter combination of the secondary collimators for a machine model in preparation for CSNS/RCS commissioning.     

Calculation of hydrogen and helium concentrations for CSNS target

The China Spallation Neutron Source(CSNS) is driven by protons whose energies are about 1.6 GeV.At such high energies, the spallation neutrons lead to the formation of large amounts of helium, hydrogen and new heavier species in the form of transmutation products. These hydrogen, helium and transmutation products have a critical effect on the mechanical properties on the one hand and exacerbate the displacement radiation damage on the other hand. In this paper, the background hydrogen/helium concentrations and the maximum hydrogen/helium concentrations near cracks in a tungsten target for CSNS have been calculated at temperatures of 100 and 300 by applying a theoretical model. For the CSNS tungsten target plate, we find the maximum hydrogen concentration near the tips of cracks ranges from 3.0×10~(-2)–2×10~(-1), which exceeds the hydrogen background concentration by 1.2–1.8 times; the maximum helium concentration near the tips of cracks ranges from 3.0×10~(-4)- 1.2×10~(-3), which exceeds the helium background concentration by 2- 4 times; the maximum hydrogen/helium concentration increases with the increase of the transfer length across the surfaces of the target and it decreases with the increase of temperature.     

Study on the effects of chromaticity and magnetic field tracking errors at CSNS/RCS

The Rapid Cycling Synchrotron (RCS) is a key component of the China Spallation Neutron Source (CSNS). For this type of high intensity proton synchrotron, the chromaticity, space charge effects, and magnetic field tracking errors between the quadrupoles and the dipoles can induce beta function distortion and tune shift, and induce resonances. In this paper, the combined effects of chromaticity, magnetic field tracking errors and space charge on beam dynamics at CSNS/RCS are studied systemically. 3-D simulations with different magnetic field tracking errors are performed by using the code ORBIT, and the simulation results are compared with the case without tracking errors.     

Calculations of radioactivity and afterheat in the components of the CSNS target station

This paper shows the calculations of radioactivity and afterheat in the components of the China Spallation Neutron Source (CSNS) target station, with the Monte Carlo codes LAHET, MCNP4C and the multigroup code CINDER'90. These calculations provide essential data for the detailed design and maintenance of the CSNS target station.     

Study on the effects of chromaticity and magnetic field tracking errors at CSNS/RCS

The Rapid Cycling Synchrotron （RCS） is a key component of the China Spallation Neutron Source （CSNS）. For this type of high intensity proton synchrotron, the chromaticity, space charge effects, and magnetic field tracking errors between the quadrupoles and the dipoles can induce beta function distortion and tune shift, and induce resonances. In this paper, the combined effects of chromaticity, magnetic field tracking errors and space charge on beam dynamics at CSNS/RCS are studied systemically. 3-D simulations with different magnetic field tracking errors are performed by using the code ORBIT, and the simulation results are compared with the case without tracking errors.     

Study on the pre-chopper in CSNS LEBT

Physical designing of the pre-chopper in CSNS LEBT is carried out, which includes the deflecting voltage, the length and the width of the deflecting plates, and the gap between the deflecting plates. The most outstanding feature of the design is that both the gap and the width vary with the beam envelope size. So both the requried deflecting voltage and the loaded capacitance are lowered. In order to avoid destruction of the space charge neutralization by the pre-chopper in the whole LEBT, an electron-trapping electrode is arranged to confine the electrostatic field of the pre-chopper to the local area. To examine the reliability of the pre-chopping design in CSNS LEBT, a similar pre-chopping design in ADS RFQ LEBT is set up and an experiment on the pre-chopper is prepared. 3-dimensional simulations are carried out to determine the loaded capacitance and the applied voltage of the electron-trapping electrode.     

Study on the pre-chopper in CSNS LEBT

Physical designing of the pre-chopper in CSNS LEBT is carried out,which includes the deflecting voltage,the length and the width of the deflecting plates,and the gap between the deflecting plates.The most outstanding feature of the design is that both the gap and the width vary with the beam envelope size.So both the requried deflecting voltage and the loaded capacitance are lowered.In order to avoid destruction of the space charge neutralization by the pre-chopper in the whole LEBT,an electron-trapping electrode is arranged to confine the electrostatic field of the pre-chopper to the local area.To examine the reliability of the pre-chopping design in CSNS LEBT,a similar pre-chopping design in ADS RFQ LEBT is set up and an experiment on the pre-chopper is prepared.3-dimensional simulations are carried out to determine the loaded capacitance and the applied voltage of the electron-trapping electrode.     

中国散裂中子源靶站重要部件的辐照损伤计算与分析

本文采用高能粒子输运程序MCNPX 2.5.0,对中国散裂中子源(CSNS)靶站重要部件所使用的钨、SS316不锈钢与Al-6061等材料,由于中子与质子辐照所引起的损伤能量截面与原子离位截面进行了计算,对钨靶体、靶的不锈钢容器、慢化器与反射体的铝容器等部件的辐照损伤量——原子离位次数(displacement per atom,DPA)进行了计算与分析,并给出了质子束斑形状对靶体及靶容器DPA峰值的影响. 这些计算与分析对正在建设的中国散裂中子源靶站的设计及参数选择具有重要的实际意义. 关键词： 中国散裂中子源 损伤能量截面 原子离位截面 DPA     

Study of the design of CSNS MEBT

The design of CSNS MEBT has two objectives: (1) to match the beam both in the transversal direction and the longitudinal direction from RFQ into DTL; (2) to further chop the beam into the required time structure asked by RCS. It is very difficult and critical to control well the emittance growth and in the meantime to match and chop the beam. Firstly, the optical design is done and optimized, and the multi-particle simulations show that the maximum emittance growth is successfully controlled within 14%. Secondly, based on the different beam envelopes obtained by TRACE-3D and PARMELA, the least deflecting angle of the chopper is determined by TRACE-3D. At last, the field of steering magnet is determined through simulations.     

Study of the design of CSNS MEBT

The design of CSNS MEBT has two objectives：（1） to match the beam both in the transversal direction and the longitudinal direction from RFQ into DTL;（2） to further chop the beam into the required time structure asked by RCS. It is very diffcult and critical to control well the emittance growth and in the meantime to match and chop the beam. Firstly,the optical design is done and optimized,and the multi-particle simulations show that the maximum emittance growth is successfully controlled within 14%. Secondly,based on the different beam envelopes obtained by TRACE-3D and PARMELA,the least deflecting angle of the chopper is determined by TRACE-3D. At last,the field of steering magnet is determined through simulations.     

Study on space charge effects of the CSNS/RCS

The Rapid Cycling Synchrotron (RCS) is a key component of the China Spallation Neutron Source (CSNS). The space charge effect is one of the most important issues in the CSNS/RCS, which limits the maximum beam intensity, as well as the maximum beam power. Space charge effects are the main source of emittance growth and beam loss in the RCS. Space charge effects have been studied by simulation for the CSNS/RCS. By optimizing the painting orbit, the optimized painting distribution was obtained. The space charge effects during the acceleration are studied and dangerous resonances, which may induce emittance growth and beam loss, are investigated. The results are an important reference for the design and commissioning of the CSNS/RCS.     

Plasma shielding of a XeCl-laser-irradiated YBCO target

Plasma shielding during the laser ablation process of YBCO high-T_c superconductor is demonstrated by observing the transmissivity of a probe beam. A plasma electron density of 1.2×10²² cm^-3 at the end of the laser pulse is estimated. Received: 7 February 2000 / Accepted: 20 April 2000 / Published online: 2 August 2000     

Effect of the crystal structure of a deuterated target on the yield of neutrons in the dd reaction at ultralow energies

The energy dependence of the neutron yield in the d(d, n)₃ He reaction proceeding in a textured titanium deuteride target with the preferred orientation of microcrystals in the [100] direction has been studied. Measurements have been performed for the energy range of incident deuterons of 7–12 keV in the laboratory system. It has been shown that the energy dependence of the enhancement factor of the reaction is described not only by the screening potential but also by the simple inclusion of channeling effects.     

Proposal for muon and white neutron sources at CSNS

The China Spallation Neutron Source （CSNS） is a large scientific facility with the main purpose of serving multidisciplinary research on material characterization using neutron scattering techniques. The accelerator system is to provide a proton beam of 120 kW with a repetition rate of 25 Hz initially （CSNSⅠ）, progressively upgradeable to 240 kW （CSNS-Ⅱ） and 500 kW （CSNS-Ⅱ＇）. In addition to serving as a driving source for the spallation target, the proton beam can be exploited for serving additional functions both in fundamental and applied research. The expanded scientific application based on pulsed muons and fast neutrons is especially attractive in the overall consideration of CSNS upgrade options. A second target station that houses a muon-generating target and a fast-neutron-generating target in tandem, intercepting and removing a small part of the proton beam for the spallation target, is proposed. The muon and white neutron sources are operated principally in parasitic mode, leaving the main part of the beam directed to the spallation target. However, it is also possible to deliver the proton beam to the second target station in a dedicated mode for some special applications. Within the dual target configuration, the thin muon target placed upstream of the fast-neutron target will consume only about 5% of the beam traversed; the majority of the beam is used for fast-neutron production. A proton beam with a beam power of about 60 kW, an energy of 1.6 GeV and a repetition rate of 12.5 Hz will make the muon source and the white neutron source very attractive to multidisciplinary researchers.