Mayank Shukla

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.

Equation of state of condensed matter in laser-induced high-pressure regime

We have simulated the shock Hugoniot of copper and uranium based on the results of first principles electronic structure calculations. The room temperature isotherm has been obtained by evaluating the accurate ground state total energies at various compressions, and the thermal and electronic excitation contributions were obtained by adopting isotropic models using the results obtained by the band structure calculations. Our calculations ensure smooth consideration of pressure ionization effects as the relevant core states are treated in the semi-core form at the ambient pressure. The pressure variation of the electronic Gruneisen parameter was estimated for copper using the band structure results, which leads to good agreement of the simulated shock Hugoniot with the measured shock data. The simulation results obtained for U are also compared with the experimental data available in literature and with our own data.

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 .

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.

Evaluation of Spatial Correction Factors for BRAHMMA Subcritical Assembly

Abstract The reactivity measurement in a subcritical assembly using the area ratio method is affected by the detector location and needs to be corrected for spatial dependence. One of the approaches to calculate the spatial correction factor is based on steady-state numerical simulation as proposed by Bell and Glasstone. This paper discusses the evaluation of the spatial correction factor for the BRAHMMA (BeO Reflected And HDPe Moderated Multiplying Assembly) subcritical assembly, India. The factors have been used to correct experimentally measured reactivity values from the area ratio method at different locations inside the core.

Associated particle technique in single-sided geometry for detection of explosives

Associated particle technique (APT) for detection of explosives is well established but has been implemented mostly for fixed portal systems. In certain situations, a portable system is required where the suspect object cannot be moved from site. This paper discusses the development of a portable APT system in single-sided geometry which can be transported to site and requires only one-sided access to the object. The system comprised D-T neutron source and bismuth germanate (BGO) detectors fixed on a portable module. Different aspects of the system have been discussed such as background contribution, time selection, and elemental signatures. The system was used to detect benign samples and explosive simulants under laboratory condition. The elemental ratios obtained by analyzing the gamma spectra show good match with the theoretical ratios.

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.