An automated fast neutron computed tomography instrument with on-line focusing for non-destructive evaluation

Abstract

A fast neutron tomography imaging instrument has been designed, built, and tested at The Ohio State University 500 kW Research Reactor on a fast neutron beamline with a peak neutron flux ≈5.4 × 107 n·cm−2·s−1 at 1.6 MeV median neutron energy. The instrument and beamline are also configurable for thermal neutron imaging. The imaging apparatus is composed of a lens coupled, water-cooled Electron Multiplying Charge Coupled Device camera, a front-surface mirror, and a high light yield plastic Polyvinyl toluene scintillator. The instrument sits on a mobile cart. A total of 5 motion-control stages are built into the system for XYZ and rotational degrees of freedom for sample positioning; the fifth stage fine tunes the focal distance between the camera and the scintillator to achieve on-line focusing. A Python code with a user-friendly graphical user interface controls the fully automated image acquisition, not requiring user interaction, yet facilitating tracking of the image acquisition. A complete fast neutron computed tomography dataset with 360 projections requires less than 3 h, with 30 s per projection. On-line focusing is accomplished with a commercial, off-the-shelf, dielectrically actuated liquid lens. Finally, tomographic reconstructions are visualized using the Livermore Tomography Tools software package. The effective pixel size (width and height) is ≈0.1058 mm, yielding a minimum voxel size of 0.1058 × 0.1058 × 0.1058 mm3, and produces a spatial resolution of 231 μm when calculated from knife-edge measurements.

Imaging of low Z masked with high Z (Pb, U) materials using 14 MeV neutron

“…Iron material was chosen because of its weak scattering for fast neutrons compared to that of polyethylene, graphite, etc. Also, Fe is easily available at a reasonable lower price compared to dense and high Z materials like tungsten (W) or tantalum (Ta) [13]. Also, Fe is a most commonly used structural material.…”

“…After the imaging of test samples of structural material, this section focuses on the imaging of low-Z materials. Fast neutrons can provide high contrast good quality images of objects containing low-Z/high-Z materials simultaneously [13,14]. This is due to the fact that in the high energy (≳ MeV) range, the neutron interaction cross section (macroscopic) is in general low and does not exhibit significant element-to-element variations.…”

“…Among them, reactors and large accelerators have been successfully used to develop advanced fast neutron imaging facilities, by virtue of their high neutron fluxes. Fast neutron imaging beam lines or test facilities for example, NECTAR: radiography and tomography station using fission neutrons [12], fast neutron tomography system designed at 500 kW research reactor, Ohio State University [13], and Fast neutron tomography (FNCT) experiments at an accelerator facility of PTB, Germany [14] have proven the potential use of these sources in producing high quality images. The neutron radiography facility relying on large neutron source has good beam quality, high imaging collimation ratio, and high beam intensity at the imaging plane, but the disadvantage is that the facilities are too large and the cost is high, which prevents them from meeting the users local testing needs.…”

Improved image stitching method for neutron imaging of large object with small beam size

“…The imaging instrument used in this experiment was developed to automatically acquire rotated radiographic views for CT for both thermal and fast neutron applications [16]. A 450 µm thick LiF:ZnS NDg scintillator (Scintacor) was used to acquire thermal neutron radiographs, which is shown in Figure 1-A.…”

“…This is used to provide known translations for radiograph stitching. Mounted to the top of the 3-axis stage is a rotation stage on which the phantom is placed atop a standoff and used to provide the rotated view for CT. Each of these devices and the EM-CCD camera are controlled by a custom tomography script [16].…”