Thin Carbon Foils Research Articles

Two-photon decay of the 2 (1)S0 state in He-like bromine.

We report a measurement of the lifetime of the 1s2s $^{1}$${\mathit{S}}_{0}$ state in two-electron ${\mathrm{Br}}^{33+}$. This state decays to the 1${\mathit{s}}^{2}$ $^{1}$${\mathit{S}}_{0}$ ground state only by the simultaneous emission of two photons. Ions in the 1s2s $^{1}$${\mathit{S}}_{0}$ level are produced by excitation of energetic bromine ions in a thin carbon foil. The lifetime is determined by measuring the change in the rate of two-photon coincidences as a function of the foil-detector separation. The lifetime measurement yields 39.32(32) ps in agreement with a theoretical value of 39.63(16) ps.

Charge state dependent energy loss of high velocity oxygen ions in the charge state non-equilibrium region

Mean energy losses of high velocity bare H-like, He-like and Li-like O ions in thin carbon foils were measured in the charge state non-equilibrium region and a clear charge state dependence was observed. The screening effect of bound projectile electrons on energy losses are deduced and compared with theoretical predictions.

The two-dimensional electron momentum density of carbon by angular correlation measurements of Compton scattering

We report on measurements of the angular correlation between inelastically scattered γ-rays and the recoiling electrons in 5 μg/cm 2 thin carbon foils. The coincidence count rate is proportional to the two-dimensional electron momentum density of the target. The experiment was done with 100 keV synchrotron radiation from the DORIS III storage ring at DESY, Hamburg.

Convoy electrons associated with neutral ejectiles

Convoy electrons are important in the understanding of the interaction processes of swift, heavy ions in matter, providing information about the influence of the projectile on the production and transport of electrons through solids. One crucial point of discussion is whether these convoy electrons are created in the bulk or only upon exit of the projectile from the solid. The observation of convoy electrons associated with neutral ejectiles can provide a stringent test to distinguish between bulk and surface models. Forward electron energy spectra in coincidence with the emergent charge states of H + and He + projectiles (100–400 keV/u) impinging upon thin carbon foils have been measured. Convoy electrons were observed associated with neutral as well as charged ejectiles. These results favour bulk models, although the observed yields for neutrals are up to a factor 18 smaller than for charged particles, which indicates that the formation of convoy electrons might occur in the last layers of the solid.

Correlated electron emission from thin carbon foils bombarded by 1.8 MeV/u Ar ions.

A correlation of secondary electrons emitted in the forward and in the backward directions has been studied for 1.8 MeV/u Ar ions bombarding carbon foils. Appreciable correlation has been observed not only for thin targets but also for thick targets where no correlation was expected. It is shown that a classical trajectory Monte Carlo simulation including conversion of bulk plasmons into electron-hole pairs which extends over the target can reproduce the observation qualitatively.

Charge exchange of low energy ions in thin carbon foils. II. Results for ions of B, C, F, Ne, Na, Si, S, Cl, Ar, K, and Fe

In order to calibrate a novel type of time-of-flight mass spectrometer to be flown in the solar wind, we have continued our investigation of the charge exchange of low energy ions passing thin carbon foils. We analyzed elements with widely different chemical properties: Ions of B, C, F, Ne, Na, Si, S, Cl, Ar, K, and Fe in the energy range 0.5–3 keV/u were passed through carbon foils with thicknesses between 1.1 and 10 μg/cm2, and their charge state distributions and residual energies were determined. It was found that (1) the charge state distribution behind the foil is independent of the charge of the incident projectile, (2) isotopes show the same charge exchange properties at equal velocities as we have found previously, (3) at the lowest energies the charge state distribution is no longer a function of the residual energy alone but depends on both residual energy and foil thickness, (4) probable differences in chemical properties between the front and back surfaces of the foil have no detectable influence on the charge exchange properties, and (5) strong electron shell effects manifest themselves when results for different elements are compared.

Doubly differential secondary-electron yields following 8-MeV/uU68+- and 3.5-MeV/uU38+-ion impact on a thin carbon-foil target

Doubly differential secondary-electron yields following the impact of 8-MeV/u ${\mathrm{U}}^{68+}$ and 3.5-MeV/u ${\mathrm{U}}^{38+}$ ions on 44-\ensuremath{\mu}g/${\mathrm{cm}}^{2}$ carbon-foil targets have been investigated experimentally. Electron emission yields were determined for ejection angles varied between 0\ifmmode^\circ\else\textdegree\fi{} and 157\ifmmode^\circ\else\textdegree\fi{} and electron energies ranging from 10 eV to 8 keV. The experimental data are compared to results from a theoretical model for production and transport of fast electrons in solids.

A discussion of the compared advantages and drawbacks of the use of the α parameters matrix effects correction method of PIXE measurements on infinite or intermediate thickness targets

The α parameters method for calculating matrix effects correction factors in PIXE has been initially established for intermediate thickness samples. It consists of the simultaneous acquisition of the PIXE spectrum along with the energy distribution of protons elastically scattered by a thin carbon foil located behind the PIXE target. These experimental data, associated with an experimental measurement of the X-rays transmission factor T, allow the calculation of the matrix correction factors CF. The main interest of the α parameters method is that contrary to other correction methods it does not require a priori knowledge of the major element composition of the matrix. We have demonstrated that the formalism of the α parameters method can be extrapolated to deal with infinitely thick samples. In this case, one obtains α's by two PIXE measurements performed either at different beam energies or in different geometric configurations. In this paper we summarize and compare the two methods for permitting one to choose between them when the type of sample to be studied allows it.

Strong inhibition effect on secondary-electron emission induced by fast hydrogen clusters.

We report on measurement of secondary-electron emission (SEE) induced by hydrogen clusters (H{sub {ital n}}{sup +}, {ital n}{le}19, odd) from thin carbon foils for beam velocities above and around the Bohr velocity. The {ital n} dependence of the SEE yield shows a strong inhibition effect with respect to the atomic case. At a given velocity, the inhibition effect is observed first to increase with {ital n} and then to reach a saturation value ({ital n}{ge}7). These results are connected to the understanding of the inhibition effects observed with molecular beams.

Charge state dependent energy loss of high velocity carbon ions in the charge state non-equilibrium region

Energy losses of high velocity bare H-like and He-like carbon ions passing through thin carbon foils were measured in the charge state non-equilibrium region. The mean energy losses for each charge state increased proportionally to the foil thickness and a clear charge state dependence was observed.

Thickness uniformity and pinhole density analysis of thin carbon foils using incident keV ions

Abstract The detection and characterization of thickness defects (nonuniformities and pinholes) in thin carbon foils is important in applications such as space-based particle analyzers. In this study we present a method of quantifying pinhole locations and sizes using scatter distributions of low energy ions which transit the foils. This technique, which we call transmitted ion mapping (TIM), is particularly suitable for analysis of ultrathin (⩽1 μg/cm2) foils in which thickness defects cannot be detected using optical microscopy due to foil transparency. Additionally, the foils are exposed to ion doses of less than 106 ions/cm2, so the method is nondestructive to the foils. As an illustration of this method, thickness nonuniformities and pinholes are examined in 0.2 and 0.5 μg/cm2 foils using incident 5 keV Ar+, Ne+, He+, and H+ ions.

Production of convoy electrons by foil-excited He ions

Convoy electrons are studied, which are produced by the transmission of 50 MeV 3He 2+ ions through two thin foils of carbon, where the downstream foil is exposed to the beam transmitted through the upstream foil. By eliminating the convoy electrons emitted from the upstream foil by applying an electric field, the convoy electrons produced at the downstream foil are studied. The yields of convoy electrons produced at the downstream foil, which has a thickness of less than the mean free path for convoy electrons (∼ 3 μg/cm 2), are about twice as large as those emitted from a single foil of the same thickness. The increase is attributed to the convoy electrons produced by electron loss to the continuum (ELC) of Rydberg states of 3He + ions produced at the upstream foil.

Energy loss of hydrogen projectiles below the Bohr velocity in amorphous carbon

Electronic stopping cross section data of amorphous carbon for hydrogen projectiles with velocities from 0.35 v Bohr to 0.63 v Bohr are presented. Experiments were done on a set of thin self-supporting carbon foils, utilizing a time-of-flight apparatus. A procedure is used which avoids the main errors inherent to transmission experiments, e.g., that due to target inhomogeneity and that due to target surface contamination. aprotons, deutrons and molecular projectiles were used. We find that incident H 2 + and H 3 + molecules experience a smaller energy loss than protons and deutrons of equal velocity.

Secondary electron emission from a C-foil resulting from the passage of orientation selected He 2+ molecular ions

We have measured the effect of the orientation of the molecular axis on average numbers and number distributions of multiply emitted secondary electrons (MUSE) from a very thin (∼ 50 Å) carbon foil both in the forward and backward directions resulting from the passage of 1 MeV/u He 2 + molecular ions. MUSE was observed in coincidence with a pair of outgoing He 2+ ions detected 3.5 m downstream from the carbon foil. Average numbers of MUSE in the forward and backward directions per incident projectile are 11.5 and 7.5, respectively, when the orientation of the molecular axis is disregarded. Number distributions of MUSE are much wider than the Poisson distribution in any case. They are compared with those which are calculated from the data for 1 MeV/u He + ions. A negative correlation of the number distributions of MUSE in the forward and backward directions was observed. The average number of MUSE in the forward directions is larger when the molecular axis is parallel to the beam direction than when it is perpendicular, but no orientation dependence was observed in the backward direction.

Convoy electrons accompanying neutral ejectiles in ion—solid interactions

Electron energy spectra ( θ = 0°) induced by H + (150 keV) and He + impinging on thin carbon and copper foils (100–500 Å) have been measured in coincidence with the emergent change states q f(H) = 0, 1 and q f(He) = 0, 1, 2 experiments were carried out in an ultrahigh vacuum chamber ( P = 1.5×10 −9 hPa), in order to minimise the charge changing of the ejectiles between the target and their detection in the novel charge state analysing detector UHLAND. The charge states were separated electrostatically and extreme care was taken to prevent them from overlapping. Convoy electrons accompanying the projectiles in low-lying continuum states were detected in coincidence not only with emerging charged ions, but also with neutral ejectiles. These measurements may help to clarify the discrepancies between recent data published by different groups.

Light emission from thin carbon foils bombarded with medium velocity heavy ions

The relative photon intensities of the continuum spectra emitted from the tilted carbon foils have been measured in the wavelength region 300–800 nm by 60 keV/amu Ne + impact at ±30° tilt angles. This result was compared with our previous observations for H + and He + impacts. It was found that, while the photon emission intensities for H + and He + are approximately proportional to the first and second power of the electronic stopping powers, respectively; for Ne + it is proportional to a higher power of the stopping power.

Study of the influence of the electron capture and loss process on the secondary electron emission induced by protons in aluminium

Experiments on the secondary emission of electrons from thin carbon foils bombarded with protons have shown that the forward-backward yield ratio increases with energy below 200 keV. Theoretical calculations for an aluminium target, on the contrary, show that this ratio decreases with energy. To explain this discrepancy, we have included in our Monte Carlo simulation model the electron capture and loss process, assuming that a proton-captured electron system does not excite electrons in the solid. We have also taken into account the partial yield due to the lost electrons. This simple model, applied to aluminium, gives results consistent with the carbon experiments. We have also studied the statistics of the number of secondary electrons accompanying an outgoing proton or hydrogen atom.

Stark beats of Lyman-? emission of foil-excited hydrogen atom

The Stark beats of Lyman-α emission due ton=1 − 2 transition of hydrogen atom have been studied by the beam-foil method. After passage through a thin carbon foil, the static electric field of 500 V/cm was applied to the beam in the direction either parallel to or anti-parallel to the the beam velocity. The linearly polarized emission was measured by using a toroidal mirror at a Brewster's angle reflection. When the direction of the applied electric field is reversed, an appreciable phase shift was observed. The analysis of the data leads to the complete determination of the density matrix of the H(n=2) atoms at the time of their production.

A mass-selective neutral particle energy analyzer with background rejection

A mass-discriminating neutral particle spectrometer has been developed for the Rutherford scattering diagnostic at the TEXTOR tokamak. The analyzer is equipped with a momentum preselector and a triple-coincidence time-of-flight detection system providing a rejection capability for background events. Entering neutral particles are stripped by means of a thin carbon foil. Electrons emitted from a second carbon foil are used to give the time-zero signal. Calibration has been performed for hydrogen and helium particles in the energy range from 10 to 90 keV. The energy loss inside the carbon foils, the absolute efficiency, and the resolution of the analyzer have been investigated. The momentum preselector has a bandwidth of ±12.5% with respect to its adjustable central momentum. For both hydrogen and helium, the energy resolution is 2.5% for energies above 30 keV. Calculations show that the analyzer can still operate in the highly radiative environments of nuclear fusion devices.

Interaction of hydrogen Hn+ clusters with thin carbon foils.

A Monte Carlo computer program was developed to calculate the angular distribution of fragments resulting from the breakup of fast hydrogen clusters ${\mathrm{H}}_{\mathit{n}}^{+}$ when traversing thin amorphous carbon foils. The three-dimensional relative coordinates of each proton in the cluster are given as input. The code begins with a generation of a random orientation of clusters. Next, the foil is divided into slabs and the effects of Coulomb explosion, multiple scattering, and energy loss on each fragment are calculated for successive slabs. The distributions of separations and relative velocities at the exit surface as well as the angular distributions of fragments are then obtained. Therefore, the cluster-stopping-power ratio is given. The use of the code is illustrated in the case of 30-, 40-, 60-, and 80-keV/p clusters, with n=2,3 to 21 odd, impinging on a 2-\ensuremath{\mu}g/${\mathrm{cm}}^{2}$ carbon foil, where experimental results are available.