Stereoscopic radiographic images with thermal neutrons

Abstract

Spatial structure of an object can be perceived by the stereoscopic vision provided by eyes or by the parallax produced by movement of the object with regard to the observer. For an opaque object, a technique to render it transparent should be used, in order to make visible the spatial distribution of its inner structure, for any of the two approaches used. In this work, a beam of thermal neutrons at the main port of the Argonauta research reactor of the Instituto de Engenharia Nuclear in Rio de Janeiro/Brazil has been used as radiation to render the inspected objects partially transparent. A neutron sensitive Imaging Plate has been employed as a detector and after exposure it has been developed by a reader using a 0.5 μm laser beam, which defines the finest achievable spatial resolution of the acquired digital image. This image, a radiographic attenuation map of the object, does not represent any specific cross-section but a convoluted projection for each specific attitude of the object with regard to the detector. After taking two of these projections at different object attitudes, they are properly processed and the final image is viewed by a red and green eyeglass. For monochromatic images this processing involves transformation of black and white radiographies into red and white and green and white ones, which are afterwards merged to yield a single image. All the processes are carried out with the software ImageJ. Divergence of the neutron beam unfortunately spoils both spatial and contrast resolutions, which become poorer as object–detector distance increases. Therefore, in order to evaluate the range of spatial resolution corresponding to the 3D image being observed, a curve expressing spatial resolution against object–detector gap has been deduced from the Modulation Transfer Functions experimentally. Typical exposure times, under a reactor power of 170 W, were 6 min for both quantitative and qualitative measurements. In spite of its intrinsic constraints, this simple technique may provide valuable information about the object otherwise available only through more refined and expensive 3D tomography.

Introduction

Spatial structure of an object can only be perceived using a suitable approach or technique. In the case of a sufficiently transparent object containing spatially distributed inserts, their positions can be recognized by direct observation, thanks to the stereoscopic vision provided by eyes. Yet, even in the absence of this dual-vision, three-dimensional character of that distribution can still be perceived by the parallax produced by movement of the object with regard to the observer. When the object is not transparent enough, and should not be submitted to an invasive assay due to safety reasons or high costs, a technique to render it transparent is used, as a non-destructive assay.

A largely employed technique is the tomographic approach, which however demands a large number of projections requiring specially tailored equipment and software, not always affordable, being moreover very time-consuming. In some circumstances expensive 3D tomography can be replaced by a stereoscopic view of the object under inspection, where instead of tens of radiographic projections, only two of them taken at suitable object attitudes are employed. This approach has been used earlier in the 1980s [1], [2] for X-ray stereoscopy employed for viewing device optical stereoscopes.

The development of image processing softwares in the last decades has made it feasible to deal with stereoscopy in a simpler fashion. As done in this work, once these projections are acquired, they are digitalized and processed in order to be viewed by a simple stereoscopic device. As they are monochromatic images, single color filters, rather than the polaroid ones, can be used.

Processing involves the transformation of black and white radiographic images to black and red and black and green ones, which are afterwards merged to yield a single image on a PC monitor. All the processes are carried out with the software ImageJ. When watched through a red and green eyeglass, the observer perceives a stereoscopic image.

In this work a beam of thermal neutrons available at the main port of the Argonauta research reactor of the Instituto de Engenharia Nuclear in Rio de Janeiro/Brazil has been used as radiation to render the inspected objects partially transparent. As for the detecting device, a neutron sensitive Imaging Plate has been employed and after exposure has been developed by a reader using a 0.5 μm laser beam, which defines the finest achievable spatial resolution of the acquired digital image. The final resolution of the radiographic images, however, is affected by other factors that spoil spatial resolution, such as the penumbra caused by divergence of the neutron beam. Due to this divergence, larger gaps between object and detector would produce more extensive penumbrae and hence higher degradation of both spatial and contrast resolutions. Therefore, an object under inspection would have different regions at different distances from the Imaging Plate, resulting in different resolutions. In order to evaluate the range of spatial resolution a Modulation Transfer Function (MTF) has been determined for several object–detector gaps.