New neutron imaging facility at the NIST

The design objective of the thermal neutron radiography facility at the National Institute of Standards and Technology (NIST) Center for Neutron Research was to provide a large beam diameter and a high fluence rate in order to produce images of dynamic systems. A thermal neutron beam with a 14 cm diameter thimble was chosen. The beam was initially filtered by a 10 cm thick single crystal bismuth filter cooled with liquid nitrogen. The beam exiting the port is shaped using either a 1 cm or 2 cm diameter pinhole to form a uniform high fluence rate beam at the sample. The resulting neutron beam at the sample has an L/D ratio of 280 with a fluence rate of 1.84×107 cm−2 s−1 and 560 with a fluence rate of 4.75×106 cm−2 s−1 uniformly spread over a 26 cm diameter beam. To capture the neutron beam image a scintillator and CCD camera is used. The current neutron camera system is limited to a 2.5 s frame rate; however, a high frame rate detector system based on amorphous silicon will allow frame rates to meet the design goal. Samples can be rotated and translated in situ for radiography and tomography applications. This facility became operational in early 2003. Since then the facility has been translated backwards by ≈2.13 m and 5 cm of bismuth was added to the filter. The design of this facility and the impact of the later changes are discussed.