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DOI : 10.22661/AAPPSBL.2014.24.2.23
Overview and Progress of the China Spallation...
FANGWEI WANG AND HESHENG CHEN
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Overview and Progress of the China Spallation Neutron Source

FANGWEI WANG AND HESHENG CHEN
CHINA SPALLATION NEUTRON SOURCE, CHINESE ACADEMY OF SCIENCES

The China Spallation Neutron Source (CSNS) is the first accelerator-based multidiscipline user facility to produce pulsed neutrons by tungsten target under collision of a pulsed proton beam with a beam power of 100 kW at a repetition rate of 25 Hz. In this paper, we focus on the physical design of the CSNS target station and the neutron instruments. Under the optimized design, the flat tungsten target and the compact target-moderator-reflector coupling enhance effective cold and thermal neutron output from moderators. Three wing-type moderators supply four different characteristics of neutrons to 19 beam lines primarily for neutron scattering applications with a beam extracted from the target directly for high-energy neutron irradiation. All designs are optimized for Phase I of 100 kW with an upgradable capacity of up to 500 kW.

1. INTRODUCTION

The China Spallation Neutron Source (CSNS) is proposed and designed as a user facility to provide multidisciplinary platforms for scientific research and applications, primarily to investigate the position and dynamics of atoms, molecules or atomic clusters in solids and liquids by neutron scattering [1, 2]. The fields of application include condensed-matter sciences such as: physics, chemistry, material, medicine, protein and biology, geology, and hydrogen-rich energy storage; and nuclear sciences such as: irradiation, rare-isotope, and radiography, based on thermal/cold neutrons, fast neutrons, and proton beams. As the first pulsed neutron facility in China, CSNS fills a 40-year gap from other world research leaders, and is strongly advocated by domestic user groups. The high-flux pulsed neutrons from CSNS will complement continuous-wave reactor neutrons and synchrotron X-rays from light sources in China. CSNS was granted final acceptance in the beginning of 2002 after three review meetings organized by the Chinese Academy of Sciences (CAS) and other scientific organizations, and approved by the Chinese Central Government with a funds allocation in 2005 after a conceptual design and feasibility investigation. In February 2007, the Chinese Academy of Sciences and the Guangdong Provincial Government agreed to co-construct the CSNS facility in Dongguan, Guangdong Province. Since then, the construction site has been selected and the preliminary geological survey as well as the environmental assessment has been completed. In September 2008, CSNS set up its first official project milestone and the proposal was approved by the Development and Reform Commission of the Chinese Central Government. After the feasibility and technical designs were approved, CSNS broke ground in October, 2011. Presently, the linac tunnel, office building and accessory buildings are completed, the bases of the synchrotron circle and neutron scattering hall are being prepared, and the linac components were to be installed in September 2013. The facility is expected to burst the first neutron beam in the second half of 2017, and to be accepted for user operations in 2018, 6.5 years from the initial construction (http://csns.ihep.ac.cn/).

2. CSNS DESIGN

CSNS is an accelerator-based short-pulse neutron facility, mainly consisting of an 80-MeV H linac and a 1.6 GeV rapid cycling synchrotron (RCS) to harvest a proton current of 62.5 μA (100 kW) at a 25 Hz repetition rate, a target station (includes the target, moderators, reflectors, and shielding with a 20 beam port) to produce pulsed neutrons with a wide energy distribution, and three dayone instruments to utilize thermal/cold neutrons to interpret the microscopic structure of condensed substances. Some utilities and maintenance systems were necessary to support the test, installation, commissioning and operation of the main machine. The architectural rendering of the CSNS is shown in Figure 1.

Fig. 1: CSNS Architectural Design at the Construction Site in Dongguan,Guangdong, China.

Table. 1: Main Basic Parameters and Requirements of the CSNS.

CSNS is the first machine working under high intensity proton beams in China. The beam power of 100 kW is a reasonable choice for Phase I to cover most applications of neutron scattering according to the requests from potential users. The facility retains the capacity to be upgraded to 500 kW (Phase II) with a peak flux in competition for 1 MW machines. The primary basic parameters and requirements of the CSNS are shown in Table 1.

3. TARGET STATION

The target station of a spallation neutron source usually functions to convert short pulse, high energy protons into lower-energy neutrons used by neutron scattering instruments. Figure 2 shows the engineering and technical designs of the CSNS target station based on and iterated with the physical design. In the core, the tungsten target at the end of target trolley produces pulsed neutrons with average energies of about MeV by a spallation reaction of 1.6 GeV proton impinging. These neutrons that emerge from target are slowed down and then thermalized by moderators located above and below the target from the MeV to the meV region. Three moderators supply diverse characteristics of pulsed neutron beams. The reflector surrounds the moderators and the target, and returns into the moderators the neutrons that miss the moderator or leak out before slowing down, for another chance to slow down in the moderator and contribute to the moderator output of slow neutrons. Outside the TMR system, are the inner shielding, the helium vessel, the bulk iron shielding (including the neutron shutter) and the heavy concrete shielding. In total, 20 beam ports transport neutrons out of the target station to the instrument suite. The mode of TMR maintenance is similar to those of the SNS and the J-PARC where the plug of moderators and reflectors (MR) are replaced vertically and the target is maintained horizontally.

Fig. 2: Engineering Design of the CSNS Target Station Based on its Physical Design. The Station Block is 12 Meters in Diameter (4.8 Meters of Iron Plus 1.2 Meters of Heavy Concrete Shielding).

Fig. 3: Schematic Layout of the CSNS Neutron Instruments. Beam Lines Extracted from Different Moderators are Marked in Different Colors (Green: CHM, Orange: DWM, Blue: DPHM, Red: Tungsten Target). PND: Powder Neutron Diffractometer; DG/RG: Direct Geometry/Reversal Geometry.

The neutronic performance in the pulse spallation source required by the neutron instruments emphasizes neutron flux and suitable neutron pulse shapes at a definite neutron wavelength (energy) range that is primarily determined by the TMR configuration and parameters. The neutron pulse width or pulse shape determines the resolution of the neutron scattering instruments, and the neutron flux is desired to be as high as possible within the constraints of the resolution requirements. Other requirements for neutron source performance include the suppression of the fast neutrons and gamma rays in the background. In the CSNS TMR configuration, cold, thermal or epithermal neutrons for 19 neutron beam lines are supplied by two cryogenic hydrogen moderators (20 K) and an ambient water moderator. The coupled hydrogen moderator (CHM) is adapted to provide high time-averaged flux cold neutrons with acceptable pulse widths. On the other hand, the decoupled and poisoned hydrogen moderator (DPHM) is chosen to provide a narrow pulse for high-resolution instruments. The decoupled water moderator (DWM) is suitable for middle resolution instruments working in the thermal and the epithermal regions.

4. NEUTRON INSTRUMENTS

CSNS’s main goal is neutron scattering using diverse neutron instruments to determine the microscopic structure and dynamics of atoms and their spin in a condensed substance. Moreover, a few specialized machines are planned for neutron physics, neutron imaging, and neutron irradiation [3]. Although neutron scattering is simply classified into elastic and inelastic scattering according to whether the energy of scattering neutrons is conservative or not, the diverse types of neutron scattering instruments can be defined by incident neutron intensity, Q- and/or E-range as well as their resolutions. Following the characteristics of the short pulse, the scientific frontier, the demands of potential users in China and experiences of spallation sources operated overseas, CSNS has planned 20 neutron scattering instruments for 20 beam lines: 19 from three moderators and another one from a direct target for atmospheric neutron irradiation, as seen in Figure 3.

Because of budget limitations, only three day-one instruments: the General Purpose Powder Diffractometer (GPPD), the Small Angle Neutron Scattering (SANS), and the Multipurpose Reflectometer (MR), are funded by the project. Currently, the physical design, including the choice of the moderator, the neutron flight path, choppers, and detectors, has been completed for subsequent technical design and component fabrication. All those parameters are optimized for resolution, flux and cost. The shielding of the beam line has already been physically designed.

5. USERS ACTIVITIES

On behalf of the Chinese Neutron Scattering Society (CNSS), CSNS hosted the 11th AONSA Executive Committee Meeting & Facility Directors Meeting, November 16-17. In 2013 the 1st National Conference on Neutron Scattering and Workshop on Applications of National Neutron Facilities was held from December 10-12, 2013 with more than 120 potential users participating. CSNS still offers a small budget for Chinese users to carry out neutron scattering experiments around the world.

References

1) Zhang J, Yan Q W, Zhang C, et al. Recent Progress Of The Project Of the Chinese Spallation Neutron Source. Journal Of Neutron Research, 2005, 13: 11-14
2) Wei J, Chen H S, Chen Y W, et al. China Spallation Neutron Source: Design, R&d, And Outlook. Nuclear Instrumentation Methods Physics Research A, 2009, 600: 10-13
3) Wang F W, Liang T J, Wen Y, et al., Physical Design Of Target Station And Neutron Instruments For China Spallation Neutron Source. Science China- Physics Mechanical Astronomy, 2013, 56: 2410-2424

Fangwei Wang has been working as a researcher in the field of magnetism and neutron scattering at Institute of Physics after getting his PhD there in 1997, and became a full professor in 2005. He investigated permanent magnetism and magentocaloric effect in rare-earth transition-metal compounds, magnetoresistance and magnetocapacity in oxides by magnetic measurements and neutron scattering. Since joining the China Spallation Neutron Source (CSNS) in 2003, he has served as the leader of experiment division, and conducted the design of CSNS target station and neutron instruments, focusing on neutron scattering instrumentation and data analysis.