Characteristics of highly charged ions produced at Kobe EBIS under modulated operation

The electron beam ion source (Kobe EBIS) has been developed to perform modification of surfaces using highly charged ions (HCIs) at the Kobe University, Japan. Recent study revealed that periodic intermission of electron beam improves charge state distribution of extracted ions. The period of intermission is in the order of 100 ms, and the width of beam-off time is 1 ms or less. This operational mode (pulse mode) makes it possible to produce Ar¹⁵⁺ to Ar¹⁷⁺ effectively, whereas the charge is limited less than 14+ under the ordinary operational mode with direct current (DC) electron beam. A spike of HCIs with a peak current in the order of nA is also observed at each moment of electron beam off. The measurement of the time evolution of Ar¹⁶⁺ intensity around the timing of mode change revealed that the intensity of extracted Ar¹⁶⁺ changes slowly after mode change with a time constant of few seconds.     

Dissociative ionization of the H2O molecule bombarded by swift singly charged and slow highly charged ions

Fragment ion energy spectra of the water molecule have been measured in conventional crossed-beam experiments by the impact of 46 keV/u energy, singly charged ions (SCIs) and 4.3 keV/u energy, highly charged ions (HCIs). Double differential cross sections have been determined and a comparative analysis has been performed. We found that the fragmentation spectra for SCIs and HCIs are very similar, indicating that both collisions lead to the same fragmentation channels. This suggests that the Coulomb explosion of the water molecule is dominantly determined by the charge state of the transient molecular ions, and it is almost independent from the primary ionization mechanism. Differences were observed not only between the SCI and HCI impact-induced fragmentation cross sections, but between those obtained by the 60 keV N⁶⁺ and 70 keV O⁷⁺ projectiles. The differences were attributed to the selectivity of the electron capture process for HCIs. Multiple target ionization cross sections have been deduced from the fragment ion spectra. We found contributions of up to fivefold ionization for SCIs and up to sixfold ionization for HCIs.     

Influence of ambient gas and its pressure on the laser-induced breakdown spectroscopy and the surface morphology of laser-ablated Cd

The ablation of Cd has been performed by employing Q-switched Nd: YAG 10 ns laser pulses with a central wavelength of 1064 nm for a pulsed energy of 200 mJ under various ambient environments of argon, air and helium. The optical emission spectroscopy of Cd plasma has been studied under different filling pressures of shield gases ranging from 5 torr to 760 torr using LIBS spectrometer system. The effect of different gases and their pressures on the intensity of spectral emission, electron temperature and density of the laser-produced plasma has been investigated. SEM analysis has been performed to investigate the dependence of surface morphological changes of an irradiated target on the nature and pressure of an ambient gas. A strong correlation has revealed the vital role of electron temperature and density of laser-induced plasma for the surface modification of Cd. These results strongly indicate that the nature and pressure of the ambient atmosphere is one of the controlling factors of the plasma characteristics, as well as the factors related to the laser energy absorption for surface modification.     

Observation of Nano-Dots on HOPG Surface Induced by Highly Charged Arq+ Impact

Highly charged ions （HCIs） have huge potential energy due to their high charge state. When a HCI reaches a solid surface, its potential energy is released immediately on the surface to cause a nano-scale defect. Thus, HCIs are expected to be useful for solid-surface modifications on the nano-scale. We investigate the defects on a highly oriented pyrolytic graphite （HOPG） surface induced by slow highly charged Ar^q＋ ions with impact energy of 20-2000qeV with scanning probe microscopy （SPM）. In order to clarify the role of kinetic and potential energies in surface modification, the nano-defects are characterized in lateral size and height corresponding to the kinetic energy and charge state of the HCIs. Both the potential energy and kinetic energy of the ions may influence the size of nano-defect. Since potential energy increases dramatically with increasing charge state, the potential energy effect is expected to be much larger than the kinetic energy effect in the case of extremely high charge states. This implies that pure surface modification on the nano-scale could be carried out by slow highly charged ions. The mean size of nano-defect region could also be controlled by selecting the charge state and kinetic energy of HCI.     

EUV spectra of europium—Chasing for spectral lines of P- to Ar-like ions

The understanding of atomic structure implies a combination of accurate measurements and a reliable theoretical framework. Atomic structure computations are employed to bridge the many gaps in the experimental data, but their results need to be tested by measurements. We have selected extreme ultraviolet spectra of europium (Eu, Z = 63) for such tests. We study the emission spectra of Eu ions produced and excited in an electron beam ion trap by observation with highly resolving spectrographs. General purpose atomic structure computations help us at disentangling the spectra and identifying the emission of specific charge state ions. At the same time, the wavelength data provide a tool to judge the quality of the computations. The present study concentrates on ions of the charge states q = 45+ to q = 49+ or P- to Ar-like ions of Eu.     

Cold highly charged ions in a cryogenic Paul trap

Narrow optical transitions in highly charged ions (HCIs) are of particular interest for metrology and fundamental physics, exploiting the high sensitivity of HCIs to new physics. The highest sensitivity for a changing fine structure constant ever predicted for a stable atomic system is found in Ir^17?+?. However, laser spectroscopy of HCIs is hindered by the large (～ 10⁶ K) temperatures at which they are produced and trapped. An unprecedented improvement in such laser spectroscopy can be obtained when HCIs are cooled down to the mK range in a linear Paul trap. We have developed a cryogenic linear Paul trap in which HCIs will be sympathetically cooled by ⁹Be^?+? ions. Optimized optical access for laser light is provided while maintaining excellent UHV conditions. The Paul trap will be connected to an electron beam ion trap (EBIT) which is able to produce a wide range of HCIs. This EBIT will also provide the first experimental input needed for the determination of the transition energies in Ir^17?+?, enabling further laser-spectroscopic investigations of this promising HCI.     

X-Rays emission by high-intensity pulsed lasers irradiating thin foils at PALS laboratory

X-rays and forward ion emission from laser-generated plasma in the Target Normal Sheath Acceleration regime of different targets with 10-μm thickness, irradiated at Prague Asterix Laser System (PALS) laboratory at about 10¹⁶ W/cm² intensity, employing a 1,315 nm-wavelength laser with a 300-ps pulse duration, are investigated. The photon and ion emissions were mainly measured using Silicon Carbide (SiC) detectors in time-of-flight configuration and X-ray streak camera imaging. The results show that the maximum proton acceleration value and the X-ray emission yield growth are proportional to the atomic number of the irradiated targets. The X-ray emission is not isotropic, with energies increasing from 1 keV for light atomic targets to about 2.5 keV for heavy atomic targets. The laser focal position significantly influences the X-ray emission from light and heavy irradiated targets, indicating the possible induction of self-focusing effects when the laser beam is focalized in front of the light target surface and of electron density enhancement for focalization inside the target.     

Enhanced soft X-ray emission from carbon nanofibers irradiated with ultra-short laser pulses

A comparative experimental study of the X-ray emission in the water-window spectral region has been performed using carbon nanofibers (CNFs) of different sizes and graphite plate targets, irradiated with ultra-short (Ti:sapphire) laser pulses. More than an order of magnitude enhancement in the X-ray yield is observed from CNFs of 60-nm diameter with respect to graphite targets. The X-ray emission from CNFs of 160-nm diameter was also high. The integrated X-ray yield of these carbon-based targets scales with the laser intensity (I _L) as I_L ^{~ 1.3-1.4}I_{\mathrm{L}}^{\sim 1.3-1.4} in the intensity range of 4×10¹⁶–4×10¹⁷ W/cm². The effect of the laser pulse duration on the X-ray emission from the CNFs was also studied by varying the pulse duration from 45 fs up to 3 ps at a constant fluence of 2×10⁴ J/cm². The optimum laser pulse duration for maximum X-ray emission increases with the diameter of the CNFs used. The results are explained from physical considerations of heating and hydrodynamic expansion of the CNF plasma in which resonance field enhancement takes place while passing through two times the critical density. The results add to the efforts towards achieving an efficient low-cost water-window X-ray source for microscopy.     

Physical and chemical modifications of PET surface using a laser-plasma EUV source

Extreme ultraviolet (EUV) radiation is the electromagnetic radiation ranging from vacuum ultraviolet to soft X-rays. A single EUV photon carries enough energy to ionize any atom or molecule. The penetration depth of the radiation in any material is very short, ranging from tens to hundreds nanometers. Intense EUV pulses can remove material from the surface or modify its morphology or/and chemical structure. In this work, the radiation from a laser-plasma EUV source based on a double-stream gas-puff target was used for surface modification of polyethylene terephthalate (PET). The PET samples were irradiated with the EUV pulses emitted from krypton plasma and focused with a gold-plated ellipsoidal collector. The spectrum of the focused radiation covered the wavelength range from 9 to 70 nm. The PET samples were irradiated for 1 s–2 min at a 10-Hz repetition rate. Surface morphology of polymer samples after irradiation was investigated using a scanning electron microscope. Changes in chemical surface structure of the irradiated samples were investigated using an X-ray photoelectron spectroscopy. Different kinds of surface microstructures were obtained depending on the EUV fluence in a single pulse and the total EUV fluence. XPS measurements also revealed a modification of the chemical structure.     

Performance and early applications of a versatile double aberration-corrected JEOL-2200FS FEG TEM/STEM at Aalto University

Applications relevant to carbon based nano-materials have been explored using a newly installed JEOL-2200FS field emission gun (FEG) (scanning) transmission electron microscope (S)TEM which is integrated with two CEOS aberration correctors for both the TEM image-forming and the STEM probe-forming lenses. The performance and utility of this newly commission hardware has been reviewed with a particular focus on operation at an acceleration voltage of 80 kV, thus bringing the primary electron beam voltage below the knock-on threshold for carbon materials and opening up a range of possibilities for the study of carbon-based nanostructures in the aberration-corrected electron microscope. The ability of the microscope to obtain both atomic TEM images and high-quality electron diffraction patterns from carbon nanotubes was demonstrated. The chiral structure of a double-walled carbon nanotube was determined from its diffraction pattern. The aberration corrected TEM imaging technique facilitates a unique approach to accurate determination of single-walled carbon nanotube diameters. On the other hand, the probe-corrected high angle annular dark field (HAADF) STEM imaging performance allows for the detection of single gold atoms at 80 kV and was used to study the graphite interlayer spacing in a multi-walled carbon nanotube.     

Influence of high-energy electron irradiation on field emission properties of multi-walled carbon nanotubes (MWCNTs) films

The effect of very high energy electron beam irradiation on the field emission characteristics of multi-walled carbon nanotubes (MWCNTs) has been investigated. The MWCNTs films deposited on silicon (Si) substrates were irradiated with 6 MeV electron beam at different fluence of 1×10¹⁵, 2×10¹⁵ and 3×10¹⁵ electrons/cm². The irradiated films were characterized using scanning electron microscope (SEM) and micro-Raman spectrometer. The SEM analysis clearly revealed a change in surface morphology of the films upon irradiation. The Raman spectra of the irradiated films show structural damage caused by the interaction of high-energy electrons. The field emission studies were carried out in a planar diode configuration at the base pressure of ∼1×10⁻⁸ mbar. The values of the threshold field, required to draw an emission current density of ∼1 μA/cm², are found to be ∼0.52, 1.9, 1.3 and 0.8 V/μm for untreated, irradiated with fluence of 1×10¹⁵, 2×10¹⁵ and 3×10¹⁵ electrons/cm². The irradiated films exhibit better emission current stability as compared to the untreated film. The improved field emission properties of the irradiated films have been attributed to the structural damage as revealed from the Raman studies.     

Surface structure of quark stars with magnetic fields

We investigate the impact of magnetic fields on the electron distribution of the electrosphere of quark stars. For moderately strong magnetic fields of B ∼ 10¹³ G, quantization effects are generally weak due to the large number density of electrons at surface, but can nevertheless affect the photon emission properties of quark stars. We outline the main observational characteristics of quark stars as determined by their surface emission, and briefly discuss their formation in explosive events termed as quark-novae, which may be connected to the r-process.     

Vision of a fully laser-driven ${\sf n\gamma}{-}{\sf m\gamma}$ collider

The use is suggested of a laser-accelerated dense electron sheet with an energy of (E=[(g)\tilde] mc²E=\tilde{\gamma} mc^2) as a relativistic mirror to reflect coherently a second laser with photon energy ħω, generating by the Doppler boost high-energy γ photons with $ \hbar \omega ' = 4\tilde \gamma ^2 \hbar \omega $ \hbar \omega ' = 4\tilde \gamma ^2 \hbar \omega and short duration [A. Einstein, Annalen der Physik 17, 891 (1905); D. Habs et al., Appl. Phys. B 93, 349 (2008)]. Two of these counter-propagating γ beams are focused by the parabolically shaped electron sheets into the interaction region with small, close to diffraction-limited, spot size. Comparing the new nγ-mγ collider with former proposed γγ collider schemes we achieve the conversion of many photon-pairs in a small space-time volume to matter-antimatter particles, while in the other discussed setups only two isolated, much more high-energetic photons will be converted, reaching in the new approach much higher energy densities and temperatures. With a γ-field strength somewhat below the Schwinger limit we can reach this complete conversion of the γ bunch energy into e⁺e^- or quark-antiquark q[`(q)]q\bar{q}-plasmas. For a Bose-Einstein condensate (BEC) [A. Einstein, Physikalisch-mathematische Klasse (Berlin) 22, 261 (1924); A. Einstein, Physikalisch-mathematische Klasse (Berlin) 22, 3 (1925); A. Griffin, D.W. Snoke, S. Stringari, Bose-Einstein Condensation (Cambridge University Press, 1995)] final state or for the Cooper pair ground state at higher densities [A.J. Leggett, Quantum Liquids, Oxford Graduate Texts (Oxford University Press, 2006)] the strong induced transition into this coherent state is of special interest for single-cycle γ pulses. Due to annihilation these cold coherent states are very short-lived. For γ beams with photon energies of 1–10 keV the rather cold e<sup>+</sup>e<sup>-</sup>-plasma or e<sup>+</sup>e<sup>-</sup>-BEC expands to a cold dense aggregate of positronium (Ps) atoms, where the production of Ps molecules is discussed. For photon energies of 1–10 MeV we discuss the production of a cold induced π<sup>0</sup>-BEC followed by the formation of molecules. For the direct population of higher q[`(q)]q\bar{q} densities we can study condensates of color-neutral mesons with enhanced population. For a γγ collider with several-cycle laser pulses the following cycles heat up the fermion-antifermion f[`(f)]f\bar{f} system to a certain temperature. Thus we can reach high energy densities and temperatures of an e⁺e^-γ plasma, where the production of hadrons in general or the quark-gluon phase transition can be observed. Within the long-term goal of very high photon energies of about 1 GeV in the nγ-mγ-collider, even the electro-weak phase transition or SUSY phase transition could be reached.     

Space-selective modification of the magnetic properties of transparent Fe3+-doped glass by femtosecond-laser irradiation

We have demonstrated spatially selective modification of the magnetic properties of transparent iron-oxide-doped glass by femtosecond- (fs-) laser irradiation and subsequent annealing. A near-infrared fs-laser beam with a wavelength of 775 nm was focused 1 mm below the surfaces of glass samples. This produces absorption peaks due to the formation of hole-trap centers in the irradiated region. Transparency was recovered after annealing at 450°C. A ferrimagnetic component was observed in the M–H curve even at room temperature, whereas the diamagnetic component dominated in the M–H curve of the as-prepared glass sample. This indicates that fs-laser irradiation enhanced the magnetization in the irradiated area. The irradiated and annealed glass sample also exhibited superparamagnetic blocking in the temperature dependence of the magnetization with a blocking temperature higher than room temperature. This change in magnetism is presumably due to local crystallization of ferrimagnetic nanoparticles, such as magnetite, induced by fs-laser irradiation and annealing. The magnetic and optical properties of glass that had been annealed but not irradiated by a fs-laser beam remained unchanged.     

Parity Nonconserving (PNC) Electroweak Radiative Corrections for Highly Charged Ions (HCI)

We derive the electroweak radiative corrections to the basic PNC atomic transition amplitude for highly charged hydrogenlike ions. In the case of highly charged ions (HCI) effects of strong fields are reflected by the momentum transfer q ² involved. It is of the order q ²≈m _e ² inHCI, while q ²≈0 inneutral atoms. This may open the possibility to search for “new physics” beyond the Standard Model and to test the Standard Model in experiments with HCI. This revised version was published online in July 2006 with corrections to the Cover Date.     

Critical role of laser wavelength on carbon films grown by PLD of graphite

The structure of thin films deposited by pulsed laser ablation (PLD) is strongly dependent on experimental conditions, like laser wavelength and fluence, substrate temperature and pressure. Depending on these parameters we obtained various kinds of carbon materials varying from dense, mainly tetrahedral amorphous carbon (ta-C), to less compact vertically oriented graphene nano-particles. Thin carbon films were grown by PLD on n-Si 〈100〉 substrates, at temperatures ranging from RT to 800°C, from a rotating graphite target operating in vacuum. The laser ablation of the graphite target was performed by a UV pulsed ArF excimer laser (λ=193 nm) and a pulsed Nd:YAG laser, operating in the near IR (λ=1064 nm). The film structure and texturing, characterised by X-ray diffraction analysis, performed at grazing incidence (GI-XRD), and the film density, evaluated by X-ray reflectivity measurements, are strongly affected both by laser wavelength and fluence and by substrate temperature. Micro-Raman and GI-XRD analysis established the progressive formation of aromatic clusters and cluster condensation into vertically oriented nano-sized graphene structures as a direct function of increasing laser wavelength and deposition temperature. The film density, negatively affected by substrate temperature and laser wavelength and fluence, in turn, results in a porous bulk configuration and a high macroscopic surface roughness as shown by SEM characterisation. These structural property modifications induce a relevant variation also on the emission properties of carbon nano-structures, as evidenced by field emission measurements. This work is dedicated to our friend Giorgio who passed away 20th August.     

Ionization density dependence of the curve shape and ratio of blue to UV emissions of Al2O3:C optically stimulated luminescence detectors exposed to 6-MV photon and therapeutic proton beams

In this work we characterized the dose and linear energy transfer (LET) (ionization density) dependence of commercial Al₂O₃:C optically stimulated luminescence (OSL) detectors (OSLDs) exposed to clinical photon and proton beams. We characterized the dose-dependence of the OSL signal, OSL curve shape, and the relative intensities of the blue and ultraviolet (UV) OSL emission bands using different readout protocols and beam qualities. We irradiated OSLDs with absorbed doses ranging from 0.1 Gy to 100 Gy in a 6-MV photon beam and from 0.1 Gy to 50 Gy in 140- and 250-MeV proton beams. Readouts were done with both continuous-wave (CWOSL) and pulsed (POSL) stimulation. The linearity of the OSLD dose–response depended on readout protocol and radiation type. Improved linearity was found for OSLDs irradiated with beams of increased LET, and for OSL signals containing only the blue emission band of Al₂O₃:C (which remained linear for doses up to 10 Gy for 140-MeV proton beam irradiations). The OSL curve shape did not vary with dose in the low-dose region (below 5 Gy depending on readout protocol), but beyond this, curves decayed more rapidly with increasing dose. Similarly, the ratio of blue to UV emission band intensities in the OSL signal did not vary with dose up to 5 Gy (depending on readout protocol), beyond which the ratio decreased with increasing dose. Because both the OSL curve shape and the ratio of blue to UV emission intensities have been investigated as potential parameters for measurements of LET, the constancy of these two quantities at doses relevant to radiotherapy is encouraging for the potential development of novel OSL methods to measure LET. Our findings are expected to contribute to the development of (i) improved readout protocols for commercially available Al₂O₃:C OSLDs and (ii) methods to measure radiation quality and LET.     

Nano-crater formation on a Si(1 1 1)-(7 × 7) surface by slow highly charged ion-impact

Using scanning tunneling microscopy (STM) and time of flight secondary ion mass spectrometry (TOF/SIMS), we observed radiation effects on a Si(1 1 1)-(7 × 7) surface in the collision of a single highly charged ion (HCI) with a charge state q up to q = 50. The STM observation with atomic resolution revealed that a nanometer sized crater-like structure was created by a single HCI impact, where the size increased rapidly with q. The secondary ion yields also increased with q in which multiply charged Si ions (Siⁿ⁺) were clearly observed in higher q HCI-collisions. The sputtering mechanism is briefly discussed, based on the so-called Coulomb explosion model.     

Evaluation of the dosimetric characteristics of a radiophotoluminescent glass dosimeter for high-energy photon and electron beams in the field of radiotherapy

We aimed to evaluate the suitability of a glass dosimeter (GD) for high-energy photon and electron beams in experimental and clinical use, especially for radiation therapy. We examined the expanded dosimetric characteristics of GDs including dose linearity up to 500 Gy, uniformity among GD lots and for individual GDs, the angular dependence, and energy dependence of 4 therapeutic x-ray qualities. In addition, we measured the dosimetric features (dose linearity, uniformity, angular dependence, and energy dependence) of the GD for electron beams of 10 different electron energy qualities. All measurements with the exception of dose linearity for photon beam were performed in a water phantom. For high-energy photon beams, dose linearity has a linear relationship for a dose ranging from 1 to 500 Gy with the coefficient of determination; R² of 0.998. The uniformity of each GD of dose measurements was within ±0.5% for four GD lots and within ±1.2% for 80 GDs. In terms of the effects of photon beam angle, lower absorbed doses of within 1.0% were observed between 60° and 105° than at 90°. The GD energy dependence of 4 photon beam energy qualities was within ±2.0%. On the other hand, the result of the dose linearity for high-energy electron beams showed well fitted regression line with the coefficient of determination; R² of 0.999 between 6 and 20 MeV. The uniformity of GDs exposed to the nominal electron energies 6, 9, 12, 16, and 20 MeV was ±1.2%. In terms of the angular dependence to electron beams, absorbed doses were within 2.0% between 60° and 105° than at 90°. In evaluation of the energy dependence of the GD at nominal electron energies between 5 and 20 MeV, we obtained responses between 1.1% and 3.5% lower than that for a cobalt-60 beam. Our results show that GDs can be used as a detector for determining doses when a high-energy photon beam is used, and that it also has considerable potential for dose measurement of high-energy electron beam.     

IR laser-induced metal ablation and dielectric breakdown in benzene

IR laser-induced irradiation of Co and Ni sheets leads to metal ablation and when carried out in gaseous benzene (1–10 Torr) to dielectric breakdown which is accompanied by metal plasma and deposition of nanostructured carbon. The metal plasma (metal atoms and ions) as well as transients of benzene decomposition (neutral and ionic carbon and C₂ species) were detected by optical emission spectra. Different features of carbon deposited at benzene pressure 5–10 Torr on a distant glass and on the irradiated metal sheets were revealed by FTIR, Auger and Raman spectroscopy and electron microscopy and explained by surface assisted carbonization. The reported process suggests a novel approach to gas-phase carbonization of organic molecules.