Mean Charge State Research Articles

Measurement of the equilibrium charge state distributions of Ni, Co, and Cu beams in Mo at 2 MeV/u: Review and evaluation of the relevant semi-empirical models

Equilibrium charge state distributions of stable 60Ni, 59Co, and 63Cu beams passing through a 1μm thick Mo foil were measured at beam energies of 1.84MeV/u, 2.09MeV/u, and 2.11MeV/u respectively. A 1-D position sensitive Parallel Grid Avalanche Counter detector (PGAC) was used at the exit of a spectrograph magnet, enabling us to measure the intensity of several charge states simultaneously. The number of charge states measured for each beam constituted more than 99% of the total equilibrium charge state distribution for that element. Currently, little experimental data exists for equilibrium charge state distributions for heavy ions with 19≲Zp,Zt≲54 (Zp and Zt, are the projectile’s and target’s atomic numbers respectively). Hence the success of the semi-empirical models in predicting typical characteristics of equilibrium CSDs (mean charge states and distribution widths), has not been thoroughly tested at the energy region of interest. A number of semi-empirical models from the literature were evaluated in this study, regarding their ability to reproduce the characteristics of the measured charge state distributions. The evaluated models were selected from the literature based on whether they are suitable for the given range of atomic numbers and on their frequent use by the nuclear physics community. Finally, an attempt was made to combine model predictions for the mean charge state, the distribution width and the distribution shape, to come up with a more reliable model. We discuss this new “combinatorial” prescription and compare its results with our experimental data and with calculations using the other semi-empirical models studied in this work.

The influence of the mean charge state on the Coulomb heating of fast diclusters through the Si〈1 1 1〉 direction

In the present work, we report a theoretical and experimental study of the Coulomb heating of H2+ and C2+ in Si〈111〉 channel, covering an energy range from 200keV/ion to 2200keV/ion. The experimental values for Coulomb heating were obtained by combining the Rutherford backscattering spectrometry (RBS) and the particle induced X-ray emission (PIXE) techniques under channeling conditions. Theoretical values were obtained by performing classical trajectory Monte-Carlo (CTMC) simulations of the ion paths inside the 〈111〉 Si channel, using Dirac–Hartree–Fock–Slater (DHFS) results for the interionic potential. As seen for the 〈110〉 case, it is shown that the use of a DHFS potential based on the ion mean charge states in amorphous targets leads to a disagreement between the Coulomb heating values and the expected potential energies stored in the dicluster prior to the Coulomb explosion. Therefore, a numerical procedure was used in order to calculate the mean charge state values for ions traveling in Si〈111〉. The use of the resulting charge states led to a linear relationship between the Coulomb heating values and the stored potential energy per ion of the diclusters. Finally, the Coulomb heating/stored potential energy ratio amounts to about 2/3, as expected from an isotropic Coulomb explosion.

X-ray spectroscopy: An experimental technique to measure charge state distribution during ion–solid interaction

Charge state distributions of 56Fe and 58Ni projectile ions passing through thin carbon foils have been studied in the energy range of 1.65–2.69 MeV/u using a novel method involving the X-ray spectroscopy technique. Interestingly the charge state distribution in the bulk shows Lorentzian behavior instead of usual Gaussian distribution. Further, different parameters of charge state distribution like mean charge state, distribution width and asymmetric parameter are determined and compared with the empirical calculations and ETACHA predictions. It is found that the X-ray measurement technique is appropriate to determine the mean charge state during the ion–solid interaction or in the bulk. Interestingly, empirical formalism predicts much lower mean charge states of the projectile ions compared to X-ray measurements which clearly indicate multi-electron capture from the target surface. The ETACHA predictions and experimental results are found to be comparable for the present energy regime.

Adding high time resolution to charge-state-specific ion energy measurements for pulsed copper vacuum arc plasmas

Charge-state-resolved ion energy–time distributions of pulsed Cu arc plasma were obtained by using direct (time-dependent) acquisition of the ion detection signal from a commercial ion mass-per-charge and energy-per-charge analyzer. We find a shift of energies of Cu2+, Cu3+ and Cu4+ ions to lower values during the first few hundred microseconds after arc ignition, which is evidence for particle collisions in the plasma. The generation of Cu+ ions in the later part of the pulse, measured by the increase of Cu+ signal intensity and an associated slight reduction of the mean charge state, points to charge exchange reactions between ions and neutrals. At the very beginning of the pulse, when the plasma expands into vacuum and the plasma potential strongly fluctuates, ions with much higher energy (over 200 eV) are observed. Early in the pulse, the ion energies observed are approximately proportional to the ion charge state, and we conclude that the acceleration mechanism is primarily based on acceleration in an electric field. This field is directed away from the cathode, indicative of a potential hump. Measurements by a floating probe suggest that potential structures travel, and ions moving in the traveling field can gain high energies up to a few hundred electron-volts. Later in the pulse, the approximate proportionality is lost, which is related to increased smearing out of different energies due to collisions with neutrals, and/or to a change of the acceleration character from electrostatic to ‘gas-dynamic’, i.e. dominated by pressure gradient.

26Al measurements below 500 kV in charge state 2+

The use of helium as stripper gas improved the measurement efficiency of compact AMS systems for many radionuclides significantly because of a higher mean charge state and reduced scattering losses compared with other conventional gases. Recent tests at the ETH 500kV AMS facility (Tandy) with aluminum have demonstrated that a transmission of more than 50% is achievable in the charge state 2+ at terminal voltages between 300 and 500kV. On the other hand the m/q interference of 13C1+ entering the detector at very high intensity has to be suppressed. Based on first positive results with a very simple absorber cell a more elaborate absorber detector configuration was designed and built in order to eliminate the carbon interference. The suppression of carbon with the new detector–absorber design has been studied extensively at 300kV (950keV) and 500kV (1550keV) and the results are compared with simulated data. With the new configuration an overall transmission for 26Al of more than 42% at 500kV and about 30% at 300kV terminal voltage is achieved, while 26Al/27Al blank ratios of aluminum targets in the range of 5–14·10−15 are measured.

Experimental characterization of a transition from collisionless to collisional interaction between head-on-merging supersonic plasma jetsa)

We present results from experiments on the head-on merging of two supersonic plasma jets in an initially collisionless regime for the counter-streaming ions. The plasma jets are of either an argon/impurity or hydrogen/impurity mixture and are produced by pulsed-power-driven railguns. Based on time- and space-resolved fast-imaging, multi-chord interferometry, and survey-spectroscopy measurements of the overlapping region between the merging jets, we observe that the jets initially interpenetrate, consistent with calculated inter-jet ion collision lengths, which are long. As the jets interpenetrate, a rising mean-charge state causes a rapid decrease in the inter-jet ion collision length. Finally, the interaction becomes collisional and the jets stagnate, eventually producing structures consistent with collisional shocks. These experimental observations can aid in the validation of plasma collisionality and ionization models for plasmas with complex equations of state.

Equilibrium and non-equilibrium charge-state distributions of 2.0 MeV/u carbon ions passing through carbon foils

Both equilibrium and non-equilibrium charge-state distributions were studied experimentally for 2.0MeV/u carbon ions after passing through carbon foils. Measured charge-state distribution established the equilibrium at a target thickness of 10μg/cm2 and this remained unchanged until a maximum target thickness of 98μg/cm2. The equilibrium charge-state distribution, the equilibrium mean charge-state, and the width and skewness of the equilibrium distribution were compared with predictions using existing semi-empirical formulae as well as simulation results, including the ETACHA code. It was found that charge-state distributions, mean charge states, and distribution widths for C2+, C3+, and C4+ incident ions merged into quasi-equilibrium values at a target thickness of 5.7μg/cm2 in the pre-equilibrium region and evolved simultaneously to the ‘real equilibrium’ values for all of the initial charge states, including C5+ and C6+ ions, as previously demonstrated for sulfur projectile ions at the same velocity (Imai et al., 2009). Two kinds of simulation, ETACHA and solution of rate equations taking only single electron transfers into account, were used, and both of them reproduced the measured charge evolution qualitatively. The quasi-equilibrium behavior could be reproduced with the ETACHA code, but not with solution of elementary rate equations.

Classical calculation of multiple ionization cross sections of noble gases near Bragg peak energies

In this paper, we extend our previous work of COBI model to study the multiple ionization and mean charge state of noble gases collided by heavy ions at energies not far from Bragg peaks of several to hundreds keV/amu. Being in good agreement with various experimental data and taking extremely small computational time make the model helpful to study the charge state distribution and multiple ionization of gases transforming to plasmas.

OBSERVATION OF HIGH IRON CHARGE STATES AT LOW ENERGIES IN SOLAR ENERGETIC PARTICLE EVENTS

The ionic charge states of solar energetic particles (SEPs) provide direct information about the source plasma, the acceleration environment, and their transport. Recent studies report that both gradual and impulsive SEP events show mean iron charge states Q Fe ~ 10-14 at low energies E ≤ 0.1 MeV nuc–1, consistent with their origin from typical corona material at temperatures 1-2 MK. Observed increases of Q Fe up to 20 at energies 0.1-0.5 MeV nuc–1 in impulsive SEPs are attributed to stripping during acceleration. However, Q Fe > 16 is occasionally found in the solar wind, particularly coming from active regions, in contrast to the exclusively reported Q Fe ≤ 14 for low energy SEPs. Here we report results from a survey of all 89 SEP events observed with Advanced Composition Explorer Solar Energetic Particle Ionic Charge Analyzer (SEPICA) in 1998-2000 for iron charge states augmented at low energy with Solar and Heliospheric Observatory CELIAS suprathermal time-of-flight (STOF). Nine SEP events with Q Fe ≥ 14 throughout the entire SEPICA and STOF energy range have been identified. Four of the nine events are impulsive events identified through velocity dispersion that are consistent with source temperatures ≥2 MK up to ~4 MK. The other five events show evidence of interplanetary acceleration. Four of them involve re-acceleration of impulsive material, whose original energy dependent charge states appear re-distributed to varying extent bringing higher charge states to lower energy. One event, which shows flat but elevated Q Fe ~ 14.2 over the entire energy range, can be associated with interplanetary acceleration of high temperature material. This event may exemplify a rare situation when a second shock plows through high temperature coronal mass ejection material.

Electronic interactions of medium-energy ions in hafnium dioxide

In this article, the electronic interaction of medium-energy ions with hafnium dioxide is studied. Stopping cross sections for He ions in the energy range from 30 to 160 keV have been measured in backscattering experiments from thin films of HfO${}_{2}$ on Si using time-of-flight medium-energy ion scattering. The observed energy loss for helium ions is found to be high compared to expectations from earlier results for hydrogen in HfO${}_{2}$, a result which indicates a contribution from energy-loss processes that is different from direct excitation of electron-hole pairs in close collisions. Furthermore, data exhibit a significant deviation from velocity proportionality. A discussion of the results, together with data from studies for H and He in SiO${}_{2}$, show characteristic differences which are traced back to different electronic structures of the target materials and their influence on the charge-exchange channels that are active in the interaction with helium. Charge exchange, in turn, via shifted mean-charge states, will influence the observed ionization along the ion track. The results can furthermore serve as reference values for ion-beam-based depth profiling at medium and low ion energies.

Coulomb heating behavior of fast light diclusters thorough the Si ⟨ 110 ⟩ direction: influence of the mean charge state

In this work we report on the results for the Coulomb heating of H+ 2, B+ 2 and C+ 2 diclusters traveling in Si 〈 110 ⟩ direction covering an energy range from 200 keV/ion to 2400 keV/ion. Those results were obtained by combining the Rutherford backscattering spectrometry (RBS) and the particle induced X-ray emission (PIXE) techniques. By comparing the present results to those obtained previously for ions traveling in the narrower Si ⟨ 100 ⟩ channel, several common features are observed for the Coulomb heating values; especially, they follow a linear relationship as a function of the stored potential per ion. However, at variance with previous results, it is shown that the use of a Dirac-Hartree-Fock-Slater (DHFS) potential based on the ion mean charge states in amorphous targets leads to a considerable disagreement between the Coulomb heating values and the expected potential energies stored in the dicluster prior to the Coulomb explosion. In order to investigate this problem, a numerical procedure was developed in order to calculate the mean charge state values for ions traveling under channeling conditions. The use of the resulting charge states led to a linear relationship between the Coulomb heating values and the stored potential energy per ion of the diclusters. Moreover, the Coulomb heating/stored potential energy ratio amounts to about 2/3, which is in full agreement with those results obtained for the Si ⟨ 100 ⟩ direction.

Heavy ion charge-state distribution effects on energy loss in plasmas

According to dielectric formalism, the energy loss of the heavy ion depends on its velocity and its charge density. Also, it depends on the target through its dielectric function; here the random phase approximation is used because it correctly describes fully ionized plasmas at any degeneracy. On the other hand, the Brandt-Kitagawa (BK) model is employed to depict the projectile charge space distribution, and the stripping criterion of Kreussler et al. is used to determine its mean charge state [Q]. This latter criterion implies that the mean charge state depends on the electron density and temperature of the plasma. Also, the initial charge state of the heavy ion is crucial for calculating [Q] inside the plasma. Comparing our models and estimations with experimental data, a very good agreement is found. It is noticed that the energy loss in plasmas is higher than that in the same cold gas cases, confirming the well-known enhanced plasma stopping (EPS). In this case, EPS is only due to the increase in projectile effective charge Q(eff), which is obtained as the ratio between the energy loss of each heavy ion and that of the proton in the same plasma conditions. The ratio between the effective charges in plasmas and in cold gases is higher than 1, but it is not as high as thought in the past. Finally, another significant issue is that the calculated effective charge in plasmas Q(eff) is greater than the mean charge state [Q], which is due to the incorporation of the BK charge distribution. When estimations are performed without this distribution, they do not fit well with experimental data.

Dependence of ion kinetic energy and charge on cluster size in multi-photon ionization of xenon clusters

Multi-photon ionization of xenon clusters created by nozzle expansion has been investigated by time-of-flight mass spectrometry. With infrared irradiation, cluster ions beyond the dimer are missing from the mass spectra, but there is a copious yield of multiply-charged atomic ions. With ultraviolet irradiation, multiple ionization is almost completely suppressed and singly ionized large cluster ions are detected. The mean cluster ion size under UV ionization shows an increasing trend with the stagnation pressure for nozzle expansion, as do the mean charge state and kinetic energy of atomic ions formed via IR irradiation. Interrogating the same cluster population by two different probes provides a corroboration between the effect of the size of the cluster in the ionization mechanism and the kinematic effects thereof. Furthermore, it provides evidence for the influence of the neutral cluster size on the formation of multiply charged ions by IR irradiation via the electron re-collision mechanism.

Classical calculation of multiple-ionization cross-sections of noble gases near Bragg peak energies

AbstractIn this paper, we extend our previous work of classical over barrier ionization (COBI) model to study the multiple-ionization and mean charge state of noble gases colliding with heavy ions at energies close to the Bragg peak region ranging up to some hundreds of keV/amu. The method we report is in good agreement with experimental data and offers the advantage of very small computation time. Therefore, this model will be extremely helpful to be included in numerical codes to calculate the charge state distribution in plasma.

Sulphur hexafluoride as a stripper gas for tandem accelerators

We have investigated sulphur hexafluoride as a stripper gas in tandem accelerators by using the ANTARES accelerator system at ANSTO to measure charge state distributions for this gas. Results are reported at 4MV terminal voltage for injected negative ions ranging from carbon to uranium oxide. For iodine and thorium the distributions are extended across a range of energies of practical use for accelerator mass spectrometry, ion beam analysis and other accelerator applications. Charge state distributions using sulphur hexafluoride are found to have mean charge states up to 1 charge unit higher than, and to be broader than, corresponding distributions for argon gas, except in the case of carbon beams. As a result, SF6 is shown to provide significantly higher yields for charge states of heavy ions above the mean charge state. We now perform actinide AMS measurements with 9% yield to the 5+ charge state, compared to 4–5% achieved previously with argon gas.

Heterogeneous behaviour of the lithium battery composite electrode LiFePO4

To improve the performance of the positive electrode in batteries, it is important to thoroughly characterize these materials during battery cycling, beyond just electrochemical characterization. Among the very few techniques available for operando studies, X-ray Absorption Spectroscopy (XAS) appears very powerful, with high spatial and time resolutions provided by synchrotron radiation. Moreover XAS is well known for its capabilities in charge transfer and structure determinations, both being the major changes induced by electrochemical cycling of electrode materials. By an appropriate combination of three XAS beamlines using a specially designed electrochemical cell, we have studied composite positive electrodes made with LiFePO4 as the active material. We directly observed the heterogeneity of the electrode during operation, some parts being delayed and others advanced, compared to the mean charge state of the electrode. A mapping of this heterogeneity was made at different length scales.

Modelling of Silicon in inertial confinement fusion conditions

In ICF experiments recently undertaken at the National Ignition Facility (NIF), the high-Z dopant in the capsule ablator material has been changed from Germanium to Silicon for some experiments [1]. In this paper two issues involving the modelling of Silicon in the ablator of an ICF capsule will be addressed. Firstly, we consider the differences between LTE and non-LTE modelling of Silicon in the prediction of the opacity and of the mean charge state. Secondly, the necessary size of the photon groups (or ‘bins’) used in the radiation transport modelling will be discussed, and radiative transport calculations with varying bin sizes will be presented in order to support the conclusions.

Energy Spectra, Composition, and Other Properties of Ground-Level Events During Solar Cycle 23

We report spacecraft measurements of the energy spectra of solar protons and other solar energetic particle properties during the 16 Ground Level Events (GLEs) of Solar Cycle 23. The measurements were made by eight instruments on the ACE, GOES, SAMPEX, and STEREO spacecraft and extend from ∼0.1 to ∼500–700 MeV. All of the proton spectra exhibit spectral breaks at energies ranging from ∼2 to ∼46 MeV and all are well fit by a double power-law shape. A comparison of GLE events with a larger sample of other solar energetic particle (SEP) events shows that the typical spectral indices are harder in GLE events, with a mean slope of −3.18 at >40 MeV/nuc. In the energy range 45 to 80 MeV/nucleon about ∼50 % of GLE events have properties in common with impulsive ^(3)He-rich SEP events, including enrichments in Ne/O, Fe/O, ^(22)Ne/^(20)Ne, and elevated mean charge states of Fe. These ^(3)He-rich events contribute to the seed population accelerated by CME-driven shocks. An analysis is presented of whether highly-ionized Fe ions observed in five events could be due to electron stripping during shock acceleration in the low corona. Making use of stripping calculations by others and a coronal density model, we can account for events with mean Fe charge states of ≈+20 if the acceleration starts at ∼1.24–1.6 solar radii, consistent with recent comparisons of CME trajectories and type-II radio bursts. In addition, we suggest that gradual stripping of remnant ions from earlier large SEP events may also contribute a highly-ionized suprathermal seed population. We also discuss how observed SEP spectral slopes relate to the energetics of particle acceleration in GLE and other large SEP events.

Effect of source tuning parameters on the plasma potential of heavy ions in the 18 GHz high temperature superconducting electron cyclotron resonance ion source

Plasma potentials for various heavy ions have been measured using the retarding field technique in the 18 GHz high temperature superconducting ECR ion source, PKDELIS [C. Bieth, S. Kantas, P. Sortais, D. Kanjilal, G. Rodrigues, S. Milward, S. Harrison, and R. McMahon, Nucl. Instrum. Methods B 235, 498 (2005); D. Kanjilal, G. Rodrigues, P. Kumar, A. Mandal, A. Roy, C. Bieth, S. Kantas, and P. Sortais, Rev. Sci. Instrum. 77, 03A317 (2006)]. The ion beam extracted from the source is decelerated close to the location of a mesh which is polarized to the source potential and beams having different plasma potentials are measured on a Faraday cup located downstream of the mesh. The influence of various source parameters, viz., RF power, gas pressure, magnetic field, negative dc bias, and gas mixing on the plasma potential is studied. The study helped to find an upper limit of the energy spread of the heavy ions, which can influence the design of the longitudinal optics of the high current injector being developed at the Inter University Accelerator Centre. It is observed that the plasma potentials are decreasing for increasing charge states and a mass effect is clearly observed for the ions with similar operating gas pressures. In the case of gas mixing, it is observed that the plasma potential minimizes at an optimum value of the gas pressure of the mixing gas and the mean charge state maximizes at this value. Details of the measurements carried out as a function of various source parameters and its impact on the longitudinal optics are presented.

The potential of He stripping in heavy ion AMS

The use of helium as a stripper gas for Accelerator Mass Spectrometry (AMS) measurements of heavy ions is presented. At ion stripping energies of about 500keV and below we observe a significant increase of the mean charge state when using helium instead of other gases. Moreover, scattering losses are reduced with helium because of its lower mass compared to other commonly used stripper gases. Thus, highly efficient AMS measurements for 41Ca, 129I and 236U with transmissions through the accelerator in the range of 40–50% are now possible.