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Khan, F.

Publications and source records attributed to Khan, F..

16 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2003

Nanoporous Silicon Carbide for Nanoelectromechanical Systems Applications

A major goal of this project is to produce porous silicon carbide (PSiC) via an electroless process for eventual utilization in nanoscale sensing platforms. Results in the literature have shown a variety of porous morphologies in SiC produced in anodic cells. Therefore, predictability and reproducibility of porous structures are initial concerns. This work has concentrated on producing morphologies of known porosity, with particular attention paid toward producing the extremely high surface areas required for a porous flow sensor. We have conducted a parametric study of electroless etching conditions and characteristics of the resulting physical nanostructure and also investigated the relationship between morphology and materials properties. Further, we have investigated bulk etching of SiC using both photo-electrochemical etching and inductively-coupled-plasma reactive ion etching techniques.

Hossain, T.↗
NASA NTRS1994

Role of intrinsic width in fragment momentum distributions in heavy ion collisions

It is demonstrated that the intrinsic widths incorporating correlations in conjunction with dynamical contributions give better agreement with experiments for collisions in the energy range of 200 A MeV to 2.4 GeV than using only intrinsic widths without correlations. The sensitivity of the intrinsic width decreases with increasing projectile mass. A simple recipe for calculating intrinsic width correlations is presented.

Townsend, L. W.↗
NASA NTRS1993

Universal characteristics of transverse momentum transfer in intermediate energy heavy ion collisions

A microscopic optical model formalism for estimating momentum transfer in intermediate energy heavy ion collisions predicts universal behavior of the transverse component. In particular, for symmetric systems heavier than niobium, it appears that values of P(perpendicular)/A are independent of the mass and charge of the colliding nuclei and vary only with impact parameter and incident beam energy. This suggests that momentum transfer per nucleon saturates to some limiting value with increasing mass.

Khan, F.↗
NASA NTRS1991

Momentum transfer in relativistic heavy ion charge-exchange reactions

Relativistic heavy ion charge-exchange reactions yield fragments (Delta-Z = + 1) whose longitudinal momentum distributions are downshifted by larger values than those associated with the remaining fragments (Delta-Z = 1, -2,...). Kinematics alone cannot account for the observed downshifts; therefore, an additional contribution from collision dynamics must be included. In this work, an optical model description of collision momentum transfer is used to estimate the additional dynamical momentum downshift. Good agreement between theoretical estimates and experimental data is obtained.

Townsend, L. W.↗
NASA NTRS1991

Calculations of hadronic dissociation of 28Si projectiles at 14.6A GeV by nucleon emission

An optical potential abrasion-ablation collision model is used to calculate hadronic dissociation cross sections for one, two, and three nucleon removal for the first time for a 14.6A GeV 28Si beam fragmenting in aluminum, tin, and lead targets. These estimates are compared with recent semi-inclusive measurements. Significant differences between some calculated and measured semi-inclusive cross sections exist which cannot be resolved without measurements of the exclusive channel hadronic cross sections. Calculations for each exclusive reaction channel contributing to the semi-inclusive cross sections are presented and discussed.

NASA Program Radiation Health↗
NASA NTRS1991

Optical model description of momentum transfer in relativistic heavy ion collisions

An optical model description of momentum transfer in relativistic heavy ion collisions, based upon composite particle multiple scattering theory, is presented. The imaginary component of the complex momentum transfer, which comes from the absorptive part of the optical potential, is identified as the longitudinal momentum downshift of the projectile. Predictions of fragment momentum distribution observables are made and compared with experimental data. Use of the model as a tool for estimating collision impact parameters is discussed.

Khan, F.↗
NASA NTRS1990

Moments of dipole oscillator-strength distribution for the helium sequence

The moments S(mu) for mu at least -6 but no more than 2 and L(mu) for mu = 0, 1, and 2 are calculated for the helium sequence for atomic numbers (Z) up to 30 under a screened hydrogenic model. The model describes the atom by single-particle hydrogenic wave functions and treats the initial and the final state as characterized by two different effective charge parameters Zi and Zf, respectively. The differential oscillator strength of the screened hydrogenic model is asymptotically expanded. Assuming the value of 287.6 for the coefficient of the term epsilon to the -7/2 for helium atoms, the parameter Zf is determined for the helium sequence.

Khan, F.↗
NASA NTRS1989

An optical model description of momentum transfer in heavy ion collisions

An optical model description of momentum transfer in relativistic heavy ion collisions, based upon composite particle multiple scattering theory, is presented. The imaginary component of the complex momentum transfer, which comes from the absorptive part of the optical potential, is identified as the longitudinal momentum downshift of the projectile. Predictions of fragment momentum distribution observables are made and compared with experimental data. Use of the model as a tool for estimating collision impact parameters is discussed.

Khan, F.↗
NASA NTRS1989

Screened hydrogenic radial integrals

The screened hydrogenic radial integral for discrete-discrete and discrete-continuum transitions is expressed in forms suitable for obtaining closed-form expressions for specific transitions. Two effective-charge parameters, for the initial state and for the final state, are retained in these formulas. As examples, explicit expressions for a few transitions are derived, and a method for obtaining a series for a discrete-discrete radial integral, suitable for large final-state principal quantum numbers, is indicated.

Khandelwal, G. S.↗
NASA NTRS1988

Differential oscillator strengths and dipole polarizabilities for transitions of the helium sequence

The dipole radial integral for an initial discrete 1s state and a final continuum state has been calculated under the screened hydrogenic model. In this model, single-electron hydrogenic wave functions are employed, and the initial and the final states are treated by two different effective-charge parameters. Numerical values of differential oscillator strengths for transitions from 1s2 1S to the continuum for the helium sequence ions are obtained. Also calculated are the dipole polarizabilities, which are found to be in excellent agreement with the results of other authors.

Khan, F.↗
NASA NTRS1988

Asymptotic screened hydrogenic radial integrals

The usefulness of the screened hydrogenic model for the transitions of the helium sequence is studied. The screened hydrogenic radial dipole integral for discrete-discrete transitions from the initial state to the final state is asymptotically expanded to the lowest order such that the final quantum number n approaches infinity. The analytical expression obtained is in terms of confluent hypergeometric functions.

Olsgaard, D. A.↗
NASA NTRS1988

1s2 1S-1s np 1P transitions of the helium isoelectronic sequence members up to z = 30

The radial integrals required to calculate oscillator strengths for the helium sequence ions have been obtained using a one-electron hydrogenic model. The present approach retains two different values of the effective charge parameter, one for the initial state, and one for the final state. The model is shown to be capable of reasonably reproducing the existing dipole oscillator strength values with minimal computational effort. Dipole oscillator strengths for certain ions are presented for the first time.

Khan, F.↗
NASA NTRS1988

1s2 1S-1s np 1P transitions of the helium isoelectronic sequence members up to Z = 30

Radial integrals have been calculated under the one-electron hydrogenic model. Two different values of the effective charge parameter, one for the initial state and the other for the final state, are retained in these formulae. The model is able to reasonably reproduce the existing dipole oscillator strength values with little effort. The dipole oscillator strength values are given for many ions for the first time.

NASA Discipline Number 22-70↗
NASA NTRS1988

Static multipole polarisabilities and second-order Stark shift in francium

The multipole polarizability of the ground state of francium is calculated by utilizing both the variational technique of Davison and the quantum defect theory underlying the Bates-Damgaard method. This approach is also shown to yield reasonable results for other alkali atoms. Second-order Stark shift for the ground state of francium is presented as a function of field strength for possible future experimental comparison.

Khan, F.↗
NASA NTRS1986

Forest canopy characterization and vegetation penetration assessment with space-borne radar

Synthetic Aperture Radar (SAR) images from the National Aeronautics and Space Administration's Shuttle Imaging Radar-B mission were used to analyze the effects of radar incidence angle on information content and vegetation penetration. Three SAR data sets using incidence angles of 26, 46, and 58 deg were acquired over the mangrove jungles of Southern Bangladesh. The data sets were digitally processed using 3 x 3, 7 x 7, and 11 x 11 spatial filters and geometrically registered to a multisource-multilevel-corraborative data base consisting of Landsat data, forest map data, and in situ acquired forest enumeration and topographic information. Analyses revealed that significant vegetation 'penetration' was found at all angles, and that tree and canopy structural morphology may exert an influence on this phenomenon.

Imhoff, M.↗