Kα Emission Research Articles

Effect of pre-ionization on Cu Kα emission from a small plasma focus

One of the most important factors for optimizing the X-ray emission from a plasma focus device is the use of pre-ionization before triggering the device. In this paper, we investigated the effect of pre-ionization on Cu Kα and total X-ray emission from a Mather-type plasma focus device. The Cu Kα and total X-ray yield is measured as a function of the deuterium-filling pressure by using PIN-diode detectors. The maximum Cu Kα emission is found to be 1.95 J for the α-source and 1.62 J without the α-source, with corresponding average efficiencies of 0.049 and 0.06% without and with the α-source, respectively. The maximum total X-ray yield in 4π geometry is estimated to be about 2.64 and 2.12 J, with corresponding efficiencies of about 0.08 and 0.064% with and without the α-source, respectively.

Characterizing the energy distribution of laser-generated relativistic electrons in cone-wire targets

Transport of relativistic electrons in a solid Cu wire target has been modeled with the implicit hybrid particle-in-cell code LSP to investigate the electron energy distribution and energy coupling from the high-intensity, short-pulse laser to electrons entering to the wire. Experiments were performed on the TITAN laser using a 1.5 mm long Cu wire attached to a Au cone tip at the laser intensity of 1 × 1020 W/cm2 which was irradiated into the cone. The simulated Cu Kα wire profile and yields matched the measurements using a two-temperature energy distribution. These modeling results show that the cold component of the energy spectrum can be determined with ±100 keV accuracy from the fit to the initial experimental fall-off of the Kα emission while the simulated profiles were relatively insensitive to the hotter component of the electron distribution (>4 MeV). The slope of measured escaped electrons was used to determine the hotter temperature. Using exponential energy distributions, the laser-to-electron-in-wire coupling efficiencies inferred from the fits decreased from 3.4% to 1.5% as the prepulse energy increases up to 1 J. The comparison of the energy couplings using the exponential and Relativistic Maxwellian distribution functions showed that the energy inferred in the cold component is independent of the type of the distribution function.

Enhancement of Kα emission through efficient hot electron generation in carbon nanotubes on intense laser pulse irradiation

Near complete absorption of the energy of intense ultra-short laser pulses (45 fs, intensity ∼1.6 × 1016 to 2.5 × 1017 W/cm2) is observed in carbon nanotubes deposited on a planar molybdenum substrate. The hollow structure of the nanotube plasma facilitates resonant electric field enhancement during its ionization phase. This resonantly enhanced localized field at a density much larger than the critical density nc leads to efficient hot electron generation, which results in enhanced Kα emission of Mo at 17.5 keV. It is observed that for nanotubes, depending on the degree of hollowness, there is an optimum laser intensity for maximum x-ray enhancement compared to a planar uncoated target.

RXTE observation of recent flaring activity from the transient X-ray pulsar 2S 1553−542

We summarize the analysis of observations made by the Rossi X-ray Timing Explorer and the Swift/Burst Alert Telescope at the time of the recent flaring activity of the transient X-ray pulsar 2S 1553−542. The binary system is characterized by the projected semimajor axis ax sin i of 168 ± 18 light-seconds, the binary orbital period Porb of 29.56 ± 0.53 d and the X-ray mass function fx(M) of 5.3 ± 1.2 M⊙. We also found Pspin= 9.2829 ± 0.0003 s with s s−1, i.e. a spin-up rate of yr−1. The spin period is found to decrease more slowly than that of most other X-ray pulsars. As the flare decays with time, the structure and X-ray counts in the pulse profile showed considerable variability. However, no significant structural variability is seen in single-peaked pulse profiles in different energy bands. Pulsed fraction decreases with the decay of the flare but its value at different energy bands showed a discontinuity. Using X-ray spectroscopy at different orbital cycles, we have detected the presence of the Fe Kα emission line in the spectra with a minimum significance of 3σ. Apart from a consistent increase in blackbody temperature, other spectral parameters are found to be stable throughout orbital cycles. From detailed spectral and timing analysis, we concluded that the X-ray flare is spectrally hard and we confirmed the nature of the compact object as a transient, accretion-powered, spin-up X-ray pulsar with a Be-type companion.

Application of a transmission crystal x-ray spectrometer to moderate-intensity laser driven sources

In the pursuit of novel, laser-produced x-ray sources for medical imaging applications, appropriate instrumental diagnostics need to be developed concurrently. A type of transmission crystal spectroscopy has previously been demonstrated as a survey tool for sources produced by high-power and high-energy lasers. The present work demonstrates the extension of this method into the study of medium-intensity laser driven hard x-ray sources with a design that preserves resolving power while maintaining high sensitivity. Specifically, spectroscopic measurements of characteristic Kα and Kβ emissions were studied from Mo targets irradiated by a 100 fs, 200 mJ, Ti: sapphire laser with intensity of 10(17) W/cm(2) to 10(18) W∕cm(2) per shot. Using a transmission curved crystal spectrometer and off-Rowland circle imaging, resolving powers (E/ΔE) of around 300 for Mo Kα(2) at 17.37 keV were obtained with an end-to-end spectrometer efficiency of (1.13 ± 0.10) × 10(-5). This sensitivity is sufficient for registering x-ray lines with high signal to background from targets following irradiation by a single laser pulse, demonstrating the utility of this method in the study of the development of medium-intensity laser driven x-ray sources.

Time-gated measurements of electron beam generated Kα emission lines from brass planar wire array implosions

The relative amount of Kα radiation emitted by partially ionized copper and zinc from planar wire arrays on the 1 MA Zebra generator at the University of Nevada, Reno, does not correspond to the composition (70% copper, 30% zinc) of the array's brass wires. For example, the copper Kα line at 8 keV was observed to be much stronger than would be expected from zinc's Kα radiation at 8.6 keV, but this ratio also reversed, more emission from zinc than from copper, during the X-ray pulse. An excess of Kα photons from copper is consistent with a beam of electrons with energies above copper's K-edge but below that of zinc, but the opposite case is perplexing. Preferential ablation of Zn over Cu early in the current pulse could be a contributing factor, but opacity effects are not. Synthetic spectra for brass computed by a non-LTE collisional-radiative model that includes an electron beam component compare well with observed K-shell spectra, and suggest the parent ions of the Kα emission. These ionization stages are consistent with L-shell spectra. This comparison also indicates that the total energy in the electron beam increases towards the end of the radiation pulse while the electron temperature decreases. The crucial diagnostic in this measurement is a time-resolved X-ray spectrometer based on a LiF crystal.

Electronic structure of the Si-C-N amorphous films

The results of the investigation of amorphous a-SiCxNy films of various compositions by the ultra-soft X-ray emission spectroscopy are presented. The Si L2, 3, C Kα, and N Kα emission spectra, which reflect the partial densities of states Si 3s3d, C 2p, and N 2p, respectively, have been measured and analyzed. It has been established that the Si-N chemical bond is similar in type to the Si-C chemical bond and completely substitutes for the Si-C bonds in thin a-SiCxNy films as the oxygen concentration increases. A similar effect is associated with the local agglomeration and subsequent clusterization of carbon atoms in the SiN-enriched regions of the internal volume of the film. A high ability of the a-SiCxNy films to oxidation in air has been established. This has been confirmed by a satisfactory approximation of the Si L2, 3 X-ray emission spectra of a-SiCN with the use of the superposition of the spectra of γ-Si3N4 and Si2N2O. The relative weight coefficients of the spectra of γ-Si3N4, which have been used to approximate the spectra of thin a-SiCxNy films, are proportional to the values of the Young’s modulus for different values of compositions (different values of x and y).

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Erratum: On the efficiency of production of the Fe Kα emission line in neutral matter

Erratum: On the efficiency of production of the Fe Kα emission line in neutral matter

Pb+ implanted SiO2 probed by soft x-ray emission and absorption spectroscopy

The results of soft-x-ray O Kα emission (XES) and O 1s absorption spectroscopy (XAS) measurements of Pb-implanted glassy and crystalline silica are presented. The x-ray O Kα (2p→1s electron transition) emission spectra of SiO2 were recorded before and after Pb-implantation with the energy of 30keV and ion fluence 5×1016 ion/cm2. It was found that XES O Kα of implanted samples is sensitive to the disordering degree of the oxygen sublattice. The transformations and peculiarities of the spectra shape of implanted samples are explained by the disordering and amorphization effects in the structure of Pb-implanted SiO2. Comparing the XES O Kα of reference a-SiO2, Pb-implanted SiO2 and binary glassy PbO–SiO2 system, it was established that the ion-beam treatment of oxide matrix does not generate an oxidized Pb as PbO4-type structural units. The energy band gap of 9.2eV well coincides with previously reported data and was evaluated qualitatively with the help of overlaying the XES O Kα and XAS O 1s to the common energy scale for Pb-implanted SiO2 and binary glassy PbO–SiO2.

Modelling the broad-band spectra of MCG-6-30-15 with a relativistic reflection model

The spectrum of the bright Seyfert I galaxy MCG-6-30-15 shows a broad Fe Kα emission line, generally interpreted to originate very close to the central black hole which must then have a high spin. The observed X-ray variability is driven by a power-law component, with little variability found in the iron line energy band. The disconnection of continuum and line variability may be a consequence of strong gravitational light bending. The X-ray spectrum of MCG-6-30-15 is however complex, strongly modified at soft X-ray energies by warm absorbers, which some workers have extended to build an absorption-dominated model of the whole source behaviour. The absorption model interprets the whole X-ray spectrum without a relativistic reflection component and attributes most variability to the warm absorbers. We re-examine the XMM–Newton, Chandra and Suzaku data taken in 2001, 2004 and 2006, respectively, and construct a model consisting of several warm absorbers confirmed by spectral analysis, together with a relativistically blurred reflection component that explains both the soft and hard excesses as well. The model works well on data from all epochs, demonstrating that the reflection model provides a consistent interpretation of the broad-band spectrum of MCG-6-30-15.

Weighing super-massive black holes with narrow Fe Kα line

It has been suggested that the narrow cores of the Fe Kα emission lines in Active Galactic Nuclei (AGNs) are likely produced in the torus, the inner radius of which can be measured by observing the lag time between the V and K band flux variations. In this paper, we compare the virial products of the infrared time lags, and the narrow Fe Kα widths for 10 type 1 AGNs, with the black hole masses from other techniques. We found the narrow Fe Kα line width is in average 2.6 +0.9−0.4 times broader than expected, assuming an isotropic velocity distribution of the torus at the distance measured by the infrared lags. We propose the thick disk model of the torus may explain the observed larger line width. Another possibility is the contamination by emission from the broad line region or the outer accretion disk. Alternatively, the narrow iron line might originate from the inner most part of the obscuring torus within the sublimation radius, while the infrared emission may be from the outer cooler part. We note the correlations between the black hole masses based on this new technique and those based on other known techniques are statistically insignificant. We argue that this could be attributed to the small sample size and the very large uncertainties in the measurements of iron K line widths. The next generation of X-ray observatories could help verify the origin of the narrow iron Kα line and the reliability of this new technique.

Evidence for a circumnuclear and ionized absorber in the X-ray obscured broad-line radio galaxy 3C 445

Here we present the results of a Suzaku observation of the broad-line radio galaxy 3C 445. We confirm the results obtained with the previous X-ray observations which unveiled the presence of several soft X-ray emission lines and an overall X-ray emission which strongly resembles a typical type 2 Seyfert galaxy despite of the optical classification as an unobscured active galactic nucleus. The broad-band spectrum allowed us to measure for the first time the amount of reflection (R∼ 0.9) which together with the relatively strong neutral Fe Kα emission line (EW ∼ 100 eV) strongly supports a scenario where a Compton-thick mirror is present. The primary X-ray continuum is strongly obscured by an absorber with a column density of NH= 2–3 × 1023 cm−2. Two possible scenarios are proposed for the absorber: a neutral partial covering or a mildly ionized absorber with an ionization parameter logξ∼ 1.0 erg cm s−1. A comparison with the past and more recent X-ray observations of 3C 445 performed with XMM–Newton and Chandra is presented, which provided tentative evidence that the ionized and outflowing absorbers varied. We argue that the absorber is probably associated with an equatorial disc wind located within the parsec scale molecular torus.

Enhanced inner-shell x-ray emission by femtosecond-laser irradiation of solid cone targets

The possibility of enhancing inner-shell x-ray emission, especially Kα emission, by femtosecond-laser irradiation of solid cones instead of foils was investigated theoretically. In a model for hot electron (HE) transport and Kα x-ray generation, Kα emission from laser-irradiated solid cones and foils is investigated. As a complementarity to the model, the contributions from electric and magnetic fields generated by the HE current in solid cones and foils are discussed. The results indicate that the efficiency of HE energy conversion to Kα photons is improved and the optimum HE temperature is increased.

Discovery of a broad OVIII Lyα line in the ultracompact X-ray binary 4U 1543.624

Abstract We report the discovery of a broad emission feature at ∼0.7 keV in the spectra of the ultracompact X-ray binary 4U 1543−624, obtained with the high-resolution spectrographs of the XMM–Newton and Chandra satellites. We confirm the presence of the feature in the broad-band MOS2 spectrum of the source. As suggested before in the literature, the donor star in this source is a CO or ONe white dwarf, which transfers oxygen-rich material to the accretor, conceivably a neutron star. The X-rays reprocessed in this oxygen-rich accretion disc could give a reflection spectrum with O viii Lyα as the most prominent emission line. Apart from the feature at ∼0.7 keV, we confirm the possible presence of a weak emission feature at ∼6.6 keV, which was reported in the literature for this data set. We interpret the feature at ∼0.7 and ∼6.6 keV as O viii Lyα and Fe Kα emission, respectively, caused by X-rays reflected off the accretion disc in the strong gravitational field close to the accretor.

Molecular orientation and optical luminescence properties of soluble star shaped oligothiophene molecules for organic electronic applications

Angle resolved near-edge absorption fine structure measurements of the C 1s edge of a film of the star-shaped molecule 4(HPBT) show a high degree of order, with the molecules maintaining a largely planar geometry in an upright configuration. X-ray excited optical luminescence measurements showed a dependence of the luminescence spectrum on the excitation energy, with (0–0), (0–1), and (0–2) vibrational modes being clearly visible when exciting off-resonance; the appearance of these features under off-resonance is attributed to a scattering process that is quenched when the excitation approaches a core absorption resonance. The HOMO–LUMO gap is estimated from the luminescence measurements (2.28 eV), and this value is confirmed by the comparison of the C 1s absorption and C Kα emission spectra.

Refluxing of fast electrons in solid targets irradiated by intense, picosecond laser pulses

The propagation of fast electrons produced in the interaction of relativistically intense, picosecond laser pulses with solid targets is experimentally investigated using Kα emission as a diagnostic. The role of fast electron refluxing within the target, which occurs when the electrons are reflected by the sheath potentials formed at the front and rear surfaces, is elucidated. The targets consist of a Cu fluorescence layer of fixed thickness at the front surface backed with a propagation layer of CH, the thickness of which is varied to control the number of times the refluxing fast electron population transits the Cu fluorescence layer. Enhancements in the Kα yield and source size are measured as the thickness of the CH layer is decreased. Comparison with analytical and numerical modelling confirms that significant refluxing occurs and highlights the importance of considering this phenomenon when deriving information on fast electron transport from laser–solid interaction experiments involving relatively thin targets.

Intense and Reproducible Kα Emissions from Micron-Sized Kr Cluster Target Irradiated with Intense Femtosecond Laser Pulses

Intense and reproducible Kr Kα X-rays (∼12.7 keV) have been generated via the interaction between a 3 TW laser pulse and a micron-sized Kr cluster target. A single-photon counting technique with an X-ray charge-coupled device (X-ray CCD) was used for measurements of the X-ray energy spectrum in a single shot. At a laser irradiance of 8 ×1016 W/cm2, the averages and standard deviations of the total X-ray yield and Kα X-ray yield were equal to (6.6 ±1.1) ×107 and (6.8 ±2.6) ×105 photons/sr, respectively. When the X-ray energy spectrum was fit using a Maxwellian with an effective temperature, the temperature was estimated to be 1.62 ±0.08 keV. These results indicate that the X-ray energy spectrum was reproducible at 8 ×1016 W/cm2.

Mapping the ionization state of laser-irradiated Ar gas jets with multiwavelength monochromatic x-ray imaging

Two-dimensional monochromatic images of fast-electron stimulated Ar Kα and He-α x-ray self-emission have recorded a time-integrated map of the extent of Ar(≈6+) and Ar(16+) ions, respectively, within a high density (10(20) cm(-3) atomic density) Ar plasma. This plasma was produced by irradiating a 2 mm wide clustering Ar gas jet with an ultrahigh intensity (10(19) W/cm(2), 50 TW) Ti:sapphire laser operating at 800 nm. Spherically bent quartz crystals in the 200 (for Kα) and 201 (for He-α) planes were used as near-normal incidence reflective x-ray optics. We see that a large (830 μm long) region of plasma emits Kα primarily along the laser axis, while the He-α emission is confined to smaller hot spot (230 μm long) region that likely corresponds to the focal volume of the f/8 laser beam. X-ray spectra from a Bragg spectrometer operating in the von Hamos geometry indicate that the centroids of the Kα and He-α emission regions are separated by approximately 330 μm along the laser axis.

X-ray Spectroscopic Study on Photoluminescence Properties of Red Phosphor SrTiO3:Pr3+,Al

The electronic structure of the practical red phosphor SrTiO3;Pr3+,Al is studied by X-ray absorption and emission spectroscopy under ultraviolet (UV) irradiation. In contrast to the invariance of the oxygen Kα emission spectra, the titanium K-edge absorption spectra show two characteristic changes depending on UV energy, one of which is proportional to the photoluminescence excitation spectrum while the other is insensitive to it. With these results, a schematic energy diagram is proposed where the titanium 4p band pulled down by oxygen valence holes overlaps Pf 4f radiative centers situated just below the conduction band. The photoluminescence properties are briefly discussed in relation to the ferroelectric phase transition from an electronic viewpoint.