Cold Neutron Imaging Instrument

The Cold Neutron Imaging Instrument (CNII) was designed to be a flexible space to test, develop, and employ novel neutron imaging methods to realize different sources of image contrast [1]. The initial motivation for the instrument was to be the home of the first neutron microscope based on Wolter optics. The instrument is capable of polarized neutron imaging, and the first neutron images of an electric field were acquired at the CNII. INFER is a large multi-disciplinary team working to develop dark-field imaging to create multi-scale images that can be thought of as “3D-SANS”. Using a double crystal monochromator, CNII provides users with Bragg-edge imaging to map strain [2] or crystalline phase fractions, for instance in transformation induced plasticity steels [3] or in lead batteries. As part of the NCNR cold source upgrade project, the NG6 guide system will be modernized.

Beam characteristics

CNII sits at the end position of the neutron guide 6 (NG6). NG6 consists of straight mirrors coated with Ni-58. There is no filter material in the beam which would lower the fluence rate. As a result the neutron beam is an intense cold beam with a thermal equivalent fluence rate (flux) of about 2 × 10⁹ cm^-2 s^-1 at the entrance to the enclosure and downstream 9 m at the sample position it is about 2 × 10⁸ cm^-2 -s^-1 for a collimation ratio of L/D about 200 with a uniform (wavelength and intensity) area of 10 cm × 10 cm. Variable aperture diameters can be installed to yield larger collimation ratios.

The energy of the beam can be selected either with a double crystal monochromator consisting of highly oriented pyrolytic graphite with mosaic spread of 0.4° (5 cm × 5 cm) or 0.7° (5 cm × 7 cm) or with a neutron velocity selector. The current velocity selector can choose neutron velocities between (920 m/s and 198 m/s) or alternatively wavelengths between (0.43 nm to 2 nm), with an input beam size of about 5 cm × 7 cm. Since the wavelength resolution of the velocity selector (~ 20 %) is much coarser than that of the mosaic crystal (~2 %), the resulting intensity is much greater and finds application in dark-field and phase imaging measurements. The velocity selector also eliminates artifacts from Bragg diffraction as it acts as a band pass filter preventing wavelengths below 0.41 nm from passing. The facility is planning to upgrade this velocity selector by Summer 2022. The new selector will have a minimum wavelength of 0.3 nm, a maximum wavelength of 4 nm, and an input beam size of 6 cm × 8 cm.

Detectors, Scintillators, and Lenses

The CNII makes use of all of the NIST neutron imaging detectors.

Sample manipulation

A wide variety of motorized stages for rotation, tip/tilt, and translation are available to align and manipulate samples during an experiment for complete 6-axis motion. All motorized stages are interfaced with the NIST neutron imaging acquisition software “Datascripting”. Sample environments developed for BT-2/NeXT are also available for use at CNII. At the moment, CNII is not capable of NeXT.

Data Acquisition

Data acquisition is fully automated through using a software package written by the Neutron Imaging Team called Data Scripting.

Data Analysis

Users of the facility have access to both the source code and compiled data analysis packages written in Matlab by members of the Neutron Imaging Team.

References

[1] D. S. Hussey et al., “A New Cold Neutron Imaging Instrument at NIST,” in Physics Procedia, 2015, vol. 69.

[2] J. W. Sowards, D. S. Hussey, D. L. Jacobson, S. Ream, and P. Williams, “Correlation of Neutron-Based Strain Imaging and Mechanical Behavior of Armor Steel Welds Produced with the Hybrid Laser Arc Welding Process,” J. Res. Natl. Inst. Stand. Technol., vol. 123, no. 123011, pp. 1–8, 2018.

[3] W. Woo, J. Kim, E. Y. Kim, S.-H. Choi, V. Em, and D. S. Hussey, “Multi-scale analyses of constituent phases in a trip-assisted duplex stainless steel by electron backscatter diffraction, in situ neutron diffraction, and energy selective neutron imaging,” Scr. Mater., vol. 158, pp. 105–109, Jan. 2019.