Narrow Energy Spectrum Research Articles

SiO2 sputtering by low-energy Ar+, Kr+, and Xe+ ions in plasma conditions

Modern plasma technologies applied for nm-size devices require localizing ions’ impact within up to atomic layer to avoid uncontrollable damage of underlying structures. The decrease in energy of ions incident on a surface is one of the ways to achieve this. In this work, SiO2 sputtering by Ar+, Kr+, and Xe+ ions at energies of 20–200 eV was studies in the low-pressure ICP-discharge at a plasma density typical for plasma processing. The rf-bias waveform tailoring due to high discharge asymmetry allowed generating and controlling ion narrow energy spectrum with FWHM 5±2 eV. Real time in-situ control over ion composition and flux as well as sputtering rate provided accurate determination of the SiO2 sputtering yield, . It is shown that at ion energy above ∼70 eV, the “classical” kinetic sputtering mechanism prevails. In this case, rapidly grows with ion energy, while decreasing with the decrease in ion mass. Below ∼70 eV, the change of is strongly slow down while the sputtering yield still stays high enough (>10-3), demonstrating the plasma impact on the sputtering mechanism. The obtained trends of under the plasma exposure are discussed in light of possible SiO2 surface modification studied by AFM and angular XPS analysis.

Fast models for the evaluation of self-induced field effects in linear accelerators

Advanced applications of modern linear accelerators (linacs) strongly rely on the quality of the particle beams produced by such machines. Key examples of this kind include particle-driven radiation sources, plasma wakefield acceleration and lepton colliders for high energy physics. Such systems require use of high brightness electron beams implying the simultaneous coexistence of high peak currents, small transverse emittances and narrow energy spectra. The associated high phase space density results in strong self-induced electromagnetic fields, causing mutual interaction of the charged particles through space-charge and wakefield forces. These two sources of collective effects may be both present at significant levels, along with the strong applied fields, both longitudinal and transverse, found in high gradient linacs. As a result, beam dynamics studies capable of investigating the performance of a given device, as well as its operational limits, are crucial. However, the modeling of collective effects in simulation codes tracking large particle ensembles often requires significant, time-intensive numerical resources. In this paper we thus present alternative, simplified approaches for the description of space-charge and wakefield effects that permit streamlined computations. The features of such models are discussed, including their limitations and validating comparisons with existing tracking codes that utilize standard but more time-consuming approaches are shown. The beamlines we consider in our analyses are part of state-of-the-art facilities now under design for which new, challenging beam physics properties due to high fields and high beam brightness are expected. Thus, in addition to the introduction of new modeling techniques, our results provide an useful insight into advanced applications.

Effect of density gradients on the generation of a highly energetic and strongly collimated proton beam from a laser-irradiated Gaussian-shaped hydrogen microsphere

An investigation is made on the influence of the sharpness of the density gradients on the generation of energetic protons in a radially Gaussian density profile of a spherical hydrogen plasma. It is possible to create such density gradients by impinging a solid density target with a secondary lower intensity pulse, which ionizes the target and explodes it to create an expanded plasma target of lower effective density for the high-intensity main pulse to hit on. The density gradients are scanned in the near-critical regime, and separate regimes of proton motion are identified based on the density sharpness. An intermediate-density gradient [npeak≈(1.5–2.5)γnc] favors the generation of high energetic protons with narrow energy spectra that are emitted with better collimation from the target rear surface. Protons with energies exceeding 100 MeVs could be achieved using such modified plasma targets with circularly polarized lasers of peak intensities I0∼1020 W cm−2 and peak energy ∼10 J.

On the Alber equation for shoaling water waves

The problem of unidirectional shoaling of a water-wave field with a narrow energy spectrum is treated by using a new Alber equation. The stability of the linear stationary solution to small non-stationary disturbances is analysed; and numerical solutions for its subsequent long-distance evolution are presented. The results quantify the physics which causes the gradual decay in the probability of freak-wave occurrence, when moving from deep to shallow coastal waters.

Examination of a chamber of a large fusion facility by means of neutron activation techique with nanosecond neutron pulse generated by dense plasma focus device PF-6

Analysis of neutron fields around large chambers of main-stream nuclear fusion facilities is an important problem. The neutron emission from Dense Plasma Focus (DPF) device is anisotropic itself. It may be additionally distorted by an architecture details of the hall and construction elements of a facility hosting the particular DPF setup. The compact plasma focus device PF-6 is a quasi-point source producing nanosecond pulse of about 108 2.5-MeV neutrons with narrow energy spectrum. It was used for examination of a chamber of the large plasma installation PF-1000. The construction of the PF-1000 chamber simulates neutron scattering and absorption taking place in a main-stream fusion facility. The silver-based activation monitors were used for a survey of the neutron emission field in all directions. Such a device can be further used for inertial confinement fusion (ICF) and tokamak chambers examination and for calibration of its neutron diagnostics.

Study of the laser-plasma acceleration of ion beams with enhanced quality: The effects of nanostructured targets

Production of high-quality ion beams by intense laser–plasma interactions represents a rapidly evolving field of interest. In this paper, a nanostructured target is proposed to generate laser-driven quasi-monoenergetic ion beams with considerably reduced energy spread and enhanced peak energy. Linearly polarized, 40-fs laser pulses of intensity 8.5 × 1020 W cm−2 were considered to irradiate simple carbon foil and nanostructured targets. The proposed target consists of a thin layer of relatively high-Z atom (Ti) with a depression on its back surface which is filled by a nanosize disc of a low-Z atom (C). Reliable and reproducible results of multi-parametric Particle-in-Cell simulations show that by using a composed nanostructured target with optimum physical properties, a quasi-monoenergetic ion beam can be generated with a narrow band energy spectrum peaking at energies higher than 20 MeV. In addition, the forward-accelerated beam of low-Z carbon ions exhibits a considerably reduced transverse emittance in comparison with the ion beam obtained in the condition of a simple foil. The proposed nanostructured target can efficiently contribute to the generation of high-quality ion beams which are critical in newly growing applications and physics of laser-plasma accelerators.

Fabrication of Fe-doped birnessite with tunable electron spin magnetic moments for the degradation of tetracycline under microwave irradiation

Manganese oxides exhibit an excellent microwave absorption performance that could increase the degradation efficiency of organic pollutants in contaminated water. Incorporation of various transition metals into manganese oxides could bring about changes in their crystal structure and improve their physicochemical performance. In this work, a better microwave absorption material was obtained by adjusting and controlling the electron spin magnetic moments of Fe-doped birnessite. The powder X-ray diffraction, inductive coupled plasma emission spectrometer, X-ray photoelectron spectroscopy, and network analyses were performed to characterize the crystal structure, chemical composition, valence and content of the elements, and the microwave absorption performance of the obtained samples. Doping Fe into birnessite resulted in little changes to their crystal structure. The narrow energy spectrum of Fe (2p) revealed that the doped Fe was in the form of Fe (III) in birnessite structure. As the content of Fe (III) increased, the content of Mn (III) decreased accordingly. Substitution of Mn (III) by Fe (III) in the birnessite crystal lattice, confirmed by combining the characterization analyses with structure refinements for each doped sample, increased the overall numbers of unpaired electrons in birnessite structure, resulting in a higher electron spin magnetic moment and better microwave response. Compared with the non-doped sample, Fe-doped birnessite improved the efficiency of tetracycline degradation, which proved that Fe-doped birnessite indeed had better response towards the microwave, and thus, could be utilized for better removal of organic pollutants under microwave irradiation.

Observation of narrow energy spectrum electron flows on the INTERBALL Tail Probe and their possible relation with the spacecraft charging

The paper describes cases of observations of narrow energy spectrum electron flows up to 500 eV on the INTERBALL Tail Probe. About 30 events were registered in 1996 on the night side of the Earth predominantly in 03h–06h local time sector. Quasimonochromatic electrons (QME) were registered by all 8 spectrometer channels oriented along the spacecraft meridian with angles of the field of view centers relative to sunward direction from 11° to 169°. Quasimonochromatic electrons were observed simultaneously with large fluxes of high temperature magnetospheric electrons. The dependences of QME energy on both fluxes and energy of high-energy magnetospheric electrons were observed in every event. The ratio of full width at half height (FWHH) to mean energy of QME was ~20%. This electron component with quasimonochromatic energy probably was originated on the spacecraft surface. The registered energy of QME was apparently due to difference of potentials between spacecraft surface from which electron beam originated and the location of electron spectrometer.

Ion acceleration with a narrow energy spectrum by nanosecond laser-irradiation of solid target.

In laser-driven plasma, ion acceleration of aluminum with the production of a quasi-monoenergetic beam has occurred. A useful device to analyze the ions is the Thomson parabolas spectrometer, a well-known diagnostic that is able to obtain information on charge-to-mass ratio and energy distribution of the charged particles. At the LENS (Laser Energy for Nuclear Science) laboratory of INFN-LNS in Catania, experimental measures were carried out; the features of LENS are: Q-switched Nd:YAG laser with 2 J laser energy, 1064 nm fundamental wavelengths, and 6 ns pulse duration.

Enhancement of proton acceleration by frequency-chirped laser pulse in radiation pressure mechanism

The transition from hole-boring to light-sail regime of radiation pressure acceleration by frequency-chirped laser pulses is studied using particle-in-cell simulation. The penetration depth of laser into the plasma with ramped density profile increases when a negatively chirped laser pulse is applied. Because of this induced transparency, the laser reflection layer moves deeper into the target and the hole-boring stage would smoothly transit into the light-sail stage. An optimum chirp parameter which satisfies the laser transparency condition, a0≈πnel/ncλ, is obtained for each ramp scale length. Moreover, the efficiency of conversion of laser energy into the kinetic energy of particles is maximized at the obtained optimum condition. A relatively narrow proton energy spectrum with peak enhancement by a factor of 2 is achieved using a negatively chirped pulse compared with the un-chirped pulse.

Particle-in-cell simulation of acceleration of ions to GeV energies in the interactions of an ultra-intense laser pulse with two-species targets

This contribution presents the results of one-dimensional particle-in-cell simulations of ion acceleration in the interactions of an ultra-intense (IL ∼ 1023 W cm−2) 130 fs laser pulse with thin hydrocarbon (CH) and hydride (CaH2, CsH, ErH3) targets. The influence of laser pulse polarization and target material on the mean ion energy and the ion energy spectrum is examined. It is shown that for circular polarization the ion energy spectra are narrower than those for the linear one and for this polarization protons from both CH and ErH3 targets of the areal mass density below 0.1 mg cm−2 have a very narrow energy spectrum of . However, for ErH3 targets, protons are clearly separated from Er ions and their mean energy equal to 9 GeV is several times higher than those for other studied targets.

Development of aC-band 6 MeV standing-wave linear accelerator

$C$-band low-energy electron linear accelerators are preferable for applications which require compact structure and accurate positioning. During the past two years, the accelerator laboratory of Tsinghua University has developed a $C$-band 6 MeV standing-wave biperiodic on-axis coupled linear accelerator. The weight of the accelerator is 7.4 kg and the length is about 39 cm, which includes the thermal-cathode gun and the flange for tests. High capture ratio, narrow energy spectrum, and small spot size have been achieved in the high power rf test, which shows good performance of this accelerator and its great potential for industrial and medical applications.

Poster — Wed Eve—13: Evaluation of Spectroscopic Compton Tomography Approach at 55 keV

The feasibility of reconstructing 2D electron density () images has been studied. The spectroscopic measurement of Compton scattered photons in a single projection computed tomography (CT) fan beam geometry at 55 keV was investigated. Images were reconstructed by backprojecting over circular paths. A line of energy sensitive detectors was placed outside of periphery of the primary x‐ray fan beam to record the number of scattered photons. The forward problem was simulated in Matlab using analytical equations containing the Klein‐Nishina scattering cross sections, electron density of the object and the solid angle between the point of scattering and the detector. A narrow energy spectrum was needed to achieve good resolution. As the FWHM of the incident spectrum increases, the location where the scatterer can be detected with high precision, decreases. The spatial resolution varies across the image and areas of higher spatial resolution are those which exhibit small angle scattering to the detectors close to the source. Thus various target/filter combinations were studied. The optimal spectrum can be generated by an x‐ray tube operated at 90 kVp with a tungsten target and filter consisting of 0.5mm tungsten and 1.2mm erbium. A point source and ideal detectors were used and no multiple scatter or attenuation was considered. Electron densities were reconstructed using this approach for two phantoms and good qualitative results were obtained. Our ultimate goal is to produce 3D images from scattered photons in a multi‐projection cone‐beam breast CT geometry.

Energy dependence of response of new high sensitivity radiochromic films for megavoltage and kilovoltage radiation energies

The purpose of this paper is to evaluate the energy dependence of the response of two new high sensitivity models of radiochromic films EBT and XR-QA. We determined the dose response curves of these films for four different radiation sources, namely, 6 MV photon beams (6 MVX), Ir-192, I-125, and Pd-103. The first type (EBT) is designed for intensity modulated radiation therapy (IMRT) dosimetry, and the second type (XR-QA) is designed for kilovoltage dosimetry. All films were scanned using red (665 nm) and green (520 nm) light sources in a charge-coupled device-based densitometer. The dose response curves [net optical density (NOD) versus dose] were plotted and compared for different radiation energies and light sources. Contrary to the early GAFCHROMIC film types (such as models XR, HS, MD55-2, and HD810), the net optical densities of both EBT and XR-QA were higher with a green (520 nm) than those with a red (665 nm) light source due to the different absorption spectrum of the new radiochromic emulsion. Both film types yield measurable optical densities for doses below 2 Gy. EBT film response is nearly independent of radiation energy, within the uncertainty of measurement. The NOD values of EBT film at 1 and 2 Gy are 0.13 and 0.25 for green, and 0.1 and 0.17 for red, respectively. In contrast, the XR-QA film sensitivity varies with radiation energy. The doses required to produce NOD of 0.5 are 6.9, 5.4, 0.7, and 0.9 Gy with green light and 19, 13, 1.7, and 1.5 Gy with red light, for 6 MVX, Ir-192, I -125, and Pd-103, respectively. EBT film was found to have minimal photon energy dependence of response for the energies tested and is suitable for dosimetry of radiation with a wide energy spectrum, including primary and scattered radiation. XR-QA film is promising for kilovoltage sources with a narrow energy spectra. The new high sensitivity radiochromic films are promising tools in radiation dosimetry.

A Multiple Scattering Theory for Proton Penetration

We extend the electron small-angle multiple scattering theory to protonpenetration. After introducing the concept of narrow energy spectra,the proton energy loss process is included in the proton deeppenetration theory. It precisely describes the whole process of protonpenetration. Compared to the Monte Carlo method, this method maintains thecomparable precision and possesses much higher computational efficiency.Thus, it shows the real feasibility of applying this algorithm toproton clinical radiation therapy.

Microdosimetric interpretation of the photon energy response of LiF:Mg,Ti detectors.

Photon energy response of MTS-N (LiF:Mg,Ti) detectors (TLD Poland) and of MTS-N detectors sensitised with 200 Gy of 60Co gamma rays, followed by UV irradiation (sMTS-N), has been determined using X rays with narrow energy spectra, in the energy range from 20 to 300 keV. The over-response of LiF:Mg,Ti detectors for X rays (relative TL efficiency eta = 1.1) can be explained as an ionisation density effect. Low energy X rays produce short electron tracks, which locally deposit a high radiation dose and, consequently, lead to an enhanced (supralinear) response. This over-response has not been observed in sensitised MTS-N where supralinearity in the response after gamma ray doses above 1 Gy is not seen. Using the dose-response curves measured for MTS-N detectors after 137Cs gamma ray irradiation and local doses calculated using Monte Carlo generated electron tracks, it was possible to predict the relative TL effectiveness for different X ray energies. The calculation procedure can be applied to predict the photon energy response of LiF:Mg,Ti detectors in an arbitrary photon field.

Determination of Photon Backscatter from Several Calibration Phantoms

The photon backscatter factor for several irradiation phantoms was determined by calculational and experimental methods and compared with a phantom constructed of International Commission on Radiation Units and Measurements (ICRU) four-element tissue. Calculations of the photon backscatter factor over the energy range from 10 to 2000 keV were performed using the MCNP 4A code for each of the phantoms. Measurements of the backscatter factor were carried out using a thin-walled ionisation chamber that was exposed to X-ray beams with narrow energy spectra. The calculations and measurements agreed to within uncertainties and demonstrated that the backscatter factors over this energy range for a water-filled phantom recommended by the International Organization for Standardization (ISO) and a phantom constructed of tissue-equivalent (RS-I) plastic are nearly the same as that of the ICRU tissue reference phantom. However, the backscatter from a polymethylmethacrylate (PMMA) phantom was up to about 8% higher. In addition, it was found that a composite phantom of polytetrafluoroethylene (PTFE) and PMMA also produced a backscatter factor quite similar to that of the ICRU tissue reference phantom.

Correlated pair conversion in heavy-ion collisions at the Coulomb barrier.

As a model for understanding the appearance of narrow lines in positron singles and electron-positron sum energy spectra found in heavy-ion collisions close to the Coulomb barrier, we propose the following scenario: After the Coulomb scattering both heavy nuclei are very likely in low (\ensuremath{\le}2 MeV) excited states, which subsequently decay by correlated monoenergetic (${\mathit{e}}^{+}$,${\mathit{e}}^{\mathrm{\ensuremath{-}}}$) pair conversion (MPC) and internal conversion (IC) processes thereby producing an asymptotic (${\mathit{e}}^{+}$,${\mathit{e}}^{\mathrm{\ensuremath{-}}}$) pair with the sum energy of the two nuclear transitions. We only consider processes where the two nuclear transitions are correlated by an intermediate bound electron state (the 1\ensuremath{\sigma} orbital) of the nuclear scattering system. Since this orbital is only weakly time dependent and persists for \ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}19}$ s, we obtain rather narrow, ${\mathrm{\ensuremath{\Gamma}}}_{{\mathit{e}}^{+}{\mathit{e}}^{\mathrm{\ensuremath{-}}}}$\ensuremath{\sim}20 keV, sum energy lines for the (${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$) pair from this process. We, however, find that correlated E0-E0-transitions have cross sections at least 8 orders of magnitude too small and can safely be excluded as the cause of the narrow sum energy spectra observed experimentally. A rough estimate of the cross section indicates that correlated E1-E1 transitions occur with a substantially larger cross section and should thus be investigated in detail.

Energy distributions of ions produced by electron-stimulated desorption

We have measured the initial kinetic energy distributions of ions produced by electron bombardment of various oxides and halides. The instrument used allows ions directly ejected from the sample surface to be distinguished from ions formed by electron impact in the gas phase. Singly and multiply charged positive ions of species present in the matrix as anions and cations were desorbed by high energy (∼ 11 keV) electron impact. Directly desorbed positive halogen ions show a narrow, low energy peak, consistent with conventional models of electron stimulated desorption (ESD). In addition, some of the cation species exhibited similar narrow energy spectra. Charge states up to +6 were observed for the halides; with the exception of F 2+ and Cl 2+, multiple charge states were due to electron impact ionization of desorbed neutrals. Charge states up to +4 were seen for silicon from electron-bombarded SiO 2; energy distributions of Si +, Si 2+ and Si 3+ showed that these species were desorbed directly from the surface. The energy distributions of O + and O 2+ ions ejected from SiO 2 are relatively wide, compared to the energy distribution of Si + ions. In contrast, O + ions ejected from TiO 2 have a much narrower energy distribution, like those observed for the halogen ions.

The physics of field emission in the context of vacuum microelectronics

It is well known that surface treatments and cathode geometry affect the stability and strength of field emission (FE). The development of sub-micron electron sources requires research on the strength, stability and controllability of emission from areas of a few nanometres in diameter. The influence of cathode radius and surface layers on FE has been studied theoretically showing strong deviations from Fowler-Nordheim behaviour at low voltages. The interaction of a tunnelling electron with the charge density in the emitter, is also responsible for modifications in the surface potential. There is a characteristic response time for changes in the charge density and therefore it is of importance to be able to estimate the time that an electron spends tunnelling. An approach to the problem using Stochastic Mechanics is introduced and discussed in the context of other theories of the tunnelling time. Electron transport in a semiconductor depends on the doping concentration. Results are presented on the narrow width of the FE energy distrubution and explained by the narrow energy spectrum of the supply of electrons to the surface for highly doped silicon.