Scintillation light produced by low-energy beams of highly-charged ions

Measurements have been performed of scintillation light intensities emitted from various inorganic scintillators irradiated with low-energy beams of highly-charged ions from an electron beam ion source (EBIS) and an electron cyclotron resonance ion source (ECRIS). Beams of xenon ions Xe^q+ with various charge states between q = 2 and q = 18 have been used at energies between 5 and 17.5 keV per charge generated by the ECRIS. The intensity of the beam was typically varied between 1 and 100 nA. Beams of highly charged residual gas ions have been produced by the EBIS at 4.5 keV per charge and with low intensities down to 100 pA. The scintillator materials used are flat screens of P46 YAG and P43 phosphor. In all cases, scintillation light emitted from the screen surface was detected by a CCD camera. The scintillation light intensity has been found to depend linearly on the kinetic ion energy per time deposited into the scintillator, while up to q = 18 no significant contribution from the ions’ potential energy was found. We discuss the results on the background of a possible use as beam diagnostics, e.g. for the new HITRAP facility at GSI, Germany.     

Single electron capture by state-selected multiply charged Arq+ ions (q = 3,4)

State-selective single electron capture by multiply charged Ar^q+ ions (q = 3, 4) has been studied in rare gas targets with the aid of double translational spectroscopy. The energy gain spectra have been measured using different projectile beams, either extracted from an electron beam ion source or produced by electron capture reactions in an additional preparation cell. The influence of metastable projectile states as well as the reaction channel identification in complex systems are discussed.     

PL and XPS study of radiation damage created by various slow highly charged heavy ions on GaN epitaxial layers

Photoluminescence (PL) spectrum, in conjunction with X-ray photoelectron spectroscopy (XPS) is used to evaluate the surface damage of GaN layer by highly-charged Xe^q+ (18 ? q ? 30), Ar^q+ (6 ? q ? 16) and Pb^q+ (q = 25,35) ions. The intensity of PL emission of GaN layer, including near band-edge peak and yellow luminescence, decreases with increasing fluence and charge state of the incident ions. Finally the PL emission is completely quenched after irradiation to high fluences at high charge state. A new peak at 450 nm appeared in PL spectra of the specimens irradiated with Xe¹⁸⁺, Ar⁶⁺ and Ar¹¹⁺, indicating that radioactive recombination within donor-acceptor pairs (DAPs) during irradiation. After irradiation, XPS spectra show N deficient or Ga rich on GaN surface and XPS spectra of Ga3d core levels indicate spectral peak evidently shifts from a Ga-N to Ga-Ga and Ga-O bond. The relative content of Ga-N bond decreases and the content of Ga-Ga bond increases with the increase of ion fluence and ion charge state. The binding energy of Ga3d_5/2 electron corresponding to Ga-Ga bond of the irradiated GaN film is found to be smaller than that of metallic Gallium (Ga⁰), which can be attributed to irradiation damage.     

The effect of internal conversion of nuclear fission products on their degree of ionization

The charge-state distribution of ²⁴¹ Pu-fission products (A = 131–145) was measured at kinetic energies of E_k = 67, 70 and 73 MeV. Smearing of the fine structure and separation of atomic and nuclear ionization effects of fission products was achieved by again changing their charge t < 5 × 10⁻¹⁰s after the fission event. The contribution of nuclear ionization effect has been shown to depend on excitation energy and on the structure of the nuclei. For some masses this contribution reaches 50%.     

Proton sputtering from polycrystalline Al surface interacting with highly charged ions at grazing incidence angle

Sputtering processes of protons from a polycrystalline Al surface interacting with Ar^q+ (q = 3-14) ions at a grazing incidence angle (∼0.5°) were investigated. The intensity of protons (I_H) detected in coincidence with scattered Ar atoms was measured as a function of q. I_H saturated at q ? 10, although it increased rapidly with q at 3 ? q ? 8. The angular distribution of protons with low kinetic energy (?2 eV) began to deviate from the cosine distribution and assumed a rather flat equidistribution as q increased. To analyze the sputtering processes of protons at the grazing incidence angle, a modified model of the “above-surface potential sputtering model” was proposed by considering image acceleration of projectile ions.     

Separation of potential and kinetic electron emission from Si and W induced by multiply charged neon and argon ions

The total secondary electron emission yields, γ_T, induced by impact of the fast ions Ne^q+ (q = 2-8) and Ar^q+ (q = 3-12) on Si and Ne^q+ (q = 2-8) on W targets have been measured. It was observed that for a given impact energy, γ_T increases with the charge of projectile ion. By plotting γ_T as a function of the total potential energy of the respective ion, true kinetic and potential electron yields have been obtained. Potential electron yield was proportional to the total potential energy of the projectile ion. However, decrease in potential electron yield with increasing kinetic energy of Ne^q+ impact on Si and W was observed. This decrease in potential electron yield with kinetic energy of the ion was more pronounced for the projectile ions having higher charge states. Moreover, kinetic electron yield to energy-loss ratio for various ion-target combinations was calculated and results were in good agreement with semi-empirical model for kinetic electron emission.     

Comparison of 41Ca analysis on 0.5 MV and 5 MV-AMS systems

A comparative study was made between the compact AMS system at the PSI/ETH Laboratory of Ion Beam Physics in Zurich with 0.5 MV terminal voltage and the 5 MV-AMS system at the Scottish Universities Environmental Research Centre (SUERC), Glasgow. Overall 34 urinary samples with ⁴¹Ca/⁴⁰Ca ratios in the range from 4 × 10^?11 to 3 × 10^?10 were processed to CaF₂ and aliquots of the same material were measured on both instruments.Measurements on the compact AMS system were performed in charge state 3+ achieving a transmission of 4% at 1.7 MeV beam energy. Under these conditions a suppression of the interference ⁴¹K is virtually impossible. However, samples with an excess of potassium can be identified by a shift of the ⁴¹Ca/⁴¹K peak in the ΔE ? E histogram of the gas ionization detector employed and a criterion for data rejection can be defined. An overall precision of ～4% and a ⁴¹Ca/⁴⁰Ca background level of 5 × 10^?12 have been reached.For studies with higher demands on the detection limit AMS systems like the one at SUERC are attractive: in charge state 5+ and using a gas stripper beam energy of 27 MeV, a transmission of 5%, a ⁴¹K suppression factor of ～500 and a ⁴¹Ca/⁴⁰Ca background level of 3 × 10^?14 are achieved.We demonstrate that both systems are well suited for large-scale ⁴¹Ca biomedical applications.     

Electron capture in Iq+ (q = 10–41) + He collisions at low energies

One electron capture processes in I^q+ (q = 10–41) +He collisions at low energies have been investigated. It is found that total cross sections for one-electron capture processes increase roughly linearly with increasing charge q of the incident ions and also increase with the square of the crossing radius R_c of the diabatic potential energy curves where the electron transfer takes place. These smooth increases are in contrast to those observed for ions with low charge q < 10.     

Polarization effects on the cross-section of ion-atom collisions

Coupled-channel cross-sections for electron capture, ionization and electron loss due to polarization effects are calculated. The maximum impact parameter for electron escape is analyzed within the classical framework. The probabilities of ionization and capture are analyzed simultaneously by a semi-empirical method. Differing from the n-body classical trajectory Monte Carlo method, the condition for electron escape is determined by Coulomb forces related to the two nuclei. This method can be used to calculate coupled-channel cross-sections rather than single-channel ones in other methods. Therefore the calculated results can be compared with experimental data directly. In the low energy range, neglecting the ionization effect, the single-capture cross-sections of hydrogen atoms induced by various partially-stripped ions were calculated. In the high energy range, neglecting the capture effect on ionization, the pure-ionization cross-sections of neon atoms induced by Ne^q+ (q = 4, 6, 8) and Ar^q+ (q = 4, 6, 8, 10) at an incident energy E = 1.05 MeV/u were calculated. Good agreement was found between our calculation and experimental data in the literature. This method had been partially applied for intermediate energy successfully.     

Single- and multiple-electron loss cross-sections for fast heavy ions colliding with neutrals: Semi-classical calculations

Extensive calculations of single, multiple and total electron-loss cross-sections of fast heavy ions in collisions with neutral atoms are performed in the semi-classical approximation using the DEPOSIT code based on the energy deposition model and statistical distributions for ionization probabilities. The results are presented for Ar¹⁺, Ar²⁺, Kr⁷⁺, Xe³⁺, Xe¹⁸⁺, Pb²⁵⁺ and U^q+ (q = 10, 28, 39, 62) ions colliding with H, N, Ne, Ar, Kr, Xe and U atoms at energies E > 1 MeV/u and compared with available experimental data and the n-particle classical-trajectory Monte Carlo (nCTMC) calculations. The results show that the present semi-classical model can be applied for estimation of multiple and total electron-loss cross-sections within accuracies of a factor of 2.From calculated data for the total electron-loss cross-sections σ_tot, their dependencies on relative velocity v, the first ionization potential I₁ of the projectile and the target atomic number Z_A are found and a semi-empirical formula for σ_tot is suggested. The velocity range, where the semi-classical approximation can be used, is discussed.     

Electron capture in intermediate-to-fast heavy ion collisions with neutral atoms

Electron capture processes of heavy ions, like Ge^q+, Xe^q+, Pb^q+, and U^q+, respectively, with the charge q ≈ 10–40, occurring in collisions with gaseous targets are considered in the E = 0.1–100 MeV/u projectile energy range. Calculations of single-electron capture cross sections are performed using the CDW and the CAPTURE computer codes. These are compared with available experimental data and CTMC (Classical-Trajectory Monte Carlo) calculations. Although the overall agreement is found to be within a factor of two, in some cases of heavy many-electron projectiles, e.g., U²⁸⁺ + N₂, Ar collisions, experimental cross sections at high energies are far smaller than theoretical predictions. Moreover, for these collision systems the observed energy dependencies are quite different from each other. Possible reasons for this behavior and how the theoretical models can be improved are discussed.     

Range distributions of 50–400 keV Hg+in amorphous silicon and Si-Ar binary targets

Range distributions of 50–400 keV Hg⁺ in amorphous Si and Si-Ar binary targets have been investigated by Rutherford backscattering spectrometry. The Si(100) wafers were amorphized by means of 150 keV Ar⁺ irradiation to a dose of 2 × 10¹⁵ ions/cm². To produce Si-Ar binary targets, the Si(100) wafers were implanted with 150 keV Ar⁺ to a dose of 3 × 10¹⁷ ions/cm². 50–400 keV Hg⁺ were introduced into amorphous Si and Si-Ar binary targets in increments of 50 keV. Parallel scanning of the Hg⁺ beams was used. The measured ranges and range stragglings have been compared to the Biersack theory. The results show that good agreements are found between the experimental and theoretical projected ranges for both Si and Si-Ar, but the predicted range straggling for both Si and Si-Ar are systematically lower than the experimental results in the case of a first order treatment. After correcting for second order energy loss terms, a better agreement for the range straggling is obtained.     

Analysis of Si crystal irradiated by highly-charged Ar ions using RBS-channeling technique

An ion-beam is one of the powerful tools applied in nanotechnology and nanoscience. It is expected that the application of highly-charged ion (HCI) beams, which have a higher reactivity in materials, would yield further developments in these fields. For effective applications of HCI beams, it is important to investigate any modifications of irradiated materials from a microscopic point of view. For this purpose, an irradiation-induced defect in a single crystal was analyzed by means of Rutherford backscattering-channeling (RBS) technique. In order to induce defects, Ar⁶⁺ and Ar⁹⁺ beams with an energy of 100 keV were irradiated onto a single crystal of Si. By means of a simple analysis, the depth distribution of disordered Si atoms induced by the irradiation was extracted from the observed RBS-C spectra. The present result implies that the productivity of defects in a Si crystal is enhanced for Ar⁹⁺ ions compared with Ar⁶⁺ ions in a limited region of the surface.     

Ionization of He,Ne and Ar atoms by 1.05 MeVamuCq+(q = 2–6) and Arq+(q = 4–14) ion impact

Total “apparent” ionization cross sections and partial cross section of the production of highly ionized recoil ions of He, Ne and Ar in 1.05 MeV/amu highly charged C^q+(q = 2–6) and Ar^q+(q = 4,6,10–14) ion impact have been measured and compared with theoretical prediction and other experimental data. The total ionization cross sections have been found to scale as the square of the effective charge of projectile ions $\text{q}{}^1$, as predicted by theories.     

Neutron dosimetry and damage calculations for the TRIGA MARK-II reactor in Vienna

In order to improve the source characterization of the reactor, especially for recent irradiation experiments in the central irradiation thimble, neutron activation experiments were made on 16 nuclides and the neutron flux spectrum was adjusted using the computer code STAY'SL. The results for the total, thermal and fast neutron flux density at a reactor power of 250 kW are as follows: 2.1 × 10¹⁷, 6.1 × 10¹⁶ (E < 0.55 eV), 7.6 × 10¹⁶ (E > 0.1 MeV) and 4.0 × 10¹⁶ (E > 1 MeV) m⁻² s⁻¹. respectively. Calculated damage energy cross sections and gas production rates are presented for selected elements.     

High-intensity variable-energy positron beam for surface and near-surface studies

We report on the construction and performance of a magnetically confined slow positron beam. The beam is designed for surface physics and near-surface disorder studies and it operates in ultrahigh vacuum with a base pressure of 3 nPa. The beam uses a Co-58 source with a backscattering W(110) single crystal moderator. We get 4.7 × 10⁶ slow positrons per second striking the sample using a 300 mCi source corresponding to a fast-to-slow positron conversion efficiency 0.28 ± 0.02%. The energy of the incident positrons can be varied from 1 eV to 35 keV. Some applications concerning positron re-emission and diffusion as well as positronium desorption from a surface state are shown.     

Low energy sputtering of eutectic Ag/Cu alloys

Experiments were carried out on the sputtering of a eutectic Ag/Cu (N_Ag/N_Cu = 1.5) alloy by normally incident 110 eV Ar⁺ ions. The collected sputtered flux was analyzed by Rutherford backscattering (RBS). Material sputtered normal to the surface at a fluence of 7 × 10¹⁹ Ar/cm² was stoichiometric (N_Ag/N_Cu ≈ 1.5), but became increasingly Cu rich at larger angles of ejection up to 75° at which N_Ag/N_Cu was about 1.09. The overall ejection was slightly Cu rich (N_Ag/N_Cu ≈ 1.3). RBS analysis of the sputtered alloy surfaces indicated they were Cu rich to a depth of about 1500 Å, with the greatest Cu content at about 450 Å depth (up to 56% Cu), the effect depending on the fluence of the bombarding Ar⁺ ions. Scanning electron micrographs of the sputtered alloy surfaces showed dense microtopographic formations.     

Isotopic composition of boron secondary ions as a function of ion-beam fluence

We have examined the effects of target mass on sputtered material using 100 keV argon and neon to sputter an elemental target comprising the two naturally occurring isotopes of boron. At low beam fluences (≈ 1 × 10¹⁵ions/cm²) a light-isotope secondary enhancement is observed relative to steady-state secondary ion yields collected at higher beam fluences. The enhancement (46.1%o for Ne⁺ irradiation and 51.8%o for Ar⁺ irradiation) is large compared to the predictions of analytical theories and is independent of variations in surface potential, chemical effects, and surface impurities. This effect is consistent with an explanation based on an energy and momentum asymmetry in the collision cascade.     

A dense electron target for the study of electron-ion collisions

A new high current (0.5 A), high density (0.3 $\text{A}\text{cm}{}^2$) electron gun has been designed for crossed beams electron-ion-experiments to yield high signal to background counting rates in the energy range 10–1000 eV. Comprehensive computer runs were performed to assure a uniform electrostatic potential in spite of the high electronic space charge. Trapped ions can be used to compensate this space charge, because their contribution to the counting rate is well discriminated below the threshold of the process to be studied. The overlap integral is determined by sweeping the whole electron gun perpendicular to both beams across the ion beam, which is possible, because the interaction region potential extends in perpendicular direction of both beams and sweeping therefore does not affect ion beam transmission. The accuracy and the reproducibility of cross section measurements with this new gun is demonstrated by a comparison with data from Harrison's group, exhibiting “perfect” agreement.