Physics Procedia 69 ( 2015 ) 55 – 59
1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Selection and peer-review under responsibility of Paul Scherrer Institut doi: 10.1016/j.phpro.2015.07.007
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1875-3892 © 2015 The Authors. Published by Elsevier B.V.
Selection and peer-review under responsibility of Paul Scherrer Institut.
Notice: This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.
10 World Conference on Neutron Radiography 5-10 October 2014
Overview of the conceptual design of the future VENUS neutron
imaging beam line at the Spallation Neutron Source
Hassina Bilheux*, Ken Herwig, Scott Keener, and Larry Davis
Oak Ridge National Laboratory, Neutron Sciences Directorate, Oak Ridge, TN 37831, USA
Abstract
VENUS (Versatile Neutron Imaging Beam line at the Spallation Neutron Source) will be a world-class neutron-imaging instrument that will uniquely utilize the Spallation Neutron Source (SNS) time-of-flight (TOF) capabilities to measure and characterize objects across several length scales (mm to μm). When completed, VENUS will provide academia, industry and government laboratories with the opportunity to advance scientific research in areas such as energy, materials, additive manufacturing, geosciences, transportation, engineering, plant physiology, biology, etc. It is anticipated that a good portion of the VENUS user community will have a strong engineering/industrial research focus. Installed at Beam line 10 (BL10), VENUS will be a 25-m neutron imaging facility with the capability to fully illuminate (i.e., umbra illumination) a 20 cm x 20 cm detector area. The design allows for a 28 cm x 28 cm field of view when using the penumbra to 80% of the full illumination flux. A sample position at 20 m will be implemented for magnification measurements. The optical components are comprised of a series of selected apertures, T0 and bandwidth choppers, beam scrapers, a fast shutter to limit sample activation, and flight tubes filled with Helium. Techniques such as energy selective, Bragg edge and epithermal imaging will be available at VENUS. © 2015 The Authors. Published by Elsevier B.V.
Selection and peer-review under responsibility of Paul Scherrer Institut.
Keywords: time-of-flight neutron radiography; instrument conceptual design; Spallation Neutron Source.
* Corresponding author. Tel.: +1-865-384-9630; fax: +1-865-574-2033. E-mail address: bilheuxhn@ornl.gov
© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
56 Hassina Bilheux et al. / Physics Procedia 69 ( 2015 ) 55 – 59
1. Introduction
Historically and for more than four decades, neutron imaging (NI) facilities have been installed exclusively at continuous neutron sources rather than at pulsed sources (Anderson et al., 2009). This is mainly due to (1) the limited number of accelerator-based facilities and therefore the fierce competition for beam lines with neutron scattering instruments, (2) the limited flux available at accelerator-based neutron sources and finally, (3) the lack of high efficiency imaging detector technology capable of time-stamping pulsed neutrons with sufficient time discrimination. Recently completed high flux pulsed proton-driven neutron sources such as the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL) in the United States and the Japanese Spallation Neutron Source (JSNS) of the Japan Proton Accelerator Research Complex (J-PARC) in Japan produce high neutron fluxes that offer new and unique opportunities for NI techniques. Over the past five years, rapid advances of boron-doped micro-channel plate (MCP) neutron detectors offer good time resolution and efficiency, with different readout options, including the noticeably high performance Medipix multipurpose ASIC readout. When coupled to an appropriate MCP, this readout can operate as a counting system, or as a higher resolution centroiding system (Tremsin et al. 2011) that can approach 15 μm position resolution with gating that is fast enough for time-of-flight (TOF) applications (Ballabriga et al. 2010).
ORNL is planning to build a TOF neutron imaging facility at the SNS, called VENUS, located on a decoupled poisoned para-hydrogen moderator, which offers the sharp neutron time pulse widths needed for high wavelength discrimination. VENUS is optimized for the measurement of micro-scale structures in radiography (2D) and tomography (3D) modes. Wavelength discrimination offers a fourth dimension, creating 4D data sets. VENUS will complement the existing “continuous source” neutron imaging capability, with no wavelength discrimination, at ORNL’s High Flux Isotope Reactor (HFIR) CG-1D beam line (Crow et al. 2011; Bilheux et al. 2014; Santodonato et al., these proceedings).
A schematic overview of the instrument is displayed in Figure 1. Neutrons from the moderator are transported through front-end optics (see section 2). They interact with the sample under investigation located in the instrument cave. Two sample positions, at 20 and 25 m, respectively are available in the VENUS cave. The 25 m position offers highest wavelength resolution while the 20 m position allows magnification capabilities with the use of Wolter mirrors (Khaykovich et al. 2011). A beam stop terminates the instrument configuration and is followed by an area that has bench tops for set-up preparation but also room for equipment and sample storage, etc. The control and visualization computers are located in the hutch of the instrument.
2. VENUS front-end optics
The VENUS optical components are optimized such that 9.5 cm by 9.5 cm of the nominal 10 cm wide by 12 cm tall moderator is used to fully illuminate detector pixels within the 20 cm x 20 cm fully illuminated field of view (FOV). The moderator is viewed at an angle that reduces its effective width to 9.5 cm. The largest FOV at 25 m is 28 cm by 28 cm and includes a penumbra corresponding to 80% of the full illumination. As illustrated in Figure 2, three aperture systems define the pinhole geometry for thermal/cold and epithermal neutrons. A permanent aperture located inside the main shutter, i.e. 2.55 m away from the moderator, defines an L/D of 400, where L is the distance between the aperture, of diameter D, and the detector at 25 m. This aperture controls the smallest L/D for thermal and cold neutrons. A set of apertures located at 4.5 m can be varied from L/D = 800 to 2000 for thermal and cold neutron imaging. This aperture set is located inside the bulk shield insert. Finally, the epithermal aperture located at 7.48 m configures the beam for the epithermal imaging capability and can be varied from L/D = 400 to 2000.
Bandwidth (BW) choppers define the incident wavelength band of neutrons arriving at the sample position and eliminate contamination from preceding and succeeding neutron pulses. They are positioned at 6, 9 and 12 m, respectively. The BW choppers are composed of two single and one double BW disks. The open angle of the double BW, located at 9 m, can be adjusted by changing the phase of the two disks relative to one another preventing wavelength contamination at either of the two sample positions.
Fig. 1. VENUS schematic drawing.
58 Hassina Bilheux et al. / Physics Procedia 69 ( 2015 ) 55 – 59
VENUS is equipped with 3 shutters: (1) the primary shutter is located upstream at ~ 2.5 m (and is 2 m thick along the beam direction) and close to the moderator, and (2) the secondary shutter is integrated with the epithermal aperture system (in closed position, 1 m thick along the beam direction) and finally (3) the fast shutter which is utilized to minimize sample exposure to thermal and cold neutrons during, for example, sample alignment. A T0 chopper is placed at 6.8 m, downstream of the first bandwidth chopper, to attenuate the high flux of fast neutrons and gammas produced when the protons strike the mercury target. Evacuated flight tubes placed between optical components are equipped with beam scrapers for collimation.
3. VENUS specifications and expected capabilities
The decoupled, poisoned para-H2 moderator provides sharp neutron pulses across the cold to thermal neutron
range providing the necessary wavelength resolution (Table 1). At 25 m, VENUS is expected to provide a wavelength resolution ¨d/d of approximately 0.12%, enabling techniques such as Bragg edge imaging and texture mapping (Kockelmann et al. 2007). Since VENUS has a direct view of the moderator, available neutron wavelengths will range from epithermal to cold neutrons, providing a unique resonance imaging capability as recently demonstrated in the epithermal range (Tremsin et al. 2014), but also the high sensitivity provided by cold neutrons. Other techniques such as stroboscopic imaging (for repetitive motion samples) and conventional “continuous source” imaging will also be available at VENUS. Developments in both the scintillator-CCD or sCMOS detectors and the MCP detectors have advanced rapidly within the last 4 years, so that the timing required for effective time-of-flight imaging is now possible. These detectors are suitable for initial operation and will be installed at VENUS.
Table 1. Beam line specifications
VENUS Beam line 10
Beam Spectrum Epithermal, Thermal, Cold
Moderator H2 decoupled poisoned
Wavelength bandwidth Resolution ¨d/d Field-of-view Source-to-detector distance Sample-to-detector distance Detector system
~ 2.4 Å (at 25 m and 60 Hz operation) 0.12%
Up to 28 cm x 28 cm 20 and 25 m
A few mm to meters (as needed for magnification)
Micro-channel plate detectors and cameras (CCDs and sCMOSs)
4. Conclusion
VENUS will truly be a unique and unprecedented tool for the U.S. and worldwide neutron sciences community and will offer advanced contrast enhancement and energy selective techniques. Future upgrades will include the installation of an X-ray source normal to the neutron beam for simultaneous neutron and X-ray 3D imaging, polarized, phase and dark field imaging techniques. The beam line will broaden the user community by attracting users from industry and other government agencies, which for the most part have not been major users of neutrons in the United States.
Acknowledgements
This research at ORNL’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. The U.S. DOE Efficiency Energy and Renewable Energy, Vehicle Technologies Office also supported this research. The authors would like to thank Prof. Anton
Tremsin at the University of California, Berkeley for the fruitful discussions about VENUS, and European colleagues such as Drs. Eberhard Lehmann, Burkhard Schillinger, Nikolay Kardjilov and Winfried Kockelmann, for their continuing support for the development of the future VENUS.
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