Search

In an effort to set up microtron based photoneutron source and in order to optimize the neutron yield, photoneutron production from beryllium has been studied for different volumes of beryllium irradiated by different peak energy bremsstrahlung radiation. The theoretical estimation of neutron yield has been carried out using the MCNP simulation for 8.75, 8.15 and 7.58 MeV peak energy of bremsstrahlung radiation. The experimental measurements were carried out using two types of detectors: SSNTD CR-39 and custom designed Silver wrapped GM detector. The neutron yield corresponding to beryllium of volume 381.70 cm 3 are found to be 2.13E+09, 2.00E+09 and 1.74E09 n/s (MCNP calculation values) for electrons of energy 8.75, 8.17 and 7.48 MeV, respectively. The experimental results are compared with the MCNP simulated results and are good agreement.

Phase contrast imaging is a new method of radiography in which the information of change in phase of the X-rays as it passes through the object gets reflected in the intensity. This leads to a better sensitivity and contrast than the conventional absorption radiography. In this paper we discuss the simulation studies of phase contrast imaging using monochromatic and polychromatic X-ray point source for simple two- and three-dimensional objects like circular and spherical objects (made up of carbon-fiber, polystyrene and lung tissue). The advantages of refraction contrast images are discussed in terms of contrast and resolution, and a comparison is made with absorption images. The result obtained shows considerable improvement in contrast with phase contrast imaging as compared to conventional absorption radiography. These results also guide us in proper selection of source to object distance, object to detector distance, etc. These results are proposed to be used in our experiment on phase contrast imaging with microfocus X-rays. The technique is going to be very useful in improving the resolution in the X-ray imaging for the composites, and in detection of cracks at micron level resolution. Moreover, if the doses can be controlled by proper selection of the detector or the source, it can have clinical application in the mammography.

Laser driven shock wave transit time in thin aluminium targets was experimentally estimated by determining the shock emergence time at the rear of thin aluminium foils of varying thickness from 5 to 35 μm. A 20 J, 5 ns Nd:glass laser was focused to produce laser intensity of 1012 to 5 × 1013 W/cm2 on the targets which were placed in vacuum. Target foil movement was measured to an accuracy of 10 μm using optical shadowgraphy technique. This technique was used to accurately measure the shock transit time by recording the optical shadowgrams at various instants of time and thus identify the instant at which the foil is just set into motion. Shock transit time measured in foils of different thickness can give the value of shock velocity at a given laser intensity. Target motion recorded by shadowgraphy can also give the target foil velocity from which shock pressure can be estimated. Experimental values of shock transit time, shock velocity and shock pressure were observed to agree well with the values using one-dimensional multi-group radiation hydrodynamic simulations.

We calculate the phonon linewidth of a Raman-active zone centre phonon in a correlated metal. The correlated metal is described by the half-filled single-band Hubbard model on a hypercubic lattice in d = ∞ . The electronic self-energy and Green function are determined within the dynamical mean field theory (DMFT) for various values of U in the metallic regime and then used to calculate the phonon linewidth. We consider only the lowest order contribution corresponding to a process in which the phonon decays into a particle–hole pair. At weak correlation we find that the electronic contribution to the phonon linewidth is negligibly small but it grows with increasing correlation strength and then reaches a maximum before showing a decrease.

3D Visualization of the interior of an object has been one of the important goals of NDT, which has many applications. It can be used for examination of defects and cracks in their full three-dimensional perspective and reverse engineering applications. This paper describes the development and characterization of a high dynamic range cooled CCD-based cone beam tomography system for industrial applications and in particular for the NDT of materials used in the nuclear industry. The novel feature of this system is its ability to distinguish minor variations in the attenuation coefficients with very high resolution. The paper also discusses the development of software implementation of Feldkamp formulation for reconstruction, testing with mathematically simulated phantom, and hardware used in the work. The system is characterized in terms of MTF both in the mid-plane and off-midplane are also discussed in this paper. We will present results of experiments conducted on a variety of samples of nuclear and industrial use.

We report first-principles full potential linearised augmented plane wave calculations of the electronic band structure of the compound MgB2C2 and its hole-doped derivatives Mg0.5Li0.5B2C2, Mg0.5Na0.5B2C2, Mg0.9Na0.1B2C2 and Mg0.5K0.5B2C2. The parent compound MgB2C2 is a band insulator, which on hole doping, is predicted to turn metallic with a large density of states at the Fermi energy. Its band dispersion shows a flat band feature close to the Fermi energy, reminiscent of MgB2. Based on our estimates of changes in the density of states at the Fermi level, we predict that hole-doped MgB2C2 is a potential candidate for high-temperature superconductivity.

For centuries, scientists and engineers dreamed of being able to peer into the interiors of microscopic and macroscopic objects. A variety of imaging techniques, both invasive and non-invasive, have been developed to accomplish this goal. The present article provides an overview of our work on non-invasive imaging techniques being used and developed for basic and applied sciences. The details of various developments in setting up X-ray and neutron imaging techniques for applications in the Indian nuclear programme, development of two- and three-dimensional tomography for industrial applications, digital medical imaging, X-ray diffraction imaging for exploring the condensed matter at normal and high pressure conditions, X-ray holography for atomic resolution imaging, phase-sensitive imaging and imaging of transient phenomenon are discussed in this paper.

We present accurate x-ray diffraction data at high pressures for AuIn2,AuGa2 and AuAl2, obtained using a diamond anvil cell with the ELETTRA synchrotron source. The resulting P–V data obtained from the d-values were used to get the universal equation of state (UEOS), which is compared with theoretical estimates. Deviation from linearity is evident in the UEOS curves of AuIn2 and AuGa2, thus verifying that some of the observed anomalies in these systems below 5 GPa are due to electronic topological transitions.

Effects of non-local thermodynamic equilibrium (non-LTE) condition on emission and hydrodynamics of typical inertial confinement fusion (ICF) plasmas are studied. The average degree of ionization at high temperatures is seen to be much lower compared to the values obtained from Thomas-Fermi scaling or Saha equation for high-Z element like gold. LTE and non-LTE predictions for emitted radiation from laser-driven gold foil are compared with the experimental results and it is seen that non-LTE simulations show a marked improvement over LTE results. The effects of one group and multigroup, LTE and non-LTE approximations of radiation transport on hydrodynamic parameters are studied for laser-driven aluminium and gold foils. It is further seen that non-LTE and multigroup effects play an important role in predicting conversion efficiency of laser light to X-rays.

The total-energy calculations are carried out to obtain the axial ratio (c/a) variation with compression for cadmium. We find two anomalies as compared to a single anomaly at V/V{sub 0}=0.85 observed in the experimental data. The first anomaly at V/V{sub 0}=0.95 is due to electronic topological transition (ETT) and correlates well with the anomaly observed in the existing resistivity data. We find the 4d-orbital overlap, coupled with transfer of electrons from d to s states as the cause of second anomaly. This observation is in contrast to the existing understanding in zinc, where the c/a anomaly is attributed to the destruction of a giant Kohn anomaly by ETT. Nondestruction of giant Kohn anomaly in cadmium at high pressures is in disagreement with the current view in the literature that its Fermi surface is identical to that of zinc. {copyright} {ital 1997} {ital The American Physical Society}

A combination of x-ray diffraction at high pressures and first-principles calculations reveals the sequence of crystal-structural phase transitions in AuGa${}_{2}$ from cubic ($Fm\overline{3}m$) to orthorhombic (Pnma) at 10 (\ifmmode\pm\else\textpm\fi{}4) GPa and then to monoclinic ($P$2${}_{1}$/$n$) at 33 (\ifmmode\pm\else\textpm\fi{}6) GPa. Neither theory nor experiment would have been adequate, on their own, in documenting this sequence of phases, but together they confirm a sequence differing from the $Fm\overline{3}m\ensuremath{\rightarrow}Pnma\ensuremath{\rightarrow}P{6}_{3}/mmc$ transitions predicted for CaF${}_{2}$ and Pnma \ensuremath{\rightarrow} $P$112${}_{1}$/$a$ transition reported for PbCl${}_{2}$ and SnCl${}_{2}$. The combined results from theory and experiment also allow us to constrain the equations of state of the three phases of AuGa${}_{2}$. Calculations on the analog PbCl${}_{2}$ predict a transition to the $P$2${}_{1}$/$n$ phase seen in AuGa${}_{2}$ that could, therefore, be a common high-pressure phase for PbCl${}_{2}$-structured compounds.

A comparative experimental study of soft x-ray emission from laser-irradiated Au–Cu mix-Z targets of different atomic compositions has been performed. Plasma was produced from planar targets using second-harmonic laser pulses from an Nd:glass laser at a focused intensity of ∼1013 W cm−2. Radiation intensity in the spectral region ∼15–150 A and integrated x-ray yield for mix-Z target were observed to be higher than those for individual elements. The maximum conversion occurred for an atomic composition of Au 0.43–Cu 0.57. These observations are consistent with the variation of Rosseland mean opacity with atomic composition calculated using a screened hydrogenic average atom model.

First-principles electronic-structure and total-energy calculations related to the 1/1 crystal approximant to the Al-Zn-Mg quasicrystal are carried out with different possible atoms at the center of the packing unit. The linear muffin-tin-orbital (LMTO) method with the atomic-sphere approximation and the muffin-tin correction to the Coulomb energy is employed. The density of states shows a very shallow minimum near the Fermi level, supporting the view that the pseudogap exists in the crystal approximants to quasicrystals even in the absence of a d-state contribution to the valence electrons. It is shown that packing units with Al or Zn at the centers are more stable than those with empty centers, supporting the recent positron-annihilation data and pseudopotential calculations, but disagreeing with qualitative conclusions from some LMTO calculations for crystal approximants to other quasicrystals.

Shock Hugoniot calculations are carried out for gold using both solid and liquid state theories. The result suggests that shock melting begins to take place at a pressure of 1.2 Mbar. At about 2.1 Mbar, the melting transition is complete. The shock temperature of gold in the liquid phase is substantially lower than that in the corresponding solid phase. The validity of the calculations is tested using equilibrium volume and bulk modulus data, deduced from diamond anvil cell experiments, Hugoniot data and the most recently reported shock temperature data at ∼6 Mbar.

We investigate through first-principles calculations the controversial observation of the high-pressure orthorhombic ($\ensuremath{\gamma}$ and $\ensuremath{\delta}$) phases of titanium. Our calculations predict the transition sequence $\ensuremath{\omega}\text{\ensuremath{-}}\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\beta}$ under pressure, and reveal that the $\ensuremath{\delta}$ phase is elastically unstable under isotropic compression. We attribute its observation to nonhydrostatic stresses present in the diamond-anvil cell experiments. We find the $\ensuremath{\gamma}$ phase to be stable in the $102--112\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ pressure range, with the upper limit of this pressure range increasing under nonhydrostatic conditions.

We present results of ab initio calculations on the high pressure phonon, magnetic anomalies, and structural instability in cobalt. Our calculations reproduce the magnetic collapse starting near $100\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and the hcp to fcc phase transition around this compression, agreeing with earlier calculations and with the experimental data. Our studies show anomaly in the ${E}_{2\mathrm{g}}$ mode phonon frequency variation near $75\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ pressure, consistent with the measurements. We have demonstrated that phonon and magnetic anomalies are interrelated in cobalt and hence, the magnetoelastic coupling should be stronger in Co than in iron, which also undergoes a high pressure bcc to hcp transition with magnetic collapse but without showing any associated anomaly in the phonon frequency variation under compression.

The E 2g mode phonon frequency anomaly, observed in high-pressure Raman measurements for the high-temperature superconductor MgB 2 and conjectured to arise due to a phonon-assisted electronic topological transition (ETT), has been corroborated by us with the details of electronic structure calculations of this compound. The present studies have been carried out with a B-atom displacement of the frozen-in E 2g -mode phonon that mimics the effects of thermal vibrations. We found a splitting of the doubly degenerate σ band along the Γ-A direction. The splitting of the band reduces with pressure, causing an ETT at 18 GPa. This ETT is expected to cause anomaly in the E 2g phonon mode by influencing the ionic system. Our calculated electron-phonon coupling constant λ , for the distorted lattice with a frozen-in displacement, also depicts an anomalous variation around 18 GPa.

High pressure Raman and angle dispersive X-ray diffraction (ADXRD) measurements on the metallic hexaboride LaB6 have been carried out upto the pressures of about 20 GPa. The subtle phase transition around 10 GPa indicated in Raman measurements is confirmed by ADXRD experiments to be a structural change from cubic to orthorhombic phase. Ab-initio electronic band structure calculations using full potential linear augmented plane wave method carried out as a function of pressure show that this transition is driven by the interception of Fermi level by electronic band minimum around the transition pressure.

Laser driven shock wave experiments were performed to study the equation of state (EOS) of Cu material using impedance-matching technique with Al as reference material. An Nd:YAG laser chain (2 Joule, 1.06 μm wavelength, 200 ps pulse FWHM) was used for generating shocks in the planar Al foils and Al-Cu layered targets. EOS of materials at shock pressure up to 11 Mbar is obtained with pressure enhancement by a factor of 1.67 at Al-Cu interface. Numerical simulations performed using one-dimensional radiation hydrodynamic code MULTI show close agreement with the experimental value of shock pressure enhancement. Simulation reveals the fact that 5 - 6 μm thickness of Al foil as a reference material is sufficient to prevent the x-ray preheating effect as well as to attain planar and steady shock wave propagation for a given laser beam used in the experiment. The experimental Hugoniot data points obtained are in excellent agreement with the existing standard SESAME data and with other reported experimental results.

The pressure enhancement due to impedance-matching at the interface between the two-step layered targets has been well studied and established technique for EOS measurement of materials in laser driven shock wave experiments. A detailed numerical simulation study of shock wave propagation through Al standard material in laser driven shock wave experiments was performed using a 1-D radiation hydrodynamic code MULTI. The definitive role of mesh thickness was noticed. The experimental results of shock pressure enhancement in Al-Cu and Al-Au, layered targets corroborated with the numerical simulation results. Simulations were subsequently extended to plane-layered targets such as CH-Al, CH-Cu an CH-Au targets. It is shown that with proper tailoring of laser and target parameters, shock pressures in the range of up to 30 to 50 Mbar could be achieved with relatively moderate intensity lasers (I L = 6 - 8x10 13 W/cm 2 ; Pulse FWHM 600 - 800 ps). The numerical simulations also enumerate appropriate conditions to maintain the steadiness of shock waves with minimal preheat effect.

Results of high-pressure Raman-scattering measurements carried out on resorcinol up to pressures of 14.5 GPa are reported. Changes observed in the Raman spectra suggest transition to a new phase $\ensuremath{\gamma}$ around $5.6\ifmmode\pm\else\textpm\fi{}0.2 \mathrm{GPa}$ besides the well-known isosymmetric $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\alpha}}\ensuremath{\beta}$ transition at 0.5 GPa. The $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\beta}}\ensuremath{\gamma}$ transition is found to be reversible with hysteresis when released from peak pressures of about 11 GPa. Raman line shapes at high-pressures suggest that the new phase $\ensuremath{\gamma}$ is disordered. Rapid pressurization of $\ensuremath{\alpha}$-resorcinol, with the suppression of $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\alpha}}\ensuremath{\beta}$ transition, leads to a new phase $\ensuremath{\delta}$ above 3 GPa which turns out to be more ordered, unlike the $\ensuremath{\gamma}$ phase, at comparable pressures. A luminescence peak centered around 2.25 eV is observed in the high pressure phase $\ensuremath{\gamma}$ as well as $\ensuremath{\delta}$ above 6 GPa. Above 11 GPa, intensity of the Raman bands weaken drastically and submerge in the luminescence background. The ambient pressure Raman spectrum of resorcinol is not recovered when released from pressures above 11 GPa.

The equation of state (EOS) of a system is a relationship between thermodynamic variables like pressure, p, and energy, E, with volume, V, and temperature, T. It has applications in a number of fields: condensed-matter physics, geophysics, astrophysics, plasma physics, and nuclear physics. Also, it is a vital input in hydrodynamic calculations for a wide spectrum of practical needs, e.g., in reactor safety simulations, design of fission and fusion energy producing devices, analysis of hypervelocity impacts, and weapon development.

Calcul des equations d'etat du molybdene cristallin (bcc) et liquide sous les conditions de pression et de temperature de 600 GPa et 1400 K, a l'aide du modele des orbitales muffin-tin lineaires (LMTO) et du modele de la sphere d'ion rigide corrige (CRIS). Les resultats sont en accord avec ceux d'un travail anterieur indiquant un deplacement electronique induit par la pression, des etats 5s vers les etats 4d, specialement au-dessus de 100 GPa. Une analyse des mesures ultrasonores et d'ondes de choc, en parallele avec les resultats theoriques, montre que la compressibilite de Mo(bcc) est renforcee pour des pressions de 100-200 GPa. Le renforcement de la compression et une possible augmentation anormale de la rigidite sont dus au transfert S→d induit par la pression. Cette etude renforce l'utilisation de l'equation d'etat de Mo comme une calibration standard pour des experiences statiques a tres hautes pressions, en particulier pour la technique de fluorescence du rubis

The shock Hugoniot equation of state for aluminum has been computed, using the pseudopotential method. The nuclear Gru neisen parameters, used in evaluating the ionic vibration contributions, have been derived from phonon frequencies. The computed electronic Gru neisen-parameter values depart considerably from the value 0.5, used often in analyzing shock-compression experiments. This leads to different Hugoniot temperatures compared with those derived by Al'tshuler et al., although the P-V curve is in good agreement with their experimental results.

The method of first principle pseudopotential is used to simulate the volume variation of nuclear Gruneisen parameter. The elements Al and Be, for which Neal’s experimental data exist are investigated. The nuclear Gruneisen obtained from the details of phonon frequencies are in better agreement with the experimental data as compared to those obtained from approximate Slater and Dugdale-MacDonald methods, often used in shock wave studies.

A knowledge of the equation of state (EOS) of matter is necessary to describe the dependence of its thermodynamic properties on its microscopic internal structure. EOS data are also vital for a variety of applications, particularly those that involve numerical simulation of dynamic processes. This has motivated intense research on EOS of materials, both in experimental and theoretical areas. Various laboratories in the world are involved in the development of techniques for the generation of increasingly higher pressures using high power lasers, electric and rail guns etc. The range of these controlled dynamic pressures is currently reaching about10 TPa. On the theoretical side, the earlier empirical methods have now been replaced by sophisticated band structure calculations. In this article, we review these first principle theoretical attempts to calculate EOS over a range of densities and temperatures, where a variety of physical phenomena such as band crossings, band closures, phase transitions, nuclear and electronic thermal excitations, core ionization, liquid disorder etc. are encountered. The emphasis in this review is on shock Hugoniot equation of state theories.

The systematics of the shock constants in shock velocity-particle velocity relations for metals have been examined by energy band theory methods. The causes of non-linearity of this relation at high pressure are discussed in terms ofs ⇌d electron transfer.

The process of laser-induced shock propagation was investigated for a number of double-layered plane targets with a one-dimensional Lagrangian hydrodynamic code using a realistic equation of state and experimentally reported laser absorption values. It was found that a suitably optimized layer of C${\mathrm{H}}_{2}$ on Pt can yield pressures in excess of 60 Mbar at its interface with a 1.06 \ensuremath{\mu}m, ${10}^{14}$ W/${\mathrm{cm}}^{2}$, 300-ps rise-time laser pulse. This should be the highest pressure that can be achieved in any single- or double-layered target with such a laser pulse. The scaling of this pressure with intensity is described by a power law having a higher exponent than usually found for any material. Suitable scaling is also investigated between the optimum thickness and peak pulse power. The results should help in designing experiments on any laser system.

A model for the equation of state of condensed matter in the "intermediate" pressure region (0.5-10 TPa) is proposed. The dominant pressure and thermal ionization effects are simulated through a modification of Saha's ionization theory. The pseudopotential method which includes core ionization effects is used to obtain the cold and lattice thermal contributions. The model is applied to Al and Mo.

Saha’s ionization equation has been solved for highZ elements with the aim of providing input for opacity calculations. Results are presented for two elements, tungsten and uranium. The ionization potentials have been evaluated using the simple Bohr’s formula with suitable effective charges for ions. The reliability of the free electron density, ion concentrations, etc., obtained from the Saha’s equation solutions has been checked by comparing thePT andET computed from them with those given by the Thomas-Fermi-Dirac equation of state. The agreement between the two is good from temperatures above 0.2 keV.

A procedure is described for computation of opacities for highZ plasmas. Bound-bound, bound-free, free-free and scattering processes are considered. The inputs for these have been obtained by solving IEEOS form of Saha’s equation. Detailed calculations of opacities have been done for tungsten and uranium up to 10 keV of temperature.

The effect of impedance mismatch on shock‐pressure enhancement in laser‐irradiated layered targets has been investigated by numerical simulation of the entire time history of shock generation. The model is a simple, one‐dimensional, single‐temperature Lagrangian hydrodynamic code incorporating a simplified laser‐absorption model and an equation of state suitable to describe the thermodynamic state of matter in a wide range of density and temperature. Effects of change in layer materials and other experimental parameters on the attainable shock pressure and compression have been studied. From an exhaustive study of such targets, it is found that it is possible to enhance the attainable values of compression (ρ/ρ0) and shock pressure with a given laser pulse by suitable optimization of the target. A pressure of more than 60 Mbar can be generated with a 1014 W/cm2, 300‐ps rise‐time laser pulse in such targets. In order to generalize the applicability of the results, within existing experimental uncertainties of measurements, and facilitate a suitable target design for any laser system, simple scaling laws are obtained in terms of parameters like intensity and rise time of the laser pulse.

Modified Saha ionization theory has recently been used to obtain the dominant pressure and thermal ionization effects in the "intermediate" region (0.5-10 TPa) of the equation of state. The present work is aimed at providing the justification for the use of this theory in the presence of interactions. The valence charges at 0 K at normal density as determined using this theory are in very good agreement with their normal valence charges for a number of elements. The equation of state as computed from it is also in good agreement with the Thomas-Fermi-Dirac equation of state. Finally the theory is shown to simulate the valence transition under pressure in Eu and Yb.

We report ${T}_{c}$ measurements on ${\mathrm{Cu}}_{2}$${\mathrm{Mo}}_{6}$${\mathrm{S}}_{8}$ disordered by shock pressure. The shock is induced by the absorption of a laser pulse. A depression in ${T}_{c}$ of up to 0.45 K is observed, while the width of the transition is broadened from 0.42 to 0.66 K. Our results are compared with earlier results of Stewart, Olinger, and Newkirk on shock-damaged ${\mathrm{V}}_{3}$Si, and also with the earlier results on particle irradiation of Chevrel-phase superconductors.

Mixed valent (MV or IV) and heavy Fermion (HF) behaviour exhibited by some Ce, Eu,Yb and U intermetallics are two aspects of the same problem with fatom spacing dependence of the f band width (real or virtual) as the control parameter1. The ground state of HF systems exhibits non-conventional superconductivity (UPt3 and Ce Cu2Si2, antiferromagnetic ordering (NpBe13)or the absence of both (CeAl3) concornmitant with high γ and large magnetic atom seperation which should normally allow localized spins. The high γ value implies a large density of states at the Fermi surface. Three types of models involving f electrons have been invoked which can result in high γ the two band models with an s−d conduction band and a narrow f band where the Fermi level is pinned to the f band, the Kondo lattice model where the conduction electrons are scattered by localized f electron spins and the Fermi liquid models with He3 as the prototype2. In the latter two models, at low temperatures narrow virtual f-bands develop at the fermi level and represent the HF systems more closely3.

A theoretical model for the calculation of the K-shell photoabsorption edge in a dense plasma is described. In the model, degeneracy and continuum lowering contributions due to free electrons and neighboring ions were treated as perturbations on the continuum and the bound state. These contributions were evaluated using first-principles solid-state methods. The model was used to interpret recently observed shifts in the K-shell photoabsorption-edge energy in shock-compressed aluminum. The validity of the model was further examined through comparisons with two average-atom, self-consistent-field calculations and an augmented-plane-wave calculation.

Two structures, thcp and modulated hexagonal structure have been proposed for the distorted fcc phase of the rare earths at high pressure. We have stabilised this phase in Ls 0.8 Th 0.2 at ambient conditions and present diffraction data to rule out the thcp structure.