Tushar Roy

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.

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.

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.

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.

Reactivity Measurement Using the Area-Ratio Method in the BRAHMMA Subcritical System

The online reactivity monitoring of accelerator-driven systems is a crucial issue in reactor control and monitoring. The area-ratio method is one of the techniques for measuring the reactivity of subcritical systems using a pulsed neutron source. This technique has been used to measure reactivity at different locations in the subcritical assembly BRAHMMA developed at the Bhabha Atomic Research Centre, India. Since the reactivity measured by the area-ratio method is spatially dependent on the detector location, the Bell-Glasstone correction factor was used to correct the measured reactivity.

Pulsed Neutron Source Measurements in the BRAHMMA Accelerator-Driven Subcritical System

Abstract The use of accelerator-driven systems for incineration of nuclear waste and energy production requires monitoring of different parameters that govern reactor safety. One of the most important parameters is the multiplication factor keff. The present paper describes the results of experiments carried out on a subcritical system (BRAHMMA) using a pulsed neutron source. The value of the multiplication factor keff obtained from time responses of the core that were measured in situ using neutron detectors after insertion of a neutron pulse matches well with the calculated value.

Neutron phase contrast imaging beamline at CIRUS, reactor, India

This paper presents the development of neutron phase contrast imaging facility at medium flux research reactor, CIRUS, India. The approach adopted for this study is innovative in the sense that both conventional and phase contrast imaging can be performed within same experiment hutch without any major modification in the experimental hutch or collimator.

High Resolution X‐ray Microscopy For Nano‐Resolution Imaging

If an object is illuminated with coherent electromagnetic radiation, e.g. by visible laser light or highly brilliant X‐rays, a diffraction pattern is formed in the Fraunhofer far field that is related via a Fourier transform to the optical transmission function of the object. The aim of X‐ray diffractive imaging or so‐called lensless imaging, is to directly reconstruct the original optical transmission function of the specimen from its measured diffraction pattern. In principle, it allows one to obtain a resolution that is ultimately limited only by the wavelength of the radiation used and not by the quality of optical lenses. In X‐ray microscopy, for instance, the resolution is presently limited to several tens of nanometers because of difficulties in manufacturing efficient high‐quality nano‐structured X‐ray optical elements. Since this technique allows the resolution to be increased beyond these limits, they are among the most promising techniques for X‐ray imaging applications in life and materials sc...