New neutron imaging facility at the NIST

doi:10.1016/j.nima.2005.01.004

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Volume 542, Issues 1–3, 21 April 2005, Pages 9-15

https://doi.org/10.1016/j.nima.2005.01.004 Get rights and content

Abstract

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.

Introduction

The new neutron radiography facility, located at Beam Tube 6 (BT-6) at the National Institute for Standards and Technology (NIST) Center for Neutron Research (NCNR), has recently been commissioned and is currently operating. The design goals of this facility are to perform dynamical studies of proton exchange membrane fuel cells (PEMFC). The region of interest in PEMFC is the membrane region, with a typical length scale ≈50 μm. The active area of a single standard testing PEMFC is ≈50 mm×50 mm. Neutron imaging facilities should be sensitive to water laminar thicknesses of 10–50 μm, at spatial resolutions less than or at 200 μm, and at image acquisition times less than 1 s. This requires an extremely high thermal neutron fluence rate and large length to aperture diameter ratios (L/D).

Section snippets

Flight path design

BT-6 views the reactor core through a 14 cm thimble, with a heavy concrete plug lined with a conically tapered collimator shown in Fig. 1. The tapered collimator is formed by 1.5 mm thick borated aluminum to collimate the thermal neutrons and 10 cm thick steel inserts to stop the fast neutrons and gamma-rays. The inserts have a through hole that gradually reduces in diameter from 14 cm to 3.4 cm at the exit. After the collimation, the high-energy neutrons and gamma-rays are attenuated by a single

Detector

Our current detector system uses a neutron to scintillation light converter screen, a mirror, a condensing lens and a CCD camera. The converter screen is an Applied Scintillation Technologies NDg screen that is a 0.3 mm sheet of ⁶LiF/ZnS doped with a Cu, Al and Ag blend with specified minimum resolution of 80 μm and homogeneity of ±5.0%. The scintillation light is peaked at 540 nm and is reflected out of the beam path at 90° by a mirror to limit the radiation damage to the CCD. A Nikon 108 mm lens

Data acquisition and analysis

We have written Visual Basic Active-X software to control the CCD camera and to control the vertical axis rotation stage to acquire Radon transforms for later reconstruction. This software also allows users to accumulate real-time radiographs. The user front end is a Microsoft Excel spreadsheet and allows the user to control all of the camera parameters.

Images are always normalized by the incident neutron beam image to produce the transmission images from which quantitative information can be

Current performance

Using ASTM standard E 545-99, we have determined the typical image quality of the radiographs. This standard uses a beam purity indicator (BPI) described in ASTM E2003 and an image sensitivity indicator (SI) described in ASTM E2023. The BPI is a Teflon block with two lead disks, two boron nitride disks and two cadmium wires, shown in Fig. 6. By comparing the densities of these regions, one measures the relative contributions to the image from thermal neutrons, gamma-rays and scattered neutrons.

Future upgrades

A Varian Paxscan 2520 amorphous silicon flat panel detector with the high energy option from HyTec, Inc. will be installed. The light sensor for this detector is radiation hard, which allows the detector to be placed in the main beam improving the light detection efficiency. The detector has a pixel size of 128 μm and an active area of 25 cm×20 cm. The detector is capable of reading out the 12 bit pixel depth at 5 frames per second and with 2×2 binning the pixels can be read at 30 frames per

Conclusions

The two facility changes, moving the imaging station back ≈2.13 m and adding 5 cm of Bi filter, has reduced the total fluence rate at the sample by ≈75%. The ASTM standard gauges indicate that the beam line has a high thermal neutron content, and excellent resolution (L/D≈285–570). The current detector system has reasonable spatial resolution (≈200 μm), enabling one to resolve the typical features of the flow fields in PEMFC. With future detector upgrades, we anticipate being able to obtain

Acknowledgment

The authors would like to thank Ivan Schröder for assistance in designing the facility and Wade Richards for kindly lending ASTM standards E 803, E 2003 and E 2023.

This work was supported by the US Department of Commerce, the NIST Ionizing Radiation Division, the Director's office of NIST, the NIST Center for Neutron Research, and the Department of Energy interagency agreement No. DE\_AI01-01EE50660.

References (6)

Certain trade names and company products are mentioned in the text or identified in an illustration in order to...
NCNR Staff, Safety Analysis Report (SAR) for License Renewal for the National Institute of Standards and Technology...
The macroscopic Bi cross section was taken from the Evaluated Nuclear Data File,...

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¹: Current address: Department of Physics, North Carolina State University, Campus Box 8202, Raleigh, NC 27695-8202, USA.

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