Browsing by Author "Pragya Bhatt"

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An apparatus to study the energy and angular distributions of electron-bremsstrahlung photons from gaseous targets
(2014) Namita Yadav; Pragya Bhatt; Raj Singh; B.K. Singh; C.A. Quarles; R. Shanker
An apparatus is developed to measure the energy- and angular distributions of bremsstrahlung generated from collisions of energetic electrons with isolated atoms and molecules. A considerable reduction of thick target bremsstrahlung (TTB) background produced by scattered electrons from the chamber wall is achieved. Details of the experimental setup with regard to design of its components, experimental technique, data acquisition and analysis etc. are given and discussed. The reliability and performance of the setup are demonstrated by obtaining some test results on angular- and energy distributions of bremsstrahlung produced in collisions of 4.0. keV electrons with free argon atoms. These results are compared with the theoretical predictions of the ordinary- and the polarization bremsstrahlung emissions. In this comparison, the experimental data for energy distributions of BS photons are found to be in reasonable agreement while they are found to have noticeable differences in shape of angular distributions. © 2013.
An experimental set up for studying the fusion edge plasma facing materials using TOF mass spectrometry
(Elsevier, 2015) Sunil Kumar; Pragya Bhatt; B.K. Singh; Ajai Kumar; R. Shanker
A new experimental set up is developed to study the fusion edge plasma facing materials under impact of low-energy (40-10,000 eV) atomic and molecular ions. The primary ions are obtained from a low-pressure D.C. gas discharge Colutron ion source. The product ions resulting from the ion-surface interactions involving different processes, for example, surface-induced dissociation (SID), surface-induced reaction (SIR), chemical- and sputtering are detected and identified by using a single-field linear time-of-flight (TOF) mass spectrometer coupled with a time- and position-sensitive dual micro-channel plate detector. The mass spectra of secondary ions are recorded by employing a pulsed extraction and acceleration technique. The experimental set up, experimental technique and data acquisition methodology are presented and discussed. A test experiment is conducted on the developed set up to demonstrate the performance and reliability of the apparatus. For this, a primary beam of 427 eV N2+ ions was chosen to interact with a hydrocarbons covered surface of platinum target kept at room temperature. SID of N2+ into N+ and TOF mass spectra of sputtered, reflected and chemically formed ions from the ion-surface interaction are analyzed. A noticeable feature in the spectra is observed which shows that the odd-electron ions of sputtered hydrocarbons are preferentially produced in the interaction in addition to the other product ions. The nominal mass resolution (M/ΔM) of the present tandem mass spectrometer system is estimated to be about 600, while energy and spatial spreads of the primary ion beam are determined to be about 518 meV and 2.8 ± 0.3 mm respectively. The survival probability of the incident ions is found to be 6.6% suggesting that the major process of ion-surface interaction at the considered impact energy is neutralization. © 2015 Elsevier B.V. All rights reserved.
Characteristic and non-characteristic X-ray yields produced from thick Ti element by sub-relativistic electrons
(2012) Namita Yadav; Sunil Kumar; Pragya Bhatt; Raj Singh; B.K. Singh; R. Shanker
Measurements are performed to study the electron impact energy dependence of doubly differential bremsstrahlung yields (DDBY) and of characteristic Ti Kα line yields produced from sub-relativistic electrons (10-25 keV) colliding with a thick Ti (Z = 22) target. The emitted radiation is detected by a Si-PIN photo-diode detector with energy resolution (FWHM) of 180 eV at 5.9 keV. The measured data of DDBY are compared with the results predicted by Monte-Carlo (MC) simulations using the general purpose PENELOPE code. A reasonable agreement is found between experimental and simulation results within the experimental uncertainty of measurements of 12%. Characteristic Ti K α yields are obtained for the considered impact energy range and they are compared with the existing theoretical results. A good agreement is found between the present measurements and the theoretical calculations. Furthermore, data are presented for impact energy dependence of the ratio K α/(Kα+ Kβ) of a thick Ti target under impact of 10-25 keV electrons. The ratio shows a very weak dependence on impact energy in the studied range. The average value of the ratio is found to be 0.881 ± 0.003. © 2012 Elsevier B.V.
Dissociation of a CO Molecule Induced by 10 keV Electrons: Kinetic Energy Release Distributions
(Nova Science Publishers, Inc., 2014) Raj Singh; Pragya Bhatt; Namita Yadav; R. Shanker
Dissociative ionization of the COq+ (q=2-4) molecular ions produced in collisions of CO with 10 keV electrons is studied using time-of-flight mass spectrometer and position sensitive detector with multi-hit ability. The kinetic energy release distributions for different channels produced from the dissociation of COq+ (q=2-4) are obtained. We found that a pure Coulomb explosion model is insufficient to explain the observed kinetic energy release distributions for the Coulomb explosion channels.
Dissociative-ionization cross sections for 12-keV-electron impact on CO2
(2011) Pragya Bhatt; Raj Singh; Namita Yadav; R. Shanker
The dissociative ionization of a CO2 molecule is studied at an electron energy of 12 keV using the multiple ion coincidence imaging technique. The absolute partial ionization cross sections and the precursor-specific absolute partial ionization cross sections of resulting fragment ions are obtained and reported. It is found that ∼75% of single ionization, 22% of double ionization, and ∼2% of triple ionization of the parent molecule contribute to the total fragment ion yield; quadruple ionization of CO 2 is found to make a negligibly small contribution. Furthermore, the absolute partial ionization cross sections for ion-pair and ion-triple formation are measured for nine dissociative ionization channels of up to a quadruply ionized CO2 molecule. In addition, the branching ratios for single-ion, ion-pair, and ion-triple formation are also determined. © 2011 American Physical Society.
Formation, structure, and dissociation dynamics of CO 2 q+ (q≤3) ions due to impact of 12-keV electrons
(2012) Pragya Bhatt; Raj Singh; Namita Yadav; R. Shanker
The electron-impact multiple ionization and subsequent dissociation of CO 2 is studied for 12-keV electron energy using a linear time-of-flight mass spectrometer coupled with a multihit, position-sensitive detector. The complete as well as incomplete Coulomb explosion pathways for CO 22 + and CO 23 + ions are examined and identified. The kinetic energy release distributions for these precursor ions are obtained. The experimental kinetic energy release values for the complete Coulomb fragmentation channels are found to be overestimated by those calculated from the Coulomb explosion model. From the angular correlation studies, it is inferred that bent geometrical states are involved for most of the fragmentation channels of CO 22 + and CO 23 + ions. The concerted and/or sequential nature of all the dissociation pathways is also assigned. This study provides the first results on energetics associated with the charge separation in dissociative ionization of CO 2 under the impact of electrons at a subrelativistic energy. © 2012 American Physical Society.
Ionic fragmentation of a CH4 molecule induced by 10-keV electrons: Kinetic-energy-release distributions and dissociation mechanisms
(2013) Raj Singh; Pragya Bhatt; Namita Yadav; R. Shanker
The dynamics of ionic fragmentation of CH4 molecules under impact of 10-keV electrons has been studied. The technique of recoil ion momentum spectroscopy is employed to obtain information about the kinetic energy release and the dissociation mechanisms of different pathways arising from the fragmentation of a CH4 dication. The results show that there are altogether eight dissociation pathways that arise from the complete and the incomplete Coulomb explosions of the CH42+ molecular ions. The kinetic energy release for these pathways is compared with earlier data from the literature for the impact of different charged particles, photons, and their impact energies. The present results indicate that mostly the lower electronic states of CH42+ are involved for the observed dissociation channels. Also, the dissociation mechanisms associated with these channels are suggested and discussed. Further, we have also estimated the relative ion intensities of different channels of fragmentation of CH 4 dication produced under impact of considered energy of electrons. © 2013 American Physical Society.
Ionic fragmentation of CO and H2O under impact of 10 keV electrons: Kinetic energy release distributions
(Institute of Physics Publishing, 2014) Raj Singh; Pragya Bhatt; Namita Yadav; R. Shanker
Dissociative ionization of COq+ (q=2-4) and H2O q+ (q=2-3) molecular ions produced from the collisions of CO and H2O with 10 keV electrons is studied using time-of-flight mass spectrometer and position sensitive detector with multi-hit ability, respectively. The kinetic energy release distributions for these channels are obtained. We found that a pure Coulomb explosion model is insufficient to explain the observed kinetic release distributions for the Coulomb explosion channels. A detail of this study is given in references [3, 4]. © Published under licence by IOP Publishing Ltd.
Ionic fragmentation of the CO molecule by impact of 10-keV electrons: Kinetic-energy-release distributions
(2013) Raj Singh; Pragya Bhatt; Namita Yadav; R. Shanker
The ionic fragmentation of a multiply charged CO molecule is studied under impact of 10-keV electrons using recoil-ion momentum spectroscopy. The kinetic-energy-release distributions for the various fragmentation channels arising from the dissociation of COq+ (q = 2-4) are measured and discussed in light of theoretical calculations available in the literature. It is observed that the present kinetic-energy-release values are much smaller than those predicted by the Coulomb explosion model. The kinetic-energy-release distribution for the C++O+ channel is suggested to arise from the tunneling process. It is seen that the peak of kinetic-energy-release distribution is larger for that dissociation channel that arises from the same molecular ion which has higher charge on the oxygen atom. Further, the relative ionic fractions for seven ion species originating from ionization and subsequent dissociation of the CO molecule are obtained and compared with the existing data reported at low energy of the electron impact. The precursor-specific relative partial ionization cross sections are also obtained and shown to be about 66.4% from single ionization, 29.9% from double ionization, 3.3% from triple ionization, and about 0.4% from quadruple ionization of the precursor CO molecule contributing to the total fragment ion yield. © 2013 American Physical Society.
Kinematics and dissociation dynamics of a water molecule under the impact of 10 keV electrons
(2013) Raj Singh; Pragya Bhatt; Namita Yadav; R. Shanker
The kinematics and dissociation dynamics of a H2O molecule induced by 10 keV electrons are studied using a time-of-flight mass spectrometer in conjunction with a position-sensitive detector in multi-hit coincidence mode. Five dissociative channels arising from the complete as well as the incomplete Coulomb explosions of H2Oq+ (q = 2, 3) ions are observed and identified. The dissociation mechanisms (concerted and/or sequential) for these channels are examined. Further, the angular correlation of different fragment ions and the geometrical structure of the precursor ion are studied. The kinetic energy release distributions for the observed channels are also determined. It is found that the pure Coulomb explosion model is insufficient to explain the observed kinetic release distributions. The mean kinetic energy release for these channels is compared with the available data reported by earlier workers who have employed different charged projectiles and sources of photons. © 2013 IOP Publishing Ltd.
Measurement of the bremsstrahlung spectra generated from thick targets with Z=2-78 under the impact of 10 keV electrons
(2010) Namita Yadav; Pragya Bhatt; Raj Singh; V.S. Subrahmanyam; R. Shanker
We present new experimental data on thick target bremsstrahlung spectra generated from the interaction of energetic electrons with bulk matter. The 'photon yields' in terms of double differential cross-sections (DDCS) are measured for pure elements of thick targets: Ti (Z = 22), Ag (Z = 47), W (Z = 74) and Pt (Z = 78) under the impact of 10 keV electrons. Comparison of DDCS obtained from the experimental data is made with those predicted by Monte-Carlo (MC) calculations using PENELOPE code. A close agreement between the experimental data and the MC calculations is found for all the four targets within the experimental error of 16%. Furthermore, the ratios of DDCS of bremsstrahlung photons emitted from Ag, W and Pt with those from Ti as a function of photon energy are examined with a relatively lower uncertainty of about 10% and they are compared with MC calculations. A satisfactory agreement is found between the experiment and the calculations within some normalizing factors. The variations of DDCS as a function of Z and of photon energy are also studied which show that the DDCS vary closely with Z; however, some deviations are observed for 'tip' photons emitted from high Z targets. © Indian Academy of Sciences.
Momentum mapping spectrometer for probing the fragmentation dynamics of molecules induced by keV electrons
(Institute of Physics Publishing, 2011) Raj Singh; Pragya Bhatt; Namita Yadav; R. Shanker
We describe a new experimental setup for studying the fragmentation dynamics of molecules induced by the impact of keV electrons using the well-known technique of recoil ion momentum spectroscopy. The apparatus consists of mainly a time- and position-sensitive multi-hit particle detector for ion analysis and a channel electron multiplier detector for detecting the ejected electrons. Different components of the setup and the relevant electronics for data acquisition are described in detail with their working principles. In order to verify the reliable performance of the setup, we have recorded the collision-induced ionic spectra of the CO2 molecule by the impact of keV electrons. Information about the ion pairs of CO+:O+, C+:O+ and O+:O+ resulting from dissociative ionizing collisions of 20 and 26 keV electrons with a dilute gaseous target of CO2 molecules has been obtained. Under conditions of the present experiment, the momentum resolutions of the spectrometer for the combined momenta of CO+ and O+ ions in the direction of the time-of-flight axis and perpendicular to the direction of an electron beam are found to be 10.0 ± 0.2 and 15.0 ± 0.3 au, respectively. © 2011 IOP Publishing Ltd.
Momentum spectroscopy of fragment ions of a multiply charged N2O molecule under impact of 10-keV electrons
(2012) Pragya Bhatt; Raj Singh; Namita Yadav; R. Shanker
The dissociative ionization of a N2O molecule is studied at an electron energy of 10 keV using the multiple-ion-coincidence imaging technique. The complete as well as the incomplete Coulomb explosion pathways for N 2O2+ and N2O3+ ions are examined and identified. The precursor-specific relative partial ionization cross sections for resulting fragment ions are obtained. It is found that about 81.8% of single ionization, 17.8% of double ionization, and about 0.4% of triple ionization of the parent molecule contribute to the total fragment ion yield. Furthermore, the relative ionic fractions for ion-pair and ion-triple formation are also determined. The kinetic energy release distributions for different coincidence channels are obtained and compared with the available data at lower energies of electron and photon impacts and with that of high-energy ion impact. From the angular correlation studies of fragment ions, it is inferred that states with bent geometries are involved for most of the fragmentation channels of N 2O2+ and N2O3+ ions. The concerted and/or sequential nature of the six dissociation pathways is also assigned. No other experimental or theoretical data exist in the literature to compare with the results obtained at the considered impact energy. © 2012 American Physical Society.
Partial ionization cross-sections of H2O molecule by 10-25 keV electron ionization
(2013) Raj Singh; Pragya Bhatt; Namita Yadav; R. Shanker
The relative ionic fractions and the precursor-specific relative partial ionization cross-sections for the fragment ions formed in direct- and dissociative ionization of H2O molecule have been measured for the first time at 10-25 keV electron energies using the multiple ion coincidence imaging technique. We have identified six ions (H2O+, OH+, H+, O+, O2+ and H 2+) from the ionization of H2O molecule and have found that the relative ionic fractions for these fragments are almost invariant of impact energy. From multiple-ion coincidence spectra, we have identified three distinct fragmentation channels: namely, H+ + OH+, H+ + O+ + H and H+ + H + + O resulting from dissociation of H2O2+ and two channels: H+ + O2+ + H and H+ + H + + O+ resulting from the dissociation of H 2O3+. From the analysis, we have estimated that about 94.4% of 'single ionization', 5.3% of 'double ionization' and 0.3% of 'triple ionization' of the parent molecule contribute to the total ion yield. It is further noted that the non-dominant fragment ions O2+ and H 2+ possess appreciably larger relative ionic fractions than those obtained at lower impact energies. © 2013 Elsevier B.V. All rights reserved.
Partial-ionization cross sections of a CO2 molecule due to impact of 10-26-keV electrons
(2010) Pragya Bhatt; Raj Singh; Namita Yadav; R. Shanker
Experimental data on total- and partial-ionization cross sections of ionic fragments of CO2 molecule produced by impact of 10-26-keV electrons are obtained on a crossed-beam apparatus in our laboratory. An ejected electron-produced ion-coincidence technique is employed together with a time-of-flight mass spectrometer for analysis of the ions. The six ionic fragments, CO2⊃+, CO⊃+, CO22⊃+, O⊃+, C⊃+, and C2⊃+, resulting from dissociative ionization of the CO 2 molecule are observed and identified; their relative ionization cross sections and branching ratios are determined as a function of impact energy. The binary-encounter Bethe model is found to overestimate the experimental data for total-ionization cross sections of the observed ions. No other experimental or theoretical data exist in the investigated energy range to make a direct comparison with the present results. © 2010 The American Physical Society.
Relative partial ionization cross sections of N 2O under 10-25-keV electron impact
(2012) Pragya Bhatt; Raj Singh; Namita Yadav; R. Shanker
The relative partial ionization cross sections for the fragment ions produced in direct and dissociative ionization of a N 2O molecule are measured for impact of 10-25-keV electrons by using an electron-ion-coincidence technique with a linear time-of-flight spectrometer. The six ionic fragments of N 2O (N 2O +, NO +, N 2 +, O +, N +, and N2 + + O2 +) are observed and identified. The impact energy dependence of the partial ionization cross sections for these ions is expressed relative to the cross section of N 2O + and is found to be nearly invariant. The relative ionic fractions for the produced ions of N 2O are also obtained and compared with the earlier reported data available at lower energies of electron impact. It is found that the relative ionic fractions for singly charged fragments are almost energy independent. However, for the doubly charged fragment ions (N2 + + O2 +), the present data are found to be higher by almost a factor of four compared to the relative ionic fraction reported earlier at a very low impact energy. © 2012 American Physical Society.
Study of K-line radiation of thick titanium produced in collisions of keV electrons
(2011) Namita Yadav; Pragya Bhatt; Raj Singh; X. Llovet; R. Shanker
The characteristic K-line yields Y(E0) of a pure thick titanium (Z=22) element target are measured for 8-18keV electron impact and compared with the simulation calculations using PENELOPE code. A fair agreement between experiment and simulation results is found within the existing experimental uncertainty of measurements. The ratio F of indirectly produced characteristic Ti K X-ray yield to its total (directly+indirectly) yield is determined by employing an approximate analytical formulation of Hanson and Cowan (Hanson, H.P., Cowan, D.J., 1961. Phys. Rev. 124, 22-26). It is found that F changes strongly with impact energy E0 for normal angle of incidence in contrast to a mild change predicted by the simulation calculations. Furthermore, experimental and simulation results for peak to effective continuum ratio R of Ti K-line are compared and discussed for the energy range of impact of the present investigation. © 2011 Elsevier Ltd.
Total M-shell X-ray yields from a thick Pt target irradiated by 10-25 keV electrons
(2012) Namita Yadav; Pragya Bhatt; Raj Singh; X. Llovet; R. Shanker
We present the measurements of total M-shell X-ray yields Y(E 0) of a pure thick Pt element produced by electron impact in the energy range of E 0 = 10-25 keV. Measurements are performed by employing a Si-PIN photo diode X-ray detector as an energy-dispersive spectrometer (EDS). The experimental data for Y(E 0) are compared with the corresponding predictions from Monte Carlo (MC) calculations using the general purpose MC code PENELOPE. A good agreement is obtained between the experiment and the MC calculations for the variation of Y(E 0) with impact energy both in shape and in magnitude within the experimental uncertainty of about 12.5%. © 2011 Elsevier B.V. All rights reserved.