P.S. Sarkar

Laboratory-based X-ray phase-contrast imaging technique for material and medical science applications

In-line X-ray phase-contrast imaging technique is an emerging method for the study of materials such as carbon fibers, carbon composite materials, polymers, etc. Similarly this technique is also well suited for the imaging of soft materials such as tissues, distinguishing between tumor and normal tissue. These represent the class of materials for which X-ray attenuation cross-section is very small. Thus this method promises a far better contrast for low X-ray absorbing substances than the conventional radiography method. We have set up an experimental facility using a combination of X-ray CCD detector and a microfocus X-ray source. This facility is dedicated to micro-imaging experiments such as microtomography and high-resolution phase-contrast experiments. In this paper, the results of X-ray phase-contrast imaging experiments are described.

Laser induced shock pressure multiplication in multi layer thin foil targets

Theimpedancemismatchtechnique has been used for shock pressure amplification in two- and three-layer thin planar foil targets. Numerical simulation results using one-dimensional radiation hydrocode MULTI in two-layer targets consisting of Al–Au and Al–Cu and three-layer target consisting of plastic–Al–Au and foam–Al–Au, respectively, are presented. These results show a pressure enhancement up to 25 and 29 Mbar for plastic–Al–Au and foam– Al–Au targets, respectively, from an initial pressure of 7 Mbar in the reference material using laser intensity of 5 × 10 13 Wc m −2 at 1.064 µm. This enhancement is more as compared with 18 and 22 Mbar found in plastic–Au and foam–Au two-layer targets, respectively. Results of laser driven shock wave experiments for equation of state (EOS) studies of Au and Cu in two-layer target are also presented. A Nd : YAG laser chain (2 J, 1.06 µm wavelength, 200 ps pulse FWHM) is used for generating shocks in the planar Al foils and Al–Au (or Al–Cu) layered targets. The EOS of Au and Cu in the pressure range of 9–14 Mbar obtained shows remarkable agreement with the simulation results and with experimental data of other laboratories and SESAME data.

Design, development and first experiments on the X‐ray imaging beamline at Indus‐2 synchrotron source RRCAT, India

A full-field hard X-ray imaging beamline (BL-4) was designed, developed, installed and commissioned recently at the Indus-2 synchrotron radiation source at RRCAT, Indore, India. The bending-magnet beamline is operated in monochromatic and white beam mode. A variety of imaging techniques are implemented such as high-resolution radiography, propagation- and analyzer-based phase contrast imaging, real-time imaging, absorption and phase contrast tomography etc. First experiments on propagation-based phase contrast imaging and micro-tomography are reported.

Characterization of pyrocarbon coated materials using laboratory based x-ray phase contrast imaging technique

In-line x-ray phase contrast is an emerging x-ray imaging technique that promises to improve the contrast in x-ray imaging process. This technique is most suited for x-ray imaging of soft materials, low atomic number elements such as carbon composite fibers, very thin coatings, etc. We have used this new emerging technique for visualization and characterization of the pyrocarbon coated materials using a combination of microfocus x-ray source and x-ray charge coupled device detector. These studies are important for characterization of coating and optimization of various process parameters during deposition. These experiments will help us to exploit the potential of this technique for studies in other areas of material science such as characterization of carbon fibered structures and detection of cracks and flaws in materials. The characterization of the imaging system and optimization of some process parameters for carbon deposition are also described in detail.

Enhancement of laser induced shock pressure in multilayer solid targets

The impedance mismatch technique was used for shock pressure amplification in two layered planar foil targets. Numerical simulation results using one-dimensional (1D) radiation hydrocode MULTI in two layer target consisting of polyethylene (CH 2 ) n -aluminium (Al) and polyethylene (CH 2 ) n -gold (Au), show a pressure enhancement of 12 and 18 Mbar, respectively (or a pressure jump of 1.64 and 2.54, respectively), from initial pressure of 7 Mbar in the reference material (polyethylene) using laser intensity of 5 × 10 13 Watts/cm 2 at 1.064 μm. The simulation data was also corroborated by experiments in our laboratory. Results of laser driven shock wave experiments for pressure enhancement studies in CH 2 -Al and CH 2 -Au targets are also presented. A Nd:YAG laser chain (2 J, 1.064 μm wavelength, 200 ps pulse duration FWHM) is used for generating shocks in the planar CH 2 foils of thickness varying from 4 to10 μm, and in two layered CH 2 -Al (or CH 2 -Au) targets with 8 μm CH 2 and 1.5 μm Al or Au .

Quantitative studies of pyrocarbon-coated materials using synchrotron radiation.

Phase-contrast imaging provides enhanced image contrast and is important for non-destructive evaluation of structural materials. In this paper, experimental results on in-line phase-contrast imaging using a synchrotron source (ELETTRA, Italy) for objects required in material science applications are discussed. Experiments have been carried out on two types of samples, pyrocarbon-coated zirconia and pyrocarbon-coated alumina microspheres. These have applications in both reactor and industrial fields. The phase-contrast imaging technique is found to be very useful in visualizing and determining the coating thickness of pyrocarbon on zirconia and alumina microspheres. The experiments were carried out at X-ray energies of 16, 18 and 20 keV and different object-to-detector distances. The results describe the contrast values and signal-to-noise ratio for both samples. A comprehensive study was carried out to determine the thickness of the pyrocarbon coating on zirconia and alumina microspheres of diameter 500 microm. The advantages of phase-contrast images are discussed in terms of contrast and resolution, and a comparison is made with absorption images. The results show considerable improvement in contrast with phase-contrast imaging as compared with absorption radiography.

Application of X-ray micro-CT for micro-structural characterization of APCVD deposited SiC coatings on graphite conduit.

SiC coatings are commonly used as oxidation protective materials in high-temperature applications. The operational performance of the coating depends on its microstructure and uniformity. This study explores the feasibility of applying tabletop X-ray micro-CT for the micro-structural characterization of SiC coating. The coating is deposited over the internal surface of pipe structured graphite fuel tube, which is a prototype of potential components of compact high-temperature reactor (CHTR). The coating is deposited using atmospheric pressure chemical vapor deposition (APCVD) and properties such as morphology, porosity, thickness variation are evaluated. Micro-structural differences in the coating caused by substrate distance from precursor inlet in a CVD reactor are also studied. The study finds micro-CT a potential tool for characterization of SiC coating during its future course of engineering. We show that depletion of reactants at larger distances causes development of larger pores in the coating, which affects its morphology, density and thickness.

Micro-structural characterization of materials using synchrotron hard X-ray imaging techniques

X-ray imaging has been an important tool to study the materials microstructure with the laboratory based sources however the advent of third generation synchrotron sources has introduced new concepts in X-ray imaging such as phase contrast imaging, micro-tomography, fluorescence imaging and diffraction enhance imaging. These techniques are being used to provide information of materials about their density distribution, porosity, geometrical and morphological characteristics at sub-micron scalewith improved contrast. This paper discusses the development of various imaging techniques at synchrotron based imaging beamline Indus-2 and few recent experiments carried out at this facility.

A variable-wavelength-based approach of phase retrieval for contrast transfer function based methods.

X-ray phase-contrast imaging has emerged as an important method for improving contrast and sensitivity in the field of X-ray imaging. This increase in the sensitivity is attributed to the fact that, in the hard X-ray regime, the phase shift is more prominent as compared with the attenuation for materials having a low X-ray absorption coefficient. Among all the methods using the X-ray phase-contrast technique, in-line phase-contrast imaging scores over the other methods in terms of ease of implementation and efficient use of available X-ray flux. In order to retrieve the projected phase map of the object from the recorded intensity pattern, a large number of algorithms have been proposed. These algorithms generally use either the transport of intensity or contrast transfer function based approach for phase retrieval. In this paper it is proposed to use multiple wavelengths for phase retrieval using the contrast transfer function based formalism.

Applications of digital neutron imaging at BARC (India) using reactor and nonreactor sources

Development in detector technology and imaging techniques have widened the scope of neutron radiography leading to several new applications which would not have been otherwise possible with the conventional film based radiography. Use of these techniques have also made possible the utilization of low strength neutron sources and thus help bring out neutron radiography from the premises of nuclear reactors. At the Bhabha Atomic Research Centre (BARC), Bombay, India, we have made attempts to exploit these developments in imaging technologies and develop new applications. This paper presents an overview of such developments being carried out at BARC.